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Cold telescope faces hot death

Herschel space observatory nears its end after unravelling star formation and tracking dust from supernovae.

After more than three years of observations, astronomy’s premier infrared space telescope is about to catch a fever and die. Later this month, Europe’s Herschel space observatory, which has helped astronomers to revise theories about the birth and death of stars, will exhaust its stores of liquid-helium coolant, and its instruments will begin to heat up. At that point, “all the scientific instruments will shut down within hours”, says Göran Pilbratt, the mission’s project scientist at the European Space Research and Technology Centre in Noordwijk, the Netherlands.

Astronomers are hailing the legacy of the €1.1-billion (US$1.4-billion) mission, which has made some 22,000 hours of observations in the far infrared and submillimetre wavelengths, a part of the electromagnetic spectrum blocked by Earth’s atmosphere. In an era when scientific spacecraft are increasingly specialized, the 3.5-metre Herschel telescope was a rare general-purpose observatory, used by more than 2,500 astronomers. “Anyone you ask who’s been involved with Herschel has their own favourite results,” says Matthew Griffin, an astronomer at Cardiff University, UK. “There’s something for everybody.”

Herschel orbits the L2 point, 1.5 million kilometres away in the cold shadow of Earth, where the combined gravity of the planet and the Sun create a ‘gravitational well’. This shady perch, together with 2,300 litres of liquid helium, allowed Herschel to cool its instruments to a chilly 2.2 kelvin. At that temp­erature, the spacecraft could observe the low-temperature glow of gas and dust in stellar nurseries and in the shells of supernovae.

The cold Universe has held surprises. For example, astronomers thought that young stars form from long filaments of gas that collapse smoothly under their own gravity. Herschel painted a more complicated picture. When it looked at star-forming regions, it saw swirling, churning flows of gas driven by turbulent winds. Researchers now think that turbulence, rather than gravity, creates dense patches in the filaments that eventually collapse into stars, says Griffin. “That’s a challenge for theoreticians.”

The deaths of stars yielded other revelations. Astronomers had thought that most of the dust in the Galaxy forms in red giants, which puff it into space as they shrink in their waning years. Instead, Herschel detected massive amounts of dust in the shock waves around supernovae, says Alexander Tielens, an astrophysicist at Leiden University in the Netherlands. “I think Herschel really nailed that super­novae make a lot of dust.”

Outside the Milky Way, Herschel enabled observations of dusty galaxies from 10 billion years ago — when most of the Universe’s stars were forming. The data show that stars tended to form evenly across these early galaxies, rather than being spurred by galactic mergers, says Gordon Stacey, an astronomer at Cornell University in Ithaca, New York. They also show that some giant black holes at the centre of galaxies, known as active galactic nuclei, hurl out jets of gas so powerful that they may prevent stars from forming in the vicinity. “It’s pretty exciting to actually see these processes in action,” says Phil Appleton, head of the NASA Herschel Science Center at the California Institute of Technology in Pasadena.

Herschel also allowed astronomers to look at a range of molecules in the Milky Way. Hydrogen fluoride worked as a tracer to reveal larger clouds of hydrogen gas, the building blocks of star formation. And water vapour turned up sometimes in unexpected places: stars made mainly of carbon and Jupiter’s atmosphere, to name but two.

A new generation of instruments will follow up on Herschel’s discoveries. From a perch in the Chilean Andes high enough to observe in the far-infrared, the Atacama Large Millimeter/submillimeter Array (ALMA) will point its dishes at distant galaxies first catalogued by Herschel (see page 156). The Stratospheric Observatory For Infrared Astronomy (SOFIA), a telescope carried by a high-flying 747 jet, will also be able to build on Herschel’s observations. So will NASA’s James Webb Space Telescope, scheduled for launch in 2018.

But “without Herschel there will certainly be a gap”, says Stacey. Infrared astronomers want another space telescope that could make the same ultra-cold observations as Herschel, but with the sensitivity to reach farther into the Universe. To that end, the European Space Agency, which built Herschel, now hopes to collaborate with Japan to build the Space Infrared Telescope for Cosmology and Astrophysics (SPICA), a 3-metre-class telescope that would chill its mirror as well as its instruments. If the project wins funding, it could be launched sometime in the 2020s, says Pilbratt.

In May, after it shuts down, Herschel will be shunted to an orbit around the Sun to eliminate the risk of it falling back to Earth (an alternative plan to send it crashing into the Moon was abandoned owing to cost). But the observatory’s public data archive will continue to lead to discoveries for years to come. “This is not the end of the mission,” says Pilbratt. “This is the end of observing.”

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Herschel overview

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Brumfiel, G. Cold telescope faces hot death. Nature 495, 151–152 (2013). https://doi.org/10.1038/495151a

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