After more than 16 years of searching the universe, the time has come for the Spitzer Space Telescope to retire: NASA's mission starring one of the largest telescopes ever built has come to an end. On Thursday 30 January the telescope has switched to safe mode, stopping all scientific operations.
Launched in 2003, Spitzer was one of NASA's four major observatories, along with the Hubble Space Telescope, Chandra X-ray Observatory and Compton Gamma-ray Observatory. Let's find out why this telescope made the history of astronomical observation.
See us better
The Universe continuously radiates a large amount of information on Earth, signals that cover a wide spectrum of electromagnetic radiation. However, not all of these messages reach the ground. Since the atmosphere of our planet blocks most of the radiation from space, it is necessary to position the telescopes beyond the boundary that keeps us alive, to try to reduce perturbations to zero and maximize the reception of these signals.
Although not the first infrared telescope built by NASA, Spitzer was the most sensitive infrared telescope in history when it was launched 17 years ago, and offered a deeper and wider view of the cosmos than its predecessors.
Infrared
Many of the "messages" from the depths of the universe are transmitted through infrared light. In space, any object that has a temperature above zero Kelvin (-273.15 degrees Celsius) emits radiation in the infrared band. But the Earth's atmosphere heavily filters this wavelength and for years astronomers have tried to position telescopes beyond the atmosphere. However, it is only in recent decades that scientists have finally managed to start and compose a team of respectable orbital observers: the program of the Great Observers.
This program has demonstrated all the benefits of using different wavelengths of electromagnetic radiation to be able to create a more complete picture of the universe.
Among the great observers is obviously NASA's Spitzer Space Telescope, but the program also includes the Hubble Space Telescope (which operates in visible light), the Compton Gamma – Ray Observatory and the Chandra X-Ray Observatory.
The success
Among the numerous scientific contributions that can be attributed to the Spitzer Space Telescope, the one given must certainly be highlighted to the study of comets and asteroids in our solar system. But his power has contributed to enrich the information regarding much more massive objects: he was the discoverer of a new ring (never identified) around Saturn.
He also studied the formation of stars and planets, the evolution of galaxies from the early universe to today and the composition of interstellar dust. It has also proved to be a powerful tool for detecting exoplanets and defining their atmospheres.
Spitzer's best-known work is related to the TRAPPIST-1 planetary system, 7 orbiting planets, which is today is the largest number of terrestrial planets ever found in orbit around a single star. In total Spitzer studied the TRAPPIST-1 system for over 1,000 hours and was able to determine the masses and densities of all the planets in the system.
In 2014, it detected collisions of asteroids in a newly formed planetary system, providing evidence that such destruction could be common in early solar systems and critical to the formation of some types of planets. In 2016, Spitzer worked with Hubble to capture an image of the most distant galaxy ever detected, GN-z11. Below is a video of NASA's Youtube channel showing some shots of the galaxy in question.
The instrumentation
The Spitzer Space Telescope is a technological marvel, with many technical solutions never used before on a space mission. It is about 4 meters high and weighs about 865 kilograms. As previously mentioned, Spitzer is designed to detect infrared radiation, which is mainly heat radiation. To do this, it uses several components.
The first is called Cryogenic Telescope Assembly and houses Spitzer's cold components, including the 0.85 meter telescope, three scientific instruments and the spacecraft, which manages the telescope controls, supplies energy to the instruments via solar panels and sends scientific information to Earth.
But what exactly is it that made the spitzer one milestone of astronomical observation? Answer: its instrumentation. Here she is.
Cryogenic Telescope Assembly
NASA's Spitzer Space Telescope needed a particular operating temperature throughout the duration of its mission. Everything inside the telescope had to be cooled down to a few degrees above absolute zero (-273 degrees Celsius). This result was possible through the use of an onboard tank containing liquid helium.
Extremely low temperatures became a necessity so that the observatory's "body heat" did not interfere with its observations of relatively cold cosmic objects. At the same time, electronic equipment instead required an average "ambient" temperature in order to function properly. In total, the CTA is made up of four parts: the Telescope, the Multiple Instrument Chamber (Infrared Array Camera, Infrared Spectrograph, and Multiband Imaging Photometer), the Cryostat and the outer shell.
The telescope
The Spitzer telescope mounts a Ritchey-Chrétien type optic, with a mirror that measures 85 centimeters in diameter. It weighs less than 50 kg and is designed to operate in extremely low temperatures.
All its parts, with the exception of the supports for the mirrors, are made of beryllium, extremely light but very resistant. Beryllium is often used in the construction of infrared telescopes, because it has one low heat capacity at very low temperatures, which means it's easy cool it quickly.
It is important that the telescope is built from a single type of material, since different materials they expand and contract to a different extent if placed near equal temperature gradients.
Building the telescope with different materials could cause deformation due to a change in temperature, causing further stress on the joints and causing it to go out of focus. The telescope is attached to the top of the cryostat.
