Webb is going to doubt his mission again? The twists and turns before the space telescope is launched

Spitzer Telescope 10th anniversary photo. Copyright / NASA

It is never easy to launch a telescope into space. Recently, NASA has once again announced the postponement of the launch of the James Webb Space Telescope. Webb's predecessor, the Spitzer Space Telescope (SST), had a long and arduous journey from the initial concept proposal, approval, construction and launch, with several design revisions, which took more than 30 years.

In the initial plan, the Spitzer Space Telescope was first called the "Spacecraft-borne Infrared Telescope" (SIRTF, S=Shuttle), and later changed its name to the "Space Infrared Telescope" (SIRTF, S=Space). It was not officially named "Spitzer" until before launch.

The name change of the Spitzer Space Telescope directly reflects its history. In the first draft of the design in 1971, it was placed on the spacecraft that had just appeared at that time as part of the payload. In the following 12 years, the Ames Research Center and commercial airlines have been designing and developing it with the 1-meter space telescope as the guiding concept.

1983 was a milestone year for the Spitzer Telescope. NASA issued a call for instruments and eventually selected three teams, led by Giovanni Fazio, Jim Houck, and George Rieke. These three teams later developed the final terminal instruments: the near-infrared photometer IRAC, the mid-infrared spectrometer IRS, and the mid-infrared photometer MIPS. SIRTF formed the initial scientific working group, most of whom persisted in the project until the Spitzer Telescope was officially launched 20 years later. Amid all the setbacks and delays, the design of the Spitzer Telescope also changed, from the original space-borne instrument to a member of NASA's "Great Space Telescope Program." With the invention and realization of high-altitude (100,000 kilometers) autonomous aircraft, the Spitzer Telescope was redesigned into a giant space infrared telescope immersed in cryogen, and the "Shuttle" in the name was changed to "Space".

The first meeting of the Spitzer Telescope (then called SIRTF) Science Working Group was held at the Ames Institute in 1984. Back row, from left: George Newton (NASA project manager), Dan Gezari (NASA Goddard Center), Ned Wright, Michael Jura (University of California, Los Angeles), Michael Werner, Fred Witteborn (Ames Institute). Front row, from left: Giovanni Fazio (Smithsonian Center for Astrophysics), George Rieke (University of Arizona), Nancy Boggess (NASA project scientist), Jim Houck (Cornell University), Frank Low (University of Arizona), Terry Herter (Cornell University). Copyright / Michael Werner

However, the SIRTF, which was originally planned to be launched in 1991, was called off at the last second. This was mainly due to the impact of the Hubble Telescope (the main mirror of the Hubble Telescope had focusing problems just after it was launched, and after many extravehicular activities by astronauts, the repair mission was completed in 1994). At this time, Johnny Kwok, a project engineer at NASA's Jet Propulsion Laboratory, proposed a change of orbit in 1992, changing the original high-altitude orbit around the earth to an orbit around the sun. This would not only keep away from the interference of the heat emitted by the earth on the infrared instrument, but also ensure that the telescope's solar panels have a fixed sunlight angle, improving the thermal stability of the instrument.

In addition, the design of the solar orbit also increases the observation range of the telescope, so that 40% of the celestial sphere can be observed at all times, creating conditions for repeated long exposures of deep space. Deeper exposures can see the darker and more distant universe, and repeatedly observing the same area of ​​the sky also helps to conduct time-domain related astronomical observations. Once this orbit update proposal was put forward, it immediately received widespread support from the astronomical community.

Later, at a meeting in 1993, after heated discussions, astronomers further decided to change the original "big and complete" design to a "small and exquisite" design, retaining only the most important instruments to answer the most urgent scientific problems. Under this principle, under the premise that the size of the telescope itself remains unchanged, the original design of being completely immersed in liquid helium soup was changed to a new design of launching at room temperature and cooling with liquid helium after reaching space orbit, which greatly saves the amount of liquid helium and the load of the launch. In the new design, all terminal equipment is firmly fixed, and only one instrument component that needs to be moved is retained (the scanning mirror of MIPS). Try to ensure the success of the telescope with the smallest variables.

A photo of the telescope assembled in a clean room. Copyright: NASA/SSC

The design of all hardware was basically completed in 1996. Huang Jiasheng, a researcher at the National Astronomical Observatory, participated in the debugging of the IRAC instrument before the launch of the Spitzer Space Telescope in the late 1990s and early 2000s. He recounted an anecdote. In the laboratory stage, the IRAC instrument was originally equipped with a shutter like an ordinary camera to accurately estimate the exposure time and measure the background under darkroom conditions. The test results in the laboratory have always been very smooth. However, just after the IRAC instrument was installed on the telescope, a malfunction occurred during the shutter test and it stopped in the closed state. If this happened after the launch, it would mean "dying before achieving success" and would directly declare that the instrument could not work properly. The IRAC team was very worried and urgently convened 10 engineers to study the cause. It was finally discovered that the abnormality of the shutter was caused by the magnetization of the metal rod, and the problem was solved by reversing the direction of the current. Despite this, in the final stage of the launch, NASA evaluated the risks and decided not to use the shutter, allowing IRAC to go to the sky "open" to further prevent possible risks.

