A Nasa spacecraft has entered orbit around a faraway asteroid which has a very small chance of hitting Earth in the 22nd century.
The Osiris-Rex spacecraft is now circling the space rock Bennu, which is 70 million miles from Earth, and is soaring roughly a mile above its surface.
Osiris-Rex arrived at Bennu in early December and flew in formation with it until the latest manoeuvre. In 2020, the Nasa craft will collect samples of the rock in 2020 and attempt to return them to Earth in 2023.
Asteroid Bennu is a 500 metre-wide monster and there’s a tiny risk it will plough into Earth and do some serious damage.
It’s also believed Bennu may contain the building blocks of life, so recovering a sample will allow scientists to test a theory that asteroids carried vital chemical to earth which sparked the genesis of living organisms.
But the behemoth could also bring death on a gigantic scale, so the tests will also help to understand our nemesis and how to make sure it doesn’t hit us.
If a disastrous impact came to pass, it would release more energy than all the nuclear weapons detonated in the entirety of human history.
Bennu is as wide as five football fields and weighs around 79 billion kilograms, which is 1,664 times heavier than the Titanic.
It has a 1 in 2,700-chance of striking Earth between 2175 and 2199 – which is really very small, so there’s no need to worry unduly for your great, great grandchildren’s safety.
It is estimated that the energy unleashed in this impact would be equivalent to 1,200 megatons, which is 80,000 times the energy released by the Hiroshima bomb.
Although it would create a crater which would be more than two miles wide, our species would probably live to fight another day.
‘We’re not talking about an asteroid that could destroy the Earth,’ Nasa Osiris-Rex principal investigator Dante Lauretta said in 2016. ‘We’re not anywhere near that kind of energy for an impact.’
However, you might not want to abandon plans to turn your mum and dad’s basement into a bunker just yet – because there’s still a chance it might hit.
And if we do discover that Bennu is coming for us, we might not be able to save ourselves very easily.
A Nasa spacecraft designed to save Earth from asteroids is ‘inadequate’ and will not be able to nudge such a huge rock on course which guarantees it doesn’t hit Earth in 2175 without using nuclear weapons.
That’s the warning from members of a US ‘national planetary defence team’ who published a study earlier this year which says it may be impossible to redirect a Bennu unless we bring out the big guns.
Nasa is already working on a craft called HAMMER (Hypervelocity Asteroid Mitigation Mission for Emergency Response vehicle) that’s designed to blow up asteroids with nuclear bombs or steer the asteroid on a trajectory which means it won’t smash into Earth.
The second option is preferred because it gives more control over the path of the asteroid and doesn’t risk showering our planet with radioactive rock fragments.
But just one of the tiny HAMMER crafts will probably be unable to deflect Bennu, scientists said, meaning the dangerous nuclear option could be our only hope.
In a study, academics who work alongside Nasa as part of the planetary defence team ‘ultimately concluded that using a single HAMMER spacecraft as a battering ram would prove inadequate for deflecting an object like Bennu’.
‘The consequences would be dire,’ said Kirsten Howley, a physicist at Lawrence Livermore National Laboratory who is part of the planetary defence team.
‘This study aims to help us shorten the response timeline when we do see a clear and present danger so we can have more options to deflect it. The ultimate goal is to be ready to protect life on Earth.’
Nasa’s HAMMER is just nine metres tall and weighs 8.8 tonnes. Which means it will need a lot of time to push a huge asteroid onto a new course.
‘The push you need to give it is very small if you deflect the asteroid 50 years out,’ Howley added.
‘The probability of a Bennu impact may be 1 in 2,700 today, but that will almost certainly change – for better or worse – as we gather more data about its orbit.
‘Delay is the greatest enemy of any asteroid deflection mission.’
The researchers said it would take at least 7.4 years to send a deflector craft to Bennu and ‘many years for the small change in speed to accumulate into a sufficient change in trajectory’.
If we do what humans normally do and leave things until the last minute, the asteroid will be impossible to deflect unless we build a vast number of spaceships – something which would simply not be possible right now.
A mission launched in 2125 would require up to 53 launches of the Delta IV Heavy rocket carrying a single HAMMER to send enough of the planet-saving crafts to Bennu.
‘When many launches are required for a successful deflection, the mission success becomes more difficult, due to the failure rate associated with each individual launch,” said Megan Bruck Syal, LLNL physicist and coauthor on the paper.
‘If we only had ten years from launch, we would need to hit Bennu with hundreds of tonnes just to barely deflect it off of an Earth-impacting path, requiring dozens of successful launches and impact at the asteroid.’
The team concluded that nuking Bennu could be the only guaranteed way of saving ourselves, as long as the mission is carried out safely.
‘Successful disruption requires ensuring that the asteroid pieces are sufficiently small and well-dispersed so that they pose a much-reduced threat to the Earth,’ Syal added.
‘Disruption carried out as late as tens of days before impact can still be very effective in reducing the total damage felt by Earth. Previous work by our research group has shown that the impacting debris is reduced to less than 1% of its initial mass by disrupting the asteroid, even at these late times.’
You’ll be able to watch a livestream of the Bennu encounter starting from 4.45pm on Nasa’s Facebook and Twitter page as well as its online television station.