Deflecting an Asteroid with a DARTFollow article
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Artist’s impression of the DART spacecraft speeding towards its destiny: collision with Dimorphos, a ‘moonlet’ of the near-Earth asteroid Didymos. On the right, a tiny CubeSat called LICIAcube, released just before impact, will observe the collision.
Image credit: NASA
Asteroid, Meteor, Meteoroid or Meteorite?
When it comes to bits of rock floating about in Space, heading towards or even impacting the Earth, these four terms seem to be used interchangeably by most people (except astronomers and scientists of course). Here is a rough guide:
An asteroid is a lump of rock, perhaps left over from the planet-formation phase of solar system construction billions of years ago. Most of them are spread out in an orbit around the sun between Mars and Jupiter. Sizes range from a few metres to hundreds of kilometres; the really large ones create enough gravity to pull themselves into a spherical shape and may be classified as dwarf planets like Ceres. At about 800m and 160m across respectively, Didymos and its ‘moon’ Dimorphos, are rather more modest knobbly lumps of rock.
The next three terms apply to bits of asteroids or comets that have been deflected onto a collision course with our planet, usually as a result of collisions.
So, a Meteoroid is a rock heading towards Earth, after the course-change but before it enters the atmosphere.
On entering the atmosphere, the rock becomes a Meteor. At this point I should say that thankfully, the vast majority of meteors are about the size of sand grains and ‘burn-up’ or vaporise, never reaching the ground. They are seen as brief streaks of light in the sky, often in predictable ‘meteor showers’ as the Earth passes through the debris trail of a comet orbiting the sun. These showers have names such as the Perseids and Leonids, according to where they ‘radiate’ from in the sky. Large meteors up to a metre or so in size will produce a much more spectacular display, not just fizzling out, but exploding in a large fireball. Small pieces that do reach the ground are called Meteorites. On the night of 28th February 2021 such a fireball was tracked across the UK by a dedicated network of cameras as well as many doorbell cameras and car dashcams. The village of Winchcombe was determined to be the likely impact site for any fragments that survived and sure enough, a sizeable chunk weighing 300 grams was discovered on the driveway of a family home. That was a bit of luck: the Japanese space agency (JAXA) managed to get 4.5 grams of material from a distant asteroid last year thanks to the Hayabusa-2 spacecraft. NASA’s OSIRIS-REx probe is due back in 2023 with a whole 60 grams. Why bother with these very expensive Sample-Return missions when chunks of asteroid can be picked up on Earth for free? It could also be asked: why the interest in the smallest of heavenly bodies anyway? In answer to the first question, in spite of an estimated 80,000 tons of meteorite debris reaching the Earth’s surface every year, individual particles are hard to find – because most are tiny. The best place to look for them is Antarctica, simply because they are highly visible against the snow! The answer to the second question is that until fairly recently, we were blissfully unaware of the truck-sized and larger asteroids in orbits that frequently see them passing uncomfortably close to our vulnerable planet. If one of those was deflected towards us, the result would be a catastrophic, perhaps extinction level event (ELE). And there have been such impacts in the past – long before modern humans evolved though.
Meteorite craters rarely remain visible on Earth, thanks to weather erosion; one big exception being the famous Meteor Crater in Arizona USA. It has survived because of the arid conditions that have remained constant over the 50,000 years since it came down. The iron rock that excavated this 1,186m diameter, 170m deep hole was about 50m across and generated an explosive force equivalent to 10Mtons of TNT. In other words, that of a large H-bomb. Fortunately, there was nobody around at the time. But what if another like it crashed into a large city today?
Meteor (Barringer) Crater, Arizona USA. Image credit: USGS
Meteor Crater was only recognised for what it was in 1891. If anyone back then began thinking about the implications, they didn’t spend too much time worrying about them. After all, what could be done?
Now, thanks to satellite imaging, ghosts of ancient impacts are being discovered all over the planet. One in particular, the Chicxulub crater buried under the Yucatán Peninsula in Mexico is associated with a well-known extinction level event – the wiping out of the dinosaurs 66 million years ago. Given the 150km diameter of the crater with a depth of 20km, the meteorite is estimated to have been about 10km wide.
