Dimorphos is a lump of space rock so far away from Earth that we don’t even know what it looks like — and on Monday, we’re going to smash it with a spacecraft. The Double Asteroid Redirection Test (DART) will be traveling at more than 14,000 miles per hour when it hits the asteroid, in what has to be one of the most metal science experiments of all time.
DART is a NASA effort to see if it can change an asteroid’s movement in space. It’s being billed as the world’s first “planetary defense test mission” — a test run to see if we have what it takes to avert a serious asteroid impact on Earth sometime in the future. You know, just in case.
To be extremely clear, neither Dimorphos, or its larger companion Didymos, pose any threat to Earth. In fact, we haven’t identified any asteroids that pose an immediate threat to our planet. These two are just good target practice. Dimorphos and Didymos are a binary asteroid system, with Dimorphos being a ‘moonlet’ of Didymos. As the tiny moonlet orbits the bigger asteroid, it passes between the bigger asteroid and Earth. This means that telescopes both on and off-world can monitor the system and see relatively quickly what a crash does to Dimorphos’ speed and trajectory.
Soon after the impact, telescopes on every continent on the planet will focus on the system to see the aftermath. Off-world, the James Webb Space Telescope, Hubble, and even the asteroid-bound Lucy spacecraft will also train their gaze on the asteroid system, waiting to see what happens when a rock meets a hard spacecraft.
Playing Planetary Defense
The impact is expected to alter the speed of Dimorphos by a fraction of a percent, researchers say, changing the time it takes to complete its orbit by several minutes. That might not seem like much, but for planetary defense scientists, those minutes are monumental. “This demonstration is extremely important to our future here on the Earth” said Lindley Johnson, NASA’s Planetary Defense Officer, at a press briefing ahead of the mission.
This moment in history is unique, Johnson said; it’s the first time that humans have both knowledge about the threat that asteroids pose, and actually have the tech to do something about it. In the event that we ever do detect a giant rock hurtling towards the planet, having a plan or two in place for how to stop said rock is a good thing — and having a few practice runs under our belt could be even better.
“DART is demonstrating what we call the kinetic impact technique for changing the speed of the asteroid in space and therefore changing its orbit” Johnson said.
There are other options in the planetary defense toolbox, including a ‘gravity tractor,’ a spacecraft that could fly next to an asteroid, gently pulling it to a safer path. There’s also the possibility of firing an ion beam at an asteroid for a long time, pushing it to a different orbit. DART is trying a more direct method first; crashing into it full speed ahead.
Bracing for impact
During its final approach, DART will be driving itself. There will be about 44 people in a control room watching telemetry and data, but starting about four hours before impact, “the spacecraft has to do everything,” said Elena Adams, DART mission systems engineer at John Hopkins Applied Physics Laboratory during a press conference. It has a smart navigation system on board that is guiding it to the Didymos/Dimorphos system. It spotted Didymos earlier this summer, but it won’t be able to see Dimorphos, the actual target, until about an hour before impact.
When it spots Dimorphos, the 163-meter-wide (530 feet) asteroid will only appear as a pixel. That will be enough for the navigation system to begin tracking toward the rock itself, instead of its companion asteroid. Two and a half minutes before impact, the navigation systems that brought the spacecraft to that point will switch off, Adams says. “We’re just going to point the camera, and take the most amazing pictures of this asteroid that we’re going to see for the first time.”
It’s not every day that scientists get to crash a $250 million spacecraft, as Adams told The Verge last November, ahead of DART’s launch. Because it’s such a once-in-a-lifetime experience, the team will be documenting the collision in detail.
In addition to the observatories in space and on Earth that will be watching, DART’s own camera will be sending back images until the last minute, beaming them back to Earth so that people can watch as the mission reaches its dramatic conclusion.
In addition, a small companion spacecraft will be documenting the action in space. The Italian LICIACube (Light Italian CubeSat for Imaging Asteroids) launched with DART and separated from the larger spacecraft on September 11th. It is following its companion, and will document the experiment’s aftermath, flying by Dimorphos about three minutes after impact. It will also have the chance to see the other side of Dimorphos, which the larger spacecraft will never get to see.
What comes next?
“This mission has two parts. The first part is hitting the asteroid, the next part is actually measuring what happens afterwards,” Adams said. The team expects the asteroid to run faster after the collision, and will be tracking that over time.
“It’s just like if you dropped your wristwatch and damaged it. It’s not going to keep necessarily the same time,” said Tom Statler, DART’s program scientist. “You might not notice it right away, but in the weeks and days and weeks to follow you will notice that your watch is running fast — and we will notice that the binary asteroid system is running fast.” Statler said.
While Statler and the other researchers have a good idea of what might happen after the crash, one of the big reasons for this test is that we don’t know exactly what will happen when we crash into an asteroid. Information about how the asteroid reacts to an impact could help calibrate future tests, and eventually inform how we might approach a threatening asteroid.
“As a scientist I fully hope to be surprised by the results of the experiment.” said Statler. “Although as a planetary defender, I don’t want to be too surprised.”