In terms of software, the Infrared Processing and Analysis Center (IPAC) of the California Institute of Technology has specially established the Spitzer Space Telescope Science Center for this purpose, which is responsible for data processing and analysis, and accepts observation project applications from all over the world. The background problem corresponding to the above shutter is estimated and removed through later data processing. Similarly, an accident occurred in the IRS instrument of the Spitzer Telescope before it went into space. A small crack appeared on the filter, and the diffraction stripes it produced would directly affect the quality of the observation data. However, the launch is imminent, and it is too late to grind a new filter. We can only pray that the thrust during the launch will not expand the crack. Fortunately, the launch and orbit entry went smoothly, and the crack did not change. The ground personnel also effectively removed the influence of this interference term on the spectrum by solving the diffraction stripe equation in the later data processing stage.

During the process of repeated demonstrations, the competitor of SIRTF at that time, the 60-centimeter infrared space observatory ISO of the European Space Agency (ESA), was launched in 1995 and retired in 1998. ISO provides images and spectral information in the mid- and far-infrared ranges of 2.4 microns to 240 microns and 25 microns to 197 microns, respectively. Although the final mirror of the Spitzer telescope is only 85 centimeters, which does not seem to have been improved much, due to the rapid technological development from receiver to control system design at the same time, the sensitivity, efficiency and accuracy of the Spitzer telescope have been improved by more than one order of magnitude compared with the initial design. From the perspective of the net speed of observation alone, the speed increase from 1986 to 2003 exceeded 1,000 times. Therefore, among the space infrared telescopes of the same period, although the Spitzer telescope was launched first and then manufactured, its scientific discoveries, especially those in galaxy cosmology, far exceeded those of its competitors, and even exceeded the imagination of the earliest design. Even astronomers associated with the Spitzer Telescope often said, "Relax, Moore's Law will help you solve most of the problems," when they later mentioned the 30 years of long wait and perseverance.

The design changes that the Spitzer telescope underwent from the early 1990s to its final launch were mainly reflected in the size of the instrument. The expected lifespan and size of the telescope remained basically unchanged, but the budget and the weight of the instrument were greatly reduced. Source / Werner/Michael/Eisenhardt/Peter/ More Things in the Heavens (Princeton Press)

Although the emphasis was placed on "small but precise" when it was designed, the Spitzer telescope's powerful infrared observation capabilities cover a wide range of sciences, from the solar system, planet formation, and exoplanets to stars and nebulae, galaxies, and cosmology, directly bringing our understanding of the universe to a new level. It more accurately measured the Cepheid variables in the Large Barley Cloud and the rate of cosmic expansion; in collaboration with HST, it detected the mass and age of galaxies in the early reionization period of the universe (650 million years after the Big Bang); by detecting the temperature and radiation of dust, it recorded the history of star formation as the universe aged, and found that our universe had reached its growth (star formation) peak 3 billion years after the Big Bang (redshift 2-3), and has been declining for the next 10 billion years; it found that many galaxy clusters existed in the early universe, which is very important for our understanding of the large-scale structure and evolution of the universe; in addition, the Spitzer telescope has also discovered a large number of dusty active galactic nuclei, which represent supermassive black holes in the center of galaxies that are frantically absorbing surrounding matter, most of which are difficult to see optically due to dust obstruction.

In the Milky Way, the Spitzer telescope has observed the formation of young stars in clouds and discovered traces of planet formation and protoplanetary disks around brown dwarfs. Among them, there may be the closest exoplanets to Earth. Throughout its life, especially in the more than 10 years of working in "thermal mode", the Spitzer telescope has focused on the identification and observation of near-Earth asteroids, comets and planetary systems outside the solar system. The most famous of these is the discovery of the Trappist-1 system with 7 Earth-sized planets. Many of these extrasolar planetary systems are very similar to our solar system. On the one hand, this proves that our solar system itself is not special in terms of structure and composition; on the other hand, it also provides a direction for exploring possible life forms and possible living environments outside the Earth.

An artist's impression of the Trappist-1 system discovered by the Spitzer Telescope, an ultra-cool red dwarf star about 40 light-years from Earth, Trappist-1, is surrounded by seven Earth-sized planets. As the distance from the star increases, water will evaporate due to excessive heat (shown as water vapor in the picture), or condense into ice due to excessive cooling. At the right intermediate distance, water can exist in liquid form. This range is considered suitable for life and is called the "habitable zone." Copyright / NASA/JPL-Caltech/R.Hurt (IPAC)

The Spitzer telescope was originally expected to operate for 5 years, and the design operating period of the refrigerant was only 2.5 years. In fact, compared with the original design of liquid helium soup, the evaporation of liquid helium caused by instrument heating and natural temperature rise was well controlled. The Spitzer telescope worked for a total of 6 years in "cold mode" at the low temperature required for operation (5 Kelvin, -268 degrees Celsius). After consuming the refrigerant, the Spitzer telescope continued to operate in "hot mode" for more than 10 years using the instrument's own radiation cooling. As the noise increased due to the increase in system temperature, after entering the "hot mode", IRS and MIPS stopped working, and the remaining 3.6 microns and 4.5 microns of IRAC remained strong for many years until January 31, 2020, when it was finally officially retired after the funds were exhausted. It has been in orbit for a total of 16 years and 4 months, far exceeding the originally designed 5-year working time.

——Selected from the November 2021 issue of China National Astronomy

About the Author/

Dai Yu is a researcher at the National Astronomical Observatory of China. His research direction is multi-band observation of the formation and evolution of galaxies.

November issue of "China National Astronomy"

Editor/Huaichen Huan

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