Will the JWST be affected by dust at L2 (gegenschein?)

Will the JWST be affected by dust at L2 (gegenschein?)

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Gegenschein is a "faint brightening of the night sky" at the anti-solar point. A naked eye limiting magnitude of about 7.6 might enable an observer to make out gegenschein.

The Wikipedia article on gegenschein suggests that interplanetary dust at the Sun-Earth L2 point might be responsible for gegenschein.

The James Webb Space Telescope will also be at the Sun-Earth L2 point.

Does this imply that the same interplanetary dust that may be responsible for gegenschein will affect the JWST?

The Wikipedia article cites "Zdeněk (1962)" for the statement that the dust responsible for the Gegenschein has a possible concentration at L2. I haven't been able to obtain that paper, but I can't really see why that would be the case, since L2 is not dynamically stable.

However, the dust consists of millimeter-sized grain (see e.g. this APOD image), which is quite large for dust. Such large grains probably has a highly anisotropic "phase function", with a high preference for backscattering. Since when you look from Earth toward L2 you have the Sun in your back, you will thus see an increased brightness compared to other directions, even though the density of that dust is the same.

If this is the case, then placing JWST in L2 is no problem (except when it looks exactly away from the Sun, but that would be a problem anywhere in the ecliptic).

Not likely. The sun shade will always be pointed towards Earth and the sun. Thinking about the design, that means the telescope will never be pointed in a direction where it will have gegeschein in its field of view, because otherwise the heating from IR radiation from Earth would cause it problems. The term for the angle between where you're looking and where the sun is is "solar elongation". I don't know what the ranges of solar elongation JWST will be limited to, but I'm sure it excludes a substantial angle around $180^circ$.

The Spitzer space telescope is similarly limited in the parts of the sky it can view at any one time by the need to keep the sun behind it's shade.

The James Webb Space Telescope vs the Hubble Space Telescope

What are the main differences between Hubble and the new James Webb Space Telescope?

The James Webb Space Telescope is planned to be launched by NASA in Spring 2019. It has often been referred to as the successor to the Hubble Space Telescope, launched in 1990, aiming to “complement and extend” its predecessor’s discoveries. However, NASA say that it is unfair to call the James Webb a direct replacement because of the several key differences between each instrument.

Priorities in Space Science Enabled by Nuclear Power and Propulsion (2006)

D Details of Selected Astronomy and Astrophysics Mission Concepts


Radio (&lambda >1 mm) interferometry on interplanetary baselines appears, at first glance, to be an ideal application of the ability of nuclear power and propulsion systems to deploy astronomical assets at great distances from Earth. However, the scattering of radio waves by the interstellar medium blurs radio images to angular sizes much greater than the resolution of a radio interferometer with a baseline of >1 AU.

The &ldquowarm, ionized&rdquo phase of the interstellar medium contains electron density fluctuations on a range of size scales, from smaller than

10 13 m. Evidence implies that the spectrum of density fluctuations is close to the Kolmogorov spectrum, familiar from characterizations of neutral turbulence. These fluctuations scatter radio waves from cosmic sources, causing frequency-dependent phase deviations that ultimately result in interference in the observer plane. This results in a variety of observed phenomena, including amplitude variations in time and frequency, akin to the twinkling of stars due to density inhomogeneities in Earth&rsquos atmosphere. Multipath scattering makes point sources of radio emission appear to have finite angular extent, of full width half maximum (FWHM) &thetas, a result of averaging over short-time-scale image wander. The size of the scattering &ldquodisk&rdquo varies inversely with &upsilon 2 and depends strongly on the line of sight.

The greatest scattering is seen toward the galactic center: Sag A* has an apparent angular extent of 1.3 arcseconds at 1 GHz. Lines of sight perpendicular to the galactic plane show the least scattering, typically

5 milli-arcseconds at 1 GHz. Out of the plane, given the frequency dependence of &thetas, it is not hard to show that regions for which &upsilon < 50 to 150 B (where &upsilon is in hertz and B is the interferometer baseline in meters) have &thetas > &lambda/B, the effective interferometer resolution. This region is shown in Figure 8.1 radio interferometry with B &ge 1 AU is clearly uninteresting.


The impact of natural and human radio emissions on radio astronomy is enormous. The entire radio spectrum from

30 MHz is strongly affected by interfering manmade signals. The region above 300 MHz&mdasheven those bands that have long been allocated specifically for radio astronomers&mdashcontinue to experience tremendous pressure for commercial use. Moreover, Earth&rsquos ionosphere absorbs and refracts radiation below

30 MHz. Additionally, natural sources of interference on Earth&mdashsuch as auroral kilometric radiation, which produces very intense

radiation in the frequency range from 50 to 750 kHz, or lightning, which produces strong interference in the range from 1 to 30 MHz and above&mdashpreclude observations below 30 MHz (the very low frequency [VLF] range) except under exceptional circumstances, or at special locations and for limited amounts of time.

A variety of astronomical phenomena are expected to emit radiation at the wavelengths affected by terrestrial radio noise. These include non-thermal emission from the Milky Way galaxy, pulsars, interstellar scintillation, active galactic nuclei, and clusters of galaxies, as well as the Sun and Jupiter. Much higher up in frequency, neutral atmospheric gases&mdashparticularly atmospheric water vapor&mdashattenuate cosmic radiation increasingly strongly above 10 GHz, with attenuation peaking around 22 GHz. Strong oxygen lines attenuate heavily near 60 and 120 GHz, and water lines around 183 GHz.


For observations at &lambda &asymp 0.2 to 3 &mum, sunlight scattered by zodiacal dust grains is the dominant source of diffuse background emissions and can, hence, be the dominant noise source for observations of faint sources. Observations from Pioneer 10 1 and Helios 1 and 2 2 spacecraft suggest that zodiacal brightness declines with heliocentric distance as Iz &alpha r &ndash2.3 or Iz &alpha r &ndash2.5 . An observatory at 5 AU could have &asymp 50× lower zodiacal background than current or planned ultraviolet/optical/infrared observatories in Earth-trailing or L2 orbits. Reducing the zodiacal background further is of limited use, as diffuse galactic emission and the mean extragalactic flux are &asymp 10 &minus2 of the 1-AU zodiacal background near 800 nm.

Point Sources

For background-limited observations of unresolved sources of specific flux f&upsilon, the signal-to-noise (S/N) ratio acquired in time T from a diffraction-limited telescope of diameter D scales as:

The last factor is the bandwidth of the observation. The lowered zodiacal background at 5 AU could increase observing efficiency by a factor of 50. This gain is realized only when the diffuse background is the dominant noise source. For brighter sources, shot noise in the source photons is dominant. For 2-meter-class visible telescopes at 1 AU, such as the Hubble Space Telescope (HST) or the proposed Supernova/Acceleration Probe, any source brighter than V &asymp 29 mag is brighter than the diffuse background&mdashnearly every star within 10 kpc, for example. The zodiacal brightness in the near-infrared is similar to that in the visible and drops precipitously into the ultraviolet, so it is unlikely that observing beyond 1 AU would be of use in observations of stars in the Milky Way.

Study of stars beyond the Milky Way, for example in elliptical galaxies, requires reaching V > 29 mag. But such observations also require very high angular resolution, much better than that afforded by the HST, to eliminate crowding of stars and resolve the population. Hence an increase in D to improve resolution (and S/N) would be much more useful than a reduction in Iz. There is hence little S/N incentive to move beyond L2 for the observation of point sources at ultraviolet, optical, and infrared wavelengths.

A major thrust of the astronomy and astrophysics (AAp) decadal survey 3 and NASA&rsquos exploration initiative is the detection and study of extrasolar planets. For such observations, there is little S/N incentive to reducing the solar zodiacal background by going to >1-AU orbits, because most of the targets will be embedded in a dust disk about their host stars that is a significantly larger and unavoidable source of background photons. Thus, the first reconnaissance and characterization of extrasolar planets will be done from a near-Earth vantage point.

