Didymos: The Monster Companion Asteroid to Dimorphos, Target of NASA's DART

NASA is crashing a spacecraft into an asteroid, in order to test how much a collision would deflect space rocks.

On September 26, the Double Asteroid Redirection Test (DART) spacecraft is due to crash into the side of Dimorphos, a small asteroid orbiting the larger near-Earth asteroid Didymos, at a speed of four miles per second [around 14,400 mph].

According to NASA, DART is the first mission to directly test how such a "kinetic impact" affects the orbit of the asteroid around its neighbor, allowing scientists to more accurately calculate how well firing an object at a space rock headed towards Earth would work at deflecting its trajectory.

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Stock image of an asteroid. Dimorphos, the asteroid neighbor of Didymos, is due to collide with a spacecraft as part of the DART mission. iStock / Getty Images Plus

Dimorphos measures around 530 feet in diameter, and orbits around the larger asteroid Didymos, which is about 2,560 feet across, in a so-called binary asteroid system.

Didymos was detected for the first time in 1996, while Dimorphos was only found orbiting it seven years later in 2003.

According to NASA, Didymos is shaped like a spinning top, with a bulge at its equator caused by the asteroid's rapid spin. This fast rotation is thought to have resulted from sunlight hitting its surface unevenly, heating up some parts more than others.

Dimorphos doesn't have this same shape, and instead is slightly elongated. Dimorphos may even have been formed as a result of Didymos's rapid spin via rotational fission, which is a process where material is flung off a fast spinning object in space.

It is currently unknown what Didymos and therefore Dimorphos are made of, although asteroids in our solar system usually fall into one of three classes of composition, depending on how far from the Sun they formed: C-type (chondrite) asteroids made of clay and silicate rocks; S-types ("stony") made of silicate materials and nickel-iron; and M-types, made mostly of metals (nickel-iron).

Didymos and Dimorphos orbit the Sun once every 2.11 years, passing closer to the Earth at some points in the orbit than others. In 2003, it passed only 0.048 AU from Earth, or around 4.5 million miles. At the time of the impact, Dimorphos will be about 6.8 million miles from Earth.

While this may seem immensely far away in Earth terms, the Moon orbits the Earth at a distance of 238,900 miles, and the Earth's closest planetary neighbor, Venus, is 38 million miles away at its closest point.

The DART team will observe the two asteroids after the impact using telescopes and a nearby flyby CubeSat called LICIACube to witness the impact, and will measure how much the impact changed the asteroid's motion in space, as well as any impact of resulting ejection on Dimorphos.

The two asteroids represent a small-scale model of the Earth and a potential near-Earth asteroid that may be headed towards Earth.

If an asteroid hit the Earth, the effects of the collision would depend on the rock's size, speed, and the angle that it hit the Earth's surface.

"Roughly an object 100 meters [330 feet] in diameter is likely to create a crater about 1 kilometer [3,300 feet] across on Earth," Gretchen Benedix, a professor of geology and geophysics with the Space Science and Technology Center and the School of Earth and Planetary Sciences at Australia's Curtin University, previously told Newsweek.

"If that hits a densely populated area, that could be quite bad but overall not devastating for the region or the Earth. An object this size is estimated to hit the Earth every 10,000 years."

Larger asteroids could have more catastrophic results if they collide with the Earth: the asteroid that led to the end-Cretaceous extinction event that wiped out the dinosaurs along with 75 percent of the Earth's species measured 6.2 miles in diameter.

However, according to BBC Science Focus, a dinosaur-level asteroid colliding with the Earth is only likely to occur once every 100 million years.