This graphic shows the size of Asteroid Bennu compared to a tiny ‘planetary defender’ spaceship (Credit: Lawrence Livermore National Laboratory)

A Nasa spacecraft designed to save Earth from asteroids is ‘inadequate’ and will not be able to nudge a gigantic space rock onto a 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 have just published a study which says it may be impossible to redirect a huge space rock called 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.

A Nasa artist’s impression of the Osiris-Rex spacecraft at the asteroid Bennu

In a new 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.’

The 500-metre-wide doomsday space rock is called Bennu and is big enough to wipe out a city.

It has a 1 in 2,700-chance of striking Earth in 2175, and 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.

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Bennu is as wide as five football fields and weights around 79 billion kilograms, which is 1,664 times as heavy as the Titanic.

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 Osiris-Rex craft being tested ahead of launch (Picture: Nasa)

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.’

Osiris-Rex will collect a sample of Bennu and bring it back to Earth (Picture: Nasa)

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.’

Nasa’s Osiris-Rex spacecraft is currently on its way to collect a sample of Bennu and bring it back to Earth.