The European Space Agency’s Proba-3 mission has achieved what no other mission or synchronized swimmer has ever done before: autonomous, precise alignment in space with millimeter accuracy.
ESA’s Proba-3 satellites – Occulter and Coronagraph, respectively – maintained a constant distance of 492 feet (150 meters) from each other in a highly elliptical orbit, simulating a single massive instrument. The same principle is being used by satellite constellations and planned space telescopes to take advantage of the laws of physics and the relatively silent space environment to collect more data than would otherwise be possible. But the goal of Proba-3 is unique: to align the spacecraft so that one of them (Occulter) can block the glare of the Sun, while the other (Coronagraph) can clearly see its outer atmosphere, the solar corona.
To do this, the two spacecraft cannot simply drift together – they must remain aligned with millimeter (0.04 inch) precision, without any micro-control from Earth. Specifically, the instruments must be aligned so that there is a distance of nearly 500 feet (152.4 m) between them, and the 4.6-foot (1.4-meter) disk on Occulter casts a 2-inch (5-centimeter) shadow on Coronagraph – which is all the latter needs to protect itself from the Sun’s brilliance and image its faint, ethereal corona.
After an initial positioning by ground control, the satellite duo’s autonomous system of cameras, LEDs and laser rangefinders brings the two spacecraft closer together using an algorithm that ensures that Coronagraph is safely in the shadow of Ocular.
The Precision Longitudinal and Longitudinal Sensor (FLLS), a laser system that detects changes in position, ensures that both spacecraft remain precisely aligned; the constant measurement of laser light is the same technology that will enable LISA, NASA and ESA’s next-generation gravitational wave observatory, to align over much greater distances in 10 years.
Proba-3’s achievement marks a level of orbital choreography that has never been achieved before. “We’re talking millimeter precision in the range and sub-millimeter precision in the lateral position,” said project manager Damien Galano in an ESA press release. “We are looking forward to completing the instrument’s calibration and delivering the first processed image of the Sun’s corona.”
Sun science is an outstanding achievement in itself, but the demonstration of the Proba-3 technology also helps to prove the accuracy of measurements that will be crucial for future spacecraft missions such as LISA.
Extremely precise science is laying the groundwork for the future of astronomy. In the meantime, ESA’s Probe 3 is preparing for its research campaign – tethered by lasers in a Kubrickian waltz, preparing to gaze directly into the Sun.