Resolved Sources

Once the telescope is large enough to resolve the astronomical source, the S/N for objects with surface brightness fainter than the zodiacal background becomes

A glance at the Hubble Ultra Deep Field shows that most of the faint, high-redshift galaxies in the universe are resolved by 2-m telescopes and are fainter than the zodiacal background in the visible. For observations of these very interesting sources, operation at 5 AU can perhaps be equivalent to a 50-fold increase in telescope area (or 7-fold increase in diameter). For a nuclear propulsion system to be useful, its cost and weight would have to be such that placing a 2-m telescope at 5 AU would be much cheaper than placing a 15-m telescope (or 50 2-m telescopes) at L2. Otherwise, one would choose the L2 observatories, which would offer superior S/N for other observations, as well as resolution.

Furthermore, a large telescope is only worth deploying at 5 AU if its purpose is limited to obtaining ultraviolet/optical/infrared spectra of high-redshift galaxies. A 0.1-arcsecond-diameter galaxy with surface brightness 10 times lower than the 1-AU zodiacal light will deliver only &asymp10 &minus3 photons per second to a 6-m aperture in a spectral element with R = &Delta&upsilon/&upsilon &asymp 1,000. Hence a measurement with S/N = 20 would take 400,000 s&mdasheven assuming no deleterious effects of detector noise or radiation events. Imaging observations (R &asymp 10) could more readily profit from the lower zodiacal background at 5 AU.

In terms purely of S/N, therefore, the value of nuclear power systems to ultraviolet/optical/infrared astronomy depends on the cost of the 5-AU location versus the cost of larger apertures at 1 AU, and in any case the S/N gains are likely to be limited to imaging of faint resolved galaxies. Certainly, the priorities of the ultraviolet/optical/infrared community will be served first by building a larger collecting area near 1 AU.


A space telescope located 1 AU from the Sun will have an ambient temperature of approximately 300 K. Observations at wavelengths longer than 1 &mum will be strongly background limited by the telescope&rsquos own thermal emissions. Cooling the telescope&rsquos optical system is clearly highly advantageous. Small, 1-meter-class telescopes can achieve operating temperatures of order of between 4 and 8 K by the use of onboard expendable cryogens. Applying such a cooling strategy to a large astronomical telescope is more problematic. The 6.5-m James Webb Space Telescope (JWST) will use a multilayer sunshield to passively achieve an operating temperature of 40 K at the Sun-Earth L2 point. This temperature is low enough to allow background-limited performance at &lambda < 20 &mum, but for observations at longer wavelengths, still lower temperatures will be required. Current models for the proposed Single Aperture Far-Infrared Observatory (SAFIR) mission&mdashenvisioned as a colder, somewhat larger (

10-m-class), far-infrared version of JWST&mdashindicate total residual heat loads of

1 W for a telescope at 5 to 10 K. Such a heat load could, in principle, be addressed with expendable cryogens, although this approach would require some 30 liters of liquid helium per day. Such a consumption rate would lead to unreasonable masses of cryogen, and active cooling with onboard refrigerators is considered to be the best way to ensure long observatory lifetime. As described below, SAFIR is taken to be representative of the requirements for a class of large thermal-infrared space telescopes.

Space-qualified cryocoolers have been developed for both infrared and x-ray applications. Such low-temperature refrigeration systems do not rely on consumables and can be understood to provide a failure-limited observatory lifetime. These systems do, however, have substantial power requirements, and nuclear power sources can be considered as potentially enabling for such missions. For the low temperatures required by SAFIR, cryocooler efficiency is low, and a ratio of compressor input power to cooling power on the order of 1,500 is expected. Using the residual heat load referred to above, this would require on the order of 1.5 kW of

NASA Engineers Complete First 'Center of Curvature' Test on James Webb Space Telescope

Engineers conducting a white light inspection of the James Webb Space Telescope, currently located in the clean room at NASA’s Goddard Space Flight Center in Greenbelt, Md. Photo Credit: NASA/Chris Gunn

The James Webb Space Telescope (JWST) has completed another significant milestone toward becoming the most powerful space telescope ever built: the finished primary mirror just underwent an optical measurement test called the Center of Curvature test. In essence, this is a “before” and “after” measurement of the mirror, both before the telescope undergoes more rigorous mechanical testing which could affect the mirror’s capabilities, and then again after.

According to Ritva Keski-Kuha, the test lead and NASA’s Deputy Telescope Manager for JWST at NASA’s Goddard Space Flight Center in Greenbelt, Md.: “This is the only test of the entire mirror where we can use the same equipment during a before and after test. This test will show if there are any changes or damages to the optical system.”

The mechanical tests are essential before the telescope is launched, scheduled for October 2018, since the telescope will experience violent sound and vibration environments inside the rocket. The shape or alignment of the mirror could be affected or even adversely affect its performance once JWST is in space.

The same optical measurement is then taken after the mechanical tests for comparison, to ensure that the telescope can survive the launch and function properly in orbit.

The five layers of the sunshield, which will protect the telescope in space. Photo Credit: Northrop Grumman

The optical measurement tests consist of using an inferometer to measure the shape of JWST’s primary mirror with incredible precision. The optics in the mirror need to be extremely accurate, even more than waves of visible light which are less than a thousandth of a millimeter long. By using wavelengths of light to make very tiny measurements, engineers can avoid physical contact with the mirror, reducing the chances of any physical damage occurring such as scratches. The inferometer records and measures the tiny ripple patterns which result from different beams of light mixing and their waves combining or “interfering” with each other.

More specifically, the Center of Curvature test measures the shape of the primary mirror by comparing the light reflected off it to a computer-generated hologram depicting what the exact shape should be the inferometer compares the two with astounding precision.

“Interferometry using a computer-generated hologram is a classic modern optical test used to measure mirrors,” said Keski-Kuha.

“We have spent the last four years preparing for this test,” said David Chaney, who is JWST’s primary mirror metrology lead at Goddard. “The challenges of this test include the large size of the primary mirror, the long radius of curvature, and the background noise. Our test is so sensitive we can measure the vibrations of the mirrors due to people talking in the room.”

After engineers make sure that the mirrors are perfectly aligned in the first Center of Curvature test, the launch environmental tests will follow. Then the Center of Curvature test will be repeated and compared to the first test to ensure that the mirrors remain aligned.

The primary mirror actually consists of 18 smaller hexagonal mirrors , making it kind of look like a giant puzzle piece. These mirrors will allow JWST to see deeper into space (and thus further back in time) than ever before, to when the first stars and galaxies were forming. Infrared sensitivity will help astronomers compare them to today’s largest galaxies.

Last month, a sunshield , consisting of five sunshield membrane layers, was completed on the telescope. This sunshield, designed by Northrop Grumman in Redondo Beach, Calif., will prevent background heat from the Sun from interfering with the telescope’s infrared sensors. Each of the five layers is as thin as a human hair and the entire sunshield is the size of a tennis court. The layers in the sunshield can reduce temperatures by approximately 570 degrees Fahrenheit, and each successive layer, made of kapton, is cooler than the one below. The final layer was delivered to Northrop Grumman Corporation’s Space Park facility on Sept. 29, 2016. Protecting the telescope when it is in space is of course just as important as during the launch.

Another view of the completed primary mirror of JWST, consisting of 18 smaller hexagonal mirrors. Photo Credit: NASA/Chris Gunn

“The completed sunshield membranes are the culmination of years of collaborative effort by the NeXolve, Northrop Grumman and NASA team,” said James Cooper, JWST Sunshield manager at Goddard. “All five layers are beautifully executed and exceed their requirements. This is another big milestone for the Webb telescope project.”

The sunshield and the rest of the telescope will fold origami-style into the Ariane 5 rocket for launch.

“The groundbreaking sunshield design will assist in providing the imaging of the formation of stars and galaxies more than 13.5 billion years ago,” said Jim Flynn, Webb sunshield manager at Northrop Grumman Aerospace Systems. “The delivery of this final flight sunshield membrane is a significant milestone as we prepare for 2018 launch.”

As Greg Laue, sunshield program manager at NeXolve, also noted, “The five tennis court-sized sunshield membranes took more than three years to complete and represents a decade of design, development and manufacturing.”

As reported earlier this year, the Integrated Science Instrument Module (ISIM), known as the “scientific heart” of JWST, completed its last round of essential cryogenic tests .

