NASA’s DART spacecraft to smash into asteroid in planetary defence system test


Step outside. Look up. Nothing but the brilliant blue of the sky. The day could not be more perfect. Hang on, what’s that black speck in the sky? It slowly grows as it sweeps through the Earth’s protective atmospheric dome until you can make out flames roaring and fragments ripping off. It vanishes out of sight, but the catastrophic boom makes it quite clear that the asteroid has hit. Life on Earth may never be the same again.

The preceding paragraph might resonate well with anyone familiar with the killer asteroid trope of science fiction exemplified by such films as Deep Impact or Armageddon. Maybe it makes you remember the Chelyabinsk meteor that exploded in the atmosphere over Russia in 2013. Perhaps the paragraph could be used to describe the fate of the dinosaurs 66 million years ago.

All of these are examples of the dire threat that asteroids pose to life on Earth, and it raises important questions – if an asteroid of the magnitude that wiped out the dinosaurs was headed to Earth now, could we do anything about it?

Previously, the answer would have been a firm no. Errant asteroid trajectories are just part of cosmic life and, whilst incredibly unlikely, they have the potential to cause unprecedented devastation to human society.

NASA’s most recent mission, named DART (Double Asteroid Redirection Test), could be the first step in establishing a planetary defence system, allowing us all to breathe a bit more easily and preventing human society from going the way of the dinosaurs.

The DART mission has launched a 610kg craft on a trajectory to intercept a system known as the Didymos system. This system consists of two asteroids, Didymos and Dimorphos, orbiting each other as they journey around the Sun.

As it stands, these asteroids do not pose any risk to Earth and are sufficiently far away to act as a useful testing ground for the next part of DART’s mission – crashing directly into Dimorphos and changing its direction and its speed.

The principle is a simple application of elementary physics. The DART spacecraft, travelling at 6.6 kilometres per second, possesses a large momentum and upon crashing, transfers almost all of this to Dimorphos. This will alter the trajectory of the asteroid in a way that is perceptible to the cameras ejected from DART ten days before collision.

The results from DART will give astrophysicists vital information about how the probe breaks apart on impact

Interestingly, the effects of the collision can be calculated completely using mathematics that have been in use since the time of Isaac Newton and when this is done, it is predicted that the impact of DART will slightly reduce the size of Dimorphos’ orbit around Didymos and bring an accompanying alteration in the orbital period by 73 seconds.

This small modification doesn’t sound particularly significant but it ushers in a resoundingly important paradigm shift when considering humanity’s place in the cosmos; if the mission succeeds, we would have engineered the astrophysical dynamics of another world to our own specifications.

Philosophical implications aside, the DART mission signifies the beginning of a potentially civilization-saving defense system capable of redirecting asteroids that would otherwise be on a direct collision course with the Earth. This is a very real threat and NASA has identified 27,000 asteroids with orbits that bring them dangerously close to Earth.

However, the real danger is those asteroids which have so far not been identified and whose chaotic orbits could one day bring them hurtling towards Earth. If an asteroid of this type were detected with sufficient time before impact, it is hoped that future missions based on DART could be launched to deflect it.

The results from DART will give astrophysicists vital information about how the probe breaks apart on impact and how this changes how the momentum is transferred to the Dimorphos. Yet sadly, it is exceedingly unlikely that any asteroid on a collision course with Earth is similar in size or structure to Dimorphos and so more systematic tests like DART must be conducted to determine the efficacy of the method on a wider range of asteroids.

Perhaps significant alteration of an asteroid’s orbit would require higher impact speed or possibly even multiple impact. As Megan Bruck Syal, an astrophysicist at Lawrence Livermore National Laboratory in California involved in the analysis of the DART data, put it in an interview with Nature magazine, “We need to do more experiments like this”.

But it becomes increasingly difficult to justify missions of this type to resource-strapped funding bodies once the initial novelty of the concept wears off and so it is incredibly important for the public to remain educated to the risks that extraterrestrial collisions pose to life on Earth so that the required research can be granted the resources it needs. It is not a time for doom-mongering, but we must be realistic about how vulnerable the Earth is. Will our DART based defence system be ready in time? My fingers are crossed.


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