v1.0.1 / chapter 8 of 10 / 01 aug 08 / greg goebel / public domain
* New space astronomy missions continue to be launched in the 21st century, including advanced cosmic-background mappers, high-energy and infrared astronomy satellites, and a follow-on to the Hubble Space Telescope, the Webb Space Telescope -- which will be the biggest space telescope ever made.
* The progress of space astronomy in the 1990s continued into the new millennium. One of the major space astronomy missions of the early 21st century was the NASA "Microwave Anisotropy Probe (MAP)", a follow-on to COBE that was designed to provide a much more detailed map of the cosmic background radiation. MAP was launched from Cape Canaveral by a Delta II 7425-10 booster on 30 June 2001; it was named the "Wilkinson MAP (WMAP)" after arriving safely into space.
WMAP had a launch weight of 439 kilograms, and carried two microwave telescopes, mounted back-to-back, with the spacecraft spinning to provide stabilization and allow the satellite to sweep the sky. The telescopes measured the microwave background at five frequency bands, using a differential system to record the difference in the signals picked up by the two telescopes.
WMAP was placed at the Earth-Sun "L2 libration point", which is a stable position 1.5 million kilometers beyond Earth's orbit where the satellite kept the same position relative to the Earth, giving the satellite a view of the cosmos not obstructed by the Earth. The observatory did not have a cryogenic cooling system, instead being chilled by a sunscreen on the rear that was kept pointed at the Sun. While COBE's map had an angular resolution of about 7 degrees, WMAP provided a resolution of 0.23 degrees, with a thermal resolution of about 20 millionths of a degree Kelvin. At last notice, WMAP was still in operation.
* The 4.1 tonne ESA "International Gamma-Ray Astrophysics Laboratory (Integral)" was launched by a Russian Proton booster on 17 October 2002. The 4.1 tonne Integral was designed to perform observation in the X-ray and low-energy gamma ray regions of the spectrum. It carried a gamma-ray spectrometer, a gamma-ray imager, and an X-ray imager, all implemented as coded mask telescopes; a small "Optical Monitoring Camera (OMC)" provided the optical context for the high-energy images. At last notice, Integral was still in operation.
* One small but interesting space astronomy mission, the Canadian "Microvariability and Oscillations of Stars (MOST)" satellite, was launched along with a set of other payloads by a Russian Rockot booster from the Russian Plesetsk cosmodrome on 30 June 2003. MOST was a suitcase-size astronomy platform with a launch mass of 60 kilograms, carrying a telescope with aperture of 15 centimeters and a sensor system capable of detecting variations in stellar brightness of only 1/10,000th of a percent.
The major objective of MOST was to detect occultations of distant stars by planets in orbit around them. It was a relatively low-cost mission -- staff referred to it as the "Humble Space Telescope" -- but it was Canada's first space astronomy satellite, and it paved the way for bigger "planet hunter" missions. At last notice, MOST was still in operation.
* Another one of the major missions of the decade was the flight of the last of the four NASA Great Observatories, the "Space Infrared Telescope Facility (SIRTF)", which was named the "Spitzer SIRTF" after its launch from Cape Canaveral on 25 August 2003 by a Delta 7920H booster.
SIRTF followed in the path of IRAS and the ISO. SIRTF had a launch mass of 865 kilograms and had a mirror 85 centimeters in diameter, giving it twice the collecting area of the ISO. SIRTF featured a precision pointing system, as well as camera system with much more resolution and sensitivity, and a longer mission lifetime of 2.5 years. The telescope system was supported by a camera, photometer, and spectrograph.
Once again learning from the experience of the Hubble Space Telescope, SIRTF was not placed in low Earth orbit; in fact, it was placed in solar orbit, trailing the Earth. This allowed to be kept on target for as long as need be, with little thermal interference from the Earth, and also allowed it to use a passive cooling system, with its reflective silver back end pointed at the Sun at all times. While the instrumentation is cryogenically cooled, the telescope itself is not, and passive cooling was a necessity. Once the coolant ran out, SIRTF would still be able to perform useful if not quite as sensitive observations. At last notice, SIRTF was still in operation.
* As mentioned earlier, the Chandra / AXAF satellite was to be complemented by a satellite to perform X-ray spectroscopy. This project was actually taken up by Japan, with the Japanese ISAS organization developing the "Astro-E" satellite, with a launch weight of 1.6 tonnes.
Astro-E featured a cryogenically cooled "X-Ray Spectrometer" built by Japan and the US. The XRS provided about five times greater spectroscopic resolution than Chandra, though only over the high end of Chandra's spectral range. The XRS also had a wider field of view than the spectroscopes on the other satellites. The XRS was expected to run out of coolant in about two and a half years, but the payload also included a low-energy "X Ray Imaging Spectrometer (XRIS)" and a "Hard X-Ray Detector (HXD)" that could continue to operate after the helium coolant ran out. All the instruments could operate simultaneously.