The Cryostat
He is mainly responsible for the very low operating temperatures of the Spitzer: he uses liquid helium vapor to keep the instruments cold. The cryostat contains approx 360 liters of liquid helium and can cool instruments down to 1.4 Kelvin (about -272 degrees Celsius) for more than 5 years. The cryostat is attached to the bottom of the telescope and consists of a helium tank, a vacuum shell, internal shields and a fluid management system.
The telescope and the cryostat shell were launched and only subsequently cooled (to about 35 degrees Kelvin) once in orbit. The vacuum shell of the cryostat was sealed during ground operations and kept closed while the telescope cooled down to protect the delicate instruments installed inside.
The Multiple Instrument Chamber
This is where all the operating tools were actually installed. In this container with a diameter of 84 cm and a height of 20 cm They housed the infrared camera, the infrared spectrograph and the multiband image photometer. It also housed the reference sensor for aiming calibration, which is part of the aiming control system, which helps ensure that the telescope is pointing in the right direction. The "Chamber" is generally built to be so narrow that no light can pass except the one coming directly from the instruments from the telescope.
The Infrared Array Camera (IRAC)
The infrared camera (IRAC) is one of Spitzer's three scientific tools. It is a machine designed to detect light at medium and near infrared wavelengths – in other words, light with wavelengths between 3.6 and 8.0 microns (1 micron is one millionth of a meter) . It is a camera used by observers for a wide range of astronomical research programs.
Unlike a normal camera, which has a single detector array and is sensitive to a wide range of different light wavelengths, the IRAC is a four-channel camera, which means it has four different detectors, each of which measures light at a particular wavelength.
Capture simultaneous images at wavelengths of 3.6, 4.5, 5.8 and 8.0 microns and each of the four detector arrays in the camera has a size of 256 x 256 pixels.
IRAC uses two different types of material in the detector matrices: the two shorter wavelength channels (3.6 and 4.5 microns) have detectors made of indium and antimony, and the two longer wavelength channels (5.8 and 8.0 microns) have detectors that have been specially treated with arsenic.
The only moving part on the whole instrument is the camera shutter and the IRAC is the only Spitzer instrument that can work even when liquid helium it is no longer in circulation.
The infrared spectrograph (IRS)
The infrared spectrograph allows to carry out both high and low resolution spectroscopy at the medium infrared wavelengths (from 5 to 40 microns).
Like a prism that breaks light into a rainbow, the spectrograph takes infrared light from a distant object and divides it into a spectrum. Each chemical element in the universe has its own signature in the spectrum, like a unique fingerprint. By studying the light spectrum of a distant object, astronomers can understand what elements and molecules it is made of.
The IRS has four different modules: a low wavelength module that detects light with wavelengths from 5.3 to 14 microns; a module for observations between 14 – 40 microns, one for the rage 10 – 19.5 microns and a module high resolution at variable wavelength for detailed observations between 19 and 37 microns. Each module has its own entrance slot to allow infrared light to enter the spectrograph.
The detector arrays are 128 x 128 pixels in size. Short wavelength detectors are treated with arsenic while longer wavelength silicon detectors use antimony. The IRS unlike the IRAC it has no moving parts.
The Multiband Imaging Photometer (MIPS)
The multiple band photometer is a machine that detects light in far infrared, at wavelengths of 24, 70 and 160 microns. MIPS it is also capable of performing simple spectroscopies, just like the IRS. The array of detectors for the 24 micron mode is 128 x 128 pixels and is made of silicon which has been specially treated with arsenic.
All these numbers translate into the fact that MIPS can watch a 5 x 5 arc-minute section of the sky with a detail of 24 microns. The only moving part in MIPS is a scan mirror which is used to map large areas of the sky. Here is what you can get from this tool:
In the end, but not entirely
Spitzer was originally built to last for at least 2.5 years, but has managed to be operational for over 5.5 years. On May 15, 2009 the coolant ran out and Spitzer's "hot phase" began.
So Spitzer's main mission ended in 2009, when the telescope ran out of supply of the helium coolant needed to operate two of its three instruments.
The mission was considered a success, having achieved all its primary and even scientific goals exceeded expectations. But the story of Spitzer it was not over. Engineers and scientists were able to carry out the mission by using only two of the four wavelength channels on the third instrument, the infrared camera. Despite growing engineering and operational challenges, Spitzer has continued to collect information for others 10 and a half years, much longer than expected in the mission plans.
In 2016, following a review of the operational missions, NASA decided to end Spitzer's in anticipation of the launch of the James Webb Space Telescope, which will also observe the universe in infrared light (we have dedicated a special to this topic).
Spitzer's legacy is immense, and is the result of decades of collective effort by the scientific community. Hundreds of people have contributed directly to Spitzer's success and thousands have instead used his scientific skills to analyze collected data.
We leave you with the words of the Spitzer mission project scientist, Michael Werner: "I think Spitzer is an example of the excellence that people manage to achieve. I feel very lucky to have worked on this mission and to have seen the ingenuity and genius shown by the people who took part in it. When you allow people to take advantage of these qualities, then incredible things happen".
