NASA’s more metallic mission will test new, more powerful electric propellers

A satellite company called Maxar recently delivered a piece of spacecraft the size of a passenger van 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, called Psyche in honor of the eponymous asteroid it will explore, is due to launch next summer on a Falcon Heavy 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 that a spacecraft has ventured into deep space using Hall thrusters and, without this technology, the Psyche mission would probably not be happening – certainly not at the cost of just $ 1 billion.

For David Oh, the large, square 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 work for Space Systems / Loral, which first put propulsion technology on large commercial satellites and would later be acquired by Maxar.

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

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

And now, the Hall propulsion technology where Oh worked as a graduate student will take NASA to an entirely new place, Psyche. No spaceship has ever visited a world like this, made up of about 60 percent metal. We really have no idea how it will be.

Electric propulsion

Chemical powered engines are great for getting rockets off the Earth’s surface when you need a strong burst of energy to get out of the planet’s gravity well. But chemical rocket engines are not the most fuel-efficient machines in the world, as they consume propellant. And once a spaceship is in space, there are more efficient ways to get around.

One is solar electric propulsion, which uses solar panels to capture energy from the sun, which in turn ionizes and accelerates a gas – usually xenon – to produce an impulse. It is not much of an impulse. In fact, it is exceptionally light. Each of the propellers of the Psyche mission reaches its maximum with approximately the same force exerted by two or three quarters in the palm of the 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 constant acceleration.

NASA has been experimenting with this technology for some time. The space agency first tested electric propulsion technology on its Deep Space 1 mission, launched in 1998, and later with the Dawn mission in 2007, which 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 constrain the flow of the propellant. These engines were invented in the Soviet Union and later adapted for commercial purposes by Maxar and other companies. Many of the largest communication satellites in geostationary orbit today, such as those providing DirecTV, use Hall thrusters to maintain the station.

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

“It’s always important when you go beyond Earth’s orbit,” said Robert Curbeam, a former astronaut who is Maxar’s senior vice president. “As you move away from the sun, it will generate less energy with the solar arrays. The radiation environment will be different. And there is the question of whether we can keep these thrusters pulsing for so long.”

The mission

If NASA had tried to develop the chemical-powered Psyche mission, it would have required about five times as much fuel. That volume would have made Psyche even larger than the Cassini spacecraft NASA used to explore the Saturn system from 2004 to 2017, said Oh. The Cassini mission cost about $ 4 billion, the type of budget that the NASA reserves only for the highest priority “flagship” missions. NASA flies only one or two of these per decade, and the scientists behind Psyche, led by Arizona State University’s Lindy Elkins-Tanton, knew they would not get that designation.

The use of Hall-powered technology allowed mission scientists and engineers to design a smaller, more accessible spacecraft. Critically, NASA was able to purchase a Maxar spacecraft chassis that was built largely with commercial technology out of the box. If NASA and the Jet Propulsion Laboratory needed to develop this technology on their own, it would have cost billions of dollars and taken years more.

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

Instead of carrying out a large burn when the spacecraft approaches Psyche – as is common in a chemical-powered vehicle – it absorbs xenon and returns to orbit around the asteroid. Over time, the spacecraft will move closer and closer, always trying to find a stable orbit around the asteroid, which has a unique shape and a non-uniform gravitational field. Having low fuel consumption engines will help with this.

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

Oh said he was anxious to know if Psyche could be the nucleus of something that could have become a planet during the early days of our Solar System, but in the end it was not. In that case, we can learn much more about the core of our own world, Earth. In addition, the findings of this mission will better inform scientists and entrepreneurs about the potential riches that can be obtained from metallic asteroids, which could be fundamental for human settlement in deep space in decades to come.

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