Observing the Earth from space has become an essential lever for protecting our planet. In particular, its gravity field reveals crucial information about the distribution of water and the mechanisms that govern climate: for example, when a glacier melts or the monsoon hits a continent, the distribution of mass and therefore the gravity field changes.
To refine measurements of the Earth’s gravity field, a technological revolution could emerge thanks to quantum technologies.
It is this ambitious challenge that the CARIOQA space mission intends to take on, by sending the very first quantum accelerometer into orbit, a key step towards new generation space gravimetry missions. The project is completing its feasibility phase at the end of the year, with a launch planned for 2030.
Space gravimetry missions
The Earth’s gravity field varies by region and fluctuates over time. Its study is essential in various fields such as geophysics (monitoring tectonic movements), oceanography (monitoring ocean levels) and navigation (guidance of boats and submarines).
Before the advent of space gravimetry, terrestrial gravity measurements were local and limited in coverage, without the possibility of globally and continuously monitoring variations in the gravitational field.
Since the 2000s, the CHAMP space mission has made it possible to measure gravity using an orbiting satellite equipped with an accelerometer. Indeed, the position of a satellite in orbit depends on the Earth’s gravity field and other types of accelerations, linked for example to friction in the atmosphere.
Thus, to precisely measure the Earth’s gravity field and its variations, we precisely measure the position of the CHAMP satellite using GNSS (GPS technology), which we correct using an on-board accelerometer measuring the non-gravitational effects experienced by the satellite.
In 2002, the GRACE mission (Gravity Recovery and Climate Experiment) provided the first temporal maps of the Earth’s gravitational field, thanks to two satellites in low orbit, each equipped with an accelerometer. By following the variation in distance between the two satellites and rejecting non-gravitational accelerations, we deduce the fluctuations of the gravitational field. In 2018, the precision of this distance measurement between the two satellites was further improved thanks to a laser interferometer on board the GRACE Follow-On mission.
Restitution of the gravity field on a global scale offers new perspectives in the field of Earth sciences, allowing a better understanding and anticipation of climate change.
Quantum accelerometers: a technological breakthrough for measuring the gravity field
Current space gravimetry missions rely on measuring non-gravitational accelerations using precision accelerometers. These instruments measure the movements of a test mass, for example a metal cylinder of around a few hundred grams, to accurately detect the forces at play. Today, this principle can be applied by replacing this mass with a cloud of gaseous atoms in a vacuum, manipulated by lasers, to develop quantum accelerometers.
The contribution of quantum physics lies in the exceptional stability of the measurement over time: like atomic clocks, quantum accelerometers use the internal properties of atoms to offer precision which remains constant, unlike conventional accelerometers, whose measurements tend to drift.
In a vacuum chamber, a gas of rubidium atoms is trapped, and the movements of the atoms within the cloud are slowed using very precisely controlled lasers. The reduction in the speed of atoms is associated with a drop in temperature: we then speak of clouds of cold atoms. In these extreme conditions, close to absolute zero, atoms reveal behavior governed by the laws of quantum physics: matter behaves like a wave. Like waves on the surface of the oceans, matter waves can add or cancel each other to create a phenomenon of quantum interference.
It is on this principle that the technology of the atomic interferometers which will be used for measuring acceleration on board CARIOQA is based. Laser pulses are used to split, manipulate and recombine cold, free-falling atoms, thereby creating interference that contains the information of interest for the measurement: the relative acceleration between the cloud of atoms, free-falling in the chamber, and the laser field which interrogates it.
If, today, the performances of quantum gravimeters are better than those of classical gravimeters in certain conditions (better resolution of low spatial frequencies for example), they are not always easy to estimate.
CARIOQA: a demonstration mission to bridge the technological gap
Atomic accelerometers have been studied since the 1990s in the laboratory, having demonstrated their capacity in fundamental physics tests in airplanes developing inertial navigation, or even studying gravity on the slopes of Etna.
The next step? Earth’s orbit!
The CARIOQA project, started in 2022, aims to demonstrate the viability of this technology on board a satellite, preparing for future space gravimetry missions. This ambitious project brings together 17 partners, including the French and German space agencies (CNES and DLR), industrial players such as Airbus, Exail, Teletel and Leonardo, as well as a consortium of European laboratories. The first part of CARIOQA makes it possible to develop a prototype with a view to the final instrument, intended for the flight phases.
It is by combining the expertise of space agencies, industry and laboratories that Europe is placing itself at the forefront of this technological revolution, opening the way to a new era of exploration and understanding of Earth’s gravity.
This article, written by Célia Pelluet, optical physics and quantum sensors engineer at CNES, is republished from The Conversation sous licence Creative Commons. Lire l’article original.