According to Begoña Vila, NASA’s Cryogenic Test Lead for the ISIM at Goddard: “We needed to test these instruments against the cold because one of the more difficult things on this project is that we are operating at very cold temperatures. We needed to make sure everything moves and behaves the way we expect them to in space. Everything has to be very precisely aligned for the cameras to take their measurements at those cold temperatures [for] which they are optimized.”

Diagram showing different parts of JWST. Image Credit: NASA

Some of the JWST team members outside a full-scale model of the telescope at Goddard Space Flight Center. Photo Credit: NASA

The JWST mission, often dubbed as the successor to the Hubble Space Telescope, will be an exciting one, allowing astronomers to learn even more about distant galaxies and exoplanets. JWST will look at distant exoplanets and the dust clouds where new stars and planets are being born, as well as search for the molecular building blocks of life. It will be able to directly image some larger exoplanets orbiting brighter stars by using coronagraphs, and it will also be able to study the atmospheres of those exoplanets. The telescope is named after a former NASA administrator, James Webb.

“The James Webb Space Telescope will be the premier astronomical observatory of the next decade,” said John Grunsfeld, astronaut and associate administrator of the Science Mission Directorate at NASA Headquarters in Washington. “This first-mirror installation milestone symbolizes all the new and specialized technology that was developed to enable the observatory to study the first stars and galaxies, examine the formation stellar systems and planetary formation, provide answers to the evolution of our own Solar System, and make the next big steps in the search for life beyond Earth on exoplanets.”

The James Webb Space Telescope is a joint project of NASA, the European Space Agency and the Canadian Space Agency. More information is available at two NASA websites, here and here .

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From Spitzer to JWST’s Early Targets

Yesterday’s post on the Spitzer Space Telescope leads naturally to the targets it produced for its successor. For when Spitzer’s mission ends on January 30, we have the far more powerful James Webb Space Telescope, also operating at infrared wavelengths, in queue for a 2021 launch. In many ways, Spitzer has been the necessary precursor for JWST, for it was the need to operate a telescope at extremely low temperatures in order to maximize infrared sensitivity that drove Spitzer design. JWST must maintain its gold-coated beryllium mirror at similarly precise temperatures.

With over 8,700 scientific papers published based on Spitzer findings, a number that will continue to grow for many years, a path has been charted that JWST will follow in the form of observations early in its mission. Consider WASP-18b, a gas giant of ten times Jupiter mass in a tight orbit around its star. Data from both Spitzer and Hubble showed in 2017 that the planet is laden with carbon monoxide and all but devoid of water vapor. No other extrasolar planet can match this one in the way carbon monoxide dominates its upper atmosphere.

What’s going on in the atmosphere of this planet merits close study because it’s extreme even for ‘hot Jupiters’ in being so close to its star that it may not survive another million years. Expect a long look from JWST into the processes at work here. Nikku Madhusudhan (University of Cambridge) was a co-author on the 2017 paper describing the WASP-18b findings:

“The only consistent explanation for the data is an overabundance of carbon monoxide and very little water vapor in the atmosphere of WASP-18b, in addition to the presence of a stratosphere. This rare combination of factors opens a new window into our understanding of physicochemical processes in exoplanetary atmospheres.”

The image below implies the method: Transmission spectroscopy. We can look at the light of the star passing through the atmosphere of the planet as it moves around the star in its orbit.

Image: A NASA-led team of scientists determined that WASP-18b, a “hot Jupiter” located 325 light-years from Earth, has a stratosphere that’s loaded with carbon monoxide, or CO, but has no signs of water. Credit: Goddard Space Flight Center.

At TRAPPIST-1, expect the fourth planet, TRAPPIST-1e, to receive early JWST scrutiny because of its density and surface gravity, both similar to Earth’s, in combination with incoming stellar flux sufficient to keep temperatures in the range needed for water on the surface. JWST should be able to tell us whether this planet does indeed have an atmosphere, and assuming it does, whether molecules like carbon dioxide or water vapor are present.

Here again Spitzer helped set the table, working with ground-based telescopes to confirm the first two candidates (found by the Transiting Planets and Planetesimals Small Telescope in Chile) and discover the other five. Our ideas of what these planets look like will change with the new data JWST, 1000 times more powerful than Spitzer, will bring in. The Hubble instrument has not been able to detect evidence for a hydrogen-dominated atmosphere on TRAPPIST-1d, e and f, making rocky composition likely. But it’s going to take JWST to further clarify the presence of atmospheres on the seven worlds and begin the study of their chemistry.

All of that should take what is a very fanciful image (below) and help us determine how far from reality it actually is. At present we’re simply injecting sparse data into the realm of art. As Nikole Lewis (Cornell University) says, “The diversity of atmospheres around terrestrial worlds is probably beyond our wildest imaginations. Getting any information about air on these planets is going to be very useful.”

Image: This artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. Credit: NASA/JPL-Caltech.

Spitzer’s work on 55 Cancri e will also inform early JWST studies of the system. Spitzer was able to produce data leading to the first temperature map of a super-Earth, in this case an apparently rocky world about twice the size of our own. Lava flows may be the cause of the extreme temperature swings between one side of the planet and the other, as noted in 2016 by Brice Olivier Demory (University of Cambridge), who was lead author of the paper in Nature:

“Our view of this planet keeps evolving. The latest findings tell us the planet has hot nights and significantly hotter days. This indicates the planet inefficiently transports heat around the planet. We propose this could be explained by an atmosphere that would exist only on the day side of the planet, or by lava flows at the planet surface.”

Spitzer put in 80 hours of infrared telescope time in the 55 Cancri e work, watching this tidally locked world move about its star and allowing the construction of the temperature map. Mission scientists pushed Spitzer hard in accumulating their data, using novel calibration techniques to extract maximum results from a detector that had not been design to work at such high precision. Now JWST will help sharpen the map’s focus to explain its unusual temperature swings, which argue against a thick atmosphere and global temperature distribution.

Image: 55 Cancri e is tidally locked to its star, just as our moon is to Earth, which means that one side always sizzles under the heat of its star while the other side remains in the dark. If the planet were covered in lava, then the hot, sun-facing side of the planet would have liquid lava flows, while the colder, dark side would see solidified lava rock. The hardened lava would be unable to transport heat across the planet, explaining why Spitzer detected that the cold side of the planet is much colder than the hot side. Such a lava planet, if it exists, would have dust streaming off of it, as illustrated here. Radiation and winds from the nearby star would blow off the material. Scientists say that future observations with NASA’s upcoming James Webb Space Telescope should provide more details about the nature of this exotic world. Credit: NASA/JPL-Caltech.

Both Spitzer and Webb are sensitive to the infrared glow of gas and dust orbiting in circumstellar rings around stars, which means JWST will be able to extend our knowledge of planetary formation. The same sensitivity will make JWST the instrument of choice in the study of brown dwarfs, an area where Spitzer has already been able to examine clouds in brown dwarf atmospheres. What’s interesting here are the differences between the distribution and motion of clouds on brown dwarfs and the atmospheric boiling seen on true stars. JWST will investigate winds that seem reminiscent of the belts of Jupiter, Saturn, Uranus and Neptune.

I’ve focused on exoplanetary targets here, but of course the handoff from Spitzer to JWST will also involve using the large surveys of Spitzer and the Hubble instrument to furnish JWST with targets like GN-z11, now the most distant galaxy ever measured. Sean Carey is manager of the Spitzer Science Center at Caltech/IPAC in Pasadena:

“Spitzer surveyed thousands of galaxies, mapped the Milky Way and performed other groundbreaking feats by looking at large areas of the sky. Webb won’t have this capability, but it will revisit some of the most interesting targets in the Spitzer surveys to reveal them in amazing clarity.”

JWST’s higher sensitivity should make it possible to find galaxies that are even older. It will also home in on luminous infrared galaxies (LIRG), which Spitzer found to be producing far more energy per second than typical galaxies, most of it in the form of far-infrared light. Star formation and galactic mergers come into focus, as does the growth of supermassive black holes. All of this depends, of course, on getting a fabulously complex telescope plagued by cost overruns into its future home at the L2 Lagrangian point 1.5 million kilometers from the Earth.

