NASA chooses two asteroid missions instead of a Venus return

For the planetary science community, the day when NASA makes the final call on new Solar System exploration missions always feels bittersweet. Yes, there are deserving winners, but that means there must be losers as well. On Wednesday, happily, the space agency announced two winners for missions to launch during the early 2020s—Psyche and Lucy, which will each explore different classes of asteroids.

Unfortunately, there were also three losers. Two of the three losers were missions to study the atmosphere and surface of Venus, a planet not visited by NASA in more than two decades. A third project to build an instrument that would identify asteroids that might one day strike Earth, NEOCAM, will receive funding to essentially keep the project on life support for future consideration.

In a video announcing the winning missions, NASA's chief planetary scientist, Jim Green, said the agency opted to study relics of the early Solar System. "These missions will help us learn about the infancy of our Solar System, a period just 10 million years after the birth of our Sun," he said. The decisions were based on the value of the science, the mission's cost, and overall risks, he said later, during a news conference.

Psyche

Launching in 2023, this mission will send a spacecraft to an asteroid in the belt between Mars and Jupiter, Psyche 16. With a diameter of 210km, Psyche 16 is among the 10 largest objects in the asteroid belt, and it's especially intriguing because it is metallic, composed largely of iron and nickel. Scientists think the intriguing object may be the exposed core of a planet that was once roughly the size of Mars but lost its outer, rocky layers due to a series of violent collisions.

It will take the spacecraft about six years to reach Psyche 16, located 3 astronomical units from the Sun (Earth is at 1 AU), after which it will spend more than a year in orbit around the asteroid to map and study its properties. Scientists aren't sure what they'll find at the asteroid, as they've never explored a metallic world, just bodies of rock and ice. They hope to find some answers about our planet as well, because this is the only known place in the Solar System where scientists can make an up-close study of a metallic core like that at the center of Earth as well as Mercury, Venus, and Mars.

The spacecraft will carry magnetometers, multispectral imagers, a gamma ray and neutron spectrometer, and a radio-science experiment to help elucidate the nature of Psyche 16. Over the course of a year it will spiral down to lower and lower orbits, before spending 70 days orbiting the asteroid at an altitude of just 105.5km to fully assess its elemental composition. After such time, scientists should have a better understanding of how planets form and basic knowledge about an entirely new kind of world.

Lucy

The second mission selected by NASA, Lucy, is slated to launch in 2021. It will visit the heretofore unexplored swarms of "Trojan" asteroids that have accumulated at the L4 and L5 points of Jupiter's orbit around the Sun. After stopping by a small main belt asteroid in 2025, the plan is for Lucy to make five flybys of Trojan asteroids from 2027 to 2033. Scientists know very little about these objects, and they aren't sure whether they are captured asteroids, comets, or even objects that have found their way into the main Solar System from the Kuiper Belt.

“This is a unique opportunity,” said Harold F. Levison, principal investigator of the Lucy mission from the Southwest Research Institute in Boulder, Colorado. “Because the Trojans are remnants of the primordial material that formed the outer planets, they hold vital clues to deciphering the history of the Solar System.”

During its flybys, Lucy will map the color, composition, geology, and inner structure of the Trojan asteroids. From this information, scientists hope to infer answers to questions about Solar System formation, such as how the giant planets accreted and moved to their present orbits and how the inner planets formed as a result. The Lucy spacecraft will also be looking for clues about possible primordial organic matter.

Led by the Southwest Research Institute in Boulder, Colorado, the Lucy spacecraft will use some heritage hardware from the New Horizons spacecraft that successfully flew by Pluto in 2015, including newer versions of the RALPH and LORRI science instruments. The name Lucy is indeed a nod to the iconic fossil that helped anthropologists pin down the origins of humanity here on Earth.

Discovery-class missions

The new missions fall within NASA's Discovery program, which funds proposals that cost $400 million to $500 million and seek out novel targets in the Solar System. Recent, successful Discovery-class missions include the Dawn spacecraft at Ceres, the Kepler exoplanet finder, and the Lunar Prospector. A dozen such missions have been funded to date. They are the least costly tier of NASA planetary science missions, with flagships such as the Curiosity rover costing the most at more than $1 billion, followed by New Frontiers missions such as New Horizons in the $1 billion range.

"This is what Discovery Program missions are all about—boldly going to places we've never been to enable groundbreaking science," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate in Washington.

So why didn't Venus make the cut this time? Sources in the planetary science community have suggested to Ars that the Venus Exploration and Analysis Group would do well to coalesce around a single, highest priority concept for a mission to Venus, rather than presenting several different ideas to NASA for funding. Green also noted that Venus, although it lost on Wednesday, has an opportunity to be selected for the next New Frontiers opportunity.