A 70-kilogram cube of metal and silicon, no larger than a large microwave oven, was launched into the void on the 3rd of December 2014. This was PROCYON, a spacecraft designed to be a low-cost experiment in deep space navigation. It rode as a secondary passenger on the same rocket that carried the massive Hayabusa2 probe, a decision that would define its entire existence. While the primary mission focused on landing on an asteroid, this smaller companion was tasked with testing a new type of ion engine and proving that a tiny, inexpensive satellite could perform complex orbital maneuvers. The University of Tokyo and JAXA had built it to be a proof of concept, a small vessel intended to fly by an asteroid in 2016. It was a gamble on miniaturization, packing a specific impulse of 1000 seconds into a frame that weighed less than a grand piano.
The Ion Engine Gamble
The heart of the mission was a novel propulsion system that fed both the main ion engine and eight attitude control cold-gas thrusters from a single tank. This shared reservoir held 2.5 kilograms of xenon gas at launch, a design choice that simplified the spacecraft but introduced a single point of failure. On the 22nd of February 2015, the ion engine roared to life, delivering 330 micro-Newtons of thrust, exceeding the designed 250 micro-Newtons. The initial results were favorable, suggesting that the small craft could adjust its orbit to perform a gravity assist around Earth. The plan was to use 20 percent of the xenon propellant for the initial correction and save the rest for the final approach to the target asteroid. The delta-v budget was set at about 500 meters per second, a modest amount for such a small vehicle, but enough to change its destiny if the engine held.The Silent Failure
On the 10th of March 2015, the ion engine failed and could not be restarted, turning a successful test into a ticking clock. The spacecraft flew past Earth on the 3rd of December 2015, but without the ability to make a controlled orbit change, it was unable to reach its intended target. The malfunction meant that the probe would never achieve the controlled flyby distance of 30 kilometers that engineers had calculated. Shortly after the Earth flyby, contact with the spacecraft was lost, leaving it to drift in a heliocentric orbit that would never intersect with the asteroid. The mission, which had begun with such promise, ended in silence, a victim of a single mechanical failure that could not be fixed from millions of kilometers away.The First Geocorona