All launches are scary, but this one more than most. We need Spitzer’s successor to fly.

Comments on this entry are closed.

“Data from both Spitzer and Hubble showed in 2017 that the planet is laden with carbon monoxide …” carbon monoxide ??
Not carbon dioxide ??

The James Webb Telescope will see Earth-like worlds

Artist conception of the James Webb Space Telescope. Credit: NASA

The James Web Space Telescope has been in the news a lot lately.  Often referred to as the replacement for the Hubble Space Telescope, its existence has been in jeopardy since a House committee voted to cut its funding this summer.  While the telescope promises to revolutionize space science, its expanding budget has caused politicians and others to wonder if the promised returns justify the cost.

The JWST is not merely an upgraded version of Hubble.  Rather than measure visible or ultraviolet light like Hubble does, JWST will detect infrared wavelengths from 0.6 (orange light) to 28 micrometers (deep infrared radiation of about 100 K (� °C or  � °F)). 

Because JWST will be looking for heat, the telescope has to be kept very cold, and shielded from radiation coming from the Sun, Earth and Moon. To keep the telescope’s temperature down to 40 degrees Kelvin (� °C or � °F), JWST will have a large sunshield and will orbit the Sun at Lagrange Point 2. The orbit of JWST will be 1,500,000 kilometers (930,000 miles) from the Earth, nearly 4 times farther than the distance between the Earth and the Moon. The balance of gravitational forces from the Earth and Sun at the L2 point will keep JWST in a stable orbit without having to expend much energy.  However, this great distance also means servicing or repairing the telescope after launch may not be possible.

“JWST’s complexity, with large deployable optics and other systems operating at 40 K in an environment precluding any repair or servicing missions, has probably created one of the world’s most complex and expensive integration and test programs,” Michael Kaplan, NASA’s first Program Executive for the James Webb Space Telescope program, wrote recently in The Space Review. While this testing protocol is a part of the ballooning budget, the Independent Comprehensive Review Panel Report, issued in late 2010, said the main problem was that the necessary development costs had not been properly estimated, and the budget therefore had been unrealistic.

The James Webb Space Telescope will be in an L2 orbit. Credit: ESA

Five years ago, the project was estimated to cost 2.4 billion dollars, but the latest reports peg the total at closer to 8.7 billion. This July, the House of Representatives’ appropriations committee on Commerce, Justice, and Science moved to cancel the project by taking $1.9 billion out of NASA’s 2012 budget. Maryland Senator Barbara Mikulski is now the project’s main defender in Congress, since, as is the case with Hubble, the Science and Operations Center for JWST is the Baltimore-based Space Telescope Science Institute. (JWST development is led by NASA's Goddard Space Flight Center, and the telescope is being built by Northrop Grumman Aerospace Systems for NASA, the European Space Agency, and the Canadian Space Agency.) While Congress deliberates the issue, NASA administrator Charlie Bolden said JWST is one of the agency’s top priorities. He added that NASA needed to look across all its programs to find funding for JWST as well as its other two priorities: sending commercial crews to the ISS, and developing the next generation Space Shuttle: the Space Launch System (SLS).

Due to the size of the budget, the journal Nature called JWST "the telescope that ate astronomy". Yet even if JWST is canceled, the money won’t be given to other astronomical projects -- instead, under the recommendations of the House appropriations committee, the funding would be entirely eliminated from NASA’s budget. 

As of this writing an estimated $3.5 billion has been spent on the JWST project, with about 3/4 of the construction and testing completed.  If JWST is not canceled by Congress, it is scheduled to launch in 2018 on the European Space Agency’s Ariane V rocket.

The goal of the JWST is to search for the first stars and galaxies that formed after the Big Bang, and study the formation and evolution of galaxies, stars, and planetary systems.  According to Matt Mountain, director of the Space Telescope Science Institute, and John Grunsfeld, former NASA astronaut and STScI deputy director, JWST also will be able to search for and study planets orbiting distant stars in a way that no other telescope can.  Astrobiology Magazine editor Leslie Mullen recently talked with Mountain and Grunsfeld about what JWST could reveal about habitable worlds in our galaxy.

Q: The James Webb Space Telescope should be able to find exoplanets, and now there’s talk that a star shade could extend this capability. Could you tell me more about that?  

Matt Mountain (MM): The whole idea with the star shade is, once we get James Webb up there and working, then you can launch the star shade and it floats in front of it, a hundred thousand miles away from us.  It’s an autonomous vehicle that you keep lined up.

Q: Can you explain what the star shade actually does?

MM: It’s like putting your thumb in front of the Sun -- it creates a shadow.  It’s very carefully shaped, so you don’t get the sort of flaring that you normally get when you use a perfect sphere, where you get all these rings and refractions. These petals are designed to create a very smooth, very deep shadow.  You basically slide in and out of the shadow, and then you can actually see the planet next to the star.  The star is in the shadow, and the planet peeks around the shadow.

Q: You can see any sort of planets with that?

MM: Any planet that’s within 1 AU, like a habitable zone, or [farther] out.

John Grunsfeld (JG): James Webb is sold as studying galaxies, but I think it’s greatest discovery may be a habitable Earth-like exoplanet.  That’s what’s going to blow everybody away.

Q: So you’d be able to directly image a terrestrial planet, which has never been done before?

JG: Exactly.  But it wouldn’t be like a Rand McNally map, it would be a spot.  But because you’d see a spot, we can then do a spectrum of that spot.

MM: You’d actually get a color.  If it’s like Earth, it’ll look blue.

JG:  And, if you had enough time, and there were seasons, with ice covering and then going away, you could study it and be able to tell the difference between winter and summer on the planet, or vegetation, in principle. Just from unresolved single pixels, because of the color changes.

MM: Or you could tell it is rotating. 

If the Hubble Space Telescope’s mirror were scaled to be large enough for Webb, it would be too heavy to launch into orbit. The Webb mirrors are made from beryllium, which make them strong but very light. The mirror segments fold up so they can fit into a rocket, and then will unfold after launch. Credit: NASA

Q: Is there a limit to the kinds of stars JWST can target for planet searches?

MM: You can only look at stars out to a certain distance, to about 10 or 20 parsecs.   But that’s ok, because the planets [farther out] are too faint anyway.  Any farther away and we can’t differentiate them, both the planet and the star will be hidden by the star shade. 

Q: Could James Webb confirm the exoplanet candidates discovered with the Kepler space telescope?

MM: Not the Kepler set, because they’re all very distant.

Q: They’re not within 10 to 20 parsecs?

JG: They picked them not because they were close to Earth, but in a sense because they were far away.  Not all of them, but in that one field [where Kepler looks], there were lots and lots of stars.  And you say why didn’t they point toward the center of the Milky Way galaxy, but that’s too many stars.  So their Goldilocks Zone is the number of stars in their field of view that was just about right so they can study 150,000 stars in one go.  In order to see the nearest thousand stars around Earth, you have to look in all directions, because they’re spherically all around us.

Q: And James Webb will be able to see that whole sphere?

A star shade flying in front of the JWST could help the telescope see details of non-transiting Earth-like planets. Credit: University of Colorado / Nothrup Grumman

MM: Yes, over the course of a year it can sample the entire sphere and tens of possible habitable stars: 20, 30, 40. You can search the nearest 20 to 30 solar systems, and based on current assumptions there is a significant likelihood you could find 5 Earth-like planets. You can make 9 to 10 repeat observations, which is what will be needed to confirm 5 Earth-like planets.

JG: But here’s the key, and this is what James Webb can uniquely do.  Let’s say these cubesats [small satellites designed to hunt for exoplanets] identify in the hundred or so stars that are close to us, based on transits [when a planet passes in front of its star], Earth-sized planets in habitable zones around those stars.  That’s all we can learn.  We may be able to learn how far away they are from their parent star, their mass and diameter.  Those are all good things.  And you say, gosh, that looks a lot like Earth.  But does it have oceans?  Does it have an atmosphere? Are there any chemical signs of interest there?  That’s where James Webb comes in, and that’s what James Webb will be able to do with or without a star shade for a subset.  The star shade expands that remarkably.  It allows you to see the atmosphere of planets that are orbiting nearby stars.

