Usually when a spacecraft goes into orbit, it orbits something — but in the case of L2, there’s nothing there. And yet the Lagrangian Points are superb anchor points for spacecraft that shun more orthodox orbits.
L2 is located in a direct sun-Earth line, 1.5 million kilometers from our planet’s surface away from the sun. It is constantly shaded from solar radiation by Earth’s shadow, making it the perfect hitching post for highly sensitive space telescopes that need a region of calm to operate.
“Lagrange points are special — it’s true there’s nothing there,” says Markus Landgraf, a mission analyst at ESOC, ESA’s operations center in Darmstadt, Germany. “They are points where the gravitational forces between two masses, like the sun and Earth, add up to compensate for the centrifugal force of Earth’s motion around the sun, and they provide uniquely advantageous observation opportunities for studying the sun or our Galaxy.”
The location of the L2 point in the sun-Earth system.
Five Lagrangian Points, or “liberation points,” can be found in the sun-Earth system. L1 is located directly between the sun and Earth, 1.5 million kilometers from our planet’s surface; L2 is located directly away from the sun-Earth line (on the opposite side of Earth from L1); L3 is on the sun-Earth line, but located on thefar side of the sun; L4 and L5 can be found 60 degrees off the sun-Earth line.
Although this gravitational island is ideal for spacecraft like Gaia, L2 is inherently unstable, meaning spacecraft need to constantly use their thrusters to avoid slipping away from stability. To avoid unnecessary fuel wastage, a “quasi-periodic orbit” can be employed to keep the spacecraft in place while preserving valuable thruster supplies. This is when orbital dynamics experts employ the use of the exotic-sounding “Lissajous orbit.”
In the case of Gaia, it is currently orbiting the L2 point in a squished 700,000 kilometer by 170,000 kilometer L2 Lissajous orbit. Needless to say, this kind of crazy looping orbit requires shedloads of modeling, computations and special care by members of the European Space Agency’s dynamics team, lest the orbit collapse into a horrid, chaotic mess.
“That is where expertise and experience are indispensable to reconsider the assumptions and then start all over,” says Frank Dreger, Head of Flight Dynamics. “There’s no commercial source for this sort of software or expertise — it’s been built up over many years at ESOC and represents a capability that is rare in the world and unique in Europe.”
This is by no means the first mission to hang out in L2. It’s joining another European mission, the Planck space telescope. Other spacecraft to sit at L2 included Europe’s Herschel infrared telescope, China’s moon-asteroid mission Chang’e 2 and NASA’s Wilkinson Microwave Anisotropy Probe, but all have since ended their L2 tenure. NASA’s flagship mission, the James Webb Space Telescope, is scheduled to be launched to the L2 point in 2018.
Gaia’s mission is to measure the location, distance and motions of one billion stars in our galaxy —it is the most ambitious astrometry mission to date. The project will monitor the same stars a planned 70 times over the next 5 years to construct a 3D map of the Milky Way.