Herschel space telescope finishes mission

By Jonathan Amos Science correspondent, BBC News

Europe's flagship space telescope has stopped working.

The billion-euro Herschel observatory has run out of the liquid helium needed to keep its instruments and detectors at their ultra-low functioning temperature.

This equipment has now warmed, meaning the telescope cannot see the sky.

Herschel, which was sensitive to far-infrared and sub-millimetre light, was launched in 2009 to study the birth of stars and the evolution of galaxies.

Its 3.5m mirror and three state-of-the-art instruments made it the most powerful observatory of its kind ever put in space.

The end of operations is not a surprise. Astronomers always knew the helium store onboard would be a time-limiting factor.

The "blind" satellite is currently located about 1.5 million km from Earth on the planet's "night side".

Controllers at the European Space Agency's (Esa) operations centre in Darmstadt, Germany, will run some final tests on the spacecraft in the coming weeks before putting it in a slow drift around the Sun.

"We will push it out into a heliocentric orbit and passivate it," said Micha Schmidt, the Herschel spacecraft operations manager.

"We will switch off the transponder and the spacecraft will go silent."

Herschel should not come anywhere near the Earth again for several hundred years.

Data legacy

The telescope will be remembered for its great vistas of gas and dust; the billowing clouds and threading filaments that trace the locations where future stars will form.

Over the course of the mission, it gathered thousands of such images. It also acquired detailed spectrographic data on many of its subjects, revealing their chemistry.

All of the information is now being assembled into a public archive.

Jonathan Amos inspected Herschel just before its launch in May 2009

This will become an important resource for future study and a starting point to plan follow-up observations with other astronomical facilities.

This is already happening with the recently opened, ground-based Alma telescope in Chile, which views the sky at frequencies that overlap those pursued by Herschel.

A US-German telescope called Sofia, which is mounted on a converted Boeing 747, can also see some of Herschel's frequencies.

"But the amazing thing about Herschel is that its maximum productivity in science terms probably won't be reached for another five years yet," said Prof Matt Griffin, the principal investigator on Herschel's Spire instrument.

"The telescope gathered images and information in such volume that astronomers have barely scratched the database," the Cardiff University, UK, scientist told BBC News.

The Herschel telescope had to be kept extremely cold to study its frigid targets

Engineers issued an alert early in March warning astronomers that observations were coming to an end.

Herschel used special light detectors in its instruments known as bolometers. Although supremely efficient at capturing light, the technology must be kept close to absolute zero (-273C) to work properly.

This was achieved with the aid of 2,300 litres of liquid helium that was held in a giant flask, or cryostat.

But as the mission progressed, the cryogen gradually boiled away, and, on Monday, the Darmstadt controllers received telemetry from Herschel confirming every last drop was gone.

Herschel Space Telescope

• Herschel was one of the largest space telescopes ever launched; its 3.5m diameter mirror perfectly captured infrared light
• It clocked more than 1,440 days of operations; making 22,000 hours of scientific observations; resulting in 600 scholarly papers... so far
• Infrared shines through gas and dust clouds that can block visible light - Herschel could see deep into dusty, star-forming regions
• The telescope was named after the astronomer William Herschel, who discovered infrared radiation while studying the Sun in 1800
• The Earth's atmosphere is an infrared absorber, so Herschel was launched in 2009 to get a clear view of the long-wavelength Universe

• Like hot tea behind a bin bag - how does Herschel let us see hidden stars?

Herschel's demise occurred close to the time forecast at the start of operations nearly four years ago.

If anything, astronomers got a few months' more observations than they were expecting.

Herschel's cryostat approach to cooling was evolved from a previous Esa mission - the Infrared Space Observatory (ISO), which operated in the 1990s.

This approach is described as a "passive" system because once initial conditions are set inside the flask, a continuing presence of helium and good insulation is all that is required to maintain those conditions.

An "active", or mechanical, cooling system was considered for Herschel in the initial feasibility studies. This would have involved a chain of Stirling units that use a cycle of compression and decompression in a fluid to get to low temperatures.

Theoretically, mechanical coolers could have given Herschel more life, but engineers considered such a design to be too risky.

"There was a competitive concept but it involved a lot of stages, a lot of machines," recalls Jean-Jacques Juillet, the director of scientific programmes at Thales Alenia Space, the company that led the industrial development of Herschel.

"If one of those stages had failed, it could have been a disaster for the continuity of the mission. The cryostat option was the safest option," he told BBC News.

With the cryostat path adopted, engineers then set about constructing the largest possible helium vessel they could fit inside an Ariane launch rocket.

Esa hopes to join a future far-infrared telescope project called Spica. This is a Japanese venture that could fly in the early 2020s.

Europe would provide important components, including the primary mirror and a spectrograph. Unlike Herschel, Spica is likely to use mechanical coolers.

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Herschel has built a huge catalogue of far-infrared and sub-mm images of the sky. This recently released image shows the layers of gas shed by Betelgeuse, one of the most familiar stars in the night sky

Herschel will be remembered as the telescope that produced great vistas of gas and dust, as here in the constellation of Cygnus. Invisible to optical telescopes like Hubble, these billowing clouds and threading filaments trace the locations where future stars will form

With its far-infrared detectors, Herschel has been able to re-visit familiar objects to provide new data. This picture shows the classic “Pillars of Creation” in the Eagle Nebula, in the constellation Serpens. At long wavelengths, the clouds of gas and dust in the pillars glow, revealing processes at play in star formation

Herschel has sought to study the evolution of galaxies. In this image taken of a small patch of sky in the constellation of Ursa Major, countless galaxies are recorded. Some of these fuzzy blobs are many billions of light-years distant

One of Herschel's instruments was called HIFI. The spectrograph's job was to untangle the chemistry of space. Its study of Comet Hartley 2 showed the object to have water-ice with a similar composition to Earth's oceans

At a relatively close distance of 2.5 million light-years, Andromeda is very similar in size to our own Milky Way Galaxy. Here, Herschel's observations (orange) are combined with the Newton XMM telescope's (blue)

Herschel's view of RCW 120, a bubble of gas and dust in space around a massive star. Another huge star, 8-10 times the mass of our Sun, is forming on the bubble's boundary - seen as a bright white object

Herschel has revealed new things about our own galaxy. Here, looking towards the centre of the Milky Way, it spies a ribbon of cold gas and dust that is more than 600 light-years across and which appears to be twisted. Scientists have yet to explain it

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