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EXPLAINED: How Nasa's DART Will Test ‘Nudge Theory’ For Dealing With Dangerous Asteroids

The Nasa DART spacecraft will be deliberately crashed into an asteroid to see if the impact succeeds in making a slight alteration to its course (Image for representation/Shutterstock)

The Nasa DART spacecraft will be deliberately crashed into an asteroid to see if the impact succeeds in making a slight alteration to its course (Image for representation/Shutterstock)

There are no known asteroids that are on a collision course with Earth, but Nasa wants to be ready for such an eventuality should it ever arise

Protecting our planet from a cosmic danger should require nothing short of planetary defenders moving heaven and earth. Well, what US space agency Nasa has set out to do would amount only to a small nudge to an asteroid, but it sure will be a huge step towards testing the ability of humans to counter the kind of threat that had wiped out the mighty dinosaurs from the face of the planet. While there are no asteroids known at present to scientists that are on collision course with Earth, Nasa clearly sees no harm in staying prepared.

What Is The DART Mission?

Short for ‘Double Asteroid Redirection Test’, it is a mission that the space agency has launched to collide a spacecraft with an asteroid to see if the impact succeeds in making a small alteration in its trajectory. However small, such an alteration, the project scientists hope, would be enough to deflect an asteroid off a collision course to Earth should one ever be found to be headed our way.

The target of the DART spacecraft is a small asteroid called Dimorphos — Nasa describes it as a ‘moonlet’ — that revolves around a larger companion asteroid called Didymos. While Didymos is about 780m across, Dimorphos, at 160m, “is more typical of the size of asteroids that could pose the most likely significant threat to Earth", Nasa said of the binary near-Earth asteroid system being used for the DART demonstration. It has stressed repeatedely though that these asteroids do not actually pose any threat to Earth, but are just the target for an experimental exercise.

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Nasa says that Near-Earth Objects (NEOs) are any comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to veer into the Earth’s neighbourhood. Nasa has clarified that no known asteroid larger than 140m in size “has a significant chance to hit Earth for the next 100 years", adding that “only about 40 per cent of those asteroids have been found as of October 2021".

How Will DART Deflect An Asteroid Off Its Path?

The DART spacecraft, which is set for liftoff on a SpaceX rocket with the launch window scheduled for November 23, is going to be intentionally crashed into Dimorphos with a view to slightly changing its orbit. Nasa hopes that DART’s “kinetic impact will prove that a spacecraft can autonomously navigate to a target asteroid and kinetically impact it".

What is kinetic impact, you may want to know. It appears its a technical term for the simple principle of hitting something hard enough to knock it off its set path. Nasa hopes that the demonstration will pave the way for “asteroid kinetic impactor computer models" that provide insights as to how “we could deflect potentially dangerous near-Earth objects in the future".

“DART will be the first demonstration of the ‘kinetic impactor’ technique in which a spacecraft deliberately collides with a known asteroid at high speed to change the asteroid’s motion in space," said Lindley Johnson, Nasa’s Planetary Defence Officer, adding that it is “thought to be the most technologically mature approach for mitigating a potentially hazardous asteroid".

Nasa said the DART spacecraft will seek to achieve the intended kinetic impact by crashing into Dimorphos at a speed of approximately 6.6 km/s or more than 23,000kmph “with the aid of an onboard camera (named DRACO, which incidentally is the only equipment on board the spacecraft whose fate it is to self-destruct) and sophisticated autonomous navigation software".

The collision should be able to change Dimorphos’ speed around its larger companion by a fraction of one per cent, Nasa said, adding that “this will change the orbital period of the moonlet by several minutes — enough to be observed and measured using telescopes on Earth". The DART spacecraft will need to travel for close to a year before it slams into Dimorphos some time in late September 2022. The twin asteroid system would be within 11 million kilometers of Earth at the time and be observable by ground-based telescopes and planetary radar.

What Is An Asteroid? Is It Different From Meteors And Meteoroids?

In terms of size, asteroids are the biggest of the space rocks orbiting around the Sun that are not classified as either a planet or a satellite. However, asteroids can range in size from a few metres wide to hundreds of kilometres in diameter. One such is Ceres, which has a diameter of more than 900km and is big enough to be classified as a dwarf planet. Ceres lies in the same area of the solar system where most of the other asteroids are found. Known as the ‘asteroid belt’, it is a region between Mars and Jupiter. However, US space agency Nasa says that asteroids can also be found in other parts of the solar system and “some asteroids orbit the Sun in a path that takes them near Earth".

Meteoroids are nothing but much smaller pieces of space rock and are described as “pebble-sized objects" formed when one asteroid collides with another and shatters. Scientists say that meteoroids can also come from comets.

A meteor is not so much an object but the phenomenon that is witnessed when a meteoroid enters the Earth’s atmosphere. Friction with the Earth’s atmosphere causes a meteoroid to burn up. The streak of light that is thus formed as the meteoroid vaporises is called a meteor. These are also referred to as a “shooting star" although, of course, they are not stars. Asteroids, and meteoroids, are mainly made of rock although some may be entirely composed of a metal like nickel or iron.

Then comes the meteorite. If a meteoroid makes the journey through the Earth’s atmosphere all the way to its surface, then its remnant is known as a meteorite.

What’s The Danger Posed By These Space Rocks?

Nasa says that on a daily basis Earth is “bombarded with more than 100 tonnes of dust and sand-sized particles" from outer space. Given their size, they burn up even before they hit the surface. Any space rock smaller than about 25 metres in diameter is most likely to burn up as it enters the Earth’s atmosphere, causing little or no damage. If a meteoroid is larger than 25 metres but smaller than 1km, it can be expected to cause local damage in the impact area. Nasa says that any meteorite “larger than 1-2km could have worldwide effects".

Most space rocks “smaller than a football field will break apart in Earth’s atmosphere… (and) typically less than 5 per cent of the original object will ever make it down to the ground", Nasa says. But it notes that if a large rock were to break through, it could have a devastating impact. Scientists say that an asteroid impact 65 million years ago — it created the 300km-wide Chicxulub Crater buried under the Yucatan peninsula in Mexico — led to the extinction of about 75 per cent of marine and land animals on Earth, including the dinosaurs.

What Is The System To Protect Against Asteroid Collision?

Since 1998, Nasa has been tracking and listing asteroids as part of its Near-Earth Object Observations Programme. By 2019, it had found more than 19,000 near-Earth asteroids with an average of 30 new discoveries said to be added each week. It says that about half of the near-Earth asteroids are larger than 140 metres in size. This threshold was determined as indicating the objects that can produce “regional effects". Anything larger would trigger “sub-global effects", it said.

Experts say that an event with an impact at the scale of the Chelyabinsk meteorite — which blew up more than 20km above Earth’s surface in skies over Russia and released the energy equivalent of around 400-500 kilo tonnes of TNT (the atom bomb dropped on Hiroshima during World War II was of 15 kilo tonnes) — takes place once or twice a century while the span for anything larger hitting Earth is once in centuries to millennia.

However, if an asteroid were headed to Earth, it “could not be shot down in the last few minutes or even hours before impact" given “the velocity at which it travels – an average of 12 miles (close to 20 km) per second". In fact, Nancy Chabot, a planetary scientist and the DART coordination lead at Johns Hopkins Applied Physics Laboratory said told a news conference that “you’d want to do this technique many years in advance, decades in advance" if it can be reliably found out that an ateroid was coming to hit Earth.

“You would just give this asteroid a small nudge, which would add up to a big change in its future position, and then the asteroid and the Earth wouldn’t be on the collision course," she said.

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first published:November 10, 2021, 08:41 IST