How altering an asteroid's path today could save Earth tomorrow

NASA’s Double Asteroid Redirection Test, or DART mission, which launched on November 24, plans to do something simple in essence and ambitious in scale. It intends to crash a small object into a much larger object at a very high velocity, and to measure how much the crash alters the movement of the larger object.

The DART impactor spacecraft is about 2 cubic metres in size (ignoring its solar array, which has a much greater wingspan). It massed about 610 kg at launch, and will be about 550 kg at impact. Its ion engine will accelerate it to a speed to 6.6 km per second (around 23,770 kmph) before it crashes into the targeted “moonlet”, Dimorphos.

Dimorphos is a roughly spherical blob of rock, about 160-170 metres in diameter, which orbits a much larger asteroid, Didymos, about 780-800 metres in diameter. This double asteroid system is about 7.8 million km away at its nearest approach to Earth. The impact is scheduled for the next time it is close to Earth, in late September or early October 2022.

If all goes well, Dimorphos, with a mass of many million tonnes, will be slightly disturbed and shift a little closer to its “parent” Didymos. Its orbital period will be altered. The kinetic energy of any object depends on the mass multiplied by the square of the velocity and although the spacecraft is dwarfed in mass by Dimorphos, it is moving at a high enough velocity to disturb the moonlet — much like a bullet weighing a few grams can knock a 100 kg man off his feet.

The mission will be photographed from LICIACube (Light Italian CubeSat for Imaging of Asteroids), a cubesat built by the Italian space agency and carried by the spacecraft. The LICIACube will be launched 10 days or so before the impact to start making observations as it makes a fly-by of the binary asteroid system. Any last-minute adjustments to the spacecraft’s trajectory will be done after LICIACube has taken close-range observations of Dimorphos. In 2024, the European Space Agency (ESA) intends to send another spacecraft on a three-year mission to carry out further observations.

You may wonder why NASA is spending $300 million on DART. The answer harks back to an age-old threat to which the Earth has never possessed a satisfactory defence. The solar system is cluttered with rocks of various sizes, moving at various speeds.

About 6,000 of these intersect the Earth every year. Most burn up through friction when they enter the atmosphere. (This is also what happens to most satellite debris.) But a big enough asteroid may not burn up completely. In that case, we have a meteor strike. A big enough meteor strike could lead to mass extinction.  

While we have the tools to detect such a large asteroid, we lack any defence to it. The mechanics of such a catastrophe would be similar to what’s known as a “nuclear winter”. Ever since the bombings of Hiroshima and Nagasaki in 1945, the spectre of nuclear winter has hung over civilisation. If you have enough in the way of large explosions, or a single explosion that’s large enough, dust thrown into the atmosphere will block off sunlight for a period that may last for years.

That would lead to a mass extinction. Plants die because they can’t carry out photosynthesis; animals die for lack of food and due to a drastic fall in temperature. There’s an ice age. While a nuclear winter hasn’t occurred, we have a pretty good idea of what could happen.

Volcanic explosions cause global cooling for similar reasons. This phenomenon has been studied many times, starting with the Krakatoa explosion of 1883 in the Sunda Strait between the Indonesian islands of Java and Sumatra. Apart from setting off tsunamis and seismic events, that volcanic eruption, which was around four times as powerful as the largest hydrogen bomb ever tested, pushed global temperatures down for a year.

The Earth has suffered at least one mass extinction caused by asteroid strike. The Cretaceous–Paleogene (K–Pg or K-T) extinction event was a mass extinction of three-quarters of all species about 66 million years ago. No land animal that weighed over 25 kg survived. This is what wiped out the dinosaurs. The Chicxulub crater in Mexico is the impact point of the asteroid, which must have been 10 km or more in diameter.

The threat of such an asteroid strike is one reason for colonising the moon and Mars, and storing DNA repositories in safe places. While no known asteroid larger than 140 metres in diameter has a significant chance of hitting Earth in the next 100 years, only an estimated 40 per cent of large asteroids have been located as of now. If DART works as advertised, it would offer some hope that an asteroid big enough to cause a mass extinction could be knocked off-course.