What size asteroid would cause an extinction event - and how Nasa has a plan to protect us

As Nasa gears up for its Double Asteroid Redirection Test (Dart) mission on Monday, University of North Dakota assistant professor of Space Studies Sherry Fieber-Beyer is thinking about the stakes involved in this first-of-its-kind mission to test the technology to deflect hazardous asteroids.

She points to the Australasian tektite event, a still somewhat mysterious asteroid or comet impact that took place around 790,000 years ago in Southeast Asia, which “cauterized a large region of Southeast Asia, wiping out perhaps 10 per cent or more of the human ancestors alive at the time.”

“This is why the DART mission is so important,” Dr. Fieber-Beyer told the Independent in an interview, “The impact of a small asteroid or comet nucleus is the only potentially preventable natural catastrophe which could threaten human civilization or the lives of a large portion of the human race.”

On Monday evening, Nasa’s Dart spacecraft will slam into a small asteroid about 6.8 million miles from Earth in an attempt to alter the asteroid’s orbit, a practice run for diverting future space rocks that might endanger Earth — the asteroid Dart will impact poses no threat to Earth.

What are potentially hazardous asteroids?

But potential threat from other asteroids or comets is real, even if relatively rare, and the outcomes vary from large impacts that could wipe out most life on Earth, to near misses that don’t quite hit the planet, but disrupt human civilization anyway.

“It’s not just a strike to Earth; we have a lot of space assets around the Earth, and those assets are sitting ducks,” Dr Fieber-Beyer said. “A small asteroid that would be flying by could disrupt a lot of our space infrastructure without necessarily impacting Earth.”

Not all asteroids are threats to Earth. Many reside in orbits that will never intersect Earth, while others are so small that they would harmlessly burn up in Earth’s atmosphere. More than 48 tons of space rocks, some the size of footballs, some  the size of grains of sand, enter Earth’s atmosphere every year, according to Nasa.

But there are about 2,259 known asteroids designated as PHA — Potentially Hazardous Asteroids, according to Dr Fieber-Beyer, defined by their closest approach to Earth’s orbit and their size. If a space rock is about 460 feet in diameter or larger, and its orbit comes within about 46 million miles of Earth’s orbit, or about 19.5 times the distance of the Earth to the Moon, it’s considered potentially hazardous.

That makes about 7.7 per cent of known near Earth objects PHAs.

How hazardous are asteroids?

It’s easy to see why the criteria for PHA status are what they are, at least in terms of size. An asteroid of about 460 feet diameter — 140 meters — traveling at the 44,000 miles per hour typical of an asteroid strike would release energy equal to 170 million tons of exploding TNT, according to Dr Fiever-Beyer, or “about three times the energy release of the largest hydrogen bomb ever exploded on the Earth’s surface, the 50 megaton Soviet Tsar Bomba.”

Even smaller space rocks can be dangerous, depending on how well they hold together while entering Earth’s atmosphere.

In 2013, Dr Fieber-Beyer notes, a small meter exploded around 97,000 feet above Chelyabinsk Oblast in Russia, generating a shockwave that broke windows and damaged buildings. That meteor was only around 20 meters, or 66 feet in diameter, “And what it released was about an energy that was equivalent to about 400 to 500 kilotons of TNT,” she said.

The nuclear bomb the US dropped on the Japanese city of Hiroshima generated an energy equivalent to just 15 kilotons, so had the Chelyabinsk meteor held together longer and exploded nearer the Earth, Dr Fieber-Beyer noted, “nearly a million people would have been impacted just from that airburst alone.”

How much should we worry about asteroids?

Buț large asteroid strikes are, thankfully, relatively rare, Nasa planetary defense officer Lindley Johnson told reporter’s at a Thursday press conference about the Dart mission.

“Maybe once a century is there any asteroid that we would really worry about and want to deflect,” he said.

The key thing, and perhaps more important than developing technologies like Dart to divert asteroids, is making sure we find them all, Dr Johnson said, and Nasa plans to launch a new satellite by 2026 to help do just that, the Near Earth Object (NEO) Surveyor space telescope.

“NEO Surveyor will be able to find the population of asteroids 140 meters and larger within about a 10 year period,” Dr Johnson told reporters. “That’s a very short period in geologic time and unless we happen to be particularly unlucky, I’m not worried about an impact in that kind of a timeframe.”

That’s also enough time for Nasa to chew on the results of the Dart mission and ascertain whether a scaled up version of the Dart spacecraft, or some other technique, could be used to deflect a major asteroid or comet threat whenever the next one appears on the space horizon.

“We won’t have a standing fleet of DART spacecraft,” Dr Johnson said. “technology will probably have moved on. In 30, 40 years from now, who can imagine what technology we might have available to deflect an asteroid?”

Technology has already come far in the 20 years Dr Fieber-Beye has been studying asteroids.

“When I started studying asteroids, this was a pipe dream,” she said of Dart. “This is our first technology test, which is really neat. We’ll see if these ideas come to fruition.”

Nasa’s Dart test will be carried live on Nasa TV beginning at 6pm EDT Monday evening.