The Double Asteroid Redirection Test (Dart) probe will be launched by the space agency in the coming months.
The plan is to crash the robot into Dimorphos at 15,000 mph and change its path. If the mission is successful, it will mean that Nasa and other space agencies can stop an asteroid from hitting Earth.
What is happening?
At 1.20am Eastern time, the Dart spacecraft is scheduled to lift off atop a SpaceX Falcon 9 rocket from a launchpad in California. If the liftoff is delayed, Nasa has an 84-day launch window to try again.
It will take 10 months to travel to Dimorphos and the collision will not happen until October of 2022.
Dimorphos is a football-field-sized asteroid that is close to Didymos. They do not pose a threat to Earth. The Didymos system is very close to Earth so it makes sense for Nasa to conduct some target practice with it.
Before the crash, the Italian-made satellite LICIACube will position itself nearby to send pictures and data back to Earth.
Will it work?
LICIA has pictures and telescope observations from Earth, which could give an indication of whether Dimorphos's orbit has been altered. Only a fraction of a percentage point is the goal.
Ground-based telescopes and radar will measure how much the moonlet changes around Didymos.
Why can't they blow it up?
It is disappointing that Nasa will not be lowering Bruce Willis and Ben Affleck to the surface of an asteroid in the foreseeable future.
If an asteroid were to threaten Earth, it would be a good idea to identify it early on and knock it off course, because the earlier that it can be altered, the better.
Lindley Johnson, a planetary defence officer at Nasa, told a media briefing that the key to avoiding a killer asteroid is to detect it in advance and be prepared to change course if necessary. He said that they don't want to be in a situation where an asteroid is headed toward Earth and then have to be testing this kind of capability.
There are lots of other ideas, including running a spaceship alongside and using a laser, ion engine jet or other beam device to exert a slight but significant-over time effect. The mass-driver effect could be used to change the course of a robotic lander. Solar sails could be hoisted nearby to reflect the sun's rays on the object.