MM:  We’ve already shown with the modeling that if there’s a SuperEarth with the right orientation, where the planet transits the star, we can detect liquid water. You can look at the spectra for water in the atmosphere and other things.

Q: Why can see only a subset of planets, but you need a star shade for the others?

JG: You can do atmospheric transmission from a transit.  Now, what if it’s not transiting?  Then what do you do?  James Webb needs a star shade for those others.

MM:  If you think of it geometrically, the orientation’s got to be right.  If you do the statistics, you’ll measure a transit for 5 to 7 percent of stars.  Well, 5 to 7 percent is already a small number, and there’s a small number of Earth-like planets, and a small number of those will be in the habitable zone.  But what you really want to do is a census of all the nearby stars, and take spectra directly, independent of the orientation.  The star shade enables that. 

Q: It seems one of the difficulties of JWST is the technology that needed to be developed beyond what had been done for previous telescopes.

MM: The technology has all been going really well.  One of the most challenging things was the mirrors.  Each individual mirror on James Webb is close to a Hubble.  We have all the mirrors.  Half of them are in boxes, and they’re all ready to roll. 

Q: The James Webb telescope is projected to cost about 8.7 billion.  How much did Hubble cost?

JG: About 6 billion in today’s dollars, not including science operations. James Webb started in earnest in about year 2000, and it’ll take about 15 years to complete, and then test and launch.  Hubble started in 1975, and it took about 15 years to assemble, test and launch.  Hubble is 26,500 pounds, and James Webb is about half of that, but more complex.  To build something brand-new, technologically advanced, pushing the envelope in its time, it’ll cost a quantum unit, and if you want to repeat that experience, inflating to current-year dollars, if it costs vastly different, more or less, you either have a breakthrough, which everyone will then want to figure out why it cost so little, or you’re doing something wrong.  And I think the miraculous thing about James Webb is, it’s much harder than Hubble.  It has to operate close to absolute zero, 40 Kelvin, a million kilometers from Earth, not in Earth orbit, it can’t be fixed, and it doesn’t fit in the rocket it launches on -- it has to unfold when it gets there.

Q: I worry about it not unfolding properly after launch. That’s always a risk for these space missions, isn’t it?

MM:  Those technologies are the ones we inherited from industry.  There’s no way you can launch a 19-meter antenna. But you need two 19-meter antennas if you want to transmit HDTV and internet from geostationary orbit, where you make real money. Such deployable antennas are the provenance for the star-shade technology. For JWST it’s this plus a whole raft of large deployable technologies aerospace companies use.

JG: And of course, everybody needs HDTV.  CNN reported that this year, the television rights for advertising just for college basketball exceeded NASA’s budget.  The rights for college football is much more.  So what are our priorities?   When you heard about bank bailouts and these kinds of things, we could have fleets of James Webbs for the AIG bail-out.  Obviously that was a national priority to prevent collapse -- or at least that was the story -- but those were very big numbers, and these are very small numbers.

Q: Would you ever want James Webb to have commercial partners?

MM: No, it’s just a government project.

Q: But why not?  Seems everything in space is going commercial these days.

MM: What is James Webb for?  It’s pure science.  It’s part of the scientific endeavor.  It’s like Hubble.  Hubble was paid for by the taxpayer.  How many kids did we turn on to science because they saw a great Hubble picture? In Hubble we make two discoveries a day.  People shouldn’t be paying for those images because the taxpayer already paid for it upfront.

JG: The whole point is, right now the commercial space industry for human spaceflight, or even commercial orbital transportation of cargo for human space flight at NASA, it’s almost fully subsidized by NASA.  Companies are saying it’s commercial.  But none of them would exist if it wasn’t for NASA putting money up front.  Not tens of dollars, but hundreds of millions of dollars.  And the reason is, and this commercial drive is, is the hope that they will spur on industries that are subsidized. 

And so for instance, the companies that are building James Webb Space Telescope, NASA is paying them to build it, and NASA is paying the other companies to develop technologies for James Webb Space Telescope, and it’s to do pure science.  It’s to discover an Earth-like planet around nearby stars, to discover the very first stars that ever formed in the universe -- these incredible things.  What does that do for us?  Well, for the company that’s building it, when they go to build the next communications satellite, or the next spy satellite, or the next commercial satellite for something else, they have all that expertise now that allows them to build it that otherwise they would not have been able to invest in.  Because they weren’t trying hard things.  If the United States doesn’t try to do hard things and really interesting things, all we’re going to end up with is college basketball, professional sports, entertainment, and service [industries]. 

MM: And the other thing is, it inspires people to say, ‘do want to work on the telescope that’s going to find the first life?’  It doesn’t matter if you work for NASA or a commercial company, that gets the engineers interested.  The company brings in very smart people, and what’s more, they can talk about what they do.  Because when they’re working on a spy satellite, they can’t even tell their kids what they do.  But to be able to tell their kids and their spouses that they are working on the forefront that’s going to change the world.   the companies always tell you they’re incredibly proud to be associated with these missions.   And they even occasionally make money.  They’re getting a huge return on their investment.  Congress several years ago worked out that for every dollar invested in space science, 7 dollars got returned to the US economy over a period of 20 years, from the sheer technology flow.  In Europe they made a similar calculation, that for every dollar they invested in big science in Europe, roughly 3 to 4 dollars got returned to the European economy over the same period because of the improvements. 

You know, if we hadn’t funded this crazy German who talked about relativity and bending the light, or this crazy guy who wanted to build a device based on that strange quantum theory thing and all he talked about was dead cats in a box -- I mean, what was he talking about?  But because they funded those things, we now have GPS. 

JG: All of news about the James Webb Space Telescope over the past year has been about how it’s expensive, it’s going to take a long time, and if only we didn’t have to spend that money, we’d be able to do all these great scientific things.  And what has been lost for some reason that I don’t understand -- maybe because I’m a newcomer to the project -- is that, yes it’s expensive, but it’s also, enormously by many factors, the most capable astronomical facility that we’ll ever have.  And in the fields of dark energy, dark matter, all of astrophysics, but specifically in exoplanets, it does all the things that everybody is saying, ‘oh, if only we didn’t have James Webb we could do this.’  But James Webb does it, and does it better than they think the new missions might.  So it’s very frustrating.

Goodbye Hubble?

  • topic starter

Death knell for space telescope

Without the shuttle, Hubble will last only a few years
Nasa is halting all space shuttle missions to service the Hubble Space Telescope - a move that will put it out of action within four years.
The shuttle craft that maintain Hubble are being retired in 2010 under the new US space goals which focus on voyages to the Moon and Mars.

"This is a sad day," said Nasa's chief scientist John Grunsfeld, but "the best thing for the space community".

Hubble has revolutionised the study of astronomy since its launch in 1990.

It has sent a steady stream of striking images of space back to Earth from its orbit.

The announcement that the telescope would be left to degrade comes as astronomers revealed details of a new image produced by Hubble of the deepest view ever of the cosmos, detecting the youngest and most distant galaxies ever seen.

The image - the result of an unprecedented long look of 80 days at just one patch of sky - will be released in February and will be a major advance in our understanding of the cosmos, says BBC's News Online's science editor Dr David Whitehouse.

The shuttle is also gradually being wound down, and virtually all remaining flights until it goes out of service in 2010 will be used to complete the International Space Station.

Scientists say Hubble captured the "best ever" image of Mars
It also:
Gave us the age of the Universe
Provided proof of black holes
Gave first views of star birth
Showed how stars die
Caught spectacular views of Comet Shoemaker-Levy 9's collision with Jupiter
Confirmed that quasars are galactic nuclei powered by black holes
Gathered evidence that the expansion of the Universe is accelerating

Servicing missions are required to the Hubble every few years to replace worn-out parts. Flights have been halted since the explosion of the Columbia shuttle a year ago, delaying replacement of the telescope's ailing gyroscopes.

Without such maintenance, Hubble should continue operating until 2008 but would eventually come back to Earth in about 2011, Mr Grunsfeld said.

