Spitzer Space Telescope

History

By the early 1970s, astronomers began to consider the possibility of placing an infrared telescope above the obscuring effects of Earth's atmosphere. In 1979, a report from the National Research Council of the National Academy of Sciences, A Strategy for Space Astronomy and Astrophysics for the 1980s, identified a Space Infrared Telescope Facility (SIRTF) as "one of two major astrophysics facilities [to be developed] for Spacelab", a Shuttle-borne platform. Anticipating the major results from an upcoming Explorer satellite and from the Shuttle mission, the report also favored the "study and development of ... long-duration spaceflights of infrared telescopes cooled to cryogenic temperatures."

The launch in January 1983 of the Infrared Astronomical Satellite, jointly developed by the United States, the Netherlands, and the United Kingdom, to conduct the first infrared survey of the sky, whetted the appetites of scientists worldwide for follow-up space missions capitalizing on the rapid improvements in infrared detector technology.

Earlier infrared observations had been made by both space-based and ground-based observatories. Ground-based observatories have the drawback that at infrared wavelengths or frequencies, both the Earth's atmosphere and the telescope itself will radiate (glow) strongly. Additionally, the atmosphere is opaque at most infrared wavelengths. This necessitates lengthy exposure times and greatly decreases the ability to detect faint objects. It could be compared to trying to observe the stars at noon. Previous space observatories (such as IRAS, the Infrared Astronomical Satellite, and ISO, the Infrared Space Observatory) were launched during the 1980s and 1990s and great advances in astronomical technology have been made since then.

Most of the early concepts envisioned repeated flights aboard the NASA Space Shuttle. This approach was developed in an era when the Shuttle program was expected to support weekly flights of up to 30 days duration. A May 1983 NASA proposal described SIRTF as a Shuttle-attached mission, with an evolving scientific instrument payload. Several flights were anticipated with a probable transition into a more extended mode of operation, possibly in association with a future space platform or space station. SIRTF would be a 1-meter class, cryogenically cooled, multi-user facility consisting of a telescope and associated focal plane instruments. It would be launched on the Space Shuttle and remain attached to the Shuttle as a Spacelab payload during astronomical observations, after which it would be returned to Earth for refurbishment prior to re-flight. The first flight was expected to occur about 1990, with the succeeding flights anticipated beginning approximately one year later. However, the Spacelab-2 flight aboard STS-51-F showed that the Shuttle environment was poorly suited to an onboard infrared telescope due to contamination from the relatively "dirty" vacuum associated with the orbiters. By September 1983 NASA was considering the "possibility of a long duration [free-flyer] SIRTF mission".

Spitzer is the only one of the Great Observatories not launched by the Space Shuttle, as was originally intended. However, after the 1986 Challenger disaster, the Centaur LH2–LOX upper stage, which would have been required to place it in its final orbit, was banned from Shuttle use. The mission underwent a series of redesigns during the 1990s, primarily due to budget considerations. This resulted in a much smaller but still fully capable mission that could use the smaller Delta II expendable launch vehicle.

One of the most important advances of this redesign was an Earth-trailing orbit. Cryogenic satellites that require liquid helium (LHe, T ≈ 4 K) temperatures in near-Earth orbit are typically exposed to a large heat load from the Earth, and consequently entail large usage of LHe coolant, which then tends to dominate the total payload mass and limits mission life. Placing the satellite in solar orbit far from Earth allowed innovative passive cooling such as the sun shield, against the single remaining major heat source to drastically reduce the total mass of helium needed, resulting in an overall smaller lighter payload, with major cost savings. This orbit also simplifies telescope pointing, but does require the NASA Deep Space Network for communications.

