On the evening of April 13, 2029, the asteroid Apophis is expected to pass within about 20,000 miles of Earth's surface – closer than satellites carrying TV broadcasts and satellite images. A NASA spacecraft still on its way to meet the asteroid will capture a moment from deep space: a speck of light sliding past the side of Earth where night has fallen, over Europe and northern Africa. Millions of people will be able to watch it with their own eyes.

An asteroid this size comes so close to Earth only once about every 7,500 years. 

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"I think that's just going to be a really powerful sequence of images to help people understand how vulnerable we are," said Dani Mendoza DellaGiustina, associate professor at the Lunar and Planetary Laboratory and the principal investigator of OSIRIS-APEX. 

The spacecraft originally flew as OSIRIS-REx, the mission that brought a sample of the asteroid Bennu back to Earth in September 2023. It is expected to reach Apophis in late spring of 2029, but the team is already keeping the spacecraft busy. 

"It's really cool getting to use this spacecraft for so many scientific investigations that nobody even imagined when it was originally proposed," DellaGiustina said. 

Earth as a distant world

Currently, the team is writing the commands the spacecraft will follow when it swings past Earth next March. That flyby is one of the several gravity assists built into the mission's route, in which the spacecraft passes close to Earth and borrows some of our planet's energy to reshape its own path around the sun. Each flyby bends the spacecraft's path to line it up with Apophis. 

The last flyby in September 2025 gave the team a chance to point the instruments to Earth – in August and September, as the spacecraft approached from millions of kilometers away, the team observed Earth as if it were a distant astronomical object, which is roughly how telescopes see planets orbiting other stars. 

"Studying Earth as a habitable world from the perspective of a distant spacecraft that is seeing our own planet as it though it's an astronomical object is a really rare opportunity," DellaGiustina said. 

She said the team has made a significant discovery in the data collected during the 2025 flyby. The team also plans to observe Earth the same way again before the March 2027 flyby, building on the 2025 results that it expects to publish. The new data will help recalibrate instruments whose behavior has drifted somewhat since the spacecraft launched in 2016.

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Closer to the sun

Getting to Apophis involved making innovations with the spacecraft itself. The route requires an orbit that carries the spacecraft much closer to the sun than it was designed to survive. It has made three of those close passes already. To get through the passes, engineers tilt the spacecraft's wing-like solar panels, which sits on wrist-like joints, so that they cast shade over its heat-sensitive parts. The panels sit on hinged mounts that were originally meant to lift them clear of boulders when the spacecraft touched down on Bennu. The engineers called the protective pose "the fig leaf."

DellaGiustina said the approach has worked so far. However, three more close passes remain. 

"I'm pretty confident after going through three passes already," she said.

Watching an asteroid shake

Once the spacecraft reaches Apophis in late spring 2029, the big question is what Earth's gravity will do to the asteroid. Although DellaGiustina's expectations haven't changed dramatically since NASA approved the OSIRIS-APEX mission in 2022, she said researchers have spent the past several years working through telescope observations of the asteroid and now have better answers on two things Earth's pull could change: how the asteroid spins and what its surface looks like. 

The team had worried, for instance, that Earth's pull might change the asteroid's rotation so drastically that the spacecraft would not know the right cadence to observe it ahead of time. Also, predictions for how much the asteroid's surface will shift ranges from meter-scale changes to millimeter-scale changes that most spacecrafts could not detect. But OSIRIS-APEX has the resolution to detect them, DellaGiustina said. 

As Earth's gravity pulls on Apophis during the close approach, the asteroid will shake hard enough that material on its surface could be thrown upward. DellaGiustina said that stretching and squeezing will shake Apophis so hard that the asteroid's weak gravity may not be able to hold everything down, with loose dust and rock tossing up off the surface. 

"OSIRIS-APEX will be in a unique geometry to observe that shaking, which is very cool," she said. 

Building the team for 2029

The mission has a few students on the team now. OSIRIS-REx trained more than 150, many of whom went on to work for NASA and its industry partners. 

"It's a really important and personal part of our story – the number of students that we've trained," DellaGiustina said. "We'll continue to do that as we start ramping up our staffing the closer we get to arriving at Apophis."