Analysis: the DART probe is part of a mission to see if future collisions between asteroids and Earth can be stopped

Just after midnight on September 27th next, NASA will crash a $300 million space probe called DART into a small asteroid called Dimorphos some 11 million kilometres from Earth. So why would the world's leading space agency build such an expensive probe, only to then crash it intentionally into a nearby celestial object?

The cosmic context

While we live on surely the most clement of the planets in our solar system, an estimated two trillion other worldlets are also orbiting the Sun. These range in size 100-metres to thousands of kilometres wide, with countless trillions more even smaller. They are organised into belts around the Sun such as the Asteroid belt between Mars and Jupiter, the Edgeworth-Kuiper belt beyond Neptune and the vast Oort cloud extending out to one light year;

Another category is the Near-Earth objects (NEOs) which orbit the Sun close to Earth. It is estimated that there may be 500 million NEOs ranging from 5m to 10km. While the vast majority are tiny, there are likely to be five million objects greater than 25m wide, tens of thousands greater than 140m across and upwards of one thousand 1km or more across.

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From RTÉ News, NASA launches DART spacecraft to kick asteroid off course

Are these asteroids harmless?

Collisions between large asteroids and the Earth are extremely rare, but they do occur and have radically altered Earth's surface throughout its history. A 10km wide object impacts Earth every 100 million years, as with the Chicxulub asteroid impact 65 million years ago that wiped out the dinosaurs.

But it isn't just such large objects that are of concern. We're currently tracking more than 26,000 NEOs 140m or larger in diameter, with thousands more still to be discovered. A single 140m object impact may only occur over thousands of years, but such a collision could wipe out a city. Even objects as small as 20m - of which there five million orbiting near Earth - can wreak havoc; an 18m wide asteroid exploded in the atmosphere in 1908, flattening 2000 square-kilometres of forest in the Tunguska region of Russia.

Overall, both NEOs and larger asteroids and comets are of concern because all maintain unpredictable orbits perturbed by the larger planets. Our fate in this regard, then, is not in our own hands.

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From RTÉ Radio 1's Brendan O'Connor Show, Frances McCarthy from the Blackrock Observatory in Cork talks about the Bernardinelli Bernstein Comet, the asteroid that hit the earth 66 million years ago annihilating dinosaurs and what causes solar flares

Planetary Defence

However, this may be changing. Thanks to dedicated astronomers who have tracked thousands of NEOs for decades, the likes of NASA and ESA now recognise the possibility of a strategy of Planetary Defence that might allow us to prohibit future collisions.

Firstly, we must detect every NEO down to 20m. As fantastical as that might sound, our latest telescopes can detect all such NEOs. Even now, NASA's Jet Propulsion Laboratory maintains a Sentry Risk Table of thousands of objects potentially of threat. We are also beginning to figure out how, with sufficient notice, we might avert a collision by deflecting any such object off its trajectory with Earth.

Here comes the DART (not that one)

Of course, it's unrealistic to think we can deflect a large asteroid at this juncture, but we need to test this idea and we have found the perfect candidate. It's a tiny double-asteroid system comprised of a 500-million tonne, 780m-wide asteroid called Didymos and its 170m, 5-million tonne moon called Dimorphos, which orbits Didymos at a distance of just 1km every 11.9 days. With our telescopes, we see both as a tiny spec that varies in brightness every 11.9 days.

The size of Didymos compared to well-known artefacts on Earth.
Image: NASA/JHUAOA

What NASA proposes is a space probe, called the Double Asteroid Redirection Test (DART), to crash into Dimorphos at 24,000 km/hour to change its orbit around Didymos. Both orbit the Sun close to Earth every 770 days, with both coming within a safe 11 million km of Earth on September 27th next.

DART is now on route to those tiny worlds and, if all goes according to plan, will crash into Dimorphos at 00.17hrs on September 27th, nudging it closer to Didymos and reducing its orbital period to 11.8 days, which we should see with our telescopes. There is no danger to Earth, but a successful mission will verify that we can move the paths of asteroids, paving the way for a strategy of Planetary Defence from future impacts.

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From NASA, coverage of the DART mission launch in November 2021

DART is carrying a suite of advanced technologies too such as autonomous navigation, experimental ion engines and ultra-efficient solar panels. These will be used to test new technologies for use in space before its inevitable fate.

Also accompanying DART is a tiny Italian CubeSat called LICIS that will trail DART by just three minutes and transmit images back to Earth of the impact, the resulting plume and the formation of a new crater. This means we are not just reliant on our telescopes to make precise measurements, but will witness this intriguing collision within minutes of it occurring. If successful, DART will herald a new era of Planetary Defence to protect the future of Earth from even the most deadly of celestial collisions.


The views expressed here are those of the author and do not represent or reflect the views of RTÉ