NASA is plunging into the solar system’s origins, selecting two planetary missions to visit mysterious asteroids. The agency has chosen Lucy, which will visit Jupiter's Trojan asteroids, and Psyche, which will orbit a large metallic asteroid, for its next Discovery missions, its low-cost planetary science mission line.
Lucy, set for a 2021 launch, will take advantage of a unique orbital moment to fly past six of Jupiter's Trojan asteroids, which precede and follow the gas giant's orbit, a region previously unexplored by spacecraft. Launching in 2023, Psyche will orbit a rare iron and nickel asteroid of the same name, believed to be the stripped-bare core of an ancient planetesimal, and will test whether such bodies could have been hot enough to have formed molten, spinning cores.
“Lucy and Psyche will take us to unique worlds that humankind has never explored before," says Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington, D.C. "With this, NASA is continuing its legacy of firsts.”
The Jupiter Trojans have long been an exploratory goal for NASA. But Harold Levison, Lucy's principal investigator and a planetary scientist at the Southwest Research Institute in Boulder, Colorado, came to the project from his theoretical work. A decade ago, he helped devise a leading model of the solar system's formation that, among its virtues, gave birth to objects with orbits like the Jupiter Trojans. These are the fossils of planet formation, he says. "If we're going to detangle what happened out there, the Trojans are the place to look."
Lucy will make a grand tour, examining its targets with an infrared spectrometer and new versions of the black-and-white and color cameras found on New Horizons, the Pluto explorer. After passing one asteroid in the main belt, in 2027 it will fly past four Jupiter Trojans over the course of a year and a half. And then, 5 years later, it will pass a rare binary asteroid system in an orbit highly tilted from Jupiter’s orbital plane. It's a complicated trajectory that was only possible with the 2021 launch, which may have helped its selection. "I've been worshipping at the feet of the celestial mechanics god for the last 30 years," Levison says, "and they're paying us back."
Little is known about Lucy's targets. The team knows their colors, from which they infer composition, and some of their reflectivity. From this limited information, it's clear they are diverse in type, and not homogenous leftovers from the formation of Jupiter. It's likely they originated farther out from their current orbit, Levison says, and found their way to Jupiter during the reconfiguration of the planets. The most interesting target is the binary pair Patroclus and Menoetius, Levison adds. Binaries are rare in much of the solar system, but common in the Kuiper belt beyond Neptune, far from the gravity of planets. That could indicate the duo are nearly pristine leftovers from that early era.
Psyche, meanwhile, has its origins in research conducted by Lindy Elkins-Tanton, the project's principal investigator and planetary scientist at Arizona State University in Tempe. With several colleagues, Elkins-Tanton identified remnant magnetic fields in meteorites that could be linked to ancient planetesimals, the building blocks of planets. Such magnetism was thought only to come from the spinning molten cores of full-fledged planets, not cooler, smaller protoplanets. But they surmised that a radioactive isotope of aluminum may have been abundant enough to heat the planetesimals from the inside-out, forming a molten core.
It was controversial. At the first meeting where they presented this idea, in 2011, "they lined up three deep at the microphones to rebut me even before I started talking," Elkins-Tanton says. But it gradually caught on, and then the Jet Propulsion Lab called, suggesting a mission to test their hypothesis.
They settled on 16 Psyche as a target. At 210 kilometers in diameter, it is one of the largest known members of the asteroid belt, and thought to be largely iron and nickel in composition, similar to Earth's core. They suspect it could have once been the molten core of a planetesimal that might have neared Mars in size. And late last year, observations from NASA's Infrared Telescope Facility also discovered signs of what looks to be water or hydroxyl on Psyche's surface, which could make it a promising resource-rich depot for human exploration in the future.
After its launch in October 2023, Psyche will arrive at its target in 2030. While orbiting, it will use a magnetometer to search for signs of a remnant magnetic field, which could have once been half as strong as Earth’s. "A little fridge magnet in space," Elkins-Tanton says. It will also employ a gamma-ray and neutron spectrometer to study radiation emitted by the asteroid, to judge its metallic composition. It's not an asteroid mission in the classic sense, Elkins-Tanton adds. "We want to understand the interior of a planet. It just happens this planet is categorized as an asteroid."
NASA has supported many missions to small bodies in recent years, but its appetite has not waned. Lucy and Psyche are different, but worthy, missions, says Michael A'Hearn, an astronomer at the University of Maryland in College Park. "Psyche is important because it will tell of the evolution of bodies in the solar system, whereas Lucy is telling us more about the origins of Trojans." Many scientists suspect the Jupiter asteroids are similar in composition to comets, he adds.
Each capped at $450 million in costs, these proposals were selected from five final candidates, all focused either on Venus or small solar system bodies. After an initial cull in 2015, the teams received $3 million to flesh out their proposals. After weeks of waiting over the holidays, the candidates learned of their selection only hours before the final announcement today.
The agency also announced it would extend another year of funding to the Near-Earth Object Camera (NEOCam), one of the finalists, though its plans beyond that were not immediately clear. “This mission is an important capability for the agency,” says Jim Green, NASA’s planetary science division director in Washington, D.C.
The final five candidates were:
- Lucy and Psyche;
- NEOCam, a space-based telescope that would discover and study 10 times more Earth-threatening asteroids and comets than known today;
- DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging), a probe that would plunge into Venus's atmosphere for 63 minutes and study its chemical and isotopic composition; and
- VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy), a Venus orbiter that would map the planet with radar at high resolutions, hunting for active volcanism.
The announcement comes as a particular rebuke for planetary scientists focused on Venus. For years, these researchers have been frustrated by NASA's refusal to return the planet, which it last directly targeted in the early 1990s. After failing to win selection in the previous Discovery round earlier this decade, scientists worked to clarify their scientific questions and needed measurements. By selecting two Venus missions among its final five candidates, NASA seemed to signal their success in coming together.
Many planetary scientists were also wary of the NEOCam mission. NASA faces a congressional mandate to identify 90% of mid-sized near-Earth objects by 2020. Although NEOCam would provide worthy scientific research, scientists feared it would take a Discovery slot away from more deserving work—fears that were ultimately unfounded. But bypassing NEOCam means it is unlikely that NASA will meet its congressional obligation, unless Congress ultimately decides to separately finance the mission.
The B612 Foundation, which once promised to launch a private asteroid telescope similar to NEOCam, has backed away from its plans. The $650 million Large Synoptic Survey Telescope (LSST), currently being built in Chile with the support of the National Science Foundation and the Department of Energy, will identify many near-Earth asteroids as it surveys the entire sky every 3 nights once it begins operations in 2023. But simulations suggest the LSST is unlikely to meet the 90% threshold on its own. According to a White House strategy memo released late last month, the best strategy may be to combine a space-based telescope with the LSST.
For 2 decades, NASA's Discovery program has supported low-cost planetary missions, with a goal of launching a new probe every 2 to 3 years. However, it has been 5 years since the last launch, and the most recent selection, the InSight probe to Mars, has been plagued by delays, with its launch falling back to 2018. NASA had a goal of selecting two Discovery missions this round to help it get back on schedule, though there had been concerns whether federal budget uncertainty would allow it. Green says missions should launch every 32 to 36 months going forward. “The program is now back, now healthy.”