PROCYON
PROCYON, which stands for Proximate Object Close flyby with Optical Navigation, launched into space on the 3rd of December 2014 at 13:22:04 Japan Standard Time, tucked in as a secondary payload alongside Hayabusa2. It was a small cube, roughly 60 centimetres on a side and weighing just 70 kilograms. For comparison, that is about the mass of an adult human. What could a spacecraft so compact and inexpensive actually accomplish? And what would happen when its ion engine, after briefly outperforming expectations, fell silent on the 10th of March 2015, leaving the probe to drift through the solar system without steerage?
PROCYON was developed jointly by the University of Tokyo and JAXA, and it embodied a philosophy of doing more with less. After separation from the launch vehicle, the probe was left on a heliocentric orbit, circling the Sun rather than Earth. Its ion engine carried a specific impulse of 1000 seconds, giving the spacecraft a delta-v budget of roughly 500 metres per second. Engineers planned to spend about 20 percent of the xenon propellant on an initial orbit correction, reserving the remainder for the long haul between an Earth flyby and a close pass of an asteroid. One of the more inventive decisions was plumbing the main ion engine and the eight attitude-control cold-gas thrusters from a single shared tank. That tank held 2.5 kilograms of xenon at launch. It was a neat solution to the problem of keeping a low-cost spacecraft as simple as possible. PROCYON also carried two dedicated scientific instruments: a small telescope for near-asteroid navigation and data gathering, and a Lyman-alpha imaging camera designed to photograph the geocorona, the outermost layer of Earth's atmosphere.
On the 22nd of February 2015, flight controllers started PROCYON's ion engine for the first time in flight. The initial readings were encouraging. The engine was delivering 330 micronewtons of thrust instead of the designed 250 micronewtons, beating its own specification by a comfortable margin. Then, on the 10th of March 2015, the engine failed. It could not be restarted. The original mission plan called for an asteroid flyby in 2016, with the probe passing within a controlled distance of 30 kilometres of its target. Without a working engine, that precision was impossible. Engineers kept the probe pointed toward Earth, and on the 3rd of December 2015, exactly one year after launch, PROCYON flew past Earth. But without propulsion, the spacecraft could not execute the controlled orbit change it needed. Shortly after that flyby, contact with PROCYON was lost entirely.
Before the engine failed and contact was lost, PROCYON's Lyman-alpha camera had already been at work. The probe observed the Lyman-alpha emission of comet 67P/Churyumov-Gerasimenko, using those ultraviolet observations to map the overall structure of the comet's coma, the cloud of gas and dust surrounding its nucleus. PROCYON also turned its camera toward Earth and captured the first complete image of the geocorona. That image confirmed for the first time that the geocorona has north-south symmetry, a geometric property that had been assumed but never shown in a single full picture. These results arrived from a spacecraft that weighed no more than a person and cost a fraction of a conventional deep-space mission.
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Common questions
When was PROCYON launched and what was its mission?
PROCYON launched on the 3rd of December 2014 at 13:22:04 Japan Standard Time as a secondary payload alongside the Hayabusa2 asteroid probe. Its mission was to perform a close flyby of an asteroid, passing within 30 kilometres of the target, using optical navigation techniques. The asteroid flyby was planned for 2016 but was abandoned after the ion thruster failed.
Why did the PROCYON mission fail?
The ion engine failed on the 10th of March 2015 and could not be restarted. Without propulsion, PROCYON could not perform the controlled orbit change needed to reach its asteroid target. The spacecraft flew past Earth on the 3rd of December 2015 but lost contact with ground controllers shortly afterward.
Who built the PROCYON spacecraft?
PROCYON was developed by the University of Tokyo and JAXA, the Japan Aerospace Exploration Agency. It was a small, low-cost spacecraft measuring approximately 60 centimetres on a side and weighing 70 kilograms.
What scientific discoveries did PROCYON make?
PROCYON captured the first complete image of Earth's geocorona, confirming for the first time that it has north-south symmetry. The spacecraft also observed the Lyman-alpha emission of comet 67P/Churyumov-Gerasimenko to determine the overall structure of the comet's coma.
How did PROCYON's ion engine compare to its design specifications?
When first started on the 22nd of February 2015, PROCYON's ion engine delivered 330 micronewtons of thrust, exceeding its designed output of 250 micronewtons. Despite this early overperformance, the engine failed permanently on the 10th of March 2015.
What propulsion system did PROCYON use?
PROCYON used an ion engine for main propulsion and eight cold-gas attitude-control thrusters, all fed from a single shared xenon tank holding 2.5 kilograms of propellant at launch. The spacecraft had a specific impulse of 1000 seconds and a delta-v budget of about 500 metres per second.