NASA admits there are thousands of 'city killer' asteroids still undetected in space - and we still have no way to deflect them

NASA's head of planetary defence has revealed what keeps her up at night – tens of thousands of 'city killer' asteroids that remain undetected.

Dr Kelly Fast, who leads efforts to find and track near–Earth objects like asteroids and comets, admitted they are still looking for around 15,000 mid–sized celestial bodies which could pose a threat.

While an impact from one of these – defined by being at least 140 metres wide – would likely not destroy the planet, they could 'really cause regional damage', she said.

And it turns out Earth still has no way to deflect one if we realised it was heading straight for us.

In 2022, NASA deliberately sent a spacecraft called Dart 6.8 million miles into space to crash into a mini moon called Dimorphos at 14,000mph in a bid to alter its orbit.

This mission was hailed as a success, paving the way for defending our planet by knocking incoming asteroids off their set path towards Earth.

But Dr Nancy Chabot, a planetary scientist at Johns Hopkins University who led that mission, said there are no other Dart–like spacecraft ready to launch if an asteroid was suddenly found to be on a collision course with Earth.

'Dart was a great demonstration,' she said. 'But we don't have [another] sitting around ready to go if there was a threat that we needed to use it for.'

She referenced YR4, an asteroid measuring up to 90 metres wide, which last year was deemed to have a 3.2 per cent chance of hitting Earth in 2032.

Eventually this chance was downgraded to zero, meaning no defensive action needed to be taken.

'If something like YR4 had been headed towards the Earth, we would not have any way to go and deflect it actively right now,' she added.

'We could be prepared for this threat. And I don't see that investment being made.'

During the discussion, which took place at the American Association for the Advancement of Science (AAAS) conference in Phoenix, Dr Fast explained the biggest asteroids – which can measure over a kilometre in diameter – are much easier to detect and track.

However, it's believed that only 40 per cent of the mid–sized asteroids have been detected so far – meaning experts don't know where thousands of them are.

When asked what keeps her up at night, she replied: 'It's really the asteroids that we don't know about.

'We're not so much worried about the large ones from the movies, because we know where they are.

'And small stuff is hitting us all the time. Not so much worried about that.

'It's the ones in–between that could do regional damage. Maybe not global consequences, but they could really cause damage.

'And we don't know where they all are. It's not something that even with the best telescope in the world you could find.'

NASA has been tasked by US Congress with finding more than 90 per cent of near–Earth objects larger than 140 metres in diameter.

To achieve this, the agency is currently constructing a space telescope designed with the goal of detecting the majority of near–Earth asteroids and comets within a decade.

It is expected their near–Earth object (NEO) Surveyor mission will launch next year, with the capability of detecting both bright and dark asteroids, which are the most difficult to find.

'We're searching skies to find asteroids before they find us, and get them before they get us,' Dr Fast concluded.