"We will get as much life as we can out of the Hubble telescope, and we will continue to support research and analysis even after re-entry," he said.

The images it has beamed back to earth have determined the age of the universe - over 13 billion years old - and discovered that a mysterious energy is causing all of objects in the universe to move apart ever quicker.

The space agency was already planning to replace Hubble with a new improved telescope in 2011, but it is unclear whether that project will be affected

#2 Guest_**DONOTDELETE**_*

  • topic starter

NASA has (had?) become a maintainer of antiques. What would have been the cost of another shuttle mission, an antique itself, to fix and upgrade the Hubble versus the flight of a new telescope? Or, a robotic mission to recover the Hubble to return to earth?

Returning it to earth has value in seeing what years in space will do to future equipment, and the PR/mueseum value of it would be incredible. As it sits today, Hubble will become upper atmospheric dust, and they won't risk people in the Shuttle to recover an artifact.

I vote for bringing Hubble home.

#3 Guest_**DONOTDELETE**_*

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#4 Peter Argenziano

I agree that HST would make a nice addition to the Smithsonian's new Air & Space museum at Dulles.

Hopefully HST will remain operational until the JWST is launched in 2009. HST is expected to end its mission when it encounters the Earth's atmosphere in 2011, if it stays operational until then without any more servicing missions.

I understand it was a tough decision. There are many projects, and limited funding. Given an either-or choice, I would vote for funding a new project instead of retrieving HST. It has served the science community well these past 13+ years. let's hope she still has another 5 in her!

Also the Spitzer Space Telescope (formerly SIRTF) could remain operational until 2008.

#5 rboe

#6 matt

Maybe they'll change their mind. A few months ago NASA's position was they were concerned that the HST would become a hazard on reentry (its two-ton mirror, for one, should arrive almost intact to the ground, flattening anything at he wrong place at the wrong time), so they could use a mission both to put a motor to control reentry (i.e. crash it where they want it to) and make a last upgrade to the HST itself to keep it running a few more years.

The scientific comunity's opinion is that Hubble should remain up there until it is 'relieved of its duties' by the Webb Space Telescope.

#7 desertstars

#8 Guest_**DONOTDELETE**_*

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Maybe they'll change their mind. A few months ago NASA's position was they were concerned that the HST would become a hazard on reentry (its two-ton mirror, for one, should arrive almost intact to the ground, flattening anything at he wrong place at the wrong time), so they could use a mission both to put a motor to control reentry (i.e. crash it where they want it to)

#9 matt

#10 rboe

#11 Guest_**DONOTDELETE**_*

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#12 Dave Mitsky

Maybe they'll change their mind. A few months ago NASA's position was they were concerned that the HST would become a hazard on reentry (its two-ton mirror, for one, should arrive almost intact to the ground, flattening anything at he wrong place at the wrong time), so they could use a mission both to put a motor to control reentry (i.e. crash it where they want it to) and make a last upgrade to the HST itself to keep it running a few more years.

The scientific comunity's opinion is that Hubble should remain up there until it is 'relieved of its duties' by the Webb Space Telescope.

There is no guarantee that the James Webb Space Telescope will function flawlessly and since it will orbit at the L2 point no repair will be possible. If the HST is allowed to deorbit and its replacement is a failure, what then NASA?

#13 rboe

#14 Guest_**DONOTDELETE**_*

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Maybe we can do something about it!

Please follow this link to an article on The S&T website that shows how to contact Congress so as to make your feelings known on the demise of the Hubble. Thanks.

#15 Guest_**DONOTDELETE**_*

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This may be heresey, but the problem with NASA is it is run by a bunch of astronauts. They have to have a manned space program to justify their existance and their salaries. I'd rather have a fleet of space probes and a couple of scopes in orbit than the ISS or a lunar base. Think of the advances in AI that could result from having better, automated probes. Imagine having a new planetary shot every few months instead of every few years. Imagine the savings if they built a universal probe chassis, and added custom sensors instead of building each one or two from scratch.

Somebody name ONE thing that's been discovered with the ISS. They've been doing the same experiments in zero-gee (or microgravity, if you prefer) that they've been doing since the 60's. No new science, no new technology, just a big giant money pit. But, oooh, we have astronauts in orbit. Who spend most of their time fixing things that are broke.

What, I ask, is the purpose of manned space travel at this point in time? There isn't one. The moon is dead, and we can't yet safely get anywhere else. Mars is most likely dead. New theories are developing that allow the formation of channels and "flood plains" on Mars without the requirement for liquid water, a warm, wet Mars, or anything of that nature. The existance of sedimentary rock would confirm the existance of liquid water on Mars at some point in its past. Lets look for sedimentary rock, with more that two probes, and if we find it, then we should think about sending astronauts to Mars (though I'd still rather send an automated, AI return mission).

In the mean time, we should be doing real science with the taxpayer's dollars, not rehashing experiments from the 60s. We should replace the Hubble and the Shuttle, though the Hubble should be kept online until its replacement is operational. We should put a few dozen smaller scopes in orbit. Let's put some 16" Schmidt-Cassegrains in orbit with digital cameras. We could be doing hundreds or thousands of oberservations of variable stars every day, and be taking spectra of thousands of other stars. NASA should be doing real science, not squandering resources.

Q & A with Dr. John Mather on the James Webb Space Telescope

The James Webb Space Telescope (JWST) is the much anticipated, long awaited “next generation” telescope. Planned for launch in 2013 October 2018, JWST has been touted as the successor to the Hubble Space Telescope. With it, astronomers hope to look back in time to when the universe was just 200 million years old, and see the first stars and galaxies. The lead scientist guiding this project is Dr. John Mather, co-recipient of the 2006 Nobel Prize in Physics for his work with the Cosmic Background Explorer (COBE), which measured the black body form and anisotropy of the cosmic microwave background.

We were understandably honored when Dr. Mather contacted Universe Today, saying he’d like to talk with us about the status of JWST. “I figured it might be time to talk about what we’re doing,” he said, “because exciting things are starting to happen.”

Dr. John Mather. Credit: (NASA/Bill Ingalls)
Universe Today: Dr. Mather, for over a decade we’ve been hearing about the Next Generation Space Telescope, which later was officially named the James Webb Space Telescope. Can you tell us how the concept for this telescope began?

John Mather: In 1989, even before the Hubble was launched, a conference was held about what the next space telescope should be. They discussed great telescopes of the future and from the proceedings, published a book. But they really didn’t consider that infrared was the great wave of the future. Then, in 1993, there was a committee called HST and Beyond. They published a lovely little report in 1996 that said there were two important things to do. One was to build an infrared telescope, unlike what the previous book had said, and the other was to build a telescope to search for Earth-like planets. At that point, astronomers were just recognizing that searching for extrasolar planets was possible. So in October 1995, NASA Headquarters called me up, gave me a list of scientists and engineers to contact and said to start planning. So we did, and we immediately came to a remarkable convergence of thought and opinion. We quickly agreed on a concept which met the wishes of the scientific community and came within the ambitions of NASA. You’ll find that the telescope we wanted to fly back then is very much like the one we’re going to fly in 2013.

UT: Can you give us an update on the status of the JWST right now?

Mather: The flight instrument hardware will be coming in from all over the world in the summer of 2010. The fine guidance sensor comes from Canada, one and a half instrument packages come from Europe and the rest comes from the US. So, in 18 months the instrument package starts going together, and then it meets up with the telescope around a year later. The four science instruments are a near-infrared camera, a near-infrared multi-object spectrograph, a mid-infrared instrument, and a tunable filter imager.

We just went through the Critical Design review for the instrument module. Last week we had hundreds of people come to look at everything and tell us if we’re doing it right. I think we passed, although I haven’t seen the official paperwork yet. But even I was impressed.

UT: The question many people ask me is, since Hubble has been so successful, why isn’t JWST going to be an optical telescope?

