Popular Mechanics (South Africa)
NASA’S NEW HOPE TO STOP DEADLY ASTEROIDS
IN 2013, A METEOR EXPLODED 22.5 KM (14 MILES) above Chelyabinsk, Russia, knocking out windows across 500 km2 and injuring more than 1 600 people. It was a wake-up call for astronomers to help defend Earth from more potentially hazardous asteroids. Of the almost one million known comets and asteroids in our solar system, more than 2 000 have the potential to be hazardous to Earth. These asteroids are typically 140 m wide or larger, on an orbit that swings them within 22.5 million km of Earth’s orbit. ‘Although asteroid deflection might sound like science fiction, it is a serious topic,’ says Flaviane Venditti, PhD, an observatory scientist at the Arecibo Observatory in Puerto Rico. ‘Out of all natural disasters, an asteroid impact is the only one we have the power to avoid.’
Proposed defensive measures against oncoming asteroids look blunt so far. Later this year, NASA will attempt its Double Asteroid Redirection Test (DART). This mission will involve slamming an ovensized spacecraft into an asteroid called Dimorphos – scheduled to make a close (but safe) approach to Earth in 2022 – as scientists measure how the impact changes Dimorphos’s trajectory. It’s risky to Hulksmash an Earth-bound, extinction-level asteroid, though. ‘In general, when we move an asteroid, we want to keep it in one piece,’ says planetary astronomer and DART co-lead Andrew Rivkin, PhD, of Johns Hopkins Applied Physics Laboratory. The rock could break apart and create a wave of several smaller ‘citykilling’ asteroids instead. (This risk also applies to an Armageddon-style nuclear solution, we’re told – there are no plans to test a space nuke at this time.)
But Venditti’s team of researchers has suggested a way to sidestep the fragmentation conundrum with something we might call the cosmic lasso method. As the team explained in a paper for The European Physical Journal, this method involves towing a smaller space rock to an oncoming asteroid and tethering the two. Attaching additional mass would displace the first asteroid’s centre of mass and shift it to a new, safer orbit. ‘Thus, no unwanted consequences related to fragmentation would happen after the deflection,’ Venditti and her researchers wrote.
To test the lasso method, the team ran a series of computer simulations targeting the potentially hazardous asteroid Bennu. Venditti and her colleagues mapped out various deflection scenarios for Bennu that unfolded over the course of 300 years. They tested different mass ratios for the smaller asteroid (1/1 000th the size of Bennu and 1/10 000th the size
of Bennu), evaluated three different tether lengths (1 000, 2 000, and 3 000 km long), and assessed possible angles at which the tethers could be attached to the asteroid.
Their simulations revealed that asteroids as small as 1/10 000th the mass of the main object would be sufficient enough to pull Bennu into a different, safer orbit – roughly the equivalent of tethering a No. 2 pencil to an orbiting astronaut. ‘The neat trick to this technique is that it changes the position of the original asteroid rather than its velocity, because attaching the two asteroids makes them into a single object, with the centre of mass in a different place than the original asteroid, pre-tether,’ Rivkin notes. ‘And because the centre of mass (and so, its position) is different but the velocity stays the same, the orbit changes.’
An asteroid 1/10 000th the mass of Bennu, with a tether 3 000 km long attached along an equatorial orbit, pulled Bennu off of its trajectory by almost 965 000 km, or 150 times the radius of Earth, according to the paper. However, an asteroid 1/1 000th the mass of Bennu, attached via the same tether, pulled Bennu off course by as much as 9.66 million km (1 500 times Earth’s radius). In the latter scenario, the team found attaching the tether at a 45° angle resulted in a shift in trajectory of about 32 million km. At a 90° angle, the deflection was about 38 million km.
The lasso plan dramatically lowers the risk of peppering Earth with asteroid fragments, but a lot needs to be worked out before we send robotic space cowboys across the solar system. Actually corralling and attaching two asteroids would be, to put it mildly, complex. ‘The first step is always to analyse the physics of the method,’ Venditti says. ‘If the maths works, then the logistics and engineering side of the method needs to be developed.’ To organise the logistics, astronomers would need to spot the potential threat far in advance. Bennu’s next close approach to Earth will happen in 2060. According to Venditti’s team, a mission launched by 2035 would give us enough time to deflect it if needed. (But don’t worry. At its closest, Bennu will still be more than 800 000 km away.)
When it comes to defending Earth from marauding asteroids, Rivkin argues that creativity is key: ‘Anything is possible, I suppose, if you’re willing to invest enough resources into making it happen. Planetary defence is a great topic for thinking about out-of-the-box solutions.’ This is the perfect time to experiment.