NASA/JPL

Venus and asteroids have emerged as top destinations for NASA's future planetary exploration. On 30 September, the agency announced a shortlist of five contenders for its US$500-million Discovery class of missions.

Two of the five proposed missions would target Venus, which NASA has not visited in more than two decades. A radar orbiter would map the planet’s cloud-enshrouded surface from above, while an atmospheric probe would descend directly through the layers of haze. “They're pretty exciting choices and focus on a body that has not received much attention,” says Steven Hauck, a planetary scientist at Case Western Reserve University in Cleveland, Ohio.

Asteroid mission concepts include a telescope to hunt for dangerous near-Earth objects; a visit to the peculiarly metal-rich asteroid Psyche; and a tour of four Trojan asteroids that orbit near Jupiter.

NASA will give each of the proposed missions $3 million to develop their ideas further, and by September 2016, the agency will select one — or possibly two — eventually fly. NASA officials say that they were impressed with the quality of the 27 proposals they received. If budgets permit, they may queue up two of the selected missions to fly as the next two Discovery missions rather than choose one and then put out another call for different ideas.

The selection capped months of anxious waiting for many US planetary scientists, who submitted their ideas in February. “It’s been an amazing day,” says Harold Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who heads the Trojan asteroid proposal. “I got the call when I was driving to work,” he says. “I pulled over.”

Outward bound

In principle, the Discovery competition is open for ideas to visit any target in the Solar System other than Earth or the Sun.

Among the mission concepts that lost out were a spacecraft to whizz past erupting volcanoes on Jupiter’s moon Io, and one to analyse the chemistry of plumes spewing from Saturn’s moon Enceladus, which many consider a promising place for extraterrestrial life. Also left on the sidelines were several proposals to study comets, and three focusing on the Martian moons Phobos and Deimos.

Women lead four of the five shortlisted missions. Suzanne Smrekar, of the Jet Propulsion Laboratory (JPL) in Pasadena, California, heads the VERITAS mission to map Venus at higher resolution and in different radar frequencies than NASA’s Magellan mission of the early 1990s. Lori Glaze, of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, is leading development of a probe that would descend through the atmosphere over about an hour, making measurements along the way.

Farther out in the Solar System, planetary scientist Lindy Elkins-Tanton heads the push to visit the asteroid Psyche. Elkins-Tanton, of Arizona State University in Tempe, says that Psyche represents a primordial not-quite-planet whose outer rocky layers have been stripped away to expose its metallic heart. “Psyche is the only core that humankind can ever see,” she says. “We’ve visited gassy things and rocky things and icy things, but we’ve never visited a metal object,” she says. The Psyche mission would launch in 2020 for a 2026 arrival.

Rocky path

Levison’s 'Lucy' — named after the famous early-human fossil — would fly past a main-belt asteroid on its way to visit the Trojans, whic h share their orbit with Jupiter. These poorly understood space rocks may have originated farther out in the Solar System. “There’s a huge diversity in this population, and that diversity is telling us about the evolution of the Solar System,” he says. Lucy would launch in 2021 and reach the end of its mission in 2032.

The asteroid mission NEOCam (for Near-Earth Object Camera) would use an infrared telescope to hunt for small and faint but potentially hazardous asteroids. Led by Amy Mainzer of JPL, it has been through the Discovery selection process twice before; NASA rejected the proposal in 2006 but gave Mainzer money in 2010 to develop the telescope's infrared detectors. “We really want to go find some asteroids and settle the question of whether one is heading our way,” Mainzer says.

Although Discovery missions are supposed to launch every couple of years, the current candidates are the first to be selected since 2010. Ongoing Discovery missions include the Dawn spacecraft, which is orbiting the asteroid Ceres, and the Kepler telescope that searches for extrasolar planets. In March, NASA plans to launch its next Discovery spacecraft, InSight, which will place a seismometer on the surface of Mars to study the planet’s deep interior.