The European Space Agency had been on the verge of launching the billion-euro Euclid satellite, which is designed to address the most pressing unsolved questions in astronomy: What are the true natures of dark matter and energy? ESA had scheduled a March 2023 launch for Euclid from French Guiana—but it was on a Soyuz rocket. The war in Ukraine brought an end to Soyuz operations from Guiana and started a period of uncertainty for Euclid’s team.

Keeping Euclid in storage could cost 100 million euros per year and put its entire scientific team and infrastructure in standby mode, potentially compromising European leadership in space-based observational cosmology. The partner ESA has used for almost all its launches, Arianespace, is building what should be a good backup launcher, the Ariane 62. But that rocket has not flown yet, and with each passing month, its test flight date slips further. Once it’s ready, Euclid would not even be the first Ariane 62 launch: at least four other satellites are in the queue before it.

Although all these options look bad, there is one company with a rocket that has spare launch capacity: SpaceX. Would ESA really drop its partner and send a flagship European science mission to space on a Falcon 9 from Arianespace’s biggest competitor?

Tracking the Universe’s expansion

In the 1990s, astronomers made the remarkable discovery that not only is the Universe expanding, but the rate of expansion itself is accelerating. To understand the nature of the acceleration, you need a ‘ruler’ to measure length scales in the Universe's distant past together with a way to map the amount of intervening dark matter, which plays a key role in determining how structures evolve. The best way to do that is from space, above the atmosphere, which blocks light from distant galaxies and distorts the light of those that pass.

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ESA’s “cosmic vision” program selected Euclid in 2011 to directly answer this question. The mission will measure precisely the positions and shapes of a billion distant galaxies using two unique instruments: /VIS/ a wide-field, high-resolution camera operating in visible light and a near-infrared camera and spectrograph, NISP. Because mass bends light, the amount of dark matter along sight lines to those distant galaxies can be measured from the very tiny shape correlations in those distant background galaxies. NISP, on the other hand, measures distances, positions, and velocities—the cosmic ruler—all of which bear imprints of the action of the accelerated expansion. The combination of these two measurements has unparalleled power to reveal the geometry of the Universe.

From the ground, below the soup of the atmosphere, shape measurements are hard. But technological advances have made it increasingly easier to collect vast numbers of distance measurements: the Dark Energy Spectroscopic Instrument experiment has now started an ambitious program to collect 30 million redshifts. For Euclid to have the maximum scientific impact, it needs to be launched as soon as possible; the main Euclid survey, covering the whole extragalactic sky, will take six years to complete.

A consortium of European research institutes, with some assistance from NASA, built Euclid’s two instruments. The spacecraft and service module were built by Thales Alenia Space, and the telescope, a key component, by Airbus. Today, spacecraft integration is finished, and the satellite will very shortly be shipped to Cannes for final testing before launch. But... on what launcher?