The primary instrument package (telescope and cryogenic chamber) was developed by Ball Aerospace & Technologies, in Boulder, Colorado. The individual instruments were developed jointly by industrial, academic, and government institutions, the principals being Cornell, the University of Arizona, the Smithsonian Astrophysical Observatory, Ball Aerospace, and Goddard Spaceflight Center. The shorter-wavelength infrared detectors were developed by Raytheon in Goleta, California. Raytheon used indium antimonide and a doped silicon detector in the creation of the infrared detectors. It is stated that these detectors are 100 times more sensitive than what was once available in the beginning of the project during the 1980s. The far-IR detectors (70 - 160 micrometers) were developed jointly by the University of Arizona and Lawrence Berkeley National Laboratory using Gallium-doped Germanium. The spacecraft was built by Lockheed Martin. The mission is operated and managed by the Jet Propulsion Laboratory and the Spitzer Science Center, located on the Caltech campus in Pasadena, California.

Spitzer ran out of liquid helium coolant on 15 May 2009, which stopped far-IR observations. Only the IRAC instrument remains in use, and only at the two shorter wavelength bands (3.6 µm and 4.5 µm). The telescope equilibrium temperature is now around 30 K (−243 °C; −406 °F), and IRAC continues to produce valuable images at those wavelengths as the "Spitzer Warm Mission".

Instruments

Spitzer carries three instruments on-board:

Infrared Array Camera (IRAC)

An infrared camera which operates simultaneously on four wavelengths (3.6 µm, 4.5 µm, 5.8 µm and 8 µm). Each module uses a 256×256-pixel detector—the short wavelength pair use indium antimonide technology, the long wavelength pair use arsenic-doped silicon impurity band conduction technology. The principal investigator is Giovanni Fazio of Harvard–Smithsonian Center for Astrophysics; the flight hardware was built by NASA Goddard Space Flight Center.

Infrared Spectrograph (IRS)

An infrared spectrometer with four sub-modules which operate at the wavelengths 5.3–14 µm (low resolution), 10–19.5 µm (high resolution), 14–40 µm (low resolution), and 19–37 µm (high resolution). Each module uses a 128×128-pixel detector—the short wavelength pair use arsenic-doped silicon blocked impurity band technology, the long wavelength pair use antimony-doped silicon blocked impurity band technology. The principal investigator is James R. Houck of Cornell University; the flight hardware was built by Ball Aerospace.

Multiband Imaging Photometer for Spitzer (MIPS)

Three detector arrays in the far infrared (128 × 128 pixels at 24 µm, 32 × 32 pixels at 70 µm, 2 × 20 pixels at 160 µm). The 24 µm detector is identical to one of the IRS short wavelength modules. The 70 µm detector uses gallium-doped germanium technology, and the 160 µm detector also uses gallium-doped germanium, but with mechanical stress added to each pixel to lower the bandgap and extend sensitivity to this long wavelength. The principal investigator is George H. Rieke of the University of Arizona; the flight hardware was built by Ball Aerospace.

Results

The first images taken by SST were designed to show off the abilities of the telescope and showed a glowing stellar nursery; a big swirling, dusty galaxy; a disc of planet-forming debris; and organic material in the distant universe. Since then, many monthly press releases have highlighted Spitzer's capabilities, as the NASA and ESA images do for the Hubble Space Telescope.

As one of its most noteworthy observations, in 2005, SST became the first telescope to directly capture light from exoplanets, namely the "hot Jupiters" HD 209458 b and TrES-1b, although it did not resolve that light into actual images. This was the first time extrasolar planets had actually been visually seen; earlier observations had been indirectly made by drawing conclusions from behaviors of the stars the planets were orbiting. The telescope also discovered in April 2005 that Cohen-kuhi Tau/4 had a planetary disk that was vastly younger and contained less mass than previously theorized, leading to new understandings of how planets are formed.

While some time on the telescope is reserved for participating institutions and crucial projects, astronomers around the world also have the opportunity to submit proposals for observing time. Important targets include forming stars (young stellar objects, or YSOs), planets, and other galaxies. Images are freely available for educational and journalistic purposes.

In 2004, it was reported that Spitzer had spotted a faintly glowing body that may be the youngest star ever seen. The telescope was trained on a core of gas and dust known as L1014 which had previously appeared completely dark to ground-based observatories and to ISO (Infrared Space Observatory), a predecessor to Spitzer. The advanced technology of Spitzer revealed a bright red hot spot in the middle of L1014.

