NASA's Psyche spacecraft has hit the gas. With its Mars gravity assist complete, the probe is now running its electric thrusters almost continuously on the way to the metal-rich asteroid it's named after, NASA said in a mission update on October 22.
"It's now 'pedal to the metal' to the asteroid Psyche," said Don Han, the mission's navigation lead at NASA's Jet Propulsion Laboratory in Southern California.
Nearly 90% thruster time
Since its October 2023 launch, Psyche had used its propulsion system only intermittently, maneuvering into position to use Mars' gravity for a propellant-saving speed boost and a slight trajectory change.
That changed on September 28, when the spacecraft began thrusting nearly continuously. It now pauses just once a week to check in with mission controllers. "The thrusters will be operating nearly 90% of the time from now until we reach the asteroid," Han said. That keeps Psyche on track to reach the asteroid in the main asteroid belt by summer 2029.
How a gentle push adds up
Psyche uses solar electric propulsion. Its Hall-effect thrusters use electromagnetic fields to push out charged xenon atoms, producing thrust and a bright blue glow. The force is tiny: up to 250 millinewtons, which NASA compares to the weight of a single AA battery in your hand.
The four thrusters run one at a time, but the push builds up over the journey. At its peak cruise speed, NASA says Psyche will be moving more than 100,000 mph relative to an observer on Earth.
Power comes from large cross-shaped solar arrays with about 800 square feet of collecting area. The spacecraft launched with nearly 2,400 pounds of xenon in seven tanks and has used about a third of it in three years, leaving enough to reach the asteroid and support its orbital mission there.
"For the next three months, we'll be thrusting at full power before we begin slowly throttling down to approximately half power as the spacecraft moves farther away and its solar arrays receive less sunlight," said Steve Snyder, the mission's lead engineer for propulsion operations at JPL.
A first-of-its-kind measurement
Even gentle thrusters need careful management. As xenon ions leave the thrusters, the magnetic field makes them swirl, creating a slight twisting force that would throw off the spacecraft's pointing if left unchecked. Psyche absorbs that momentum with its reaction wheels, then periodically shuts off its thruster, rotates and fires a different thruster at low power for a few minutes to clear the built-up momentum.
"In fact, this is the first time anyone has been able to measure this effect on a Hall thruster as a function of power level and of operating time, and for such a long duration in space," Snyder said.
What Psyche will study
After arriving in 2029, the spacecraft will enter orbit around the asteroid, which is about 173 miles across at its widest point. It will map Psyche over a two-year prime mission, flying a series of lower and higher orbits.
Scientists think the asteroid may be the partial core of a planetesimal, one of the building blocks of early planets. If so, it could offer a rare look at the kind of metal interior hidden deep inside rocky planets like Earth.
"Now we are cruising with confidence deeper into the solar system, powered by sunlight, ready to explore a mysterious new world in the asteroid belt," said Lindy Elkins-Tanton, the mission's principal investigator at the University of California, Berkeley.
