To be clear, Didymos and Dimorphos are not on a collision course with Earth. At the time of DART’s arrival, the pair will be at their closest approach, a safe 6.7 million miles — or 28 times farther than the moon.
Targeting a double asteroid
Didymos and Dimorphos are a binary asteroid system, gravitationally bound to each other as they orbit the sun — a situation that NASA is taking advantage of for this test.
By giving a shove to the smaller Dimorphos, the tiny change in the moonlet’s orbit around its larger companion will be relatively easy to observe, compared to the far greater challenge of measuring the deviation in a solitary asteroid’s orbit around the sun. This is why a binary system was chosen as the target for the DART mission.
At 535 feet across, Dimorphos is still about the size of the Great Pyramid of Giza — so it might seem like a collision with a minuscule, 4-foot cube like DART would have little effect on it. But DART’s punch is in its speed, not its size.
Hurtling into the asteroid eight times faster than a bullet, DART will impart a punch of kinetic energy that should knock Dimorphos slightly closer to Didymos, thus decreasing the period of its orbit.
Dimorphos circles its larger companion once about every 12 hours. Scientists anticipate that DART will change the orbital period by a few minutes, which telescopes on Earth will be able to measure with precision.
Planetary protection
DART’s mission is to test the idea of altering the course of an asteroid bound for a collision with Earth, thus preventing a disaster of potentially global proportions.
But how can a tiny deviation in the path of a cosmic juggernaut make a difference if the goal is to make it miss an entire planet?
The answer is: time.
Slapping a barreling bowling ball with a flyswatter probably won’t change whether it hits any bowling pins — but imagine if the bowling lane were 10 times longer than in a regulation alley. Then, even a tiny deviation in the ball’s path, if applied early enough, might grow to make the difference between a strike and a gutter ball.
A series of radar images of the asteroid Didymos and its satellite Dimorphos, captured by the Arecibo Observatory in Puerto Rico. (Arecibo Observatory/NASA)
With advance warning of an impending asteroid collision, there can be time to mount a DART-like mission and give the incoming doomsday rock enough of a swat to miss the Earth when it finally arrives. Even a slight course correction can grow to hundreds or thousands of miles given enough time — a change that can make all the difference.
NASA and other observers are constantly on the lookout for potentially hazardous asteroids and comets, and have discovered and track the motions of thousands of near-Earth objects.
Vigilance and advance planning are the keys to protecting our planet from major asteroid impacts, and the DART mission is a first step in figuring out how to avoid disaster once we see it coming.
Lessons from the past
Earth has been the recipient of numerous impacts by asteroids and comets in the past, many of them small, though some large enough to cause global devastation. The asteroid that scientists suspect led to the extinction of the dinosaurs was around 6 miles across and struck the northern end of the Yucatan peninsula over 65 million years ago.
Collisions by asteroids of that size are extremely rare, occurring on average every hundred million years, but impacts by smaller asteroids happen more often — the smaller the object, the more frequent the collision.
DART and LICIACube
Though DART’s terminal encounter with Dimorphos may seem a lonely end for a deep space robot, DART will not be alone at its demise. Like Didymos and its small moonlet companion, DART is accompanied by its own partner, a tiny “cubesat” named LICIACube.
LICIACube, the small ‘cubesat’ accompanying NASA’s DART spacecraft on its collision course with the asteroid Dimorphos. Built by the Italian Space Agency, LICIACube will observe DART’s impact with Dimorphos and send the images back to Earth. (NASA/Johns Hopkins APL/Ed Whitman)
Built by the Italian Space Agency, LICIACube piggybacked on the DART spacecraft for most of its journey, and on September 11 was released to a separate destiny.
LICIACube will spend the time all the way to the September 26 collision event gaining distance from DART and taking pictures of the asteroids as they approach.
Then, when DART strikes Dimorphos, it will snap pictures of the collision and the cloud of material blasted from the impact, sending the images back to Earth for scientists to study. The observations will tell us more about the structure and composition of Dimorphos to inform future asteroid-swatting missions.
