NASA’s most metal mission will test new, higher-power electric thrusters

A satellite company named Maxar recently delivered a passenger van-sized chunk of spacecraft to NASA’s Jet Propulsion Laboratory in California. This chassis will serve as the backbone for a robotic spacecraft that will explore a metallic asteroid for the first time. This ambitious mission, named Psyche after the eponymous asteroid it will explore, is due to launch next summer on a Falcon 9 rocket.

Once in space, the spacecraft will use an innovative means of propulsion, known as Hall thrusters, to reach the asteroid. This will be the first time a spacecraft has ventured into deep space using Hall thrusters, and absent this technology the Psyche mission probably wouldn’t be happening—certainly not at its cost of just less than $1 billion.

For David Oh, the large, boxy chassis represents one of those “full circle” moments in life. More than two decades ago, he worked on Hall thruster technology as a graduate student at the Massachusetts Institute of Technology. He would go on to work for Space Systems/Loral, which first put the propulsive technology on large commercial satellites and would later be acquired by Maxar.

After working on the first launches of commercial satellites powered by Hall thrusters, Oh left the private sector in 2003 to come to NASA’s Jet Propulsion Laboratory, where he has since worked on a number of missions, including the flight of Curiosity to the red planet in 2011. Now, he serves as technical lead for the Psyche mission.

“I’ve been working on electric propulsion for more than two decades,” he said in an interview.

And now, the Hall thruster technology Oh worked on as a graduate student will take NASA to an entirely new place, Psyche. No spacecraft have ever visited a world like this, composed of about 60 percent metal. We really have no idea what it will look like.

Electric propulsion

Engines powered by chemical propulsion are great for getting rockets off the surface of the Earth when you need a brawny burst of energy to break out of the planet’s gravitational well. But chemical rocket engines are not the most fuel-efficient machines in the world as they guzzle propellant. And once a spacecraft is in space, there are more fuel-efficient means of moving around.

One of these is solar electric propulsion, which uses solar panels to capture energy from the Sun, which in turn ionizes and accelerates a gas—typically xenon—to produce a thrust. It’s not much of a thrust. Actually, it’s exceptionally light. Each of the thrusters on the Psyche mission maxes out at about the same force as that exerted by two  or three quarters in the palm of one’s hand. But because they are so fuel-efficient, solar electric thrusters don’t burn for a few minutes at a time. They burn for months, producing a steady acceleration.

NASA has been experimenting with this technology for a while. The space agency first tested electric propulsion technology on its Deep Space 1 mission, which launched in 1998, and later with the Dawn mission in 2007 that visited Vesta and Ceres in the asteroid belt.

These spacecraft used ion thrusters. Hall thrusters, by contrast, use a simpler design, with a magnetic field to confine the flow of propellant. These thrusters were invented in the Soviet Union and later adapted for commercial purposes by Maxar and other companies. Many of the largest communications satellites in geostationary orbit today, such as those delivering DirecTV, use Hall thrusters for station-keeping.

But now, for the first time, they’re being used for a deep space mission. NASA and Maxar believe the technology is ready, but it still needs to be proven out in a new environment.

“It’s always a big deal when you go beyond Earth orbit,” said Robert Curbeam, a former astronaut who is a senior vice president at Maxar. “As you get further from the Sun, you’re going to generate less power from the Solar arrays. The radiation environment is going to be different. And there’s the question of whether we can keep these thrusters pulsing for that long.”

The mission

If NASA had tried to develop the Psyche mission with chemical propulsion, it would have required about five times as much fuel. This bulk would have caused Psyche to be even larger than the Cassini probe that NASA used to explore the Saturn system from 2004 to 2017, Oh said. The Cassini mission cost about $4 billion, the kind of budget that NASA reserves only for the highest-priority “flagship” missions. NASA only flies one or two of these a decade, and the scientists behind Psyche, led by Lindy Elkins-Tanton of Arizona State University, knew they weren’t going to obtain this designation.

Using Hall thruster-based technology enabled the mission’s scientists and engineers to design a smaller and more affordable spacecraft. Critically, NASA was able to buy a spacecraft chassis from Maxar that was built largely from commercial, off-the-shelf technology. Had NASA and the Jet Propulsion Laboratory needed to develop this technology on their own, it would have cost billions of dollars and taken years longer.

Each of the Hall thrusters on Psyche will generate three times as much thrust as the ion thrusters on the Dawn spacecraft and can process twice as much power. This will allow the spacecraft to reach the Psyche asteroid, located in the main belt, in January 2026, after a 3.5-year journey.

Psyche is still a big spacecraft, complete with large solar arrays to collect sunshine way out in the asteroid belt.
Psyche is still a big spacecraft, complete with large solar arrays to collect sunshine way out in the asteroid belt.

Instead of performing a big burn when the spacecraft nears Psyche—as is customary with a chemical-powered vehicle—it will sip xenon fuel and back into orbit around the asteroid. Over time the spacecraft will move progressively closer, always trying to find a stable orbit around the asteroid, which has an odd shape and a non-uniform gravity field. Having fuel-efficient thrusters will help with this.

Eventually, toward the end of this 21-month period, the spacecraft will come to within about 100 km of the surface of Psyche, which has a diameter of about 220 km. In this lowest orbit, Psyche will use a spectrometer to map the various elements that make up Psyche and answer the question of just what, exactly, metallic asteroids are made of.

Oh said he was eager to learn whether Psyche may be the core of something that could have become a planet during the early days of our Solar System, but ultimately did not. If so, we might learn a lot more about the core of our own world, Earth. Additionally, findings from this mission will better inform scientists and entrepreneurs about the potential riches that might be had from metallic asteroids, which could be pivotal for human settlement in deep space decades from now.

Listing image by NASA