Euclid (spacecraft)

Euclid (named after the ancient Greek mathematician Euclid of Alexandria, the "Father of Geometry"), is a space mission currently under development by the European Space Agency (ESA). The objective of Euclid is to better understand the geometry of dark energy and dark matter by accurately measuring the acceleration of the universe. To achieve this, the spacecraft will measure the redshift of galaxies at varying distances from Earth and investigate the relationship between distance and redshift. Dark energy is generally accepted as contributing to the increased acceleration of the expanding universe, so understanding this relationship will help to refine how physicists and astrophysicists understand it. Euclid's mission advances and complements ESA's Planck mission, and other contemporary space missions.

Euclid is a medium-class ("M-class") mission and is part of ESA's "Cosmic Vision" (2015–2025) scientific program. This class of missions have an ESA budget cap at around €500 million. Euclid was chosen in October 2011 together with Solar Orbiter, out of several competing missions. The Launch date is currently planned for 2020.

Scientific objectives and methods

Euclid will probe the history of the expansion of the universe (thought to be governed by dark energy) and the formation of cosmic structures by measuring the redshift of galaxies out to a factor of 2, which is equivalent to seeing back 10 billion years in the past.^[7] The link between galactic shapes and their corresponding redshift will give a look into how dark energy contributes to the increased acceleration of the universe. The methods employed exploit the phenomenon of gravitational lensing, measurement of Baryon acoustic oscillations, and measurement of galactic distances by spectroscopy.

Gravitational lensing (or gravitational shear) is a consequence of the deflection of light rays caused by the presence of matter that locally modifies the curvature of space-time: light emitted by galaxies, and therefore observed images, are distorted as they pass close to matter lying along the line of sight. This matter is composed partly of visible galaxies but it is mostly dark matter. By measuring this "shear", the amount of dark matter can be inferred, furthering the understanding of how it is distributed in the universe.

Spectroscopic measurements will permit measuring the redshifts of galaxies and determining their distances using the Hubble's Law. In this way one can reconstruct the three-dimensional distribution of galaxies in the universe.

From this data, it is possible to simultaneously measure the statistical properties concerning the distribution of dark matter and galaxies, and measure how these properties change as the spacecraft looks further back in time. Highly precise images are required to provide the most accurate measurements, as any distortion inherent in the sensors themselves must be accounted for and calibrated out, otherwise the resultant data will be of questionable use.^[7]

Mission characteristics

ESA selected Thales Alenia Space Italy for the construction of the satellite. Euclid is 4.5 metres long with a diameter of 3.1 metres and a mass of 2.1 tonnes.^[8]

The Euclid payload is managed by Airbus Defence and Space. It consists of a Korsch telescope with a primary mirror 1.2 meters in diameter, which covers an area of 0.5 deg². An international consortium of scientists will provide a visible instrument (VIS) and an infrared instrument (NISP). These large format cameras will be used to characterise the morphometric, photometric and spectroscopic properties of galaxies:^[9]

• a camera operating at visible wavelengths (550–920 nm) made of a mosaic of 6x6 e2v Charge Coupled Detectors, containing 600 million pixels, allows measurement of the deformation of galaxies;
• a camera composed of a mosaic of 4x4 Teledyne H2RG detectors sensitive to near-infrared light radiation (1000–2000 nm) with 65 million pixels to:

1. provide low precision measurements of redshifts, and thus distances, of over a billion galaxies from multi-color photometry (photometric redshift technique); and
2. use a spectrometer to analyse the spectrum of light in near-infrared (1000–2000 nm), to acquire precise redshifts and distances of million galaxies, with an accuracy 10 times better than photometric redshifts, and to determine the baryon acoustic oscillations.

The telescope bus includes solar panels that provide power and stabilises the orientation and pointing of the telescope to better than 35 milliarcseconds. The telescope is carefully insulated to ensure good thermal stability so as to not disturb the optical alignment.

The telecommunications system is capable of transferring 850 gigabits per day. It uses the Ka band to send scientific data at a rate of 55 megabits per second during the allocated period of 4 hours per day to the 35-m dish Cebreros ground station in Spain, when the telescope is visible from Earth. Euclid will have an onboard storage capacity of at least 300 GB.

Mission execution and Euclid data

Euclid will be launched on a Soyuz rocket from Kourou. Following a travel time of 30 days it will be stabilised to travel a Lissajous path of large amplitude (about 1 million kilometres) around the Sun-Earth Lagrangian point L2.^[4]

During its mission, which will last at least 6 years, Euclid will observe about 15,000 deg², or about a third of the extragalactic sky (the sky facing away from the Milky Way). The survey will be complemented by additional observations about 10 times deeper pointing toward two different fields located closed to the ecliptic poles and covering 20 deg² each. The two fields will be regularly visited during the whole duration of the mission. They will be used as calibration fields and to monitor the telescope and instrument performance stability as well as to produce scientific data to observed the most distant galaxies and quasars in the universe.

To measure a photometric redshift for each galaxy with sufficient accuracy, the Euclid mission depends on additional photometric data obtained in at least 4 visible filters. This data will be obtained from ground-based telescopes located in both northern and southern hemispheres to cover the full 15,000 deg² of the mission. In total each galaxy of the Euclid mission will get photometric information in at least 7 different filters covering the whole range 460–2000 nm.

About 10 billion astronomical sources will be observed by Euclid, of which 1 billion will have their gravitational shear measured with a precision 50 times more accurate than is possible today using ground-based telescopes. Euclid will measure spectroscopic redshifts for 50 million objects.

The scientific exploitation of this enormous data set will be carried out by a European-led consortium of more than 1200 people in over 100 laboratories in 15 countries (Austria, Belgium, Denmark, Finland, France, Germany, Italy, the Netherlands, Norway, Portugal, Romania, Spain, Switzerland, UK and the US). The Euclid Consortium is also responsible for the construction of the Euclid instrument payload and for the development and implementation of the Euclid "ground segment" which will process all data collected by the satellite. The laboratories contributing to the Euclid Consortium are funded and supported by their national space agencies, which also have the programmatic responsibilities of their national contribution, and by their national research structures (research agencies, observatories, universities). Overall, the Euclid Consortium contributes to about 30% of the total budget cost of the mission until completion.

The huge volume, diversity (space and ground, visible and near infrared, morphometry, photometry and spectroscopy) and the high level of precision of measurements needed demand considerable care and effort in the data processing making this a critical part of the mission. ESA, the national agencies and the Euclid Consortium are spending considerable resources to set up top level teams of researchers and engineers in algorithm development, software development, testing and validation procedures, data archiving and data distribution infrastructures. In total, 7 Science Data Centres spread over countries of the Euclid Consortium will process more than 10 petabytes of raw input images over 10 years to deliver by 2028 a public data base of the Euclid mission to the whole scientific community.

With its wide sky coverage and its catalogues of billions of stars and galaxies, the scientific value of data collected by the mission goes beyond the scope of cosmology. This database will provide the worldwide astronomical community with abundant sources and targets for the future missions such as JWST, E-ELT, TMT, ALMA, SKA or LSST.

References

External links

• Euclid homepage