Nasa will launch two new missions to asteroids in search of clues about the early solar system, the space agency announced today.

The first mission, scheduled to launch in 2021, will send a probe to study the Trojan asteroids that swarm around Jupiter and are thought to be relics of the earliest days of the solar system. The project has been dubbed "Lucy," in honour of the 3.2-million-year-old Australopithecus who is humanity's most famous ancient relative.

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 Programme, launched in 1992 to promote what then-Nasa administrator Daniel 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 US$500 million.


But they still do some pretty cool science. Mars Pathfinder - which successfully set 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."

Psyche 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 with a convoluted acronym) would have sent a probe on a 63-minute journey to the surface of Venus to study the planet's thick atmosphere. 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 Psyche 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. They are made of the same "primordial material that formed the outer planets," said Harold Levison, a planetary scientist at the Southwest Research Institute and the principal investigator for the Lucy mission, and may even contain important organic molecules.

Psyche, meanwhile, can provide clues about what happens inside a planet's core. The 210km-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 humans will ever visit a core," said principal investigator Lindy Elkins-Tanton of Arizona State University in Tempe. "We learn about inner space by visiting outer space."