The Lucy space probe blasted off into space atop an Atlas V rocket launched from Cape Canaveral, Florida on 16th October 2021. Lucy is heading for a group of asteroids, small rocky worlds, called the Trojans. Never before visited by space probes, the Trojans do not lie in the main Asteroid Belt. Instead they share Jupiter's orbital path around the Sun. One "swarm" follows behind the planet, the other travels ahead of it. Astronomers think that the Trojan asteroids were originally formed in different regions of the Solar System at the time of its formation 4.6 billion years ago, before being drawn together by the enormous gravitational pull of the giant planet Jupiter, where they remain today. If this is the case, astronomers expect the asteroids to have preserved a record of the conditions 4.6 billion years ago when Earth and all the other planets were forming.

18th October 2021
​​​​​​​

The journey to the Trojans will take six years. Lucy will use the Earth's gravity to act like a slingshot, hurling it outwards into deep space with extra energy. Passing through the main Asteroid Belt in 2025, it is due to fly by four individual Trojans during 2027 and 2028, zipping past each at speeds of between 18,000 and 32,400 km/h (10,800 to 21,600 mph). Then Lucy will return to Earth's orbit to receive an extra boost before it heads out into the second swarm and fly past another Trojan in 2033. The space probe will then continue to circle between the two Trojan swarms every six years.

It is thought the migrations of the giant planets, each exerting their own massive gravitational force, caused nearby smaller objects—rocky, not-yet planets known as planetesimals—to crash into one another, sending fragments scattering across the Solar System. These fragments, and smaller fragments produced by later collisions, make up the asteroids we know today. Some scientists think that as Uranus and Neptune moved outwards, they encountered a host of small bodies, which are thought to be the source of today’s Kuiper Belt objects which orbit at the edge of the Solar System. Some of these bodies were scattered inwards by the arrival of the two planets, with a few of them becoming trapped by Jupiter's gravity as Trojan asteroids.

The other flybys Lucy is scheduled to make are coincidences—visits to objects that Lucy would fly close enough to which are worth making a detour for. These include the first flyby, in April 2025: a Main Belt asteroid called Donaldjohanson, in honour of the American scientist Donald Johanson (born 1943). He discovered the fossil of an early human, Australopithecus afarensis, who lived in what is now Ethiopia 3.2 million years ago and whom he named "Lucy", after the 1967 Beatles song "Lucy in the Sky with Diamonds". It is this fossil Lucy that gives "her" (it was later discovered to be male) name to the current mission, likened to the search for "fossils" of the ancient Solar System.
​​​​​​​
​​​​​​​

Lucy's second encounter will be first of its principal targets, Eurybates. This Trojan asteroid is surrounded by a host of smaller asteroids, showing that it was shaped by a collision that took place after it arrived in Jupiter's orbit. One of these is a tiny moon measuring about 1 kilometre (0.6 miles) across, called Queta. It orbits Eurybates every 84 days.

​​​​​​​Next, Lucy will fly by Polymele, a small red Trojan. After that comes a slow-spinning Trojan called Leucus, before Lucy goes on to Orus, its second main target. All of these Trojans are in the L4 swarm, which runs ahead of Jupiter in its orbit.
​​​​​​​