Doomed – Should we be worried that NASA is sending a mission to investigate a potentially hazardous asteroid?

NASA have recently announced a mission to send a probe to the asteroid Bennu (RQ36) which has an orbit that intersects with the Earth’s every 6 years. The fact that Bennu is considered as a “Potentially Hazardous Asteroid” has led to quite a few overexcited articles playing up the idea that this rock could destroy life on Earth. So what are the chances and should we really be worried?

The worst offenders, at least from the UK press, are probably the stories from the Independent and the Huffington Post. Both of these are basically borrowing from the earlier article in the Times (paywalled) which is more measured.

There are several irritating simplifications, such as the statement in the HuffPo article that Bennu “travels through space at an eye-watering 63,000 mph”. This is the orbital speed of Bennu as it travels around the Sun, which seems huge, but remember the Earth’s orbital speed is 66,000 mph. The Huffington Post partly redeems itself with a later article with an interview with Professor Dante Lauretta, who is principal investigator on the OSIRIS-REx mission which will visit asteroid Bennu and return samples to Earth in 2023.

Prof. Lauretta explains in this that there is a small (1 in 2,700) chance that Bennu could hit the Earth in 2182 if its course is affected by passing between the Earth and the Moon in 2135. The ABC News puts this into context (for their US audience at least) by stating that “Your chance of being killed by firearm discharge is roughly 1 in 7,944, according to the National Safety Council.” This is probably not a fair comparison as these are lifetime odds of death by that cause, compared to a one off chance of this asteroid hitting the earth.

Bennu isn’t thought to be possible of causing a so-called “extinction level event” due to its size, about 0.5 km in diameter. The Chicxulub impact (at the end of the Cretaceous period, thought to have contributed to the death of the dinosaurs) was around 10 km in diameter. Obviously, an impact by Bennu could potentially cause a lot of damage. The energy released would be considerable, estimated at 1,450 megatons of TNT.

So is this why NASA has chosen to send this mission; to find out how much of a threat Bennu poses to the Earth? Actually no – the main reason for visiting this asteroid to take samples from its surface is because it is one of a very small number of “carbonaceous asteroids” within touching distance (relatively speaking) of the earth. This means it potentially contains organic material which is of great interest for those studying the early solar system and the origins of life on Earth. According to the OSIRIS-REx mission website, out of hundreds of thousands of known asteroids, this is one of only 5 carbonaceous objects of the right size within reach of this mission.

Another objective of the mission is to explore the Yarkovsky effect, which slowly changes the orbit of small rotating objects in space as they are heated by sunlight. Properly understanding this effect could be important in the future for predicting the course of asteroids that could hit the earth.

So, while we shouldn’t be worried about Bennu – at least for another century or so – it doesn’t mean we shouldn’t ignore the possibility. Earth and the other solar system planets have been struck many times by asteroids and comets, and impacts will continue to happen in the future. It’s clear that this is a good reason for this type of research to continue.