Summary
After its Mars gravity assist, NASA’s Psyche spacecraft began using its solar-electric propulsion system nearly continuously. The mission is targeting arrival at asteroid Psyche in summer 2029.
NASA’s Psyche spacecraft has shifted to nearly continuous use of its solar-electric propulsion system after completing a Mars gravity assist. The spacecraft began this phase on Sept. 28, with the thrusters expected to operate nearly 90% of the time until it reaches asteroid Psyche in the main asteroid belt in summer 2029, according to NASA’s Jet Propulsion Laboratory (JPL).
The Mars flyby provided a propellant-saving boost in speed and changed the spacecraft’s trajectory. Psyche had previously used its electric propulsion intermittently to position itself for the encounter. It now pauses about once a week to check in with mission controllers and for necessary engineering activities.
How sunlight and xenon propel Psyche
Unlike chemical rockets, which produce powerful thrust for relatively short periods, Psyche’s Hall-effect thrusters generate a small, steady push. Solar arrays provide electricity to electromagnetic fields that accelerate charged xenon atoms, or ions, out of the thrusters. The escaping ions create thrust and a blue glow.
Each of the spacecraft’s four thrusters operates one at a time and can produce up to 250 millinewtons of force—roughly the weight of an AA battery. That force is slight, but sustained over a long journey it gradually changes the spacecraft’s speed. At its maximum cruise speed, Psyche is expected to travel more than 100,000 mph (160,900 kph) relative to an observer on Earth.
Two five-panel, cross-shaped solar arrays provide power. Their sunlight-collecting area is 800 square feet (75 square meters). The spacecraft launched with nearly 2,400 pounds (1,090 kilograms) of xenon and had used about one-third by the time of JPL’s report. The agency says the remaining supply is sufficient for the journey and for the spacecraft to assist its orbiting operations during a two-year prime science mission.
Solar power output will decrease as Psyche moves farther from the Sun. JPL says the spacecraft is scheduled to thrust at full power for the next three months, then gradually reduce to about half power as its arrays receive less sunlight.
Managing a subtle twist in flight
Operating Hall thrusters also creates a small twisting force, or torque. As xenon ions swirl in the thruster’s magnetic field, they can affect the spacecraft’s pointing. Psyche uses reaction wheels to manage the resulting angular momentum. After several hours of thrusting, engineers can switch off a thruster, rotate the spacecraft and briefly run a different thruster at low power in another direction to reduce the wheels’ accumulated momentum.
JPL’s propulsion operations lead says the mission has been able to measure this torque in space across different power levels and operating durations. The long cruise gives engineers an opportunity to study how the effect behaves during extended Hall-thruster operation.
A two-year survey of asteroid Psyche
Psyche is about 173 miles (280 kilometers) across at its widest point. The spacecraft is due to enter orbit around it in 2029 and map the asteroid while collecting science data from a sequence of orbits at progressively lower and then higher altitudes.
The asteroid is thought to be the partial core of a planetesimal, a building block of an early planet. If that interpretation is borne out, studying Psyche could provide a window into the interiors of rocky planets. The spacecraft’s Mars encounter also gave the mission team a chance to calibrate instruments and practise procedures ahead of the asteroid rendezvous.