Scientists around the world were fooled this week by a fictitious asteroid heading for Earth. A group of experts from the US and European space agencies took part in a week-long exercise led by NASA, in which they were confronted with a hypothetical scenario. An asteroid 35 million kilometres away is approaching Earth and has the potential to strike within six months.
As the days went by, the participants learned more about the size, trajectory and chances of impact of the asteroid. They then had to cooperate and use their technological knowledge to see if anything could be done to stop the space rock. They were unsuccessful. The group determined that none of the existing technologies on Earth could stop the hypothetical asteroid from hitting within the six-month time frame of the simulation. In this alternate reality, the asteroid crashed into Eastern Europe.
As far as we know, no asteroids currently pose a threat to the Earth. But it is estimated that two thirds of the asteroids of 140 metres or more, which are large enough to cause considerable damage, have not yet been discovered. That's why NASA and other agencies are trying to prepare for such a situation.
Lindley Johnson, NASA's head of planetary defence, said in a press release:
These exercises help the planetary defence community communicate with each other and with our governments to ensure that we are all coordinated if a threat is identified in the future.
6 months is too short a time to prepare for an asteroid impact
The fictitious asteroid was called 2021PDC. In NASA's scenario, it was first 'spotted' on 19 April and, after a week, scientists were able to calculate that it had a 5% chance of hitting our planet on 20 October, six months after its discovery date.
But the second day of the exercise was fast-tracked to 2 May, when new impact trajectory calculations showed that 2021PDC would almost certainly hit Europe or North Africa. The simulation participants considered various missions in which spacecraft could attempt to destroy the asteroid or deflect it from its path.
But they concluded that such missions would not be able to take off in the short time before the asteroid impact.
If we were faced with the hypothetical 2021PDC scenario in reality, we would not be able to launch a spacecraft in such a short time with current capabilities.
They also considered trying to blow up or disrupt the asteroid with a nuclear explosive device.
Deploying a nuclear disruption mission could significantly reduce the risk of impact damage.
However, the simulation stipulated that 2021PDC could be anywhere from 60 cm to 8 metres in size, so the chances of a nuclear bomb making a dent were uncertain.
On the third day of the exercise, 30 June, the future of the Earth looked bleak: the impact trajectory of 2021PDC indicated that it was heading towards Eastern Europe.
On the fourth day, which was a week before the asteroid's impact, there was a 99% chance that the asteroid would hit near the border between Germany, the Czech Republic and Austria. The explosion would bring as much energy as a large nuclear bomb. All that could be done was to evacuate the affected areas in advance.
Most asteroids fly under the radar and many are spotted too late
It is tempting to think that, in the real world, astronomers would spot an asteroid similar to 2021PDC with much longer notice than six months. But the world's ability to monitor near-Earth objects (NEOs) is woefully incomplete.
Any space rock that orbits within 201 million kilometres of the sun is considered a NEO. But as NASA said in July, 'we have found only about one-third of the asteroid population that could pose a risk of impacting Earth.'
Of course, humanity hopes to avoid a nasty surprise like the one the dinosaurs had 65 million years ago, when a 9-kilometre-wide asteroid crashed into the Earth. But in recent years, scientists have not spotted many of the large, dangerous objects that have come close to our planet.
Comet Neowise, a 4.8 km wide piece of space ice, passed within about 103 million km of Earth in July. No one knew the comet existed until a NASA space telescope discovered it while it was approaching four months earlier.
In 2013, a meteor measuring about 19.8m in diameter entered the atmosphere at a speed of 64,000km/h. It exploded over Chelyabinsk, Russia, without warning, sending a shockwave that shattered windows and damaged buildings across the region. More than 1,400 people were injured.
In 2019, a 130-metre-wide 'city killer' asteroid flew within 72,000 km of Earth. NASA had hardly been warned. At present, the only way for scientists to track a NEO is to point one of Earth's limited number of powerful telescopes in the right direction at the right time.
To solve this problem, NASA announced two years ago that it would launch a new space telescope dedicated to monitoring dangerous asteroids. This telescope, called the Near-Earth Object Surveillance Mission, together with the Test-Bed telescope recently launched by the European Space Agency and the Flyeye telescope being built in Italy, should eventually increase the number of NEOs we can track.
NASA is testing ways to immobilise an asteroid
NASA explored the options scientists would have if they found a dangerous asteroid on a collision course with Earth. These include detonating an explosive device near the space rock, as suggested by exercise participants, or firing lasers that could heat and vaporise the asteroid enough to alter its trajectory.
Another possibility is to send a spacecraft to hit an approaching asteroid and knock it off course. This is the strategy NASA is most seriously considering. Later this year, the agency is expected to launch a test of the technology. The Double Asteroid Redirection Test (DART) will send a spacecraft towards the Dimorphos asteroid and deliberately collide with it in autumn 2022. NASA hopes that this collision will change the orbit of Dimorphos. Although this asteroid does not pose a threat to Earth, the mission could prove that reorientation of an asteroid is possible with sufficient time.