Euclid will be injected into a direct transfer orbit by a Soyuz ST 2-1b launch vehicle departing from Europe's Spaceport in French Guiana. The transfer phase to the target orbit around the second Sun-Earth Lagrange point, L2, will last approximately 30 days. A correction manoeuvre will be performed about two days into the flight, once sufficient tracking data have been acquired to evaluate the radial velocity error, which arises due to uncertainties in the conditions during launch. Commissioning of the spacecraft and instruments will start during the transfer phase. No insertion manoeuvre is needed to reach the chosen orbit.
A large-amplitude (~1x106 km) halo orbit around the second Lagrange point of the Sun-Earth system (L2) has been selected because it offers optimum operating conditions for Euclid: a benign radiation environment, which is necessary for the sensitive detectors and very stable observing conditions, which are sufficiently far away from the disturbing Earth-Moon system. In addition, the amount of propellant necessary is very favourable compared to alternative orbits.
To cope with the unprecedented science data volume generated by Euclid in L2, K-band (25.5-27 GHz) communications will be used, which offer a transfer rate of 55 Mbit/s. As a consequence of the large variations in the Sun-spacecraft-Earth angle, a two-degrees-of-freedom mechanism for the antenna is needed to maintain the science telemetry link to Earth.
The selected orbit is eclipse-free. Trajectory-correction manoeuvres will be performed every 30 days.
Euclid sky scanning strategy
Euclid will survey the sky in 'step-and-stare' mode: the telescope will realize imaging and spectroscopic measurements by successive adjacent pointing of ~0.5 deg².
The sky coverage strategy is driven by the wide-survey requirement to cover 20,000 deg² of extragalactic sky during the mission lifetime of 5 years. The main considerations behind the survey strategy are:
The L2 orbit and spacecraft viewing constraints; the Sun-spacecraft line will turn of 1 degree per day in the plane of the ecliptic.
Maintaining the thermal stability of the spacecraft constraints the pointing direction to be as much as possible perpendicular to the Sun-spacecraft axis. This means that on a given day in the mission the viewing area is mostly confined to a great circle perpendicular to the Sun-spacecraft axis.
The fundamental exposure times of the instruments and the size of a field, which is 0.5x1.0 deg².
For the imaging channels, dithering is required to over-sample the PSF (point-spread-function) to fill the gaps between the detectors, and to ensure that the field is completely covered.
On a daily basis, Euclid will observe strip: these are observations of adjacent sky fields along a great circle of (roughly) constant ecliptic longitude. A strip of order 15-20 degrees long can be covered, depending on the geometry of the instruments field of view and the integration time per field. During the assessment study, a field of 0.5 degree wide and an exposure time per field of 2400 s has been assumed, in that case the daily coverage would be a strip of about 18 degrees.
On approximately a 3-4 week basis a patch will be observed: this is a square area of about 400 deg². The geometry of a patch is roughly 20x20 deg², but significant deviations are expected depending on the precise ecliptic of galactic latitude of a patch. After 6 months, the spacecraft pointing direction is flipped (along a great ecliptic circle) to observe patches in the opposite hemisphere.
The extragalactic sky is presently defined by the regions covering the latitudes over 30 degrees. The alignment of the galactic plane with respect to the ecliptic plane causes the extragalactic sky to be poorly accessible during the equinoxes (i.e. around 21 March and 21 September).
The deep survey will cover ~40 deg², and consists of patches of at least 10 deg² which are about 2 magnitudes deeper than the wide survey. The deep survey is obtained by regular visits to the same areas on the sky at regular intervals during the mission. The same observing mode as for the wide survey is used to monitor the temporal stability of the system.