In the original Star Trek television series, Captain Kirk and the crew of the USS Enterprise sought out M-class planets—balls of rock and metal with Earth-like atmospheres. Psyche, a NASA mission scheduled for launch on 12 October, is set to go where no spacecraft has gone before: to an M-class asteroid.
If everything goes as planned, the craft will orbit the asteroid, also named Psyche, for more than 2 years. Scientists said its observations will tell them more about the early days of the solar system, including the formation of Earth and the other inner planets.
“This is a whole new kind of world,” said Lindy Elkins-Tanton, a professor of Earth and space exploration at Arizona State University and principal investigator for the Psyche mission. “We don’t even know for sure just what Psyche is,” she said. “We’re used to going to Mars, where we know a lot about it already—we have a lot of context. We just don’t have that for Psyche.”
Psyche (formally 16 Psyche, indicating it was the sixteenth asteroid discovered) is a large metal-rich asteroid (the “M” in M-type), with an average diameter of about 222 kilometers. It’s a member of the main asteroid belt, located between Jupiter and Mars, and orbits the Sun at roughly 3 times the Earth–Sun distance, on average. Psyche will be the first metal-rich asteroid viewed up close; all previous asteroid encounters, including three sample return missions, were with rocky bodies.
“Psyche is the poster child for M-type asteroids,” said Zoe Landsman, a planetary scientist at the University of Central Florida who studies asteroid surfaces. “Getting that up close to it, the data will be really amazing.”
“Every asteroid is a unique snapshot of planetary formation,” said Rhonda Stroud, also a professor at Arizona State but who is not involved with the mission. “Metal asteroids should give us our best look at the cores of planets like Earth.”
Better Observations, New Models
When the Psyche mission was conceived a decade ago, scientists thought they knew its target pretty well. Radar observations showed that Psyche is especially bright, suggesting an almost pure metal composition. Optical observations revealed a spectrum consistent with that of iron meteorites.
The composition suggested that Psyche is the remnant core of a differentiated body—one large enough to melt and form layers, with heavier iron and nickel settling in the core. Thus, studying Psyche from close range would be like looking into the core of Earth or one of the other inner planets, perhaps providing details about how metallic cores take shape.
“We’d like to visit Earth’s core, but we can’t get there,” said Bill Bottke, director of the science directorate at Southwest Research Institute in Boulder, Colo., and a Psyche mission scientist. “The way to get around that is to go to a place where nature has exposed a possible core,” he said. “That’s one possibility that gets us excited.”
Continued observations of Psyche, some inspired by NASA’s approval of the mission, contradicted that early model of formation. They provided a more detailed profile of the asteroid, including better measurements of its surface composition, shape, and density.
“It’s sort of roundish with some dents,” Stroud said. “I like to think of it as avocado-shaped instead of potato-shaped, as a lot of people describe it. It has to have some surface texture, and it’s very bright, very reflective, and avocados are shinier than potatoes.”
Regardless of which foodstuff you prefer, the refined model showed that Psyche is about two thirds as dense as expected—roughly 4,000 kilograms per cubic meter. (Early estimates suggested 6,000 kilograms per cubic meter.) It’s still the densest known asteroid, with 30%–60% metal by volume. So although it’s still metal rich, it either contains a lot of rocky material or is a “rubble pile”—a body that was fractured and then reassembled, leaving lots of empty space between chunks.
That doesn’t eliminate the core remnant model, but it makes it less likely, Bottke said. Instead, Psyche might have formed with its present composition and structure from the solar nebula—the cloud of gas and dust that gave birth to the Sun and its attendant planets and smaller bodies. If correct, that model would tell scientists just where and when Psyche was born.
“The solar nebula was making planetesimals—the building blocks of planets,” Bottke said. “Certain regions might have become naturally metal rich, making metal-rich planetesimals. Psyche may be just such an object. If so, it was made late in the life of the solar nebula—maybe a ‘last gasp.’”
Radar observations showed that most of Psyche’s surface is covered with a layer of loose, iron-rich regolith about 1 meter thick, with a couple of bright spots that could be exposed patches of almost pure metal.
“Our observations suggest that early asteroids might have had enough volcanoes to generate volcanic centers—places where iron was being brought up from the interior,” said Michael Shepard, a planetary scientist at Bloomsburg University of Pennsylvania.
Although the Psyche mission was designed with a remnant core model in mind, the change in theories isn’t a problem, Elkins-Tanton said. “If anything, it makes the science better.”
A Long Journey to the Asteroid Belt
After a delay of almost 15 months to resolve software issues and other problems, Psyche (the spacecraft) is scheduled for launch at 10:38 a.m. EDT on 5 October, with additional launch opportunities through 22 October.
It will use an ion propulsion system that has never flown beyond the Moon to help send it to Psyche (the asteroid). After a journey of 3.6 billion kilometers, the spacecraft is scheduled to arrive in July 2029 and enter orbit 1 month later. Its primary mission, which extends through November 2031, will see the tennis court–sized craft drop into a series of ever lower orbits, culminating with a minimum altitude of 75 kilometers.
Its instruments include an imaging system that’s sensitive to optical and near-infrared wavelengths to map the entire asteroid and measure its mineral composition. A spectrometer will determine the chemical composition of its surface, while a magnetometer will map its magnetic field. (A strong remnant field would support the fractured-core model.) Scientists will use the craft’s radio system to plot the asteroid’s gravitational field, which will refine measurements of its mass and density.
“We’ll use all those data sets to piece together when and where Psyche formed,” Elkins-Tanton said. “Personally, I’m fixated on how it formed. My fondest hope is that it turns out to not be a core, but be something unusual and surprising that would change the way we think about planetesimals.”
“It’s a missing piece of the bigger puzzle,” Shepard said. “Psyche will help us get a better picture of the zoo that is the asteroid belt.”
—Damond Benningfield, Science Writer
5 October 2023: This article has been updated to reflect the current scheduled launch date.
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