Scientists from the University of Texas at Austin, who discovered the object, believe the hot spot to be an example of early star development, with the young star collecting gas and dust from the cloud around it. Early speculation about the hot spot was that it might have been the faint light of another core that lies 10 times further from Earth but along the same line of sight as L1014. Follow-up observation from ground-based near-infrared observatories detected a faint fan-shaped glow in the same location as the object found by Spitzer. That glow is too feeble to have come from the more distant core, leading to the conclusion that the object is located within L1014. (Young et al., 2004)

In 2005, astronomers from the University of Wisconsin at Madison and Whitewater determined, on the basis of 400 hours of observation on the Spitzer Space Telescope, that the Milky Way galaxy has a more substantial bar structure across its core than previously recognized.

Also in 2005, astronomers Alexander Kashlinsky and John Mather of NASA's Goddard Space Flight Center reported that one of Spitzer's earliest images may have captured the light of the first stars in the universe. An image of a quasar in the Draco constellation, intended only to help calibrate the telescope, was found to contain an infrared glow after the light of known objects was removed. Kashlinsky and Mather are convinced that the numerous blobs in this glow are the light of stars that formed as early as 100 million years after the Big Bang, redshifted by cosmic expansion.

In March 2006, astronomers reported an 80-light-year long (25 pc) nebula near the center of the Milky Way Galaxy, the Double Helix Nebula, which is, as the name implies, twisted into a double spiral shape. This is thought to be evidence of massive magnetic fields generated by the gas disc orbiting the supermassive black hole at the galaxy's center, 300 light-years (92 pc) from the nebula and 25,000 light-years (7,700 pc) from Earth. This nebula was discovered by Spitzer and published in the magazine Nature on 16 March 2006.

In May 2007, astronomers successfully mapped the atmospheric temperature of HD 189733 b, thus obtaining the first map of some kind of an extrasolar planet.

Since September 2006 the telescope participates in a series of surveys called the Gould Belt Survey, observing the Gould's Belt region in multiple wavelengths. The first set of observations by the Spitzer Space Telescope were completed from 21 September 2006 through 27 September. Resulting from these observations, the team of astronomers led by Dr. Robert Gutermuth, of the Harvard–Smithsonian Center for Astrophysics reported the discovery of Serpens South, a cluster of 50 young stars in the Serpens constellation.

Scientists have long wondered how tiny silicate crystals, which need high temperatures to form, have found their way into frozen comets, born in the very cold environment of the Solar System's outer edges. The crystals would have begun as non-crystallized, amorphous silicate particles, part of the mix of gas and dust from which the Solar System developed. This mystery has deepened with the results of the Stardust sample return mission, which captured particles from Comet Wild 2. Many of the Stardust particles were found to have formed at temperatures in excess of 1000 K.

In May 2009, Spitzer researchers from Germany, Hungary and the Netherlands found that amorphous silicate appears to have been transformed into crystalline form by an outburst from a star. They detected the infrared signature of forsterite silicate crystals on the disk of dust and gas surrounding the star EX Lupi during one of its frequent flare-ups, or outbursts, seen by Spitzer in April 2008. These crystals were not present in Spitzer's previous observations of the star's disk during one of its quiet periods. These crystals appear to have formed by radiative heating of the dust within 0.5 AU of EX Lupi.

In August 2009, the telescope found evidence of a high-speed collision between two burgeoning planets orbiting a young star.

In October 2009, astronomers Anne J. Verbiscer, Michael F. Skrutskie, and Douglas P. Hamilton published findings of the "Phoebe ring" of Saturn, which was found with the telescope; the ring is a huge, tenuous disc of material extending from 128 to 207 times the radius of Saturn.

GLIMPSE and MIPSGAL surveys

GLIMPSE, the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire, is a survey spanning 300° of the inner Milky Way galaxy. It consists of approximately 444,000 images taken at four separate wavelengths using the Infrared Array Camera.

MIPSGAL is a similar survey covering 278° of the galactic disk at longer wavelengths.

On 3 June 2008, scientists unveiled the largest, most detailed infra-red portrait of the Milky Way, created by stitching together more than 800,000 snapshots, at the 212th meeting of the American Astronomical Society in St. Louis, Missouri. This composite survey is now viewable with the GLIMPSE/MIPSGAL Viewer.

