Earth aims DART to knock asteroid off course; now all eyes are on the sky

Scientists will be watching through the next several weeks to see how effective the impact was

Today, early morning, the National Aeronautics and Space Administration (NASA) successfully crashed a vehicle into a double asteroid system. Scientists will wait with bated breath to see if those two rocks are shifted from the orbit they were following.

This DART (Double Asteroid Redirection System) mission is a first attempt to develop the capability to protect the Earth from being hit by large meteors. It is a proof of concept demonstration – the target asteroids are not on a path that intersects the Earth’s.

Hundreds of meteors slice into the Earth’s orbit on a daily basis. Most burn up harmlessly due to friction as they enter the atmosphere, offering lovely son et lumiere (sound and light) shows. At certain times of the year, when the Earth’s orbit passes through places where such rocks are abundant, we see spectacular meteor showers.

On average, about 17 of these are big enough for some bits and pieces to reach the ground every day. Once in a while, there could be a really large rock that hits with substantial force. If it’s big enough, such a collision could trigger instant mass extinction, or at the least, catastrophic loss of life.

Asteroids and comets are classified as potentially hazardous objects, or PHOs, if they are 30-50-metre in diameter or larger, and their orbits bring them to within 8 million km of Earth’s orbit. We have no defences against collisions with such PHOs.  

Human beings became a dominant species because such an incident occurred about 66 million years ago. A meteor estimated to be 10 km in diameter crashed into what is now the Yucatán Peninsula in Mexico. That impact created a crater 180 km across and 2 km deep, and triggered tsunamis and earthquakes.

Dust and debris in the atmosphere cut off sunlight and probably damaged the ozone layer. This led to a sudden sharp drop in global temperature, causing a winter that lasted years. This meant mass extinction.

Plant life (which generally depends on photosynthesis) and reptilian dinosaurs (reptiles depend on external heat to keep them warm) died. Small mammals with their own internal heating systems survived, and evolved to take over empty ecological niches.

The Earth remains vulnerable to a repeat. We may have dodged a celestial bullet in June 1908, when something flattened trees across more than 2,000 square km in an uninhabited part of Siberia. The Tunguska incident, as it’s known, caused the kind of devastation equivalent to a 12 megaton nuclear explosion. (Hiroshima and Nagasaki atomic attacks were in the 2 megaton range.)

How DART hit its target

DART has multiple components. The smaller member of the double asteroid system is Dimorphos, which is about 160 metres in diameter. Dimorphos orbits its larger brother Didymos (780-metre diameter), and this dual system orbits the sun.

DART had to navigate 11 million km to reach the system, and change course when it was four hours away to ensure it hit Dimorphos.

The 19-metre, 550-kg spacecraft crashed into Dimorphos (estimated mass of 5 billion kg) at a speed of over 6 km per second. This caused a big crater – studies of the debris ejected will give us a clearer picture of the composition of the rock.

Despite differences in mass, the kinetic energy transferred in the crash would, by Newton’s Third Law, lead to a small change in the asteroid’s orbit around Didymos. In turn, that should lead to disturbances in the double asteroid system’s orbit around the sun.

Before the collision, DART released Liciacube, a tiny cube satellite, to observe this event closely. Everything from the James Webb Space Telescope to the Hubble and Lucy will be watching through the next several weeks to see how effective the impact was.

The mission uses a lot of untested technology, and it will be followed up in 2024 by another even more ambitious mission to do an in-depth study of the system.

If DART works as planned, it could be tied into global asteroid watch programmes such as those run by NASA’s Center for Near-Earth Object Studies.

Assuming we pick up PHOs quickly enough, we may be able to mitigate impact risks. This would mean launching many months in advance, and if it’s an even larger object, you may need to use explosives or nuclear devices. But that is better than waiting with fingers crossed if a large asteroid looks to be on a collision course.

Harmless or hazardous: Know your meteors

• Hundreds of meteors slice into the Earth’s orbit on a daily basis. Most burn up harmlessly
• On average, about 17 of these are big enough for some bits and pieces to reach the ground every day
• Asteroids and comets are classified as potentially hazardous objects, or PHOs, if they are 30-50-metre in diameter or larger, and their orbits bring them to within 8 million km of Earth’s orbit
• We have no defences against collisions with PHOs

Tunguska event

• June 1908: Something flattened trees across more than 2,000 sq km in an uninhabited part of Siberia
• The event caused devastation equivalent to a 12-megaton nuclear explosion. (Hiroshima and Nagasaki atomic attacks were in the 2-megaton range)