Why NASA has deliberately crashed a spacecraft into an asteroid

The 11 million km (about 300 times the distance to moon) is the closest that this asteroid, Dimorphos, comes to Earth while going around in its orbit. There was absolutely no danger that it would have come and collided with Earth. Tuesday’s collision was, therefore, just a technology demonstration, and an experiment to assess the capabilities to do such maneouvers in future should a need arise.

IMPACT SUCCESS! Watch from #DARTMIssion’s DRACO Camera, as the vending machine-sized spacecraft successfully collides with asteroid Dimorphos, which is the size of a football stadium and poses no threat to Earth. pic.twitter.com/7bXipPkjWD

— NASA (@NASA) September 26, 2022

Asteroid collision is real

Though there was no threat to Earth from this particular asteroid — and NASA says there is no real danger to Earth from asteroids for the next 100 years at least — asteroid collisions are real, and can happen. The dinosaurs, and most other life forms at that time, are known to have become extinct following an asteroid collision about million years ago.

As recently as 2013, an asteroid entered the earth’s atmosphere and exploded over Russia, causing injuries to hundreds of people, and causing widespread damage.

Small asteroids — millions of them orbit the Sun – keep entering the earth’s atmosphere fairly regularly, but burn out due to friction before they reach the surface. Some of them do drop to the surface but are not large enough to cause harm. The danger is from bigger asteroids. The one that destroyed the dinosaurs was about 10 km in width. According to NASA, an asteroid that big comes towards the Earth only in about 100 to 200 million years.

But smaller ones are more frequent. There is a probability that an asteroid of the size of 25 metres would come once every 100 years. The one that exploded over Russia in 2013 was a little smaller, about 18 metres in size.

The problem is that these calculations are based on asteroids that we know about, only about 26,000. There are many asteroids that we haven’t discovered yet. And these could surprise us.

DART

Tuesday’s mission was called Double Asteroid Redirection Test, or DART. The targeted asteroid Dimorphos is actually a moon to a slightly larger asteroid called Didymos. While Didymos is 780 m at its widest, Dimorphos is about 160 metres. Dimorphos orbits around Didymos, and this two-body system go around the Sun.

One of the reasons scientists chose to target Dimorphos was because of its relatively shorter orbit around Didymos. A deviation in this orbit was likely to be more noticeable, and thus easier to measure, than say, if Didymos itself was targeted and an attempt was made to measure the change in its orbit around the Sun.

The DART mission was launched in November last year. The collision is likely to create a crater on Dimorphos. Whether the impact is able to make a noticeable alteration to its orbit would become known much later as telescopes around the world take measurements.