An illustration showing the Lucy spacecraft passing one of the Trojan Asteroids near Jupiter.

NASA is about to launch an ambitious mission to explore swarms of asteroids as old as our solar system.

Those asteroids, called Trojans, orbit the sun alongside Jupiter. One group of them leads the gas giant along its orbital path, and another group trails behind. These space rocks formed during the birth of our solar system nearly 4.6 billion years ago and have remained largely unchanged ever since.

Scientists want to find out what those time capsules hold, so NASA is sending a 16-metre-wide spacecraft, called Lucy, to investigate. The probe is set to launch aboard an Atlas V rocket from Cape Canaveral, Florida, on October 16.

Lucy is scheduled to visit eight asteroids over the next 12 years. One of those space rocks is in the solar system's main asteroid belt, between Mars and Jupiter, which Lucy will pass on its way to the other seven asteroids, which are part of the Trojans.

"These objects we view as being the fossils of planet formation," Hal Levison, the mission's principal investigator, said in a briefing last month.

An illustrated model of the Trojan asteroids - one swarm leading Jupiter in its orbit and the other following it.

That's where Lucy gets its name - it's a reference to the famous 3.2-million-year-old fossilised human ancestor. At the time of her discovery, in 1974, Lucy was the oldest, most complete hominin skeleton ever found. She was evidence that human ancestors walked upright, which helped paleoanthropologists piece together humans' evolutionary history. Scientists hope that Trojan asteroids can do the same for the outer solar system.

"Some of the most important planetary science questions we're trying to answer are focused on the origin and the evolution of the solar system. Asteroids and other small bodies are really important keys to understanding that history," Lori Glaze, NASA's director of planetary science, said in the briefing.

Workers inside the Astrotech Space Operations Facility in Titusville, Florida, move the first half of the rocket's payload fairing toward NASA’s Lucy spacecraft on September 30, 2021.

Lucy's planned journey would lead it to visit more asteroids than any prior spacecraft. It will also venture further from the sun than any solar-powered probe has ever gone.

Liftoff is scheduled for 5:34am ET on October 16. If weather delays the launch, NASA has another 20 days in its window.

Close encounters of the asteroid kind

An illustration of the Trojan asteroids leading and following Jupiter.

Lucy's mission is seven years in the making. The original plan called for visits to just two asteroids, but NASA engineers and scientists grew more ambitious as they designed the spacecraft and plotted its journey.

Now, the probe has a record-breaking lineup. Some of its eight stops are two-for-ones: One asteroid has its own satellite - a smaller rock trapped in its orbit - and two of Lucy's targets are a pair of binary asteroids that circle each other.

But Lucy's encounter with each primordial space rock will be brief. The spacecraft can't slow down or land on its targets - that would require too much propellant - so it will zip within 965 kilometres of their surfaces, moving 5 to five 8 kilometres per second.

During the few hours in which it approaches and passes the asteroids, Lucy's scientific instruments will collect data on their composition, density, and size. It could even discover rocks or rings circling the asteroids - features too small to see from Earth.

Lucy has to revisit Earth 3 times to hit its targets

A diagram illustrating Lucy's orbital path, including three Earth flybys.

Scientists have identified over 7,000 Trojan asteroids, split into three major types. One group resembles the space rocks in the asteroid belt between Mars and Jupiter, while the other two resemble icy objects in the Kuiper Belt on the outskirts of the solar system.

This suggests that the various types of Trojans formed through different processes, and in different parts of the solar system. Scientists aren't sure how they ended up together along Jupiter's orbit. So Lucy will visit asteroids of each type in an attempt to find out.

"Covering this diversity is key. And finding a trajectory that actually will allow us to visit all these types of objects has been a real chore," Levison said.

To reach all of its destinations, in fact, Lucy has to circle back to Earth three times to get a momentum boost from our planet's gravity. That will make it the first spacecraft to travel to Jupiter's orbit and back.

NASA is going all in on asteroids

An illustration of the DART spacecraft approaching its target asteroid.

NASA is sending probes to asteroids scattered all over the solar system.

"Lucy is part of a collection of ambitious missions to study the diversity of these asteroid populations that will help us fill in more pieces of that cosmic puzzle," Glaze said.

NASA's New Horizons spacecraft has been studying asteroids and other objects in the Kuiper Belt, beyond Neptune, since it flew past Pluto in 2015.

The agency's OSIRIS-REx spacecraft, meanwhile, landed on an asteroid last year, punched into its surface, and scooped a sample of rock. That probe is on its way back to Earth with the sample in tow. Japan recently completed a similar mission and brought its own asteroid samples to Earth, so NASA and Japan's space agency plan to trade bits of their samples.

The OSIRIS-REx spacecraft's sample arm touches down on the surface of asteroid Bennu.

Other missions aim to prepare for the possibility that an asteroid could strike our planet. In November, NASA plans to launch a spacecraft to slam into a nearby asteroid. That mission, called the Double Asteroid Redirection Test (DART), is testing a method that could deflect space rocks off of a collision course with Earth. NASA is also working on a new space telescope, called NEO (near-Earth object) Surveyor, which would help scientists catalogue dangerous asteroids in our neighbourhood.

Next year, NASA plans to launch yet another probe, called Psyche, to a metallic asteroid that could be the leftover core of an ancient planet.

"All of these are incredibly interesting destinations. And in each case, we're exploring places no spacecraft has ever been, so we won't know for sure what we'll discover till we get there," Glaze said. "But we know that whatever Lucy finds will give us vital clues about the formation of our solar system.

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