The Microscope satellite carried two cylinders of metal that were designed to fall at exactly the same rate, defying the expectation that their different compositions might cause them to drift apart. This mission, operated by the French space agency CNES, aimed to verify the universality of free fall with a precision one hundred times greater than any experiment possible on Earth. Launched on the 25th of April 2016, the spacecraft was a minisatellite built to test the equivalence principle, a cornerstone of Albert Einstein's general theory of relativity. The principle states that all objects fall at the same rate in a vacuum, regardless of their mass or composition. If the experiment showed even a tiny difference in acceleration between the two test masses, it would shatter the foundation of modern physics and suggest that a new force of nature exists. The satellite was decommissioned on the 18th of October 2018, having successfully completed its science objectives and paving the way for a final report published in 2022.
The Twin Masses
At the heart of the mission was the Twin-Space Accelerometer for Gravity Experiment, known as T-SAGE, which was constructed by the French aerospace research center ONERA. This instrument contained two identical accelerometers, each holding concentric cylindrical masses that served as the test subjects. One accelerometer acted as a reference and held two masses made of a platinum-rhodium alloy. The other accelerometer was the test instrument, holding one mass of the same platinum-rhodium alloy and a second mass made of a titanium-aluminium-vanadium alloy, designated TA6V. These two alloys possess different neutron-to-proton ratios, meaning they are composed of different proportions of subatomic particles. The experiment relied on electrostatic repulsion to keep these masses motionless relative to the satellite, creating a perfect vacuum-like environment where gravity was the only force acting upon them. If the equivalence principle held true, both sets of masses would experience identical acceleration despite their different internal structures.A Thermal Sanctuary
To ensure the delicate measurements remained accurate, engineers had to construct a thermally benign environment that shielded the accelerometers from the slightest temperature fluctuation. The satellite was placed in a Sun-synchronous orbit, which provided constant illumination and allowed the experiments to be mounted on the end of the satellite bus, far away from the direct heat of the Sun. Thermal isolation was achieved by modeling the modes of thermal connection and minimizing wire connections to reduce heat transfer from the satellite bus to the sensitive instruments. This careful design was crucial because even minute changes in temperature could cause the materials to expand or contract, introducing noise that would drown out the subtle signals the mission sought to detect. The engineering challenge was to create a system where the satellite itself did not interfere with the free fall of the test masses, allowing the experiment to function as a pure test of gravity.