The first launch attempt of Astro-E was on 10 February 2000, with a Japanese H-II booster to place the spacecraft into low Earth orbit, but the booster failed. A replacement, "Astro-E2", with an improved XRS that doubled spectral resolution, was launched by an H-IIA booster on 9 July 2005. The spacecraft made its proper orbit, to be given the name "Suzaku", but during checkout the coolant leaked away, rendering the XRS useless. The secondary instruments, the XRIS and HXD, remained operational, but the failure was still a great disappointment.
* There were several other space astronomy missions in the first years of the 21st century. The NASA "High Energy Transient Explorer 2 (HETE-2)" was put into space by an air-launched Pegasus booster on 9 October 2000, following the launch failure of HETE-1 in November 1996. The 124 kilogram spacecraft was designed to quickly pin down the sources of gamma ray bursts and cue other observatories to inspect them.
HETE 2's main instrument was a French gamma-ray spectrometer array, which was complemented by a wide-field hard X-ray coded-mask telescope. These two instruments cued two coded-mask soft X-ray imagers that would be used to determine the precise spatial coordinates of the burst. The mission was a collaboration between NASA and US, Japanese, and French universities. At last notice, HETE-2 was still in operation, though its batteries were fading and it was operating on a reduced schedule.
* The NASA Small Explorer "Rheuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)" satellite was launched by a Pegasus XL air-dropped booster from over the Atlantic off Cape Canaveral on 5 February 2002. RHESSI had a launch mass of 293 kilograms and carried a payload consisting only of an X-ray / gamma-ray imaging spectrometer, using a multilayer collimating grid -- a more advanced descendant of the collimator grid used on Uhuru -- on top of a germanium detector array. The satellite's mission plan was to observe solar flares, obtaining video imagery of these explosions in both the X-ray and the gamma ray spectrum. At last notice, RHESSI was still in operation.
* The "Cosmic Hot Interstellar Plasma Spectrometric (CHIPS)" smallsat was launched as a secondary payload from Cape Canaveral using a Delta II 7320 booster on 12 January 2003. The 60 kilogram CHIPS was the first NASA low-cost, simple "University Class Explorer" mission; its sole payload was an EUV grating spectroscope. The CHIPS mission ended in April 2008.
* The NASA Small Explorer "Galaxy Explorer (GALEX)" spacecraft was put into orbit by a Pegasus XL air-dropped booster from over the Atlantic Ocean on 28 April 2003. GALEX had a launch mass of about 276 kilograms. It was designed to perform an ultraviolet survey of the Universe and carried a 50 centimeter Cassegrain-configuration Richey-Chretien telescope with a field of view of 1.2 degrees, feeding near-ultraviolet and far-ultraviolet microchannel image intensifier detectors. At last notice, GALEX was still in operation.
* The NASA Medium Explorer series "Swift Gamma Ray Burst Explorer" satellite was put into orbit from Cape Canaveral by a Delta II 7320-10 booster on 20 November 1994. As its name implied, it was designed to hunt for GRBs. It could detect a burst, zero in on it immediately to take observations, and tip off other observatories linked to the "GRB Coordinate Network (GCN)".
The Swift spacecraft normally observed the cosmos with its "Burst Alert Telescope (BAT)", a coded aperture mask telescope, which could see a sixth of the sky at one time. When BAT picked up a GRB, Swift reoriented itself to bring its "X-Ray Telescope (XRT)" and "Ultraviolet / Optical Telescope (UOT)" to bear on the target. The XRT was a grazing-incidence instrument with a CCD imager, while the UOT was a Cassegrain-configuration Richey-Chretien instrument with a microchannel image intensifier / CCD camera. The spacecraft could zero in on a target within 75 seconds. At last notice, Swift was still in operation.
* A JAXA M-5 booster was launched from Kagoshima on 21 February 2006 to put the "Astro-F" infrared astronomy satellite into orbit. It was named "Akari (Light)" once it was checked out in orbit. Astro-F had a launch weight of 950 kilograms and carried a 68.6 centimeter infrared telescope, cryogenically cooled by liquid helium. The helium supply was expected to last for 18 months. Mechanical coolers are to be used to continue the mission at degraded sensitivity for several more years.
* As a follow-up to the planet-hunting Canadian MOST smallsat, the French CNES space agency launched a space observatory named "COROT (Convection, Rotation, & Planetary Transits)" on a Russian Soyuz booster from Baikonur on 27 December 2006. COROT had a launch mass of 630 kilograms and carried a 30 centimeter telescope with a dual-camera wide-field imaging system. It was designed to search for planetary occultations, as well as perform detailed measurements of stellar oscillations to obtain a better understanding of their internal structures. It continues in operation.
