A NASA mission to deliberately smash a spacecraft into an asteroid - a test run should humanity ever need to stop a giant space rock from wiping out life on Earth - blasted off last night from California.
It may sound like science fiction, but the DART (Double Asteroid Redirection Test) is a real proof-of-concept experiment, which lifted off at 10.21pm Pacific Time yesterday (6.21am Irish time) on board a SpaceX rocket from Vandenberg Space Force Base.
"Asteroid Dimorphos: we're coming for you!" NASA tweeted after the launch.
Asteroid Dimorphos: we're coming for you!— NASA (@NASA) November 24, 2021
Riding a @SpaceX Falcon 9 rocket, our #DARTMission blasted off at 1:21am EST (06:21 UTC), launching the world's first mission to test asteroid-deflecting technology. pic.twitter.com/FRj1hMyzgH
The goal is to slightly alter the trajectory of Dimorphos, a "moonlet" around 160 metres (or two Statues of Liberty) wide that circles a much larger asteroid called Didymos (762 metres in diameter). The pair orbit the Sun together.
Impact should take place in the autumn of 2022, when the binary asteroid system is 11 million kilometres from Earth, almost the nearest point they ever get.
"What we're trying to learn is how to deflect a threat," NASA's top scientist Thomas Zuburchen said of the $330 million project, the first of its kind.
To be clear, the asteroids in question pose no threat to our planet.
But they belong to a class of bodies known as Near-Earth Objects (NEOs), which approach within 30 million miles.
NASA's Planetary Defense Coordination Office is most interested in those larger than 460ft in size, which have the potential to level entire cities or regions with many times the energy of average nuclear bombs.
There are 10,000 known near-Earth asteroids 460ft in size or greater, but none has a significant chance to hit in the next 100 years. One major caveat: scientists think there are still 15,000 more such objects waiting to be discovered.
Impact scheduled to occur between September and October, 2022
Planetary scientists can create miniature impacts in labs and use the results to create sophisticated models about how to divert an asteroid - but models are always inferior to real world tests.
Scientists say the Didymos-Dimorphos system is an "ideal natural laboratory", because Earth-based telescopes can easily measure the brightness variation of the pair and judge the time it takes the moonlet to orbit its big brother.
Since the current orbit period is known, the change will reveal the effect of the impact, scheduled to occur between 26 September and 1 October, 2022.
What is more, since the asteroids' orbit never intersects our planet, they are thought safer to study.
The DART probe, which is a box the size of a large fridge with limousine-sized solar panels on either side, will slam into Dimorphos at just over 15,000 miles an hour.
Andy Rivkin, DART investigation team lead, said that the current orbital period is 11 hours and 55 minutes, and the team expects the kick will shave around 10 minutes off that time.
There is some uncertainty about how much energy will be transferred by the impact, because the moonlet's internal composition and porosity are not known.
The more debris that is generated, the more push will be imparted on Dimorphos.
"Every time we show up at an asteroid, we find stuff we don't expect," said Rivkin.
The DART spacecraft also contains sophisticated instruments for navigation and imaging, including the Italian Space Agency's Light Italian CubeSat for Imaging of Asteroids (LICIACube) to watch the crash and its after-effects.
"The CubeSat is going to give us, we hope, the shot, the most spectacular image of DART's impact and the ejecta plume coming off the asteroid. That will be a truly historic, spectacular image," said Tom Statler, DART programme scientist.
Scientists estimate 460ft asteroids strike once every 20,000 years
The so-called "kinetic impactor" method is not the only way to divert an asteroid, but it is the only technique ready to deploy with current technology.
Others that have been hypothesised include flying a spacecraft close by to impart a small gravitational force.
Another is detonating a nuclear blast close by - but not on the object itself, as in the films Armageddon and Deep Impact - which would probably create many more perilous objects.
Scientists estimate 460ft asteroids strike once every 20,000 years.