2010s

Spitzer observations, announced in May 2011, indicate that tiny forsterite crystals might be falling down like rain on to the protostar HOPS-68. The discovery of the forsterite crystals in the outer collapsing cloud of the protostar is surprising, because the crystals form at lava-like high temperatures, yet they are found in the molecular cloud where the temperatures are about −170 °C (103 K; −274 °F). This led the team of astronomers to speculate that the bipolar outflow from the young star may be transporting the forsterite crystals from near the star's surface to the chilly outer cloud.

In January 2012, it was reported that further analysis of the Spitzer observations of EX Lupi can be understood if the forsterite crystalline dust was moving away from the protostar at a remarkable average speed of 38 kilometres per second (24 mi/s). It would appear that such high speeds can only arise if the dust grains had been ejected by a bipolar outflow close to the star. Such observations are consistent with an astrophysical theory, developed in the early 1990s, where it was suggested that bipolar outflows garden or transform the disks of gas and dust that surround protostars by continually ejecting reprocessed, highly heated material from the inner disk, adjacent to the protostar, to regions of the accretion disk further away from the protostar.

In April 2015, Spitzer and the Optical Gravitational Lensing Experiment were reported as co-discovering one of the most distant planets ever identified: a gas giant about 13,000 light-years (4,000 pc) away from Earth.

In June and July 2015, the brown dwarf OGLE-2015-BLG-1319 was discovered using the gravitational microlensing detection method in a joint effort between Swift, Spitzer, and the ground-based Optical Gravitational Lensing Experiment, the first time two space telescopes have observed the same microlensing event. This method was possible because of the large separation between the two spacecraft: Swift is in low-Earth orbit while Spitzer is more than one AU distant in an Earth-trailing heliocentric orbit. This separation provided significantly different perspectives of the brown dwarf, allowing for constraints to be placed on some of the object's physical characteristics.

Reported in March 2016, Spitzer and Hubble were used to discover the most distant-known galaxy, GN-z11. This object was seen as it appeared 13.4 billion years ago.

Spitzer Beyond

On 1 October 2016, Spitzer began its Observation Cycle 13, a  2 ¹⁄₂ year extended mission nicknamed Beyond. One of the goals of this extended mission is to help prepare for the James Webb Space Telescope, also an infrared telescope, by identifying candidates for more detailed observations.

Another aspect of the Beyond mission are the engineering challenges of operating Spitzer in its progressing orbital phase. As the spacecraft moves farther from Earth on the same orbital path from the Sun, its antenna must point at increasingly higher angles to communicate with ground stations; this change in angle imparts more and more solar heating on the vehicle while its solar panels receive less sunlight.

Planet hunter

Spitzer has been put to work studying exoplanets thanks to creatively tweaking its hardware. This included doubling its stability by modifying its heating cycle, finding a new use for the "peak-up" camera, and analyzing the sensor at a sub-pixel level. Although in its "warm" mission, the spacecraft's passive cooling system keeps the sensors at 29 K (−244 °C; −407 °F). Spitzer can use the transit photometry and gravitational microlensing techniques to perform these observations. According to NASA's Sean Carey, "We never even considered using Spitzer for studying exoplanets when it launched. ... It would have seemed ludicrous back then, but now it's an important part of what Spitzer does."

Examples of exoplanets discovered using Spitzer include HD 219134 b in 2015, which was shown to be a rocky planet about 1.5 times as large as Earth in a three-day orbit around its star; and an unnamed planet found using microlensing located about 13,000 light-years (4,000 pc) from Earth.

In September-October 2016, Spitzer was used to discover five of a total of seven known planets around the star TRAPPIST-1, all of which are approximately Earth sized and likely rocky. Three of the discovered planets are located in the habitable zone, which means they are capable of supporting liquid water given sufficient parameters. Using the transit method, Spitzer helped measure the sizes of the seven planets and estimate the mass and density of the inner six. Further observations will help determine if there is liquid water on any of the planets.