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Astro Bob: NASA's DART probe to slam into asteroid Monday

Scientists will test one way we might defend our planet from a future asteroid impact. Here's how to watch it live.

Didymos and Dimorphos
On Sept. 26, NASA's Double Asteroid Redirection Test or DART will intentionally crash into Dimorphos (left), the asteroid moonlet of Didymos. While the asteroid poses no threat to Earth, this is the world’s first test of the kinetic impact technique, using a spacecraft to deflect an asteroid for planetary defense. This image of the two orbs is a composite of 243 images taken by the spacecraft on July 27, 2022.
Contributed / NASA JPL DART Navigation Team
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On Monday, Sept. 26 at 6:14 p.m. CDT, NASA's Double Asteroid Redirection Test (DART) probe will deliberately crash into Dimorphos, a tiny asteroid moon 525 feet (160 meters) wide. Dimorphos orbits larger Didymos, which measures a kilometer (0.6 mile) across. The binary asteroid passes relatively near the Earth when closest and just beyond Mars at the far end of its orbit.

Didymos and Dimorphos by radar
These are radar images of Didymos (the larger object) and its satellite Dimorphos in 2003. They orbit just one kilometer (0.6 miles) apart. Didymos is shaped like a spinning top with a ridge of material around its equator. We don't know much about Dimorphos except that it appears to be elongated.
Contributed / Arecibo Observatory, NASA

We're disrupting the unsuspecting asteroid's day to test an impact avoidance strategy. Should we one day discover a giant space rock on a collision course with Earth, slamming it with a spacecraft may nudge its orbit just enough to send it safely off course. Astronomers call the strategy the kinetic impactor technique, and it's a lot safer than detonating a thermonuclear bomb on a potential doomsday object.

Dimorphos scale
This diagram compares the sizes of the two asteroids in the Didymos system relative to objects on Earth.
Contributed / NASA, Johns Hopkins APL

While this pair doesn't pose any immediate hazard to the planet, it's makes an ideal testing ground. Assuming it works, nothing untoward will happen. Astronomers hope to measure a slight change in the orbit of Dimorphos, which would spell success and proof of concept.

Hit me!

DART
This illustration depicts the DART spacecraft flying towards the asteroids Dimorphos and Didymos.
Contributed / NASA, Johns Hopkins, APL

DART will hit hard. Traveling in the opposite direction, it will strike Dimorphos head on at 14,000 miles an hour (22,530 kph), creating an impact crater and slowing the asteroid's speed by a hair. This will lower its orbit, edging the moonlet slightly closer to Didymos and increasing its orbital speed. The impact is expected to shorten the time it takes the moon to orbit parent from the current 11.9 hours to 11.8 hours, a difference of only six minutes.

DART impact simulation
This simulated view of the mission shows the DART impact on Dimorphos. Post-impact observations from Earth-based telescopes, planetary radar and orbiting telescopes (left) will measure the change in the moonlet’s orbit about its parent.
Contributed / NASA, Johns Hopkins Applied Physics Lab

We'll get to see it all unfold in photos that will be sent from a shoebox-sized, tag-along satellite called LICEACube . LICEACube's two cameras will photograph the collision and capture the evolution of the impact plume from 31 miles (50 km) away and send them back to Earth. Don't expect a speedy delivery. The full upload is expected to take months due to the satellite's low data rate.

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The Webb Space Telescope, Hubble Space Telescope and many others across the globe will be trained on the crash scene hoping to see the system brighten during impact and later, to measure the hoped-for shift in the moonlet's orbit. DART's final transmissions will take 38 seconds to reach the Earth traveling at the speed of light.

DART orbit change
This graphic shows the new orbit of asteroid Didiymos’s moon Dimorphos following DART’s impact.
Contributed / NASA, JPL, Planetary Defence

Although altering an asteroid's orbit seems a heavy-handed thing to do, NASA deserves an attaboy for thinking ahead. We're desperately in need of intel on how asteroid threats can be mitigated. Wayward space rocks have struck Earth in the past and undoubtedly will again. Just ask any dinosaur. If you can find one. What we learn from the test may one day save humanity.

Dimorphos is so tiny that DART's cameras will only distinguish it from larger Didymos an hour before impact. Four minutes prior to collision we should be able to clearly distinguish the shapes of both asteroids. Twenty seconds before impact we'll see refrigerator-sized boulders on Dimorphos. Seconds before the end, DART will discern rocks down to about 4 inches (10 cm).

Watch it live

Live coverage on NASA TV begins at 5 p.m. CDT (6 p.m. EDT; 4 p.m. MDT; 3 p.m. PDT) on Monday, Sept. 26. The agency's media channel will stream still images as they arrive beginning at 4:30 p.m. CDT.

You might wonder about the origin of the moons' curious sounding names. They're Greek. The larger, Didymos, was discovered in April 1996, with Dimorphos confirmed in 2003. Didymos means "twin" due to it being a binary asteroid, while Dimorphos means "having two forms" and represents how the form (shape) of the moonlet's orbit will change after the crash.

Normally, I'd say "break a leg," but for this mission, break everything seems more appropriate.

Read more from Astro Bob
The Red Planet is only 50.6 million miles away — almost walking distance! It won't get this close again until May 2031.

Related Topics: SCIENCE AND NATURE
"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at nightsky55@gmail.com.
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