The second, slated for 2023, will send an orbiter to 16 Psyche, a massive metallic object in the asteroid belt that is thought to be the exposed iron core of a protoplanet.
The missions are part of NASA’s Discovery Program, launched in 1992 to promote what then-NASA administrator Daniel S. Goldin called “better, faster, cheaper” solar system exploration. Discovery projects are shorter, more focused and smaller in scale than the average mission, and their costs are capped at around $500 million.
But they still do some pretty cool science. Mars Pathfinder — which successfully launched the first rover to explore Mars — was a Discovery mission. So were MESSENGER, the first (and so far, only) orbital survey of Mercury; Dawn, which is studying the two biggest objects in the asteroid belt, Vesta and Ceres; and the Kepler space telescope, which has found thousand of exoplanets orbiting far-off stars, including nearly two dozen in the “habitable zone.”
“We’ve explored terrestrial planets, gas giants, and a range of other bodies orbiting the sun,” Jim Green, NASA's planetary science director, said in a statement. “Lucy will observe primitive remnants from farther out in the solar system, while Psyche will directly observe the interior of a planetary body. These additional pieces of the puzzle will help us understand how the sun and its family of planets formed, changed over time, and became places where life could develop and be sustained — and what the future may hold.”
Pysche and Lucy were selected from a shortlist of five proposals. Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (or DAVINCI, because there’s no better way to win over NASA than a convoluted acronym) would have sent a probe on a 63-minute descent through the atmosphere of Venus. Another Venusian mission, the Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy mission (VERITAS), would map the planet’s surface and search for water and signs of geologic activity.
The last, Near Earth Object Camera, would have launched an infrared space telescope to seek out potentially hazardous near-Earth asteroids. Though not selected, NEOCam will get an additional year of funding, NASA said, suggesting that the telescope could be built someday.
Both Lucy and Pysche will seek to reveal the secrets of the solar system’s beginnings.
The six Trojan Asteroids to be explored by Lucy are dark bodies thought to have been pulled into orbits near Jupiter during the early days of the solar system, when planets were still forming and migrating into their current positions.
“These small bodies really are the fossils of planet formation,” said Harold Levison, a planetary scientist at the Southwest Research Institute and the principal investigator for the Lucy mission. They are made of the same material that existed in the early days of the solar system, and are thought to contain important organic molecules.
Psyche, meanwhile, can provide clues about what happens inside a planet’s core. The 130-mile-wide asteroid is made of mostly iron and nickel, not ice and rock like other asteroids. Scientists think it may be the exposed core of an early planet that lost its rocky exterior during a series of violent collisions not long after it was formed. There is no other object like it in the solar system.
“This is the only way humankind ever can visit a planetary core,” said principal investigator Lindy Elkins-Tanton of Arizona State University in Tempe. “We learn about inner space by visiting outer space.”
The selection of two asteroid-focused missions also means that NASA will not be exploring any bodies with atmospheres (aside from Mars, which has negligible one). That’s worrying to scientists working on the search for life beyond Earth, since organic molecules on an airless asteroid can only offer so much insight into how organisms arose. A place like Venus — which endures incredibly high pressures and is unbearably hot, but also has the kind of thick protective atmosphere we know life needs to survive — could help scientists understand how atmospheres operate. But NASA hasn’t sent an orbiter to our neighboring planet since the 1970s, and seems unlikely to venture there anytime soon.
What asteroids do have going for them is the presence of water, organics and valuable minerals — all of which could potentially be used for life support, rocket fuel and commercial endeavors. NASA’s interest in Lucy and Psyche, along with OSIRIS-REx, which is en route to the asteroid Bennu, and the Dawn mission, suggests that the agency is interested in worlds that might one day be mined.
Asked why two more asteroid missions were chosen over a return to Venus, Green demurred, saying only that the two selected missions were the best of the five finalists.
But that doesn’t mean the space agency has abandoned the second planet from the Sun. NASA’s call-out for ideas for its next New Frontiers mission (a class of medium-cost projects that includes New Horizons, OSIRIS-REx and the Jupiter probe Juno) includes a request for proposals for a Venus atmospheric probe and lander.
Thomas Zurbuchen, the associate administrator for NASA’s science mission directorate who was part of the selection committee, explained that as he reviewed the finalists, “I was imagining what the future would look like, where are the textbooks going to be rewritten.”
Viewed in that light, it seemed only right that two complimentary asteroid missions be chosen.
“I think about Pysche and Lucy [as] two chapters of a book on the early solar system,” he said. “They really belong together.”
This post has been updated.