Aiming to find important clues to the earliest history of the solar system, NASA has announced two missions — one to explore Jupiter's mysterious Trojan asteroids and the other to study a unique metal asteroid.

The missions, known as Lucy and Psyche, were chosen from five finalists and will proceed to mission formulation, with the goal of launching in 2021 and 2023, respectively, NASA said in a statement on Wednesday.

"Lucy will visit a target-rich environment of Jupiter's mysterious Trojan asteroids, while Psyche will study a unique metal asteroid that's never been visited before," said Thomas Zurbuchen, Associate Administrator for NASA's Science Mission Directorate in Washington. 

"This is what Discovery Program missions are all about – boldly going to places we've never been to enable groundbreaking science," Zurbuchen added.

Lucy, a robotic spacecraft, is scheduled for October 2021 launch. It is slated to arrive at its first destination, a main belt asteroid, in 2025. 

From 2027 to 2033, Lucy will explore six Jupiter Trojan asteroids. 

These asteroids are trapped by Jupiter's gravity in two swarms that share the planet's orbit, one leading and one trailing Jupiter in its 12-year circuit around the sun. 

The Trojans are thought to be relics of a much earlier era in the history of the solar system, and may have formed far beyond Jupiter's current orbit.

"This is a unique opportunity," said Harold 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. Lucy, like the human fossil for which it is named, will revolutionize the understanding of our origins," Levison noted.

The Psyche mission will explore one of the most intriguing targets in the main asteroid belt – a giant metal asteroid, known as 16 Psyche, about three times farther away from the sun than is the Earth. 

This asteroid measures about 130 miles (210 kilometers) in diameter and, unlike most other asteroids that are rocky or icy bodies, is thought to be comprised mostly of metallic iron and nickel, similar to Earth's core. 

The mission will help scientists understand how planets and other bodies separated into their layers – including cores, mantles and crusts – early in their histories.

Psyche, also a robotic mission, is targeted to launch in October of 2023, arriving at the asteroid in 2030, following an Earth gravity assist spacecraft maneuvre in 2024 and a Mars flyby in 2025.