MICROSCOPE
MICROSCOPE was a 300 kilogram class minisatellite built to answer a question that has shadowed physics for centuries. Do two objects of different composition really fall at the same rate? Operated by CNES, the satellite tested what physicists call the equivalence principle, the universality of free fall. Its full name in French is the Micro-Satellite a trainee Compensee pour l'Observation du Principe d'Equivalence. The promise was extraordinary. From orbit, the experiment could check this principle 100 times more precisely than anything achievable on Earth. Launched on the 25th of April 2016, MICROSCOPE carried two sets of masses and a single obsession. If the principle held, both sets would feel identical acceleration. If it broke, the universe would have to be rewritten. How do you weigh a falling object against another with such delicacy? And why send the test all the way to space?
The Twin-Space Accelerometer for Gravity Experiment, known as T-SAGE, was the heart of the mission and was built by ONERA. It held two identical accelerometers, each wrapped around concentric cylindrical masses. One accelerometer served as a reference, carrying two masses of the same platinum-rhodium alloy. The other was the test instrument, and here the trick lived in the metals. It paired one platinum-rhodium mass against another made of titanium-aluminium-vanadium alloy, known as TA6V. The two materials carry different neutron-proton ratios, which is exactly the difference the experiment wanted to probe. To keep the masses from drifting, electrostatic repulsion held them in place, rendered motionless with respect to the satellite. Two differential accelerometers were used in succession, and the logic was stark. Same acceleration meant the principle survived. Any difference in the acceleration that had to be applied meant the principle was violated. That reading would only be trusted if the instruments stayed undisturbed by something as ordinary as heat.
A Sun-synchronous orbit gave the accelerometers something hard to find anywhere else, constant illumination. Temperature was the enemy here, since the experiment demanded a thermally benign environment to read its tiny signals. The mission's engineers mounted the experiments on the end of the satellite bus that faced away from the Sun. To hold thermal isolation from the satellite itself, the modes of thermal connection were modelled in advance and the wire connections were kept to a minimum. Every link to the rest of the spacecraft was a path heat could travel, so fewer wires meant a quieter instrument. Steadying the temperature was only half the problem. The masses also had to be shielded from the forces pushing on the spacecraft as a whole.
A Drag-Free Attitude Control System, abbreviated DFACS and also called the Acceleration and Attitude Control System or AACS, gave MICROSCOPE an unusual way of moving. Rather than steer the masses, the system flew the satellite around them. It relied on a double-redundant primary and backup set of four microthrusters, sixteen in total, to make those constant adjustments. The forces it had to cancel were not trivial. Residual atmosphere produced aerodynamic drag even in orbit, and photons striking the spacecraft created solar pressure. Electromagnetic forces acted on it within the Earth's magnetosphere, and gravity pulled across the whole Sun-Earth-Moon system. By absorbing all of that, the spacecraft let the test masses float as if nothing touched them. That isolation is what made the eventual measurement believable.
At 21:02:13 UTC on the 25th of April 2016, MICROSCOPE lifted off from the Guiana Space Centre outside Kourou, in French Guiana. A Soyuz ST-A booster carried it, topped by a Fregat-M upper stage. The satellite did not travel alone. Sharing the flight was the European Space Agency's Sentinel-1B Earth observation satellite. Three CubeSats rounded out the manifest, each from a different university. OUFTI-1 came from the University of Liege, e-st@r-II from the Polytechnic University of Turin, and AAUSAT-4 from Aalborg University. A single rocket lofted a precision physics experiment and a cluster of student satellites in one ascent. Once in orbit, MICROSCOPE had a verdict to deliver.
On the 4th of December 2017, the first results were published, and the answer leaned toward Einstein. The equivalence principle was measured to hold true, and the measurement improved prior results by an order of magnitude. No violation appeared at the precision the satellite could reach. The final report, gathering the full weight of the mission's data, was published in 2022. Confirming a principle is quieter than overturning one, yet this confirmation pushed the known limits ten times sharper than before. With its science objectives met, the satellite still had one task left, and it involved getting out of the way.
On the 18th of October 2018, the decommissioning of MICROSCOPE was announced, after the spacecraft had completed its mission goals and run through its supply of nitrogen fuel. Engineers first passivated the spacecraft, neutralising its stored energy. Then came an unusual farewell. Two inflatable booms, each 4.5 metres long, were deployed from the Innovative DEorbiting Aerobrake System, or IDEAS. The booms widened the spacecraft's drag profile so the thin upper atmosphere would pull it down faster. The payoff was measured in decades. Instead of lingering in orbit for 73 years, MICROSCOPE is expected to re-enter Earth's atmosphere within 25. A satellite built to study falling will, in the end, fall back to the planet it came from.
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Common questions
What was the MICROSCOPE satellite designed to test?
MICROSCOPE was designed to test the universality of free fall, known as the equivalence principle, which holds that two bodies of different composition fall identically in the same gravity field. The satellite could check this principle 100 times more precisely than is possible on Earth.
When was the MICROSCOPE satellite launched and decommissioned?
MICROSCOPE was launched on the 25th of April 2016 at 21:02:13 UTC. Its decommissioning was announced on the 18th of October 2018 after it completed its science objectives, and the final report was published in 2022.
Who operated the MICROSCOPE satellite?
MICROSCOPE was operated by CNES. Its main experiment, the Twin-Space Accelerometer for Gravity Experiment, or T-SAGE, was built by ONERA.
How did the MICROSCOPE satellite test the equivalence principle?
MICROSCOPE used two differential accelerometers in succession. A reference instrument held two platinum-rhodium alloy masses, while the test instrument paired a platinum-rhodium mass with a titanium-aluminium-vanadium alloy mass called TA6V, comparing whether the differing metals fell at the same acceleration.
What were the results of the MICROSCOPE mission?
On the 4th of December 2017, the first results were published, showing the equivalence principle held true and improving prior measurements by an order of magnitude. No violation appeared at the precision the satellite could reach.
How was the MICROSCOPE satellite de-orbited?
After being passivated, MICROSCOPE deployed two 4.5 metre inflatable booms from the Innovative DEorbiting Aerobrake System, or IDEAS, to create a higher drag profile. This is expected to bring the satellite back into Earth's atmosphere within 25 years instead of 73.