Gaia in orbit, running rings around L2
Sen—ESA's billion-star surveyor Gaia is now in its operational orbit around a gravitationally stable virtual point in space called 'L2', 1.5 million km from Earth.
Just after 15:30 GMT on January 14 Gaia made a short thruster burn, nudging the galactic survey craft onto its planned scientific orbit about L2 point which consists of precisely nothing.
"Lagrange points are special, it's true there's nothing there," says Markus Landgraf, a mission analyst at ESOC, ESA's operations centre in 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."
As seen from this Lagrange point (there are a total of five such points in the Sun - Earth system), the Sun, Earth and Moon will always be close together in the sky, so Gaia can use its sunshield to protect its instruments from the light and heat from these three celestial bodies simultaneously.
The five Lagrangian points for the Sun-Earth system. An object placed at any one of these 5 points will stay in place relative to the other two. Image credit: NASA.
L2 provides a moderate radiation environment, which helps extend the life of the instrument detectors in space. However, orbits around L2 are fundamentally unstable.
"We'll have to conduct stationkeeping burns every month to keep Gaia around L2, otherwise perturbations would cause it to "fall off" the point," says Gaia Operations Manager David Milligan.
"In terms of the math, the thruster burns in January 2014 are moving Gaia onto what's known as a 'stable manifold' – a pathway in space that will lead the spacecraft to orbit around L2," says Mathias Lauer, one of the flight dynamics specialists at ESOC working on the Gaia mission. "Gaia is now moving in a so-called Lissajous orbit around L2, once every 180 days."
Gaia's Lissajous orbit. Image credit: ESA.
The name Lissajous refers to the shape of the path traced out by the orbit as seen from Earth, which will rise then fall above and below the ecliptic plane (the plane of Earth's orbit around the Sun) while sometimes leading and sometimes lagging the Earth.
To maintain this orbit for Gaia's planned 5-year mission requires extremely careful work by ESA's flight dynamics team, the "spacecraft navigators" who predict and determine trajectories, prepare orbit manoeuvres and determine satellite attitudes and pointing.
Flight Dynamics at ESA. Image credit: ESA/J. Mai.
To plan the orbit, the team applies mathematical models to generate an initial guess for the target orbit and how to get there. This guess must account for the requirements and constraints of the launcher and the needed telecommunications links. Those initial guesses are then fed into simulation software to see if the results would violate any of the constraints. Often, no solution is possible.
"That is where expertise and experience are indispensable to reconsider the assumptions and then start all over," says Frank Dreger, Head of Flight Dynamics.
Gaia's mission is to make the largest, most precise three-dimensional map of our Galaxy by surveying more than a thousand million stars. It will monitor each of its target stars about 70 times over a five-year period.