Two years ago, a NASA spacecraft smashed into a small asteroid millions of miles from Earth to test a technique that could one day prove useful to deflect an object off a collision course with Earth. The European Space Agency launched a follow-up mission Monday to go back to the crash site and see the damage done.

The nearly $400 million (363 million euro) Hera mission, named for the Greek goddess of marriage, will investigate the aftermath of a cosmic collision between NASA's DART spacecraft and the skyscraper-size asteroid Dimorphos on September 26, 2022. NASA's Double Asteroid Redirection Test mission was the first planetary defense experiment, and it worked, successfully nudging Dimorphos off its regular orbit around a larger companion asteroid named Didymos.

But NASA had to sacrifice the DART spacecraft in the deflection experiment. Its destruction meant there were no detailed images of the condition of the target asteroid after the impact. A small Italian CubeSat deployed by DART as it approached Dimorphos captured fuzzy long-range views of the collision, but Hera will perform a comprehensive survey when it arrives in late 2026.

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Enlarge / This artist's concept shows the possible appearance of ESA's RAMSES spacecraft, which will release two small CubeSats for additional observations at Apophis. (credit: ESA-Science Office)

For nearly 20 years, scientists have known an asteroid named Apophis will pass unusually close to Earth on Friday, April 13, 2029. But most officials at the world's space agencies stopped paying much attention when updated measurements ruled out the chance Apophis will impact Earth any time soon.

Now, Apophis is again on the agenda, but this time as a science opportunity, not as a threat. The problem is, there's not much time to design, build, and launch a spacecraft to get into position near Apophis in less than five years. The good news is there are designs, and in some cases, existing spacecraft, that governments can repurpose for missions to Apophis, a rocky asteroid about the size of three football fields.

Scientists discovered Apophis in 2004, and the first measurements of its orbit indicated there was a small chance it could strike Earth in 2029 or in 2036. Using more detailed radar observations of Apophis, scientists in 2021 ruled out any danger to Earth for at least the next 100 years.

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