NASA's interplanetary probe Psyche, launched in October 2023, has switched to almost continuous engine operation. The news was reported by NASA's Jet Propulsion Laboratory (JPL) in Southern California, which is managed by the California Institute of Technology. The spacecraft is flying to the metal asteroid Psyche in the main asteroid belt.
Initially the mission ran the electric propulsion system only from time to time: it was needed to take the right position so that Mars's gravity could give the spacecraft a push and slightly change its trajectory. Such a gravity assist saves fuel. Since September 28 the engines have been running almost without stopping, and only once a week does the probe pause for a communications session with Earth.
"Now we are pressing on the gas all the way to asteroid Psyche," JPL quotes Don Han, the mission's navigation lead. "The engines will run almost 90% of the time until arrival."
Unlike conventional chemical engines, which deliver a powerful impulse over a short time, Psyche's solar-electric system produces thrust using electromagnetic fields. The maximum force is up to 250 millinewtons, roughly the weight of an AA battery resting on a palm. It is based on the Hall effect: charged atoms (ions) of the inert gas xenon are ejected outward, creating thrust and a bright blue glow.
The four engines are switched on one at a time. The thrust is tiny, but over a long journey it accumulates and the spacecraft accelerates. Electricity for the systems and engines is provided by two cross-shaped solar arrays of five sections each with a total area of 75 square meters.
At launch the spacecraft carried about 1,090 kilograms of xenon in seven tanks. Over three years roughly a third of the supply has been used up; the rest should be enough both for the journey and for two years of scientific work in orbit around the asteroid. According to Steve Snyder, lead engineer for propulsion operations at JPL, for the next three months the spacecraft will operate at full power, and then it will be gradually reduced by about half: as it moves farther from the Sun, the arrays receive less light.
There is also a technical subtlety. The ejected xenon ions are swirled by the magnetic field and create a weak torque, a twisting force that without compensation would throw off the probe's orientation. To cancel the accumulated torque, reaction wheels that change their rotation speed are used. Every few hours the engine is briefly switched off, the spacecraft is turned, and another engine is switched on for a few minutes in a different direction. Snyder notes that this effect is so small that it can only be measured in space, and Psyche for the first time makes it possible to trace how it depends on power and operating time over such a long stretch.
The asteroid Psyche, about 280 kilometers wide, is thought possibly to be part of the metal core of a planetesimal, a "building block" of an early planet. If so, it will offer a rare opportunity to look into the internal structure of rocky planets like Earth. In 2029 the probe will enter orbit around the asteroid, map it and collect scientific data at various altitudes.
"The Mars flyby was a great opportunity to calibrate the instruments and rehearse procedures before meeting the asteroid," said Lindy Elkins-Tanton, the mission's principal investigator from the University of California, Berkeley. "Now we are confidently heading deeper into the solar system on the energy of the Sun."