It’s been a year since NASA launched its Lucy spacecraft on a mission to Jupiter’s Trojan asteroids. On Sunday, Lucy will wave hello to Earth for the first time during a brief, but potentially dangerous, rendezvous. In the unlikely event of a collision, the mission team has prepared maneuvers to protect the spacecraft from satellites and space junk.
Lucy’s first flyby of Earth will place the spacecraft a mere 220 miles (350 kilometers) above the surface, according to a NASA statement. That’s closer than the 254 miles (408 kilometers) that separates the International Space Station from the ground. NASA says this layer of low Earth orbit is packed with more than 47,000 satellites, space debris, and other objects spinning rapidly around our planet, and therefore not a very safe place for a lone spacecraft.
To protect Lucy, NASA engineers have been analyzing the spacecraft’s chances of a collision in the week before the planned flyby. It might seem like they’re cutting it close, but the closer the collision assessment is to the flyby, the better the accuracy of the predictions. “The further you’re predicting into the future, the more uncertain you are about where an object is going to be,” Dolan Highsmith, chief engineer for the Conjunction Assessment Risk Analysis group at NASA’s Goddard Space Flight Center, said in the statement.
The team is trying to predict the position of the Lucy spacecraft during the flyby, as well as the positions of the surrounding objects. Where these objects will be in the future also depends on the Sun’s activity. Our star flings plasma and radiation into space, increasing the density of the atmosphere and tugging on spacecraft, which causes them to slow down.
The collision assessment team will send Lucy’s position to the Space Force’s 18th Space Control Squadron, which monitors objects in low Earth orbit. The team is prepared to perform swerving maneuvers if Lucy has more than a 1 in 10,000 chance of colliding with another object. “With such a high value mission, you really need to make sure that you have the capability, in case it’s a bad day, to get out of the way,” Highsmith said.
Lucy’s navigation engineers have prepared two maneuvers in case the spacecraft needs to move out of the way, altering the time of closest approach with another object by either two or four seconds. “This is a small correction, but it is enough to avoid a potentially catastrophic collision,” Coralie Adam, Lucy deputy navigation team chief from KinetX Aerospace, said in a statement. The two maneuvers require engine burns to speed up the spacecraft as it swings past Earth, traveling at a speed of about 8 miles (12 km) per second.
The flyby will be Lucy’s first gravitational assist, using Earth to place the spacecraft on a new trajectory beyond the orbit of Mars. Lucy won’t be visible to observers from Earth before its planned flyby since it will appear to be approaching our planet from the direction of the Sun. But on October 16 at around 6:55 a.m. ET, Lucy will be visible for people in Western Australia before ducking behind Earth’s shadow. The spacecraft will reappear again at 7:26 a.m. ET, where observers in the western United States may be able to catch a glimpse of Lucy using binoculars, according to NASA. During its flyby, Lucy will take pictures of Earth and the Moon. These images will help mission scientists calibrate Lucy’s instruments.
The new trajectory will place Lucy on a two-year orbit around the Sun, at the end of which Lucy will return to Earth for yet another gravity assist. From there, Lucy will still have about three years to go before reaching its first target, asteroid Donaldjohanson. Later in August 2027, Lucy will begin its Trojan tour by visiting Eurybates and its binary partner Queta, followed by Polymele and its binary partner, Leucus, Orus, and the binary pair Patroclus and Menoetius.
The Trojan asteroids are two swarms of rocky bodies that orbit the Sun, one leading ahead of Jupiter and another trailing behind. These space rocks are held in a gravitational balancing act between Jupiter and the Sun and are believed to be the remnants of the primordial material that formed the outer planets. Lucy will be the first spacecraft to visit the Trojans, potentially unlocking the mystery of how the solar system’s outer planets formed billions of years ago.
“The last time we saw the spacecraft, it was being enclosed in the payload fairing in Florida,” Hal Levison, Lucy principal investigator at the Southwest Research Institute (SwRI), said in a statement. “It is exciting that we will be able to stand here in Colorado and see the spacecraft again. And this time Lucy will be in the sky.”