Mather: Why did the committee change from optical to infrared? It was twofold. One was that Hubble was getting so good, they could see it would be hard to beat it, no matter how big you built a telescope. Another thing happening was that people were seeing you could build big optical telescopes on the ground. The Keck Telescope was working really well, and people were starting to talk about adaptive optics, which meant even bigger telescopes on the ground were worthwhile. So those two things pointed us towards an infrared telescope. Also all the scientists of the JWST said we needed infrared. From the little capability we did have at the time, infrared was fascinating, finding that the most distant universe is exciting and is red-shifted from the visible. It starts out in ultraviolet and gets to infrared because of the great distances of these objects and the huge red shift they have. So if you want to do ultraviolet astronomy at the almost-edge of the universe you need an infrared telescope.
A full-scale model of the JWST. Credit: NASA
UT: Now that the infrared Spitzer Space Telescope is up and running so well, has that changed anyone’s mind, or does that make scientists want to go to the next level with infrared?

Mather: Yes, Spitzer has proved this is actually a fascinating territory. Spitzer is actually a little bitty telescope by modern standards it’s only 3 feet wide, 85 cm. But it has been producing some astonishing surprises. They can see things out to very, very high red shifts, and none of those things were expected. So, that tells us infrared is where the wonderful discoveries will be. We now know we can do the technology, so let’s get a better telescope. The science is way, way exciting, and there’s so much out there waiting to be discovered.

UT: In your opinion, what will set the JWST apart from previous space telescopes?

Mather: Every telescope says, “I’m better than the one before me,” and we say the same. Of course, this telescope will see farther back in time with its infrared capability and its huge aperture it will see through dust clouds to see where stars are being born it will see things that are room temperature, like you and me, planets, or young stars being born. All those things can be seen directly with the infrared capability we have on this new telescope. Most of the work will be done in infrared, with some capability in the visible range.

But we’ve built a general purpose telescope. After launch, scientists can write proposals as they do for Hubble, for what they would like to observe, so they can observe whatever the hot topic is at that time.
Fully functional, 1/6th scale model of the JWST mirror in optics testbed. Credit: NASA
UT: With your experience with COBE and the subsequent honors you received, how have you applied that to the JWST?

Mather: It wasn’t so much the honors that affected my life, it was the fact of having been through the process from the very beginning to the very end for a very radically designed observatory, which COBE was, that gave me the nerve to think big things. So when NASA Headquarters said they wanted a successor for Hubble, I thought that would be interesting, and I had enough nerve to say yes, I’d like to try that. COBE was very ambitious for the time, but small enough that I knew the engineers personally and I could talk to them any day about anything. So I thought I could graduate to a bigger project.

UT: And now you’re working with people from all around the world?

Mather: Yes, this is a huge deal. Our science team is about 19 people, from Europe, the US and Canada. The engineering team is over 2,000 people who are spread out all over the world. Clearly, I don’t know all of them. I work with the scientists most closely and talk with them about what we want to accomplish, and make sure we are accomplishing that. So I have a different role now. I don’t have hands-on responsibility for any hardware, but I work with the people who do. We have access to some of the very best people in the world on every topic.

UT: Can you talk about of the problems this telescope has had to overcome, the cost overruns and the delays it has had?

Mather: Number one, the cost overrun is not as big as is being portrayed by some folks that would like the money for their own project ideas. Originally Dan Goldin was the head of NASA when we started, and he said, “We want you to think of a way to do this observatory for a half a billion dollars in 1996 dollars.” We said we’d try. But we quickly realized building this was going to be hard. By the time we got ready to present it to the decadal survey in 2000, the cost was more like one billion dollars. Then, three years ago, we saw that the job was getting harder and we had to replan and rebudget. Now, if you count the entire NASA cost from beginning in 1995 to the end, somewhere after 2019 with inflation and civil servants (which we weren’t counting before) now it is roughly $4.5 billion in actual real dollars, not 1996 dollars. So there is cost growth, but we have had excellent success and we’re on track to launch this wonderful machine, which will be used by thousands of astronomers. And we haven’t had to change our plan or our total budget in three years, thanks to steady leadership from NASA HQ and brilliant technical work by the teams.

UT: That’s good to know. I think people have a general concept that the JWST has had a huge cost overrun.

Mather: Well, it’s not something small, and we wish we could have done better on that. But it’s about a factor of two growth, and not the factor of five that has been advertised by some people who should know better. This telescope will work for a long time. The requirement is five years, but we’re hoping to run it for ten. So, our project spans from 1995 to maybe 2024 when operations would end.
Engineers from Ball Aerospace inspect the first James Webb Space Telescope mirror segment upon its arrival at Marshall Space Flight Center, Huntsville, Al. for cryogenic testing. Credit: NASA
Let me give you some idea of what we needed to do to get ready and what we’ve been up to all this time. We developed a list of ten major technologies that we needed. The hardest thing was to develop the mirrors. That required twelve different contracts just for developing the competitors to where their designs were good enough, so that took quite a few years. The detectors clearly had to be improved over what we have on the Spitzer and Hubble telescopes. So now we have bigger and better detectors, and they are fabulous. One measure astronomers have is how many stray electrons do you get from the detectors. If you close off all the light you should get zero. We now have detectors that give off a few stray electrons per pixel per hour, which is almost perfect. It would be good to be even better, but this is fabulous. I’m impressed.

We needed to improve on the refrigerators in space. We started off saying we need to get a radiatively cooled telescope so that it would be cool enough by itself, and that’s mostly true. But it turns out we still need an active refrigerator to keep the longest wavelength detectors cold, so we had to develop that.
So, those are just some of the things we had to design, and all the technology development was finally finished in 2007 and passed the review board’s approval, who said,”Yes, those things are finally ready now to be built.”

So, just getting to 2007 was a long time, and I don’t think people have really appreciated what it takes to get new technologies ready. On the other hand, we’ve been blessed by not having to “back up.” We put enough planning and effort into these technologies that they work now. That was one of the things we learned from the Hubble project, which was, don’t finish your design until you know what you’re supposed to build.

UT: How about your testing process. Is it pretty rigorous?

Mather: That is another lesson we had to learn from Hubble. If you don’t test it, it’s not going to work. We’ve learned to have a very determined and rigorous process. They did enough testing on the Hubble that they could have known about the mirror focus problems. The mirror manufacturer had two tests that didn’t agree and they decided to ignore one of them instead of tracking down the reason, and that turned out to be foolish and expensive.

We have a generalization that if anything really matters, do it twice. We will actually test the telescope cold in the big vacuum tank down at Johnson Space Center. So it will be a full-scale end-to-end “light-in-at-the-beginning, light-out-at-the end” sort of test, something they could not do for Hubble. But they knew they could go fix Hubble in space, and we know we can’t fix JWST, since the telescope will be at the L-2 point, about 1.5 million kilometers away from Earth, which is about four times further away from the Earth than the Moon.

This is a complicated project, but our approach for doing a complicated project is dramatically different from when I was a young feller. When I got here to Goddard, we used pencils and slide rules, and computers were pretty new and most people didn’t have them. Now we have computers everywhere that keep track of our documents. We can do systems engineering, and even can do very accurate, complete simulations to know if something will fit together and work before we even built it. So the world has changed, and it’s a wonderful thing to see. So this is why we are now able to build this observatory for about the same real cost as it took to get the Hubble launched and working. But JWST is so much bigger and more powerful.
JWST primary mirror. Credit: NASA
UT: Can you tell us about the design for the mirror for the JWST?

Mather: The hardest thing to build was the mirror, because we needed something that is way bigger than Hubble. But you can’t possibly lift something that big or fit it into a rocket, so you need something that is lighter weight but nonetheless larger, so it has to have the ability to fold up.
The mirror is made of light-weight beryllium, and has 18 hexagonal segments. The telescope folds up like a butterfly in its chrysalis and will have to completely undo it self. It’s a rather elaborate process that will take many hours. The telescope is huge, at 6.5 meters (21 feet), so it’s pretty impressive.

The sun shield is completely new, and it too will have to deploy. So, what was wrapped up into a small cylinder, relatively speaking becomes a giant shield about as big as a tennis court. It’s huge. All this happens in multiple stages and will take days. We hired a company, Northrop Grumman that had experience unfolding things in space, and they tell us this is definitely not the most complicated thing they’ve unfolded in space, which is reassuring.

Video of the JWST deploying in space:

UT: Has there been any discussion of first light and what the JWST will look at first?

