DART mission will hit and move an asteroid

DART mission targets Didymos B

NASA’s DART mission (Double Asteroid Redirect Test) launched successfully from California’s Vandenberg Space Force Base on November 24, 2021. It went up aboard a SpaceX Falcon 9 rocket. DART is NASA’s first planetary defense test mission. Its goal is to strike a small asteroid and minutely change its orbit. In recent decades, it’s become clear that asteroids do have the potential to strike Earth and cause damage. The DART mission is a test run for when Earth is faced with an incoming asteroid that’s threatening our planet.

DART will arrive at its target asteroid in late 2022 with a impact, hopefully becoming the first Earth mission in our history to deflect an asteroid, albeit by a tiny amount.

DART’s target asteroid is a moonlet of a larger asteroid. The large asteroid is Didymos, 2,500 feet (780 m) in diameter. Its companion, Didymos B (or, sometimes, Dimorphos) is 525 feet (160 m) in diameter.

That’s more typical of the size asteroid that might unexpectedly threaten Earth, these astronomers said. That’s because the larger asteroids are easier to see, and their orbits are better known.

Animation showing a blinking satellite, high above Earth.
NASA’s DART mission and its booster – animation – via the Virtual Telescope Project.

Didymos and Didymos B

The asteroid Didymos and its companion Didymos B were unknown until 1996, when the Spacewatch survey at Kitt Peak National Observatory in Arizona noticed this binary asteroid.

Didymos B snuggles close to its parent, orbiting at a distance of just over 1/2 mile, or 1 kilometer. It takes the moonlet about 12 hours to complete one orbit of Didymos. Like our moon, the moonlet is tidally locked to Didymos, always showing the asteroid the same face.

Didymos and its moonlet Didymos B are classified as a Potentially Hazardous Asteroid. By definition, such asteroids have a minimum orbit intersection distance with Earth of 0.05 AU (Earth-sun units of distance) or less. And they have an absolute magnitude of +22 or less, indicating a size of greater than about 500 feet (140 m) in diameter.

In other words, asteroids that don’t sweep closer to the Earth than 0.05 Earth-sun distances – roughly 4,650,000 miles (7,480,000 km) – or are smaller than about 500 feet (140 m) in diameter are not considered PHAs.

Being a Potentially Hazardous Asteroid doesn’t mean Didymos or Didymos B is on a collision course with Earth. On the contrary, they stay far away from us. But, in 2003, the pair did pass only slightly closer than the minimum distance for Potentially Hazardous Asteroids. They passed only 0.048 AU from Earth. Although still millions of miles away from us, Didymos or Didymos B were thus categorized as Potential Hazardous Asteroids.

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Two rows of fourteen grainy spheres and a tiny companion that the DART mission will target.
These 14 radar images are from November 24, 2003, when Didymos and its moonlet Didymos B passed relatively close to Earth. Astronomers used the erstwhile Arecibo telescope in Puerto Rico to perform radar imaging. Image via Arecibo/ NASA.

This is only a test

The DART mission is only a test in what might someday become a full-blown planetary defense program. Didymos and its moonlet Didymos B are not a threat to Earth at this time.

The DART spacecraft is on a kamikaze mission to Didymos (NASA calls it a kinetic impactor mission). It’ll deliberately crash itself into the moonlet at a speed of approximately 4.1 miles/second (6.6 km/s). DART will use an onboard camera (named DRACO) and sophisticated autonomous navigation software to measure the size and shape of Didymos B and to provide detailed views of the site where it will slam into the asteroid.

I know what you’re thinking. If NASA pushes Didymos B off its regular orbit, could it cause the moonlet – or its larger parent asteroid Didymos – into a new path that could possibly be a bigger threat to Earth? These scientists said no, neither Didymos nor its moonlet will become a threat to us because of DART. But that doesn’t mean small pieces of debris from the impact won’t encounter Earth and strike our atmosphere. More about meteors from DART’s impact below.

This test by NASA isn’t intended to do harm to Earth. It’s part of a decades-long thought process on how we might keep ourselves safe from an asteroid that might someday be on a collision course with Earth. The effect of DART on Didymos B will be minimal, but still enough for scientists to measure. As NASA said:

Scientists think the collision will change the speed of the moonlet by a fraction of one percent and alter its orbital period around the larger asteroid by several minutes, enough to be observed and measured by telescopes on Earth.

The idea is that, if we were ever to discover a small asteroid on a collision course with Earth, we’d have had some practice in deflecting asteroids away from us.

Graphic of a moonlet around an asteroid and a spacecraft impacting it, with Earth in distance.
After DART crashes into the moonlet Didymos B, scientists on the ground will measure the change in its orbit around its parent body, Didymos. Image via NASA.

Meteors from DART

Here’s a cool aspect of the DART mission. Some scientists believe bits of the debris might eventually strike Earth. A March 23, 2020, paper in The Planetary Science Journal said:

Because the closest point of approach of Didymos to Earth’s orbit is only 6 million km (about 16 times the Earth–moon distance), some ejected material will make its way sooner or later to our planet, and the observation of these particles as meteors would increase the scientific payout of the DART mission.

The DART project may also represent the first human-generated meteoroids to reach Earth.

The paper also pointed out:

The DART impact will create a new meteoroid stream, though probably not a very dense one. However, larger, more capable asteroid impactors could create meteoroid streams in which the particle flux exceeds that naturally occurring in the solar system, with implications for spacecraft safety.

It seems that wherever we humans go, even into the vast space of our solar system, we have the potential to leave messes behind us.

Cube-shaped spacecraft with two long solar panels out to the sides.
The DART mission will head to the moonlet of asteroid Didymos, where scientists will crash it and attempt to move the moonlet. Image via NASA.

Watching from afar

At present, scientists said, the Didymos binary asteroid is being intensely observed using telescopes on Earth. They want to obtain precise measurements of its properties before DART arrives.

After nearly a year of cruising through space, it will intercept Didymos’ moonlet in late September or early October 2022, when the Didymos system is within about 7 million miles (11 million km) of Earth. The relatively close distance between Earth and the asteroid at that time will let scientists observe the asteroid at the time of impact with ground-based telescopes and planetary radar.

They’ll be watching and hoping to measure a change in momentum imparted to the moonlet.

Bottom line: The DART mission, set to launch November 23, 2021, at 10:20 PST, will arrive at asteroid Didymos and its moonlet Didymos B in late September or early October 2022. Scientists plan to crash it into Didymos B to see how much the moonlet moves, as practice for dealing with potential future dangerous space rock encounters.


November 24, 2021

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Kelly Kizer Whitt

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