The threat of an Earth-destroying asteroid collision has proved irresistible to disaster movie makers, the most in(famous) being the imaginatively-titled Meteor (1979). A comet collides with an asteroid in the asteroid belt and a huge fragment is sent hurtling towards the Earth. The only solution is to destroy it with re-purposed nuclear missiles. Most of the rocket science that follows is complete bilge, but somebody must have mentioned that ground-based ICBMs do not have the power to achieve Earth orbit let alone reach escape velocity and head towards a distant target (the clue is in B for Ballistic Missile). They got around that by having both the USA and USSR possess secret (and invisible) orbiting missile platforms. Groan. The movie ends with the asteroid being vaporised leaving no nasty fragments flying towards Earth. In reality, there are three possible outcomes, vaporisation not being one of them:
- The asteroid is impacted head-on, splitting it into two or more large fragments. Plus, loads of smaller ones. Life on Earth may or may not survive the blizzard of meteorites.
- The impact splits it in two, each half passing safely on either side of the Earth. It’s the solution proposed in the movie Armageddon (1998), except astronauts have to detonate H-bombs placed at the bottom of deep shafts they’ve just drilled in the asteroid. Deep Impact (1998) combines the elements of both other movies: ICBMs which fail and an H-bomb carried deep underground which succeeds. Both movies require the astronauts to die heroically in suicide missions.
- The impact changes its course just enough to avoid the Earth. At last, an approach that might just work. Too boring for Hollywood though.
The problem is, nobody in the movie business ever bothered to ask just how much force would be required to ‘destroy’ an asteroid or even just split it in two. Because of all the Cold War rhetoric surrounding the destructive power of nuclear bombs, we all came to see the hydrogen bomb as the ‘ultimate weapon’ capable of reducing anything to rubble. So naturally it could vaporise an asteroid just a few kilometres across. However, we also knew that the Earth itself seemed completely unaffected by even the largest explosions. If you weren’t present at the impact site of a large meteorite yourself, you might have heard about the appalling carnage on the news, and then forgot about it. Just because H-bomb test explosions didn’t cause great cracks to appear in the Earth’s crust followed by whole chunks of land flying off into Space, didn’t mean our lives were unaffected. Before they were moved underground, bomb tests created vast amounts of irradiated dust, ’fall-out’, which once spread around the world by winds, rendered everything ‘radio-active’ to some extent. The massive impact at Chicxulub didn’t break the world either; it just created a vast cloud of dust that blotted out the Sun for years. The dinosaurs died out as a result. If it happened again, would humankind be able to survive this ‘Nuclear Winter’?
Can an asteroid be deflected?
If the threat to life on Earth cannot be eliminated by vaporising or breaking-up a wayward asteroid, can its course be changed? NASA’s Double Asteroid Redirection Test (DART) mission to asteroid moon Dimorphos aims to find out. Didymos and its moon orbit the sun on an elliptical path taking them from the asteroid belt to a point just outside the Earth’s orbit and back in just over two years. Didymos is not a threat, but it is ideal for testing the feasibility of a kinetic orbit change. However, it’s not Didymos that will receive the impact from DART. Given all the unknowns, there would be a small risk of moving the 780m rock onto a collision path with Earth. A somewhat embarrassing test outcome to say the least. The idea is to change the orbit of the 160m moon around Didymos instead. That way, the worst outcome would see Dimorphos attempting an orbit 0m above its parent’s surface. If all goes according to plan, the head-on impact at 6.6km/sec will be equivalent of a spacecraft in Earth orbit firing its retro-thrusters to take it into a lower orbit or return all the way down to the surface (Fig.1).
The scientists have done all their calculations and have determined that the collision will decrease the speed of the moonlet in its orbit around the main body by less than one percent, changing the orbital period of the moonlet by several minutes. This should be observable using telescopes and planetary radar on Earth with the actual event recorded by the accompanying CubeSat LICIAcube. The DART spacecraft will intercept Dimorphos in September 2022, when the Didymos system is within 11 million kilometres of Earth.
The observation of a shift in Dimorphos’ orbit will at least prove that a deflection is possible, but a lot more data will need to be gathered and analysed before a viable defence weapon can be built. For a start, no two asteroids are alike in shape, mass and composition; any defence will have to handle all variations. Composition could be the toughest unknown to crack. It’s tempting to assume that an asteroid is just a big slab of solid rock, broken off in some cataclysmic planetary collision, and many may be just that. On the other hand, it could just be a pile of dust and larger particles created and held loosely together by its own weak gravity. The asteroid belt could consist of leftover bits (rockpiles) from the formation of the solar system, or the remains of a destroyed planet. The solid asteroid may be the easier of the two types to deflect reliably; like hitting a single snooker ball. Hitting a rockpile might be analogous to the opening break in a game: balls scattered in all directions. That’s why so much data is needed. Another spacecraft, this time from the European Space Agency, called Hera will be sent to meet Didymos and carry out a ‘Crime Scene Investigation’. Hera will head out in November 2024 and arrive on site in late 2026.
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