Evrim Yazgin
Cosmos science journalist
NASA spacecraft Lucy is just days away from its second close encounter with an asteroid.
Lucy was launched in 2021 and is on its way to Jupiter’s orbit to explore a group of asteroids known as the 10 Trojans.
The spacecraft has to pass through the solar system’s main asteroid belt to get to Jupiter’s orbit, which is more than 5 times further from the Sun than Earth’s orbit. The asteroid belt is between the orbits of Mars and Jupiter, on average about 2.8 times further from the Sun than Earth.
Lucy’s flybys of asteroids in the belt are a “dress rehearsal” for its main mission.
Observations made by Lucy in 2023 made headlines after it was revealed that its target, the asteroid Dinkinesh, was actually 2 asteroids. This was its last close flyby.
The next flyby is the asteroid Donaldjohanson at about 1:51pm EDT on April 20 (3:51am Australian eastern time on April 21). At its closest, Lucy will be just 960km from the asteroid – about the distance from Adelaide to Canberra or London to Berlin.
Donaldjohanson is a carbon-rich asteroid about 4km in diameter. It is a fragment from a collision of asteroids 150 million years ago – a time when on Earth the first birds and mammals were evolving in the shadows of dinosaurs.
About 30 minutes before closest approach, Lucy will orient itself to track the asteroid. During this time, its high-gain antenna will turn away from Earth, suspending communication.
“If you were sitting on the asteroid watching the Lucy spacecraft approaching, you would have to shield your eyes staring at the sun while waiting for Lucy to emerge from the glare,” says encounter phase lead Michael Vincent from the Southwest Research Institute (SwRI) in Boulder, Colorado. “After Lucy passes the asteroid, the positions will be reversed, so we have to shield the instruments in the same way.”
“These instruments are designed to photograph objects illuminated by sunlight 25 times dimmer than at Earth, so looking toward the sun could damage our cameras.”
This is the only of Lucy’s 7 targets which has this glaring problem.
“One of the weird things to wrap your brain around with these deep space missions is how slow the speed of light is,” Vincent says. “Lucy is 12.5 light minutes away from Earth, meaning it takes that long for any signal we send to reach the spacecraft. Then it takes another 12.5 minutes before we get Lucy’s response telling us we were heard. So, when we command the data playback after closest approach, it takes 25 minutes from when we ask to see the pictures before we get any of them to the ground.”
“Every asteroid has a different story to tell, and these stories weave together to paint the history of our solar system,” says NASA mission scientist Tom Statler. “The fact that each new asteroid we visit knocks our socks off means we’re only beginning to understand the depth and richness of that history. Telescopic observations are hinting that Donaldjohanson is going to have an interesting story, and I’m fully expecting to be surprised – again.”
