The first launch window for the European Space Agency’s (ESA) Euclid satellite will come into view on Saturday July 1st at 16.12 BST. Euclid should originally have been launched on a Soyuz launcher from French Guyana, but because of the Russian invasion of Ukraine, all joint ventures with the Russian Space Agency were suspended and will now be launched using Space-X’s Falcon 9 launcher from Florida. The launch window will last 90 mins, but if thunderstorms prevent the launch, it will be delayed until the following day This will be the 5th launch using the Falcon 9, so hopefully all technical issues have been identified and resolved before this flight!  

The Euclid satellite being attached to the Falcon 9 adaptor in preparation for launch (ESA/SpaceX).

The main goal of the €600M budget mission is to find out what dark matter and dark energy are made of. Dark matter is `stuff’ which doesn’t appear to emit light nor reflect it and was first proposed in the first half of the last century to explain the velocities of stars orbiting galaxies. Dark energy was proposed to account for the observational evidence for the expanding universe. Currently astronomers can only account for 5% of what makes up the universe. This is a huge problem which Euclid aims to answer over its nominal six-year mission where it will survey about 1/3 of the sky.

Euclid will survey 1/3 of the sky away from the plane of the Milky Way (its gas and dust can obscure distant galaxies) and will be making several surveys over the course of its six-year nominal mission.

Euclid has two science instruments. The VIS camera will operate in the red part of the optical spectrum and has 36 CCDs which will be able to resolve fine details of groups of distant galaxies. The VIS team is led by UCL’s Mullard Space Science Laboratory, where I worked for many years, and was led by my long-time colleague Prof Mark Cropper. It will use the effect of weak lensing which distorts the appearance of distant galaxies by the presence of dark matter between us and the distant galaxy. By imaging billions of galaxies, it will be possible to test various models of the universe and therefore find out what makes up the missing mass of the universe.   

The other instrument is the Near-Infrared Spectrometer and Photometer (NISP), led by several French groups, is an infra-red imager and low-resolution spectrograph which will obtain the redshift for billions of galaxies. Euclid will also be searching for an effect called Baryon Acoustic Oscillations (BAO) which is the imprint from the early universe and is seen on very large scales. Galaxies are expected to form in the `ripples’ of the oscillations and the size of these ripples will be used to test models of dark energy. Euclid will also observe faint objects in our Solar System; stars in the outer halo of the Milky Way and determine the masses and star formation rates of galaxies in the relatively near universe. 

A schematic diagram showing the principle of gravitational weak lensing (NASA/ESA/Calcada).

Unlike satellites such as HST or JWST, where discoveries were made not long after launch, we won’t know whether Euclid will have answered what dark matter or dark energy until after the nominal six-year mission. This might seem a long time, but it took nearly 3 years after launch before the Gaia mission, which is measuring the distance and velocities of stars, released its first set of data, and it won’t be until 2030 at the earliest that the full results of Gaia will be released.  

For details of how to watch the launch of Euclid live, see  

ESA is funded by the Governments of 22 nations including the UK and Ireland. The UK led the development of the VIS instrument led by UCL’s Mullard Space Science Lab, supported by the Open University. Many members of the Euclid Science Consortium are based in the UK. 


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