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The ultra-dense asteroid Psyche is thought to be made almost entirely of iron and nickel metal. It could be the remnant core of a planetesimal that was stripped of its mantle (artist's impression).

Corby Waste/JPL

Venus and a bizarre metal asteroid are leading destinations for low-cost NASA missions

Venus is back on NASA’s agenda. Today, NASA winnowed down the contenders for the agency’s next low-cost planetary science mission. Five finalists were announced from among 27 proposals in Discovery, a competitive mission line with a $500 million cost cap, and two of them are missions to Venus, not visited by a NASA spacecraft since 1994. The other three finalists would study asteroids.

“It sends a very positive message that it’s time to go back to Venus,” says Lori Glaze, a planetary scientist at Goddard Space Flight Center in Greenbelt, Maryland, and the leader of one of the two Venus mission proposals.

Typically, NASA picks just three finalists in its Discovery competitions, which take place every few years. But this time the agency may choose two winners instead of the usual one, says Michael New, Discovery program scientist at NASA headquarters in Washington, D.C. The two winners’ development and launch would be staggered. “It depends on what our budgets in the out years look like,” he says. “Based on what we’ve seen to date, it looks like we’ll be able to do two.” Each of the five finalists will now get up to $3 million to pursue a more detailed proposal for the final selection about a year from now.

The five finalists are:

  • VERITAS (Venus Emissivity, Radio Science, InSAR Topography and Spectroscopy) a mission to map Venus’ surface with radar;
  • Psyche, a mission to explore an asteroid that could be made up almost entirely of iron and nickel;
  • Lucy, which would tour five Trojan asteroids, which follow the orbit of Jupiter either ahead or behind the giant planet;
  • NEOCam (Near Earth Object Camera), which aims to discover 10 times more near-Earth objects than have been discovered to date; and
  • DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging), which would study the chemical composition of Venus’ atmosphere during a 63-minute descent.

VERITAS and DAVINCI represent a vindication for Venus scientists in the United States, who have not sent a probe to the planet since the Magellan orbiter mission ended in 1994. Radar, the primary tool of VERITAS, allows scientists to see through Venus’ thick clouds. Able to map the surface at higher resolution than Magellan, the spacecraft should be able to add to the mounting evidence Venus’s surface is dotted with active volcanoes. The mission is led by Suzanne Smrekar, of the Jet Propulsion Laboratory in Pasadena, California.

DAVINCI would drop a spherical metal ball through the Venusian atmosphere. Studded with sensors, the probe would relay its measurements to Earth via the carrier spacecraft. It would also make the first images of Venus’ surface since the Soviet Venera landers of the 1970s. Glaze says her team will aim DAVINCI at Venus’ “tesserae,” regions of crumpled terrain that are thought to be the remnants of continents. “They’re really mysterious -- we don’t know what they are,” she says. “We’ll be taking pictures of these for the first time.”

Psyche’s destination, a metallic asteroid of the same name, “is not just another asteroid,” says principal investigator Lindy Elkins-Tanton of Arizona State University, Tempe. She says the body, which appears to be 90% iron and nickel, may be the remnant core of a planetesimal that was stripped of its mantle and crust by an impact. “This is the only way that humankind will ever be able to visit a core,” she says. She says that Psyche is also suspected to be strongly magnetic. “It could almost be like a little fridge magnet in space,” she says. The mission would launch in 2020 and arrive in 2026 for a year of science.

Lucy, the mission to five Trojan asteroids, would launch in 2021 and arrive in 2027 to visit three of them, says principal investigator Hal Levison, of the Southwest Research Institute in Boulder, Colorado. After those flybys, Lucy would swoop back by Earth and return to the vicinity of Jupiter’s orbit to visit the last two asteroids, which orbit each other as a binary. While other asteroid types are represented in meteorite collections, no Trojan-derived meteorites have ever been conclusively identified, leaving their compositions a mystery. “We’ve never been able to study them,” Levison says. Some of the Trojans are believed to be captured Kuiper Belt objects, comet-like objects from beyond the orbit of Neptune that formed in cold conditions and have not changed much in the past 4.5 billion years. Levison says the Lucy mission thus offers a chance to study the solar system’s building blocks.

NEOCam, led by Amy Mainzer of the Jet Propulsion Laboratory, is a space telescope that would find near-Earth asteroids from a position 1 million kilometers from Earth. In 2005, Congress mandated that NASA identify 90% of objects larger than 140 meters across by the year 2020. NASA will almost certainly fail to meet that mandate if it can only search for these potentially hazardous bodies from the ground. “NEOCam was selected because of its science,” says New. “The fact that it will also help us fill our congressional mandate was considered an extra benefit.”

NEOCam competed in the last round of Discovery, but it had some competition from outside NASA: the B612 foundation. The nonprofit organization, dedicated to finding hazardous asteroids, said it would raise private money to build its own space telescope, Sentinel. But B612 has struggled to meet its fundraising goals and scheduled objectives, and, earlier this week, it was reported that NASA had ended a cooperative agreement with B612. Hap McSween, a planetary scientist at the University of Tennessee at Knoxville, says NEOCam’s selection is not unrelated to the end of the B612 agreement. “The choice of NEOCam here is perhaps a reflection of harsh reality,” McSween says. “If this is going to happen, NASA is going to have to pay for it.”

Of the 27 Discovery proposals that were evaluated (28 were proposed in February but one was non-compliant), the vast majority were missions to study so-called “small bodies.” Three aimed to study the small moons of Mars, four to use space telescopes (like NEOCam) to study small bodies, and 11 to visit comets and asteroids. There were four proposals to target Venus, two proposals that targeted the Moon, and one to Mars. One proposal would study Jupiter’s moon Io.

Just one group proposed to venture beyond the orbit of Jupiter—to Saturn’s moon Enceladus. That may be because solar power is scarce at those distances. At the time of the last Discovery competition in 2012, NASA was developing a new plutonium-238 isotope power source, called the Advanced Stirling Radioisotope Generator. But the project was canceled in 2013. New says that NASA is now trying to revamp an older radioisotope generator, like the one riding on the Mars rover Curiosity, which could be offered in future Discovery competitions.

Planetary scientists hail Discovery, which began in 1996 with the launch of NEAR Shoemaker, an asteroid probe, as one of the most cost-effective mission lines at NASA. But in recent years, NASA has struggled to sustain the planned two-to-three year rhythm of Discovery launches. In 2016, the latest Discovery mission, called InSight, will head to Mars, 5 years after the previous Discovery launch, of the moon mission GRAIL. McSween says that NASA’s goal of selecting two winners next year may be aimed at getting the program back on track. “This may well be a response that they are trying to keep a regular cadence in the Discovery program,” he says.