In 1998, NASA sent an offshore oil rig crew to space to drill down into and explode an asteroid on a direct path toward Earth. There was no world-ending impact and most of the Earth survived. Oh, and this event was purely fictitious. Hollywood played out the scenario on the big screen in Armageddon, a film starring Bruce Willis, Ben Affleck, and Liv Tyler, among others. But, what if? What if that scenario were true? How would the world respond to an Armageddon-level asteroid event? Or any asteroid-meets-Earth encounter? Would we be as blindsided as the dinosaurs? NASA, it turns out, has been preparing.

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Introducing NASA’s DART Mission

In 2020, while the world was in the middle of shutdowns due to the pandemic, NASA was busy performing a touch-and-go landing on an asteroid. The OSIRIS-REx mission was not the first of this type of mission; Japan’s asteroid samples landed in Australia just a few months later. However, the OSIRIS-REx mission was a first for the US and it should return to the planet in 2023. The question soon became “now what?” We proved we could reach out and touch an asteroid, but what comes next?

Lindley Johnson, a NASA Planetary Defense officer, provided the next step details during a news briefing earlier this month. Hitching a ride on a SpaceX Falcon 9 rocket, NASA’s next asteroid mission will make history. The mission, called Double Asteroid Redirection Test (DART), is directed by the Johns Hopkins Applied Physics Laboratory (APL). It will be the first test mission sent to change an asteroid’s course. 

In addition to the asteroid-specific mission, DART will test a new engine and help the Italian Space Agency (ASI) launch their first deep-space mission. Though powered by traditional thrusters, DART will prove NASA’s new ion propulsion engine. Then, just before reaching the asteroid, DART will deploy ASI’s LICIA CubeSat, which will follow the mission and take video and images before returning to Earth.

Mission Details: The Target

Though Armageddon dropped a nuclear warhead into and blew up an asteroid, NASA’s first test at changing a giant rock’s trajectory will not be as explosive. The intention will be to slow the velocity of the target and alter its orbit. The only destruction anticipated is DART itself. In essence, NASA plans to test the laws of physics as we understand them. 

The target in question is actually a binary asteroid, a two-bodied object with a known orbit around the Sun. The large half of the binary is called Didymos and it has a smaller satellite asteroid, called a moonlet, identified as Dimorphos. Dimorphos orbits Didymos the same way the Moon orbits Earth. Though it is about 5-times smaller, the moonlet, according to NASA, is a typical size that could pose a likely threat to Earth. It is for this reason that it has been chosen as the target for DART.

Mission Details: The Plan

Earth-based observatories will watch Didymos and its moonlet as DART travels toward them. The mission should take about 10 months from launch to asteroid contact. DART will intercept Dimorphos at approximately 6.6 kilometers per second, which is just under 15,000 miles per hour. At that same speed, a person could travel from San Francisco, California, to Tokyo, Japan, in only 20 minutes!

Once contact has occurred, the same Earth-based researchers will monitor Didymos and its moonlet to see what effect the impact has on Dimorphos’ orbit of Didymos. A shift of less than one percent is anticipated, which is a result of a “several minutes” difference in its orbit. Currently, it takes just under 12 hours to make one trip around Didymos. 

Schematic of the DART mission shows the impact on the moonlet of asteroid (65803) Didymos. Post-impact observations from Earth-based optical telescopes and planetary radar would, in turn, measure the change in the moonlet’s orbit about the parent body. Credits: NASA/Johns Hopkins Applied Physics Lab

Schematic of the DART mission shows the impact on the moonlet of asteroid (65803) Didymos. Post-impact observations from Earth-based optical telescopes and planetary radar would, in turn, measure the change in the moonlet’s orbit about the parent body.
Credits: NASA/Johns Hopkins Applied Physics Lab

The Physics of DART

Momentum is the method of quantifying an object’s motion. The formula for any body’s momentum is defined as the body’s mass times its velocity. Conservation of momentum means that even if two objects come into contact, the total momentum before collision would remain the same after. This holds true no matter if those objects bounce off each other or deflect or shatter into several pieces or stick together. DART will collide with Dimorphos and the majority of the debris should stick to the asteroid. Researchers will watch to see what is knocked off Dimorphos, as well.

Conservation of momentum aligns with Newton’s third law of motion: for every action, there is an equal and opposite reaction. This means that the force with which DART hits the moonlet should have a predictable result more than just altering its momentum. This is why the orbital period of Dimorphos around Didymos will be monitored. In addition, Dimorphos’ reaction to the impact will be closely observed.

The DART mission is, therefore, a proof-of-concept that physics is physics, especially out in space. The ASI’s LICIA CubeSat will document the impact up close and report back to Earth for further scientific review.

Watch the NASA / APL Plan 

NASA released an animation of the DART mission, from launch to asteroid-connection. Watch the clip below. This multi-phase project lifts off later this month. The launch window begins in the early hours of November 24 with any delays being revisited daily until conditions improve. This means there could be a Thanksgiving Day rocket to watch! Will you be following DART’s mission to crash into Dimorphos?


Featured image credit: NASA/Johns Hopkins, APL/Steve Gribben

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