ASTRONOMY BLOG, Stardome Observatory and Planetarium
Asteroid deflection – a sci-fi reality?
DART – The first planetary defence space mission.
One of the most daring space missions in recent years was launched in November 2021 from the Vandenburg Space Force Base in California. Strapped atop a Falcon 9 rocket, NASA’s Double Asteroid Redirect Test, or DART, blasted off from Earth on a year-long trek to a double asteroid system known as Didymos and its smaller moon Dimorphos.
Unlike other missions that have visited asteroids, DART is not there to take photos or collect samples from the asteroids. Instead, it will accelerate and then smash into the smaller asteroid Dimorphos at speed to see if such an impact could deflect an asteroid from a possible collision with Earth. It sounds like something from the plot of Armageddon or Deep Impact, but NASA says that planetary defence is a real issue for Earth, and such a technology may be needed in the future to prevent catastrophic impacts that could cause serious damage and loss of life.
The Didymos asteroid poses no threat of impacting Earth, but testing such technologies is crucial to ensure that we could deflect a dangerous asteroid if one was ever discovered. No such asteroids are known to be heading close to Earth within the next 100 years, but we estimate that we have only discovered 40% of near-Earth asteroids to date. Scientists say it’s not a matter of if, but when.
Why is NASA launching DART?
Earth has always been pummelled by asteroids, especially in the early stages of our solar system’s formation. A collision between Earth and an asteroid some 10km wide 66 million years ago caused widespread destruction and was responsible for the Cretaceous-Paleogene extinction event that eventually wiped out almost all dinosaurs on Earth.
Large collisions are extremely rare, but it is almost certain that such an event will occur again. Recent astronomical events, like the 2013 Chelyabinsk meteor in Russia, have renewed concern about such events happening without warning. The Chelyabinsk meteor exploded in the sky above Siberia and caused a powerful airburst that caused widespread damage and injured thousands of people on the ground. The meteor was only a mere 20 meters across, so it is easy to imagine how much more destructive a larger one would be. The Chelyabinsk meteor also occurred without warning, and we had no idea it was headed for Earth until it hit.
NASA warned the public in 2016 that such an event will certainly occur again at some point, and the late Stephen Hawking also warned that asteroid impacts pose one of the biggest threats to humanity in his final book Brief Answers to the Big Questions.
The Doomsday asteroid
An infamous 370-meter-wide asteroid known as Apophis caused a brief panic in 2004 when it was calculated of having a 2.7% chance of impacting Earth in 2029. It caused a media frenzy that quickly gave it the nickname of the ‘doomsday asteroid’ but further calculations into Apophis’ orbit in recent years have ruled out that chance altogether, and it now poses virtually zero threat to Earth in the next 100 years.
Astronomers have catalogued objects that orbit in close proximity to us as Near-Earth Objects, or NEOs. We have discovered over 27,000 such objects to date, but that number only represents a fraction of the total that is calculated to exist. Only around 2,000 of the known NEOs are considered hazardous to Earth.
In the 90s, space agencies around the world began to scan the skies for such objects in an effort known as ‘Spaceguard’ to give us warning of an asteroid on a direct collision course. Thousands of new asteroids are discovered every year, and NASA believes it is imperative that we develop a plan to deflect one should we need to.
What will DART do?
The mission’s target is Dimorphos – a small moon of the larger asteroid Didymos – both considered to be in a binary system. Didymos is about 780 meters across, while Dimorphos is about 120 meters across. DART will smash into the smaller moon to see if the kinetic energy from the impact will alter the course of the moon around Didymos. Astronomers will measure the change in orbit using Earth-based telescopes to see how much of an impact it has on Dimorphos.
NASA has estimated that the impact of DART would cause a velocity change of 0.4mm per second for Dimorphos. It doesn’t sound like a lot, but over time it would lead to a much larger shift in the orbit. This would change the orbital speed of the moon by almost 10 minutes. If Dimorphos were on a collision course with Earth, this change would be enough to successfully deflect it from hitting Earth.
These are all calculations, however, and we won’t know the true outcome until DART hits the asteroid.
In a collaborative mission, the European Space Agency intends to send their own probe to the double asteroid five years later to further measure the changes. The mission, known as Hera, will arrive in 2027 and will conduct detailed reconnaissance of the asteroid system and any changes caused.
The DART mission probe carries no scientific instruments at all - they are practically useless since the spacecraft will intentionally hit an asteroid and destroy itself. It has a star tracker and a small camera for navigation, along with solar panels for power and an ion thruster to move. The probe is described as roughly the size of a vending machine with a mass of 670kg. DART is due to collide with the smaller moon of Didymos on the 3rd of October 2022.
Will it work?
As the T in the name implies, the DART mission is an experiment – Double Asteroid Redirect Test. The mission has been designed with precise calculations as to how big, how fast, and how heavy a spacecraft must be to move an asteroid, but nothing is guaranteed.
We simply don’t know if it will work because we have never tested such technologies before. Even if the mission is successful, it does not mean that we are automatically protected from all Earth-bound asteroids. NASA has stated that such missions can take around five years to build, launch, and reach an asteroid. If we were to spot an asteroid, we’d need several years to plan and prepare another mission to deflect it. If it was discovered too close to impact, it would be too late to launch a deflection spacecraft.
The other issue is we simply cannot see every single asteroid that exists. Just like the Chelyabinsk meteor that took us by surprise in 2013, some asteroids strike without warning. Another problem is size; Dimorphous is 120 meters, so the success of DART might mean that only asteroids that are roughly the same size or smaller could be deflected. If we discovered an asteroid say, several kilometres across, it would likely be too large for a spacecraft impact like DART to have any noticeable effect.
If DART fails, it will still provide us with a wealth of information about how we can improve future missions to deflect asteroids. Nothing is guaranteed, but it is an exciting mission that brings very sci-fi concepts into reality. In the words of a wise astronomer, “The dinosaurs went extinct because they didn’t have a space program”.