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Artist impression of the Herschel spacecraft
 

Herschel: ESA’s supercool heat seeker

On 14 May 2009, two of the most sensitive space observatories ever built were launched from Kourou in French Guiana. On board Europe’s Ariane 5 rocket were Herschel, the world’s most powerful heat-seeking space telescope, and Planck, which was designed to study the faint radiation that filled the Universe immediately after the Big Bang.

Everything about ESA’s Herschel Space Observatory was big. The satellite was about 7 metres high and 4.3 metres wide, with a mass of 3.25 tonnes. Its telescope had a main mirror that measured 3.5 metres across, the largest ever built for a scientific spacecraft.

The observatory was named after another scientific giant, the German amateur astronomer and musician William Herschel, who discovered the planet Uranus in 1781. 19 years later, he also discovered a form of invisible light that scientists know as "infrared radiation". We often just call it "heat".

Andromeda Galaxy in infrared and X-rays
 
The Herschel observatory was sent to a special orbit 1.5 million kilometres away, far enough to avoid problems caused by heat from the Earth interfering with observations. Its three instruments were cooled by liquid helium so that their temperature was close to absolute zero (-273°C).

Herschel was used to study the ‘cold’ Universe. Its supercool 'eyes' were able to see through dust clouds to study star forming regions and galactic centres, as well as detect hidden stars and planets. It was also able to seek out distant galaxies in order to throw new light on how they formed, billions of years ago.

Herschel’s groundbreaking scientific mission ended on 29 April 2013, when its supply of helium coolant ran dry. However, the satellite continued to be used for orbital tests until the final command to shut it down was sent on 17 June.

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