You might have spotted the story in the media yesterday about an asteroid called Nereus which makes a close approach to the Earth today. \u00a0It’s described as the size of the Eiffel Tower and will zip past us pretty close, coming within 3.9 million kilometres. \u00a0Are we all doomed? \u00a0Of course not. \u00a0Here’s why.<\/p>\n
Asteroid Nereus (or to give it its full designation, asteroid 4660 Nereus<\/a>) is a lump of rock about 330 metres in diameter. \u00a0It was discovered in 1982<\/a>, about a month after it passed within 4.1 million kilometres of the Earth. \u00a0It’s not spherical, more egg shaped, according to radar observations. \u00a0In late 2001\/early 2002, astronomers used a radio telescope at\u00a0Goldstone<\/a>\u00a0to send radio waves at the asteroid during one of its previous close approaches, using the reflected signals to measure its size and shape. \u00a0They found<\/a> that it has dimensions of \u00a0510 by 330 \u00a0by 241 metres, and it rotates on its own axis every 15 hours or so.<\/p>\n There are\u00a0actually\u00a0many thousands of lumps of rock this size in the inner solar system. \u00a0None of them are known to be on a direct collision course with the Earth, but we haven’t found all of them yet – not by a long way. \u00a0The current count of known asteroids is\u00a01,113,527 (according to this NASA page<\/a>), and this number is increasing all the time as we find more of them. \u00a0Some of these space rocks are large and easy to spot from Earth, but most are much smaller and are quite hard to find.<\/p>\n The problem with hunting for asteroids is that they are mostly (a) small, and (b) made of rock. \u00a0Small things reflect less sunlight, so they are fainter and need a bigger telescope to spot, and things made of rock tend not to be terribly reflective. \u00a0You’ve probably seen snow on mountains – the snow is much brighter than the rocky areas because white things reflect more sunlight than black\/brown things.<\/p>\n Nereus is quite reflective<\/a> for a rocky asteroid, but still tricky to spot because of its small size. \u00a0One estimate puts its maximum apparent magnitude (how bright it will appear to an observer on the Earth) at 12.6 which is pretty faint. \u00a0If you are lucky enough to live somewhere with dark skies, you can probably see stars an faint as about magnitude 6. \u00a0[This is one of those annoying astronomical measurements that doesn’t make intuitive sense – bigger numbers relate to fainter objects, so the Sun has an apparent magnitude of -26.74 while faint distant galaxies can have magnitudes of +20 or more. \u00a0The scale is also logarithmic as well, which \u00a0means that a difference of one magnitude is actually a factor of 2.5 in brightness.] \u00a0At its brightest, at closest approach, Nereus will be magnitude 12.6 which is more than 400 times fainter than the faintest stars you can see unaided.<\/p>\n Now, most of these floating space rocks are never likely to cause us a problem. \u00a0They orbit the Sun in the asteroid belt<\/a>, a region between Mars and Jupiter where there is a concentration of asteroids. \u00a0Not all of them are in the asteroid belt though, many have elliptical orbits around the Sun, rather than the almost circular orbit of the Earth, and sometimes those orbits can bring an asteroid close to the Earth. \u00a0 (The trick is to spot them coming!)<\/p>\n Nereus has an elliptical orbit<\/a> within the inner solar system, taking 1.8 years to complete each orbit. \u00a0This orbit causes it to pass close to the Earth from time to time, but it also comes pretty close to Mars as well. \u00a0This actually makes it an excellent candidate for a sample return mission, and it was originally one of the candidates for the Hayabusa mission<\/a> – but a delay meant that probe ended up visiting asteroid Itokawa<\/a> instead.<\/p>\n OK, that’s the science background. \u00a0Should we worry about it coming so close?<\/p>\n Well, first some perspective. \u00a0At its closest on this occasion, Nereus will be 3.9 million kilometres from the Earth – that’s about ten times the distance between the Earth and the Moon. \u00a0Just on that count, we don’t really have anything to worry about. \u00a0This distance may be small when you compare it to the size of the entire solar system, but it’s still a long way. \u00a0Nereus will actually come even closer in the future; in 2060 its orbital path will bring it within 1.2 million kilometres of Earth. \u00a0That’s three times the Earth-Moon distance, so still not a threat. \u00a0If you are interested, here are the predictions<\/a> for future encounters (including a close approach to Mars in 2089.<\/p>\n So no, we don’t need to worry.<\/p>\n I’ve talked about other asteroids though, and how the inner solar system contains an awful lot of them. \u00a0If we didn’t spot this one until it was already a month past a close approach, how easy might we miss another one that might come closer? \u00a0Well this is a risk.<\/p>\n In planetary terms, an asteroid impact could potentially do a lot of damage. \u00a0How much damage depends on the size of the impactor, the relative velocity of the asteroid and the Earth, and what the asteroid is made of. \u00a0There are lots of impact simulators around, but this one<\/a> suggests that a collision with a 330-m object on land would result in a crater more than 4 km in diameter, and an earthquake of magnitude 7.8. \u00a0Their estimate is that\u00a0an\u00a0impact on this scale occurs on Earth (statistically) every 18,000 years.<\/p>\n There are plenty of efforts ongoing to find these asteroids and determine their orbits<\/a>. \u00a0Many telescopes do this sort of work, and it’s something the Vera C. Rubin Observatory<\/a> will be able to do really well (if the light from all the satellites being sent into low Earth orbit doesn’t cause too much of a problem… but that’s a topic for another day).<\/p>\n If we do find an asteroid that could impact the Earth, we need to have some way of dealing with it. \u00a0We can’t rely on Bruce Willis<\/a>,\u00a0so\u00a0just last month NASA launched the DART mission<\/a> to the binary asteroid system Didymos and its smaller companion\u00a0Dimorphos. \u00a0The idea with DART is to test planetary defence techniques by, well, literally smashing a 500 kg projectile into Dimorphos at ~6.6 km\/s (~15,000 mph) and watching how its orbital path changes<\/a>.<\/p>\n The physics is simple (its just conservation of momentum) and, if all goes well, following the impact in October 2022 we should be able to observe a small change in the orbital period of Dimorphos. \u00a0It’s not the first time humans have visited as asteroid, and it’s not the first time we’ve deliberately bashed into one<\/a>, but it is the first time we’ve actively tried to alter the trajectory of a solar system object. \u00a0If we do spot something dangerous headed our way, it makes sense to know how to make it less of a threat!<\/p>\n