A NOVEL PROPULSION SYSTEM WILL TAKE US TO A MASSIVE METAL ASTEROID IN DEEP SPACE

Imagine being able to move your car with your breath. One long blow out the back window, and your car begins speeding down the highway. Now imagine that if you keep blowing, your car accelerates to over 124,000 miles per hour. 

Of course, if you actually did this on Earth, you would turn blue in the face and your car would remain still. But in the vast vacuum of empty space where there is little gravity and no atmospheric drag, that tiny amount of thrust can be very effective. 

On October 12, NASA’s Jet Propulsion Laboratory in La Cañada Flintridge will launch a spacecraft toward Mars and Saturn that will put this idea to the test. The agency is sending a 3,600-pound vehicle into space propelled by futuristic solar electric thrusters that deliver a force equivalent to the mass of about two quarters. They also happen to emit a cool blue glow that looks like something out of Blade Runner. 

The spacecraft, called Psyche, launched aboard a SpaceX Falcon Heavy rocket from the historic Launch Complex 39 at NASA’s Kennedy Space Center. The spacecraft will initially be set on a trajectory to fly by Mars, where it will receive a gravity assist, catapulting it further out into the solar system. In late 2026, the spacecraft will enter into orbit around a rare metal asteroid called Psyche 16 (hence the spacecraft’s name). The journey to the asteroid will take over three and a half years and cover over 1.5 billion miles. 

Perhaps the most intriguing — and ultimately beneficial — components of the Psyche mission will be its use of solar electric thrusters. Also known as Hall Effect thrusters, the novel propellant system was designed to be efficient and cost-effective. Solar energy will be generated from a five-panel, cross-shaped solar array that will unfold and immediately begin harvesting energy from the sun. At 800 square feet, they are the largest panels ever installed at JPL, and when fully deployed, will extend about the area of a singles tennis court.  

The resulting energy will be used to turn xenon, a dense, colorless, odorless noble gas into xenon ions, atoms that carry a charge because the number of electrons does not equal the number of protons. Xenon is found in Earth’s atmosphere in trace amounts and is used in car headlights and plasma TVs. As the xenon ions are accelerated out of the thruster, they create thrust, propelling the spacecraft forward. The amount of thrust, however, will be minuscule compared to that of chemical-based propulsion systems normally employed on missions like those to Mars, Jupiter, and Saturn. 

”They operate at a low thrust level,” says David Oh, Psyche’s project system engineering manager. “You can’t use it to launch from the earth. But in space, you operate these thrusters over a long period of time and you can get to very high speeds.” 

In other words, in space, a force equivalent to a hastily expelled deep breath is enough to move a ton and a half of metal through space at a speed more than one hundred times that of a fired bullet. 

Psyche will carry over 1000 kilograms of xenon in its tanks, more than enough to get the spacecraft to Psyche and complete its 21-month mission. JPL engineers estimate that the spacecraft would burn through about 15 times that amount of propellant by weight if it had to use traditional chemical thrusters.  

“We did try conventional chemical propulsion, and we determined if we did that, we would have quadrupled the mass of the spacecraft. It would have been very difficult to launch and very expensive to build. But this technology was mature and ready to go,” says Oh.

This is not the first time an ion propulsion system has been used in space. Communication satellites orbiting the earth use them regularly. Colorado-based company Maxar Technologies developed and built the Hall thrusters for near-earth orbit, and NASA has purchased them from the company and made some modifications, but this will be the first time they will be used to venture into deep space.

“We needed advanced propulsion to get into orbit. We were looking at what could we buy rather than building our own thruster from scratch,” says Oh.

Because they’re so efficient, Psyche’s Hall thrusters can operate nearly nonstop for years without running out of fuel, says Oh. When its mission is over, the spacecraft might have lots of fuel left over, and they will have to decide whether to find other puzzles to solve. If the mission proves a success, Psyche’s Hall thrusters could play a major role in propelling future missions into deep space. 

A Metal Asteroid?

Scientists are giddy at what they might find once Psyche, propelled by the Hall Thruster system, arrives at the asteroid.  

“It’s a kind of world that humans have never visited before,” says Arizona State University’s Lindy Elkins-Tanton, principal investigator in charge of the mission. “Most of the exploration we do is going and learning more about a body we’ve already visited. Psyche, we have no photos of it, no one has ever done a flyby or really studied it. It’s unlike every asteroid we know, as far as we can tell.”   

Astronomers have been aware of Psyche’s existence ever since since it was first discovered on March 17, 1852, by the Italian astronomer Annibale de Gasparis. The asteroid, computer models of which resemble a potato, was named after the Greek mythological figure Psyche, the goddess of the soul. It is the largest and most massive of the known M-type asteroids (M stands for metal), and is one of a dozen of the most massive objects in the asteroid belt between Mars and Saturn. 

Scientists believe that Psyche may be part of the core of a body called an early planetesimal—, a moon-sized type of protoplanet that may have once been much larger, but was perhaps struck many years ago by other heavy orbiting objects, shattering it into pieces, but leaving this particular blob of metal and rock behind. Such collisions were common when the early solar system was forming.

If Psyche was once the heart of a planet with a strong convective current and a molten iron-nickel core at its center, then scientists expect it will still have a magnetic field. A magnetometer aboard the spacecraft will measure its pull, while cameras will photograph and map the surface, collecting high-resolution multispectral images. Because of the asteroid’s unique composition, scientists have no idea what to expect when the images first start rolling in. 

“What does an impact crater into metal look like? We do not know?” says Elkins-Tanton. 

In a unique twist for this mission, JPL plans to release the images captured by Psyche immediately onto the internet so that anyone can view them within a half-hour of being received. 

“We’re not going to edit them or curate them. We’re going to send them out so that everyone can be looking at this funny object for the first time,” says Elkins-Tanton.

In addition, a series of spectrometers will help us understand what the asteroid is made of by measuring the gamma rays and neutrons emitted from it. Depending on what scientists discover, the mission could help answer fundamental questions about the formation of our solar system. 

That will depend, of course, on whether the propulsion system functions as designed. As we stand on the precipice of a new era in space exploration, Hall thrusters aren’t just rocket science—they’re the closest thing we have to cosmic alchemy, promising to redefine how we navigate the vast tapestry of our solar system.