The largest infrared space telescope ever launched has run out of cryogenic coolant, permanently ending its science operations.
|Update: On June 17th mission controllers sent Herschel into its "disposal" orbit. The bon voyage marks the mission's official end. Read more in this ESA announcement.|
Herschel launched May 14, 2009 with the Planck satellite and was the first space observatory to cover far-infrared to submillimeter wavelengths, its observations spanning 55 to 672 microns. Both spacecraft went to L2, the second Lagrange point in the Sun-Earth gravitational system where a small mass can basically “hover” without being pulled this way or that.
Herschel’s 3.5-meter primary mirror worked with two cameras (both with spectroscopic abilities) and a high-res spectrometer, all of which were cryogenically cooled by superfluid liquid helium to a couple of degrees above absolute zero, -271°C (-456°F) — as least until Herschel ran out of coolant.
Mission planners estimated that Herschel would exhaust its helium supply in late March. It lasted one month beyond that projection. But without coolant its science operations are now impossible.
Herschel’s main objectives were to study the formation of stars and early galaxies, the universe’s molecular chemistry, and the chemical composition of solar system atmospheres and surfaces. Observing in infrared is key for this work: infrared wavelengths reveal stuff a few tens of degrees above absolute zero (such as nebulae) and also pass through the dusty clouds enshrouding young stars. In addition, cosmic expansion has shifted visible light from the early universe to infrared wavelengths, meaning studies of that era must be conducted in infrared. (Incidentally, that’s why the infamous James Webb will focus on infrared, too — the infrared range is possibly the most information-heavy of the entire electromagnetic spectrum.)
For far-infrared observers, Herschel was a major upgrade from NASA’s Spitzer Space Telescope, which has a primary mirror only 0.85 meters wide and observed from 3 to 180 microns while it had coolant. (Spitzer is still working in “warm mode” and observes at 3.6 and 4.5 microns — i.e. it only sees the toastier stuff now.)
Herschel allowed astronomers to zoom in on the formation of massive stars, those that will someday cataclysmically die to form neutron stars and black holes. Its observations revealed that these stars grow on the margins of rapidly expanding bubbles in the interstellar medium, where the expanding bubbles squeeze surrounding gas. In the distant universe, Herschel detected evidence that galaxies' star formation spiked within the first few billion years, and that it's the availability of gas, not the fireworks of galaxies crashing into each other, that fuels such bursts.
It also revealed that the water ice of Comet 103P/Hartley 2 has the same hydrogen-deuterium ratio as Earth’s water — a much-sought data point for those who argue Earth’s water comes from comets. However, Hartley 2 is the only comet with the right ratio, and other data suggest that the comet is actually not such a great match to Earth’s composition after all. Increasingly, astronomers are arguing that Earth formed with its water and didn’t need alien delivery.
Another water discovery came in late April from Herschel: the detection of water in Jupiter's upper atmosphere, left there by Comet Shoemaker-Levy 9 in 1994. Even though it's been nearly 20 years since the comet smashed into Jupiter, Herschel's observations show more water in the southern hemisphere (where the comet hit) than in the north and suggest that 95% of the planet's current water comes from that comet.
Using Herschel, astronomers found out that what they’d long thought was a dark cloud hiding star formation next to the reflection nebula NGC 1999 is actually a hole in space. While black in visible light, the purported cloud should have shown up in infrared — and it wasn’t there. Instead, nearby stars somehow blew out that section of gas.
I could make a long list of Herschel’s work, but hey, the ESA has already done that for me: check out the full list of Herschel press releases, which go back even before its launch. You can also read the ESA obit for Herschel, which provides a nice wrap-up.
With its usefulness ended, Herschel will have to leave L2. The mission team will send the spacecraft into a “no-return heliocentric orbit,” which means it will take at least 300 years to return to the Earth-Moon system’s gravitational playground. Maybe by then, space retrieval systems will be so good that astronauts will grab it and bring it back for 24th-century children to gawk at in the Smithsonian.