Which orbits are used for space telescopes?

Launching into space is enormously costly and difficult, so deciding where to locate an observatory must be a compromise between astronomical advantages and the payload capabilities of the launch systems.

Purely from the viewpoint of the energy required for launch, the “Low Earth Orbit”, at an altitude of about 500 km, is the most economical. Much lower than that, drag in the residual atmosphere would cause an observatory to fall after just a few months. To benefit from the impulse supplied by the Earth’s rotation, the orbit should be nearly equatorial. This is why launching facilities are preferably built at low latitudes: 28.5◦ for the USA (Cape Canaveral, Florida) and 5◦ for Europe (French Guyana). Both facilities are on the eastern side of a continent so that, in case of failure, the launcher will fall

◦ latitude because into the ocean. The Russian launch center (Baikonour) is inland at 45 that country’s ocean shores are not far enough south. The period of a low Earth orbit is given by Kepler’s third law. At an altitude of 600 km a satellite completes one orbit around Earth in 96 min.

The Low Earth Orbit is protected from cosmic rays and solar wind particles by the magnetosphere, an important advantage for astronomical observations. Nevertheless, there are several drawbacks:

1 Roughly half of each orbit is unusable because the Earth occults the sky field under observation.

Principal orbits and locations for astronomical observatories. Distances to Earth, expressed in km and also in light-travel time, are not to scale.

2 Telescopes must be carefully protected from the strong sunlight reflected from Earth during the orbital day (even with a 4 m-long baffle in front of it, the HST cannot observe at less than 80◦ from the horizon of sunlit Earth). 3 Heat from the Earth nearby creates a strong parasitic background detrimental to

infrared observations (an effect that can only be eliminated by being much further away from Earth).

A much better solution is to locate observatories at the second Lagrange point, L2 , of the Sun–Earth system, where all these problems completely disappear. Travel time from the Earth to L2 is about 100 days, and the launch can be assisted by passing close to the Moon. Several observatories, planned or already in operation, are located at L2 , including WMAP, JWST, Planck, and Herschel.

Another solution is to launch an observatory at escape velocity – that is, with just enough acceleration to escape the pull of gravity, then let it slowly drift away from Earth. This a very economical solution from the viewpoint of launch energy since no orbital insertion maneuver is required. The observatory is then on a heliocentric orbit called an “Earth-trailing orbit.” The drawback is that the observatory drifts gradually away from Earth at about 0.1 AU/year. This is because of the uncertainty about the exact amount of impulse the launching system really provides: to avoid the risk of the spacecraft falling back to Earth, the launch impulse margin must be positive. The problem is that after several years, the observatory has floated so far away that communication become impossible.


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