Lucy aims to visit eight asteroids - a record number of encounters.
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 52-foot-wide spacecraft, called Lucy, to investigate. The probe is set to launch aboard an Atlas V rocket from Cape Canaveral, Florida, on Saturday morning.
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.
That's where Lucy gets its name - it's a reference to the famous 3.2-million-year-old fossilized 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.
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.
Watch live as Lucy launches into space
Liftoff is scheduled for 5:34 a.m. ET on Saturday. NASA plans to start its live launch broadcast at 5 a.m. in the video embedded below.
If weather delays the launch, NASA has another 22 days in its window, with an hour-long launch opportunity each morning.
Close encounters of the asteroid kind
Lucy's mission is seven years in the making. The original plan called for visits to just two asteroids, but NASA grew more ambitious as engineers and scientists 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 space rock will be brief. The spacecraft can't slow down or land - that would require too much propellant - so it will zip within 600 miles of their surfaces, moving 3 to five 5 miles 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.
By the end of Lucy's journey, NASA expects to have spent $981.1 million on the mission, according to Glaze.
Lucy has to revisit Earth 3 times to hit its targets
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
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.
Other missions aim to prepare for the possibility that an asteroid could strike our planet. Next month, 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 neighborhood.
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."
This post has been updated. It was originally published on October 7, 2021.
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