Why are telescopes positioned in Lagrange points?

Question

In this Wikipedia article about the list of space telescopes to be launched (which I assume is exhaustive), of the 11 telescopes yet to be launched, 6 will be positioned at the Sun-Earth L2 Lagrange point. Why is that so?

I read that it has something to do with lower fuel consumption, but I would like a more detailed answer.

Answer 1

tldr;

L2 is a very stable thermal environment as well as good instantaneous sky visibility and high observing efficiency.

The main reason space telescopes are placed in an L2 orbit is because L2 is a stable thermal environment. Telescopes in Earth orbit can receive sunlight and earthlight in different directions, meaning that the telescope would have to shield in most directions if it wanted to keep itself cool. At L2, the Sunlight and Earthlight come from the same direction, meaning the telescope has to only shield itself from that direction. This is very important for infrared telescopes, as heat radiation acts as interference to measurements. Of the 6 space telescopes going to L2 listed, 3 are infrared telescopes.

For example JWST, one of the infrared telescopes, has to cools some of it's detectors to 7 kelvin to operate properly.

JWST L2 Halo Orbit

The other main benefit from being in L2 is that being so far away from Earth means that there is a much greater available field of view as the Earth blocks a very minor portion of the sky. This increases sky visibility and observation efficiency.

Other Benefits:
- Ease of communication
- Delta-V requirements low (2-4 m/s per year)

Answer 2

For this answer, I'll consider space telescopes to be telescopes that operate in space and that are intended to look at objects at the extremes of the solar system and beyond. This excludes Earth observation satellites, satellites that monitor the Sun, and satellites sent to another planet to observe that other planet as "space telescopes".

Every space telescope has two primary objectives. One is to view objects in space, as commanded from the Earth. The other is communicating with the Earth (receiving commands, relaying observations back to Earth). There are many alternatives regarding where to place such devices. These include

• Low Earth orbit

  • Description:
    • The satellite that contains the space telescope is injected into a low Earth orbit shortly after launch.
  • Examples:
    • Hubble Space Telescope
  • Advantages:
    • This is the cheapest of all alternatives in terms of launch ΔV.
    • Repair / replenishment is possible at this altitude. The Hubble is proof of that.
  • Disadvantages:
    • The very high orbital velocity complicates viewing, communications.
    • Being so close to the Earth means that it obscures a large portion of the sky. The closeness also means the Earth is a significant heat source for infrared and microwave astronomy.
    • Significant external torques result from high Earth gravity gradient and atmospheric drag and must be countered. LEO is probably the most expensive of all alternatives in terms of attitude maintenance.
    • This, and to a much less extent, geosynchronous orbits, are the only alternatives where debris poses a threat.

• Geosynchronous / geostationary orbit

  • Description:
    • The satellite is injected into a geostationary transfer orbit shortly after launch. Later, when the satellite reaches geostationary altitude, another burn is performed to raise perigee to geostationary altitude.
  • Examples:
    • International Ultraviolet Explorer
    • Hipparcos. That was the intent; Hipparcos's apogee boost motor failed to fire, so it remains in a geostationary transfer orbit.
  • Advantages:
    • The Earth isn't as big (compared to LEO), but it's still large compared to other alternatives.
    • Communication is easy; this is the cheapest of all alternatives regarding communications -- assuming the vehicle does make it to the intended geosynchronous orbit.
  • Disadvantages:
    • Surprisingly, this alternative is rather expensive in terms of launch ΔV. The ΔV cost to go from the Earth's surface to GEO is higher than is the ΔV cost to achieve escape velocity from the Earth's surface.
    • Fuel is needed for orbit maintenance as well as for attitude control.
    • Fuel should be reserved for end-of-life activity.

• Highly eccentric Earth orbit

  • Description:
    • The satellite is injected into a highly eccentric orbit shortly after launch. Later, when the satellite reaches apogee, another burn might be performed to raise perigee a bit (but not a whole lot).
  • Examples:
    • Chandra X-ray Observatory
  • Advantages:
    • Can be cheap in terms of launch ΔV, not as cheap as LEO, but cheaper than GEO.
  • Disadvantages:
    • Low perigee means orbit might cross Van Allen radiation belts and might even descend into altitudes where atmospheric drag is present.
    • Dives toward perigee typically interrupt operations.

• Very high Earth orbit

  • Description:
    • The satellite is injected into a highly eccentric orbit shortly after launch. Later, when the satellite reaches apogee, another burn is be performed that raises perigee by a large amount.
  • Examples:
    • I know this option has been discussed, but I can't find any.
  • Advantages:
  • Disadvantages:
    • Very expensive in terms of launch ΔV.
    • Can be expensive in terms of orbit maintenance ΔV due to perturbations from the Moon and the Sun.

• Earth-trailing heliocentric orbit

  • Description:
    • The satellite is injected into an Earth escape orbit shortly after launch. The resulting orbit has a period that is a bit longer than one year. Later, when the satellite does escape the Earth's gravity field, another burn might be performed that ensures the vehicle won't come back to Earth years / decades later.
  • Examples:
    • Spitzer Space Telescope
    • Kepler Space Telescope
  • Advantages:
    • This alternative is surprisingly cheap in terms of launch ΔV. The only alternatives with a lower launch ΔV are LEO and highly eccentric Earth orbits.
    • Zero orbit maintenance ΔV.
    • Low attitude maintenance costs.
    • Thermal radiation from the Earth more or less is a non-issue.
  • Disadvantages:
    • Communication systems are not cheap. The Deep Space Network (or equivalent) is needed on the Earth, and a simple fixed antenna is not sufficient on the satellite.

• Finally, the Sun-Earth L2 point

  • Description:
    • The satellite is injected into a complex transfer orbit shortly after launch that eventually carries the vehicle close to the Sun-Earth L2 point. When it gets close, the vehicle injects itself into a pseudo orbit (either a halo orbit or a Lissajous orbit) about the Sun-Earth L2 point.
  • Examples:
    • Lots and lots of them.
  • Advantages:
    • The telescope can always be pointed so that all three of the Sun, Earth, and Moon are always behind the telescope. This alone is a huge advantage.
    • Communication is simpler than communicating with a satellite that is receding from the Earth. The Deep Space Network is not needed to receive data from a satellite orbiting Sun-Earth L2, and the antenna on the spacecraft can be fairly simple.
  • Disadvantages:
    • Somewhat expensive in terms of launch ΔV.
    • Fuel is needed for stationkeeping.

Answer 3

Satellites positioned at L2 has the sun, earth, and moon all behind it so it gets a continuous view of deep space. The Webb Space Telescope will be positioned there. A telescope at L1 would have a continuous view of the Sun and the SOHO satellite is currently there. This link explains some of the benefits in general terms.