* An Indian Space Research Organization (ISRO) Polar Satellite Launch Vehicle put the Italian Space Agency (ASI) "AGILE (Astro-rivelatore Gamma a Immagini Leggero / Gamma-Ray Detector & Light Imager)" high-energy astronomy satellite into orbit on 23 April 2007. AGILE had a launch mass of 352 kilograms and carried a gamma-ray imager, an X-ray detector, and a thermal detector called a "mini-calorimeter" to spot GRBs and other high-energy transient events. It continues in operation.
* A NASA A Delta-2 Heavy booster was launched from Cape Canaveral on 11 June 2008 to put the "Gamma Ray Large Area Space Telescope (GLAST)" astronomy satellite into orbit. GLAST had a launch mass of 4,277 kilograms (9,429 pounds); was powered by twin solar arrays; and carried a primary payload of two instruments, the "Large Area Telescope (LAT)" and the "GLAST Burst Monitor (GBM)".
LAT detected gamma rays using a grid of 16 stacks of thin sheets of tungsten sandwiched with silicon layers, with gamma ray impacts on the tungsten sheets generating electron-positron pairs and currents through the silicon layers tracking their paths. The energy of the particles was measured by a cesium iodide scintillation detector at the bottom of the stack. The array was surrounded by plate detectors that were sensitive to cosmic rays but not gamma rays to screen out cosmic ray events. The LAT was effectively a follow-on to the CGRO EGRET instrument, but its stacked detector configuration gave it far more detecting area and so sensitivity. That increased sensitivity also allowed it to pick up gamma rays with energies of up to 300 GeV, substantially more powerful than those picked up by any gamma-ray instrument flown in space before GLAST. LAT was built by the Stanford Linear Accelerator in California.
The GBM, which as its name suggests was designed to spot GRBs, was made up of 14 detectors, including 12 sodium iodide X-ray / low-energy gamma ray detectors and two bismuth germanate high-energy gamma ray detectors. In combination, the two detectors gave GLAST the capability to detect a much broader energy range of gamma rays than any previous orbital platform. GLAST remains in operation at last report.
* The NASA Kuiper Astronomical Observatory was retired in the mid-1990s to make way for a much more sophisticated flying observatory, the "Stratospheric Observatory for Infrared Astronomy (SOFIA)", a collaboration between the US and Germany. It consist of a surplus Boeing 747SP "short jumbo" jetliner, known as the "Clipper Lindbergh" in commercial service, carrying a German-built 2.5 meter infrared telescope. The program ran into difficulties and there was some discussion about its cancellation, but the decision was made to proceed. Full operational capability is expected in 2009.
* A number of new space astronomy missions are now in planning. The ESA is now working on a space telescope named "Herschel" that will be launched on an Ariane 5 booster in 2009. Herschel will be the most powerful space telescope launched to that time, with a 3.5 meter mirror, substantially bigger than the Hubble's 2.4 meter mirror. However, Herschel will be an infrared / submillimeter telescope, more properly a successor to the ESA ISO satellite and NASA SIRTF spacecraft than the Hubble.
Herschel will have a launch mass of 3.3 tonnes. The spacecraft's mirror is being fabricated from silicon carbide (SiC), which is a rigid, lightweight material with excellent machining and thermal properties. The Herschel mirror will only weigh 300 kilograms, compared to the 1,000 kilograms of the Hubble mirror. The SiC mirror was fabricated in twelve pieces and brazed together in a vacuum environment. A small SiC mirror was flown on the ESA Rosetta comet probe. Herschel's telescope will be cooled to 70 degrees Kelvin using a helium-filled Dewar bottle; the coolant is expected to last three and a half years.
Herschel will be placed at the Sun-Earth L2 libration point, along with the ESA's answer to MAP, the "Planck" cosmic background mapper, with the two payloads to be launched on the same Ariane 5 as a dual payload. Planck will have a 1.5 meter mirror and an array of sensitive microwave receivers, all cooled by a cryogenic system. Planck will obtain a CMB map with greater resolution than those available before.
* The ESA's Herschel telescope will not keep the title as the biggest space telescope ever for long. NASA is now working on a new space telescope, the "Webb Space Telescope (WST)", named after James Webb, the NASA administrator who led the organization to put men on the Moon. The Webb, originally the "Next Generation Space Telescope (NGST)", will not actually be a direct replacement for the Hubble. Since ground-based telescopes are doing a pretty good job for optical astronomy, the Webb will be an infrared telescope.
NASA's original requirements specified that the Webb would have a reflecting mirror eight meters in diameter, but this proved overambitious, and so the Webb was rescoped to be fitted with a mirror 6.6 meters in diameter, cutting its collecting area in half. This and other changes made at the time were done not only to reduce cost and risk, but to permit adequate ground testing of the spacecraft.
