The DART mission will be the first to change the motion of a significant body in our Solar System.

It will hit an asteroid.

The test of a technique that could be used in the future will be provided by the mission.

You can watch a live stream below.

A binary pair of space rocks

The DART was launched in November of 2016 and is in the vicinity of two asteroids.

Didymos is just under half a mile in diameter and is the larger of the two asteroids. The small asteroid is called Dimorphos. One of the circles takes close to 12 hours and the other takes less than an hour.

DART's orbit around Didymos and Dimorphos
DART is expected to alter the orbit of the smaller asteroid. (NASA/Johns Hopkins APL)

There is no risk to Earth from these asteroids and they have been chosen as the target for DART. Astronomers will be able to assess the results of the impact because the asteroids are a pair.

The sun reflects off the asteroids in different ways over the course of a 12-hour cycle.

Astronomers can use telescopes on Earth to see how the variation changes after a collision.

The physics is simple, the mission is not

The physics is easy to understand. Change its motion by hitting one thing with another thing. The mission execution is very difficult.

It will take DART 10 months to reach the asteroids. The asteroids it acquires as it approaches are used to target the craft.

DART needs to know the asteroids and adjust its trajectory to hit them. The speed at which this is moving is close to 15000 miles per hour.

It's not easy to predict the results of the impact. Exactly where DART hits and how hard will affect the outcome.

Some of these factors are not certain. The main outcome of the DART mission will be the comparisons of the simulations, predictions, and measured results.

An up- close view of the impact will be possible from an Italian Space Agency CubeSat that was deployed from a spring loaded box on September 11. The collision will be photographed by LICIACube.

The Lowell Discovery Telescope, located in northern Arizona, one of the facilities that will measure the impact of the DART collision. (Lowell Observatory)

The results will show us a lot about asteroids and how they work. In the future, this knowledge could be used to plan planetary defense missions that attempt to stop asteroids from hitting the Earth.

What is the level of threat?

If a small asteroid hits the atmosphere, it could cause injuries from an airburst explosion. It is estimated that 5 million such objects exist in our Solar System, and that we have discovered around 4% of them.

It is thought that such a hit will happen once every 100 years. The risk is low and the impact risk is low.

39 percent of the objects in the Solar System are known to hit Earth every 20,000 years. Mass casualties would be caused by an object hitting a populated area.

Asteroid statistics and the threats posed by asteroids of different sizes. (NASA)

There are less than a thousand asteroids in the Solar System and they may hit Earth every 500,000 years. 95 percent of these objects have been found by us.

Potential asteroid crashes with Earth range from benign to catastrophic. There are asteroids greater than 100 meters that are being tested.

If DART succeeds, it will set the stage for future missions to target asteroids to prevent them from colliding with Earth.

The earlier we can identify asteroids that are a potential threat, the better.

There is a well-worn premise that an asteroid is coming and we need to change it.

Steven Tingay is a professor of radio astronomy at the university.

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