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The Herschel Mission


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The Herschel Space Observatory is the European Space Agency's (ESA) state of the art infrared space telescope. The satellite was launched into space on 14 May 2009, at 13:12 UTC by an Ariane 5 launcher from the ESA Space Centre in Kourou, French Guiana, together with the Planck satellite (right, video) into a special orbit one and a half million km away from the Earth and is currently observing the Universe at infrared wavelengths.

The satellite is named after Sir William Herschel (1738-1822, below) who discovered infrared radiation. Herschel is also famous for his discovery of the planet Uranus in 1781 along with two of its moons. In addition to his astronomical interests, he was also an accomplished classical composer of 24 symphonies.

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Herschel stands approximately 7.5m tall and 4m wide weighing around 3.4 tonnes at launch.

The cut away (below) shows the major components of the satellite. To protect its delicate instruments, Herschel always points perpendicular to the Sun and is shielded by a large sunshade. On the same side of the spacecraft, solar panels supply power.

 

Light is collected by Herschel's large 3.5m primary mirror and reflected up to a smaller secondary mirror before being sent down Herschel's throat to the instruments at the base of the spacecraft.

Since Herschel is specifically design to look at cold and dusty objects, the entire spacecraft has to be cooled to very low temperatures of almost absolute zero (-273 Celsius). This cooling is achieved by seating the telescope atop of a huge tank of over 2000 litres of liquid Helium. The Helium slowly evaporates into space taking excess heat with it. After about 3 years on scientific observations the Helium tank will be empty and the mission will be finished.
Herschel carries and suite of three instruments to make detailed observations of our cosmos that sit within the base of the cryostat at extremely low temperatures.
At the base of the spacecraft there is a service module that encloses the control system and electronics.

 


 

Orbit

Since Herschel is an infrared telescope it is very sensitive to heat radiation, therefore the best place to put it is a far away as possible from the Earth, Moon and Sun. In fact the Herschel satellite is not even in orbit around the Earth but rather orbits a special gravitational equilibrium point 1,500,000 km away from the Earth know as the L2 point. At this point, the orbit of Herschel is about a million kilometres in diameter.






The L2 point is one of a family of five such equilibrium points known as the Lagrangian points. At a  Lagrangian point  a small object affected only by gravity can theoretically be stationary relative to two larger objects (such as a satellite with respect to the Earth and Moon). The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centrifugal force required to orbit with them and can be compared to geostationary orbits around the Earth which always orbit over a fixed position. The L1 point lies between the Sun and the Earth, L2 lies beyond the Sun-Earth-Moon system on the Earth side, while L3 lies on the far side of the Sun. The L4 and L5 points lie at the tops of two imaginary equilateral triangles where the Sun-Earth masses balance each other.


The Sun–Earth L2 point, is a good spot for space-based observatories since an object around L2 will maintain the same orientation with respect to the Sun and be shield from emission and straylight from the Sun, Earth and Moon. The L2 point will become valuable real estate for many forthcoming long-wavelength missions.