Mather: Yes, a little. That will be the fun part after we get the thing put together.

UT: Do you have any favorite suggestions?

Mather: I think we should start with easy targets that will be pretty, that will enable the public to say, “Oh, I see its working!” Some of the first observations can be done when we’re setting up the telescope, even before it’s fully adjusted. Because it is deployed after launch and the mirror isn’t close to the right shape at first, we’ll be working up to this gradually. There’s a test model at Ball Aerospace in Boulder Colorado, where we get to practice putting the 18 mirror segments into position. Each segment has 7 motors on it to control the position and curvature, so we have to rehearse this one.

This is something they couldn’t do with Hubble. They wished they could, and it did have motors but they couldn’t push hard enough. That’s an interesting story. We learned from Hubble how to correct the optics based on the images we were getting, so we’re doing it on purpose for this telescope.

UT: There has been some controversy about how the JWST will be launched.

Mather: We’ll take the telescope to French Guiana and load it in the rocket down there. The ESA is buying the launch vehicle for us it’s the Ariane 5 rocket, a commercial product from Europe and they’ve had a good run lately, so it’s very reliable.

Naturally that caused a lot of controversy. Even if Europe was giving us the launch vehicle, so to speak, there were people here who did not want to accept it. It took headquarters two years to accept it. That cost us money. The only reason it was accepted was that we got a new administrator who would accept it. That was Mike Griffin, so I want to say, “thank you very much Mike Griffin!”
Artist concept of the JWST. Credit: NASA
UT: Your team still has a lot to do before 2013, which will probably be here before you know it!

Mather: Yes, I know. It’s been over 13 years now since NASA contacted me about this, but now the end is coming up fast. We have plenty of technical challenges ahead of us in putting everything together. And we haven’t gotten far enough to along yet find out how many things we broke or how many mistakes we made, but I think we’re pretty good at figuring them out before we make them.

It’s going to be very exciting to put the equipment together for the first time. We’ve got the pieces, we’ve got the picture on the box to show where they go, and pretty soon we get to prove that they work together, or not. By the time we receive all the parts here at Goddard, they will all have been tested individually, so they’re supposed to play together just fine. But nature doesn’t like arrogance, so we have to test the whole thing from beginning to end, just as we’re going to use it in flight. After we put it together here, we take it down to Johnson Spaceflight Center, and put it in the giant vacuum tank there. That will be an extraordinary process.

UT: Thank you so much for talking with us.

Mather: This has been fun. I love telling my story and I’m glad you want to tell it with us. I figured it might be time to talk about what we’re doing because exciting things are starting to happen. Magnificent things are happening. We’ve got the Kepler Observatory up now, and hopefully they’ll find a handful of Earth-like planets to track down and we’ll take a closer look at them.


From the ground, the variety of chemical compounds present at the surface of TNOs have been detected through the overtones and combination bands of O-H, C-H and N-H bounds up to 2.5 microns. NIRSpec is expected to open a new window into our understanding of TNO surface composition through the identification of their fundamental absorption bands in the 3𠄵 micron region. We simulated TNO spectra observed by JWST/NIRSpec using ideal mixtures of ices, without any dust or coloring compound that would change the depths of absorption features compared to the results we have shown. We have seen that for an object with a J-magnitude similar to Orcus' and relatively small exposures, the SNRs achieved for most TNOs' brightness will be sufficient to detect shallow absorption bands corresponding to 5�% of ice at the surface. We argue that the key aspect to advance our understanding of TNO surface composition will be related to the spectral resolution. Obviously, the highest spectral resolution will address the unexpected: molecules we may not anticipate, dilution and ice phases, as well as a detailed investigation of the surface temperature. This aspect will be tested with the GTO observations of Orcus.

This is however expensive, even for bright objects like Orcus (whose surface composition is already well constrained from the ground). From our experience of preparing multiple observations, we see that the efficiency of NIRSpec observations of solar system moving targets is typically 50�%. Therefore, achieving a high SNR as well as a high spectral resolution may lead to very long observing programs that may prove difficult to get through a time allocation committee.

The medium spectral resolution may be an alternative strategy: we see that we typically achieve higher SNRs when using the same exposure times as for the high spectral resolution. Using slits instead of the IFU will also allow an increase of the SNR. This will be tested with the GTO observations of 2003 AZ84, for which both slits and medium-spectral resolutions will be used. In addition, the PRISM mode will be used for both objects and will allow direct comparison of the results achieved by the different spectral resolution modes.

From our simulations, we anticipate that an efficient strategy may be to systematically observe the objects with the PRISM mode in order to get the full spectral coverage in one shot and with a good SNR. This will already allow the disentanglement of most species expected at the surface of TNOs. Then, instead of adding high-spectral resolution observations for the complete wavelength range, observations could be limited to the G395M or G395H gratings (medium- or high-spectral resolution) that cover the key 3𠄵 micron wavelength range). This combination of spectral configurations appears very promising on paper, for detailed investigations of the physical nature of components present at the surface of TNOs. The NIRSpec GTO program on Orcus and 2003 AZ84 will provide an early, real-life test of this strategy.


Paul A. Lightsey has over 40 years of experience in physics, mathematics, and engineering, much in the area of optical systems analysis and design. He has worked on JWST since 1997 and is currently the chief engineer for the James Webb Space Telescope program at Ball, and the optical thread lead for the combined Northrop Grumman/Ball/ITT/ATK system engineering team. He has contributed to all phases of development from new business through design, fabrication, alignment, test, calibration, and on-orbit operations while at Ball. He worked on several of the Hubble Space Telescope (HST) instruments built by Ball and before that, the Relay Mirror Experiment (RME) and the Retroreflector Assisted Imaging Laser Experiment (RAILE). He received his BS in physics from Colorado State University in 1966, and his doctorate in physics from Cornell University in 1972. He has received the William H. Follett, Jr. Award for Excellence in System Engineering at Ball, and the Distinguished Public Service Medal from NASA.

Charlie Atkinson is the deputy telescope manager and manager of the East Coast Office for the James Webb Space Telescope with Northrop Grumman Aerospace Systems, having worked on the program since its inception. Among other duties, he helped orchestrate the technology demonstrations for the large, cryogenic, deployable, stable structure necessary to meet JWST’s demanding science requirements. Before joining Northrop Grumman, he worked at Kodak in Rochester, NY on several programs including the Chandra X-Ray Telescope, launched in 1999. He was the operations manager for the high-resolution mirror assembly, responsible for the integration and alignment of the grazing incidence cylindrical mirrors that serve to form the x-ray images, and managed the telescope integration and test activities. He received his bachelor’s degree in physics, math, and geophysics from Washington and Lee University in Lexington, VA, where he currently serves as a member of the Science Advisory Board. He has one patent on a telescope system using a discontinuous pupil corrector and has written numerous papers on the Chandra and James Webb Space Telescopes.

Mark Clampin is currently the JWST Observatory project scientist at GSFC. He was a coinvestigator on the Attitude Control System (ACS) science team, where he served as the ACS detector scientist with responsibility for delivery of the ACS flight focal planes. Previously, he was a scientist at STScI, where he gained 10 years experience with HST servicing missions, instrument commissioning, and science operations at STScI. He served as manager of the ACS instrument group at STScI from inception through the successful completion of the SM3B servicing mission, orbital verification, and start of science operations. He is the principal investigator of the Exosolar Planetary Imaging Coronagraph (EPIC), a technology demonstration for exoplanet missions (TDEM). His science interests lie in the study of the formation and evolution of planetary systems, and he is a member of the team that discovered the exoplanet Fomalhaut-B using the ACS on the Hubble Space Telescope.

Lee D. Feinberg is the NASA optical telescope element manager for the James Webb Space Telescope and the chief large-optics systems engineer in the Instrument Systems and Technology Directorate at the Goddard Space Flight Center in Greenbelt, Maryland. Lee is an SPIE fellow and spent a decade working on the optical correction and instruments for the Hubble Space Telescope. In 1998, Lee received an MS in applied physics from Johns Hopkins University and in 1987 graduated with a BS in optics from the University of Rochester.

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