NASA successfully crashed a spacecraft into an asteroid on September 26.

The collision marked the end of part one of the DART mission, a 10-month space journey to autonomously impact and deflect a non-hazardous asteroid. But it was also be the start of streams of new data on what scientists call "planetary defense."

DART slammed into Dimorphos, an asteroid of around 160 meters (525 feet) in diameter which orbits Didymos, another, larger asteroid of around 780 meters in diameter.

DART stands for Double Asteroid Redirection Test.

Test mission to deflect an asteroid

"This is just a test," said Nancy Chabot, the DART coordination lead at Johns Hopkins Applied Physics Laboratory during a press briefing on September 15. "This asteroid is not a threat to the Earth. In fact, there are no known asteroid threats to the Earth for the foreseeable future."

There are still asteroids that scientists have yet to find. But Chabot said they wanted to do this test now "to be ready for when we might potentially need it."

10-month 'cruise' to Dimorphos

The DART spacecraft launched on November 24, 2021, on a 10-month "cruise" toward the pair of asteroids called Dimorphos and Didymos. 

On September 26 (or September 27, depending on where you live), DART crashed into Dimorphos at roughly 6.1 kilometers per second (3.8 miles per second).

Dimorphos was about 11 million kilometers (6.8 million miles) from Earth at the time of DART's impact.

As DART got closer and closer to the asteroid, we were able to see a stream of live images. Then at the time of impact, another, Italian-made spacecraft, called LICIACube, filmed the impact from the side. 

"Crashing this small spacecraft [DART] into a much larger asteroid is only going to cause a small change in how Dimorphos goes around Didymos. The change will only be about 1%. That makes this a very safe and efficient way to do this test," said Chabot before the impact. 

DART faces many technological challenges

The DART mission had to distinguish between the two asteroids in the final hour before impact. It used images from an onboard camera to autonomously identify Didymos and Dimorphos and then — also autonomously — fire thrusters to make sure it stayed on an "intercept course with Dimorphos."

"These algorithms are [among the main technologies] being demonstrated by DART," Chabot said earlier.

They are called SMART Nav and were developed Chabot and others at the Johns Hopkins Applied Physics Laboratory.

"It's autonomous navigation that uses these images to ensure you can target a small asteroid in space, accurately, and it's one of the main challenges," said Chabot.

These very same images were streamed back to Earth at one per second and shown live on a NASA broadcast.

How big is the threat from asteroids for Earth?

The asteroid that wiped out the dinosaurs was about 10 kilometers (6.2 miles) in size.

"That sort of asteroid changed the whole planet, but we don't know of any asteroid of that size today," said Thomas Zurbuchen, associate administrator at NASA's Science Mission Directorate, in an interview with DW when DART launched in November 2021.

"If a 160-meter asteroid [like Dimorphos] hits a city, it will a bad day for that city, leaving a crater of more than a kilometer, but it won't change the whole planet" he said.

However, Zurbuchen said bombardments by smaller asteroids are a much bigger threat to life on Earth.

"We've had a similar bombardment from asteroids over millions of years, but you just don't see it because we have such an active geology on Earth," Zurbuchen said.

DART is the first test of future planetary defense against asteroid bombardments. 

First of future missions

Before the impact, the European Space Agency had said Dimorphos would be the first object in the solar system to have its orbit shifted by human effort in a measurable way. 

ESA is looking ahead to its own mission to study Dimorphos up close. It will use a spacecraft called HERA, which is planned for launch in 2024.

Meanwhile, China is also reportedly planning a planetary defense mission for 2026.

Edited by: Zulfikar Abbany