Editor's note: This has been previously published on the author's website.
What sort of solar system object hasn't undergone close inspection by spacecraft or humans?
The sun? Monitored continuously. Planets? All visited by probes, including Pluto, which I sentimentally insist remains a planet. Rocky moons? Never forget July 20, 1969. Icy moons? Numerous loops around Jovian and Saturnian examples. Asteroids? OSIRIS-REx orbits asteroid Bennu with instructions to bring back a sample in 2023, while earlier this month the Japanese probe Hayabusa 2 dropped an explosive charge on asteroid Ryugu to expose samples that will be nabbed for the ride to Earth. Comets? Been there, sampled that.
The only major type remaining, apparently, is a metallic core. The rocky planets have them; possibly the gas giants do also, though nobody knows for sure. Earth's core of molten metal reaches from the center of the planet to within 2,800 miles of the surface, too deep for direct observation.
Now NASA is well on the way to visiting what scientists believe may be an exposed metallic core. Or, the Jet Propulsion Agency says, it could be merely "unmelted material," rather than the remains of a liquid core. Whichever it is, no spacecraft has yet visited such an object.
The largest metallic asteroid, and the one with the highest metal content, is Psyche, which measures 130 miles across and is a denizen of the outer region of the asteroid belt. It is about 280 million miles from the sun on average.
NASA wants to discover if Psyche is a protoplanetary core. Protoplanets are rock and metal objects that clumped together soon after the solar system began forming some 4.5 billion years ago. And rather than stemming from a protoplanet, Psyche could even be the core of a now-demolished planet. "Scientists wonder whether Psyche could be an exposed core of an early planet, maybe as large as Mars, that lost its rocky outer layers due to a number of violent collisions billions of years ago," notes NASA.
Lindy Elkins-Tanton, director of the Arizona State University School of Earth and Space Exploration, Tempe, described the project she heads — to study Psyche by spacecraft — during the University of Utah's Frontiers of Science lecture on April 23. She is one of the first women named principal investigator of a major NASA exploration project.
The asteroid is believed to be 90 or 95 percent metal and the rest rock, the highest metallic content of any asteroid, which puts it in the running to be an exposed planetary core. It is so massive that it makes up 1 percent of the entire asteroid belt, a vast junkyard of rubble left over from the formation of the solar system.
When protoplanets formed, they accumulated the radioactive element aluminum-26 from the dust and gas of the primitive solar system. In some cases, enough of it was present to melt the conglomeration. The metals, which are heavier, sank to the center of the mass. Rocks and soil-like rubble covered the surface. "This molten metal core froze starting on the outside," she said.
Then with Psyche, collisions would have caused the rocky covering to fly off, exposing the core.
"What will impact craters be like when they go into metal?" she asked. Materials specialists think pits from such collisions would be much deeper than when objects hit the rocky surface of asteroids. Giant cracks may have developed around craters. Volcanoes may have spewed liquid sulfur onto the surface.
The craft is scheduled to launch from Kennedy Space Center, Florida, on Aug. 20, 2022, aboard either an Atlas IV or a Falcon Heavy rocket, Elkins-Tanton said. It will cruise by Mars in May 2023, picking up speed as it slingshots past the planet. It is to begin orbiting Psyche around the first week of January 2026. During a 21-month visit, the orbits will become tighter while the spacecraft's cameras and other instruments send back pictures and data. The final orbits will be at about 50 miles altitude.
Researchers promise to pop photos onto the internet as soon as they arrive.