The asteroid moon Didymous just before the Dart spacecraft crashed into it. (NASA)

As a test of our ability to damage a potentially hazardous asteroid heading our way, or perhaps to give it enough of a push that the asteroid’s path is changed enough to render it harmless, a NASA spacecraft tonight successfully collided with an asteroid some 6.8 mllion miles away.

The Dart spacecraft – short for Double Asteroid Redirection Test – crashed at high speed into the asteroid Dimorphos and self-destructed yesterday evening.

It was unclear yesterday exactly how much damage was sustained by the asteroid, which is the size of a football stadium. But images taken aboard the 1,200-pound spacecraft showed that it got closer and closer to the asteroid and then the camera froze — presumably on impact.

The spacecraft was going at 14,000 miles-an-hour and hit the moon of a gravitationally-bound pair of near-Earth asteroids.

Asteroid 65803 Didymos is a binary near-Earth asteroid. The primary body has a diameter of around a half mile and a rotation period of 2.26 hours, whereas the Didymoon secondary body has a diameter of around 525 feet and rotates around the primary at a distance of around 9 miles from the primary surface in around 12 hours. (ESA)

With that impact, the orbit of Dimorphos around the larger asteroid is expected to be slightly altered, resulting in a change in the direction of the two asteroids.

While cameras and telescopes watched the crash, it will take days or even weeks to find out if it actually altered the asteroid’s orbit.

To calculate how much the moon’s orbit is altered over time it’s ‘light curve’ will be measured by observing the sunlight reflected from it with telescopes on the ground, and using this to calculate the change in the orbital period of the double-asteroid system. Satellites in orbit, including the Hubble and James Webb space telescopes will also join the effort.

This was the first  full-scale planetary defense test by the NASA, with others on the way.  Dart was launched in November, 2021.

Planetary defense experts have not found any decent-sized asteroids likely to head our way for at least a  century and likely much longer. But they also report that as many as 15,000 smaller, undetected asteroids are in the near-Earth region and their potential paths are not known.

This is part of the logic behind the planetary defense program:  The risks of an asteroid of any size hitting the Earth are extremely small, but they are not well defined and, of course, a large asteroid crash on Earth could be cataclysmic. 

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