The Webb's mirror will be made of beryllium, a material that provides high strength and light weight along with thermal stability, and will have 18 segments. The segmented design will allow the mirror to be folded up for launch. It will be folded like a table in three sections, with three segments on the folded side sections. Each segment will be controlled by seven actuators -- six to establish position, the seventh to establish curvature. The Webb will be the first segmented-mirror telescope to be put into space. Ball Aerospace, the prime contractor, is working on a 1/6th scale demonstrator mirror made of glass as a risk reduction exercise.
Like Herschel, the Webb will be placed in the Earth-Sun L2 libration point. Early concepts envisioned the spacecraft with a large sunshade to cool it to 35 degrees Kelvin, but after the rescoping the Webb team decided that it was unlikely the sunshade would work very well, and elected to keep the telescope at a higher temperature of 45 degrees Kelvin, using heaters to keep the temperature stable.
The Webb will require infrared detector arrays of unprecedented size, as well as new "nanomirror arrays" that will deflect individual targets in the field of view, allowing the spectra of a hundred different objects to be measured simultaneously.
The near-infrared / visible-light image detector arrays for the Webb are a major challenge. The arrays will have an unusually large field of view of four arc-minutes by four arc-minutes, using an array of 4,096 by 4,096 picture elements, or "pixels". The detector technology itself is available, since detector arrays based on indium antimonide or mercury cadmium telluride are now available, but current sensor arrays only have resolutions of 256 by 256 pixels or 1,024 by 1,024 pixels, with arrays with resolutions of 2,048 by 2,048 pixels now in the works. Building a detector array four times that big will be difficult, particularly in terms of ensuring that detector sensitivity remains uniform across the array.
The new detector array will be the basis of two of the three main instruments now planned for the Webb, which include:
The nanomirror arrays for the multiobject spectrometer are being derived from the "micromirror array display systems" developed by Texas Instruments for digital movie projectors and large-screen projection TVs. The micromirror arrays are a silicon chip with micromachined mirrors that can be tilted electronically. Using a nanomirror array with the Webb would make far better use of observing time by allowing simultaneous examination of as many as a hundred different targets in the field of view.
The camera-spectrometer system, intended to hunt for extrasolar planets, is particularly challenging, since it will have a silicon infrared detector that will have to be cooled to eight degrees Kelvin. The camera-spectrometer system will have its own cryocooler. Current schedules envision the launch of the Webb in 2013 on an Ariane 5 booster.
* As a follow-up to the French COROT mission, NASA is also working on a stellar occultation mission named "Kepler", to be launched in 2009. NASA has a more ambitious planet-hunting mission in the works, the "Space Interferometry Mission (SIM)", also known as "PlanetQuest". SIM will be something like an improved follow-on to the ESA Hipparcos, and will pinpoint the location of the hundred or so nearest stars to an accuracy of 4 microarcseconds, about 250 times better than the best current star catalog, and will perform other observations of stars and distant galaxies.
The primary goal of the observations is to observe small "wobbles" in the motion of nearby stars to see if these motions reveal the presence of planets in orbit around them. Ground-based telescopes have already discovered several possible extrasolar planets in this fashion, but SIM will be accurate enough to reveal the presence of planets much smaller than could be detected from the ground.
SIM will perform its measurements with two sets of three 30 centimeter telescopes, mounted in assemblies at the ends of a 10 meter truss. Each telescope assembly also contains a fourth telescope as a spare. Using interferometry techniques, SIM has the resolving power (though not the light-gathering capacity) of a ten-meter telescope. Light from the assemblies is sent down the truss and combined to provide precision information through interference effects. To operate properly, the optical path lengths through the truss, which changes with temperature, has to be validated to less than the width of a hydrogen atom. Due to budget cuts, there is no schedule for SIM launch at the present time.
Over the longer run NASA is interested in actually observing extrasolar planets, using a large space observatory tentatively named the "Terrestrial Planet Finder (TPF)". The TPF will be able to take pictures of extrasolar planets the size of Jupiter or larger, and obtain their spectroscopic signatures to determine atmospheric compositions. A planet with an oxygen atmosphere would be very likely to harbor life. The ESA is investigating a mission of their own named "Darwin" for planet hunting. Schedules for SIM and the TPF are not solid at present; NASA and the ESA may end up combining their efforts.
The ESA is also working on the "X-ray Evolving Universe Spectroscopy (XEUS)" spacecraft, which will actually be two spacecraft -- one carrying a grazing-incidence mirror assembly and the other carrying a detector array, flying in precision formation 50 meters apart. The dual spacecraft will allow a much longer focal length than could be provided by a single X-ray telescope. XEUS will be 200 times more sensitive than XMM-Newton and will be placed in the Earth-Sun L2 point. The project is in development but launch date is unclear.
