NASA will fly to an asteroid we once thought could strike Earth

An asteroid we once thought was on a collision course with Earth will now be visited by a repurposed spacecraft when the space rock passes close in 2029.

When asteroid Apophis was discovered in 2004, it appeared to be headed towards Earth with a risk of impact in 2029 that would have had potentially catastrophic results. This 340 metre-wide boulder is much smaller than the one that wiped out the dinosaurs 66 million years ago, but it is large enough to obliterate a large city.

So it was a great relief when further observations of the orbit of Apophis allowed astronomers to calculate that on its encounter with us in 2029 it will pass by about 30,000 km, or one tenth the distance to the moon. This means that there's no risk that it will strike or cause any damage, but it is still a close call by astronomical standards. It will be close enough that people in Europe and parts of Africa may be able to see it with the naked eye as it flies by.

And since Apophis will be coming to the Earth, scientists at the University of Arizona have come up with a clever plan to take advantage of the opportunity to study this visitor in closeup detail.

Right after that close encounter in 2029, a spacecraft that is already in space and has already completed its primary mission will be tasked with catching up with Apophis and flying alongside it for 18 months.  

This robot probe, OSIRIS-REx was launched in 2016, and in 2018 it visited an asteroid named Bennu, where it picked up a sample that it will bring home to Earth. When it gets here in 2023, it will drop off the sample completing its primary goal. 

But NASA has now extended funding through the end of the decade to keep the spacecraft running. The new mission is being dubbed OSIRIS-Apophis Explorer, or OSIRIS-APEX.

The probe will rendezvous with Apophis and go into orbit around the asteroid. It won't retrieve a sample as it did from Bennu, but will come extremely close, almost touching it, then fire thrusters to kick up surface material and reveal what is underneath. It will also measure how much the orbit of the asteroid was altered by the Earth's gravity to determine how close it might come to us again in the future.

The longer mission is part of an overall NASA initiative to extend eight planetary exploration missions, including the two rovers on Mars, Curiosity and Perseverance, the Mars Insight lander, orbiters around Mars and the moon, and the New Horizons program, which flew past Pluto in 2015 and is now exploring the Kuiper Belt at the edge of our solar system. 

Extending space missions is not new, as most of these robots are over-built to such an extent they last far beyond their primary goals. The Mars rovers, Spirit and Opportunity, which landed in 2004, were to drive around on the surface for 90 days, but ended up operating for years. The Voyager mission, launched in 1977, involved twin spacecraft sent out to explore the giant planets Jupiter and Saturn, but Voyager 2 continued on to Uranus and Neptune and both are still operating today in interstellar space. 

Prolonging the life of existing spacecraft is cheaper than building new ones and because they run for many extra years, it is an opportunity for the next generation of young scientists to join the team and take over leadership roles.

In the case of these rendezvous with asteroids, it's important to know what they are made of — and not just because they are extremely old and contain information about the early solar system. If we do spot one that does turn out to be headed right for us and need to nudge it to avoid a collision, it would be good to know what we are dealing with.