NASA’s Double Asteroid Redirection Test (DART) spacecraft aims to be a one-hit wonder. It will end its days when it hits an asteroid at 24,000 kilometers per hour on September 26. Launched from Earth in November 2021, DART is about the size of a bus to test and prove our ability to protect Earth from dangerous asteroids.
It is not easy to hit a target directly from 11 million kilometers away. While that sounds far away, the asteroid was actually chosen by NASA because of its relatively close proximity to Earth. This will give engineers the opportunity to test the spacecraft’s ability to operate on its own in the final stages before impact, as it crashes automatically.
The target asteroid, called Dimorphos, is a 163-meter-diameter object orbiting a 780-meter-wide asteroid called Didymos. This “binary asteroid system” was chosen because Dimorphos is in orbit around Didymos, which makes it easier to measure the outcome of the impact as its orbit changes. However, the Dimorphos system does not currently pose any risk to the planet.
Regardless, NASA is trying to change the path of an asteroid with a full-scale planetary defense experiment. The technique being used is called “kinetic impact,” which alters the asteroid’s orbit by impacting it. This is basically what is called a snooker safe shot, but at the planetary level between the spacecraft (as the cue ball) and the asteroid.
A tiny deflection is enough to prove that this technique can actually change the path of an asteroid on its collision course with Earth.
But the DART spacecraft will be completely blown up by the collision, as it will have an impact equivalent to about 3 tons of TNT. By comparison, the atomic bomb dropped on Hiroshima was equivalent to 15,000 tons of TNT.
So, given this level of damage and the distances involved, how are we going to see the crash? Fortunately, the DART spacecraft isn’t traveling alone, it’s carrying LICIACube, a shoebox-sized mini spacecraft known as a CubeSat, developed by the Italian space agency and aerospace engineering firm Argotec. This little buddy recently separated from the DART spacecraft and is now traveling alone to witness the impact from a safe distance of 55 kilometers.
There has never been a CubeSat orbiting an asteroid before, so this opens up new potential ways to explore space in the future. The effect will also be observed from Earth using telescopes. Combined, these methods will allow scientists to confirm whether the surgery was successful.
However, it could take weeks for LICIACube to send all the images back to Earth. This time will be utterly nerve-wracking—waiting for good news from a spacecraft is always an exciting time for engineers.
What happens next? An investigative team will investigate the aftermath of the crash. The scientists’ goal was to measure changes in the motion of Dimorphos around Didymos by looking at Didymos’ orbital period. This is the time that Dimorphos elapses before and after Didymos, which happens every 12 hours.
When that happens, ground-based telescopes will be dedicated to capturing images of the Dimorphos eclipse. In order to induce a sufficiently large deflection, DART must produce a change in orbital period of at least 73 seconds after impact — which can be seen in changes in the frequency of solar eclipses.
These measurements will ultimately determine the effectiveness of the “kinetic impact” technique in deflecting potentially hazardous asteroids — we don’t know yet.
This is because we actually know very little about the composition of asteroids. The enormous uncertainty of how powerful Dimorphosis is makes designing a bullet spacecraft a truly enormous engineering challenge. Based on ground observations, the Didymos system is suspected to be a rubble pile composed of many different rocks, but its internal structure is unknown.
There is also a great deal of uncertainty about the outcome of the impact. The material that pops up afterwards will help with the impact of the crash, providing extra power. We don’t know if the impact would have created a crater, or if the asteroid itself would have been significantly deformed, meaning we can’t be sure how much force the collision would have released.
Future Missions Our exploration of asteroid systems does not end with DART. The European Space Agency is scheduled to launch the Hera mission in 2024 and arrive in Didymos in early 2027 to take a closer look at the remaining impact effects.
By observing the deformation caused by DART’s impact on Dimorphos, the Hera spacecraft will gain a better understanding of its composition and formation. Understanding the internal properties of objects like Didymos and Dimorphos will also help us better understand the dangers they could pose to Earth upon impact.
Ultimately, the lessons learned from this mission will help validate the mechanics of high-velocity impacts. While laboratory experiments and computer models can already help validate scientists’ impact predictions, full-scale experiments in space, such as DART, are the closest we can get to the big picture. Learning as much as possible about asteroids will help us understand what force we need to hit them with to deflect them.
The DART mission has fostered a global collaboration among scientists looking to address global planetary defense issues, and together with our colleagues on the DART investigation team, we aim to analyze impact impacts. My own focus will be on studying the motion of the material ejected from the impact.
The spacecraft impact time is scheduled for September 26 at 19:14 EDT (September 27 at 00:14 BST). You can follow the impact on NASA TV.