NASA’s Lucy spacecraft has crossed into territory no probe has visited before. Five years after launching on October 16, 2021, and after traveling more than 2.2 billion miles, Lucy has become the first spacecraft to enter the region of the Jupiter Trojan asteroids, NASA said on October 28.

Here is what that milestone means and what comes next, based on NASA’s mission update.

What are the Trojan asteroids?

The Trojans are a huge population of primitive asteroids that share Jupiter’s orbit around the Sun. They gather in two swarms, one about 60 degrees ahead of Jupiter and one about 60 degrees behind it, around points known as Lagrange points 4 and 5. At those spots, the combined gravity of the Sun and Jupiter creates what NASA calls “islands of stability,” where thousands of objects can remain for billions of years.

Those swarms sit as far from Jupiter as Jupiter is from the Sun, about 484 million miles. That distance is why no previous mission has explored them. Spacecraft that visited Jupiter, or used it for a gravity assist, simply had no reason to make the long detour.

How NASA knows Lucy has arrived

Space has no signposts, and there is no sharp boundary around a Trojan swarm. Mission engineers determine that Lucy is approaching the leading L4 region by tracking the spacecraft’s radio signals and by taking images of the surrounding stars to pin down its position.

Even inside the region, Lucy will not be surrounded by boulders. NASA notes that Trojans are separated by more than 6 million miles on average, so they will look like pinpricks of light moving against the stars until the spacecraft closes in on a specific target.

The encounter schedule

Lucy’s itinerary is one of the most ambitious in planetary science. According to NASA, the spacecraft will fly past:

  • Eurybates on August 11, 2027
  • Polymele in September 2027
  • Leucus in April 2028
  • Orus in November 2028

After that, Lucy will return to Earth’s vicinity in 2030 for a third and final gravity assist, which will send it toward the trailing L5 swarm. There it will study the binary pair Patroclus and Menoetius in March 2033.

Why scientists care

The Trojans are thought to be leftovers from the early solar system, material that has changed little since the planets formed about 4.5 billion years ago. Studying them up close should help scientists test theories about how the giant planets formed and how they migrated into their current orbits. The mission is named after the famous fossil human ancestor, a nod to the idea that these asteroids are fossils of planet formation.

How the spacecraft is holding up

Lucy is now more than five times farther from the Sun than Earth is, where sunlight is dim and temperatures are low. NASA says the spacecraft was designed for these conditions and that it and its instruments continue to operate as expected.

With less than a year until the Eurybates flyby, the team is finalizing encounter plans so that the spacecraft and its operators can make the most of the few hours Lucy will spend studying each asteroid at close range.

A mission already full of surprises

Lucy has made headlines before it ever reached the Trojans. On its way out, it flew past main belt asteroids, including Dinkinesh in 2023, where it discovered a small moon that turned out to be a contact binary. Those early flybys gave the team valuable practice with the spacecraft’s tracking and imaging systems.

The bottom line

After five years of cruising, Lucy has reached one of the solar system’s last unexplored frontiers. The real science starts next August, when the spacecraft sweeps past Eurybates and gives humanity its first close-up look at a Jupiter Trojan.

Image: Lucy Mission Trajectory with Jupiter Trojans and Main Belt Asteroids (SVS5686 – 02510) by NASA's Scientific Visualization Studio – Science and Technol via Wikimedia Commons, Public Domain Mark 1.0