The Euclid mission will be launched at the start of the third quarter of 2023 by a Falcon 9 vehicle from Cape Canaveral, Florida (USA).
Euclid will be injected into a direct transfer orbit, from which it will take about 30 days for the satellite to reach its target orbit around the L2 Lagrange point, 1.5 million kilometres from Earth.
Lagrange points, named after the mathematician Joseph Louis de Lagrange (1736-1813), are specific points on Earth’s orbit where the gravitational attraction of the Sun and our planet cancel each other out, in such a way that these points remain fixed with respect to Earth and therefore rotate about the Sun at the same angular velocity as Earth. Newton’s law and some maths enable us to determine the position of such points, of which there are five.
The five Lagrange points in the Sun-Earth system. Credit: NASA/WMAP Science Team
L4 and L5 are more stable than the other three points, which is why for example asteroids—called trojans—accumulate at Jupiter’s L4 and L5 points.
L2 is aligned with the Earth and Sun, but lies on the opposite side of the Sun, some 1.5 million kilometres away. By calculating the orbital velocity of an object in an orbit beyond Earth using Newton’s law, we can easily determine if it is slower than Earth’s. But along the Earth-Sun axis, beyond Earth orbit the gravitational attraction of Earth and Sun reinforce one another, so the velocity is higher. The L2 distance is determined so that an object there orbits at an angular velocity identical to that of our planet.
After launch, the transfer to L2 will take 30 days. A trajectory correction manoeuvre (TCM) will be performed two days into the flight, once sufficient data have been acquired to evaluate the radial velocity error due to uncertainties related to launch conditions. The satellite will then undergo checkout during its 30-day journey to L2, followed by a two-month performance verification phase (PV). No orbital insertion manoeuvre will be needed.
At L2, Euclid won’t be alone, as the Gaia and JWST space telescopes are already there. It will go into a large-amplitude Lissajous halo orbit (approximately 1 million kilometres from Earth). This point has been chosen to optimize operating conditions for Euclid, with a radiation environment conducive to its sensitive detectors and highly stable observing conditions, far enough from the Earth-Moon system. This orbit will also save a significant amount of propellant compared to alternative orbits.
The Euclid telescope at the L2 Lagrange point. Credit: Patrice Amoyel
The chosen orbit is also eclipse-free. TCMs will be performed every 30 days. The nominal mission lifetime is six years.