You might have spotted the story in the media yesterday about an asteroid called Nereus which makes a close approach to the Earth today.  It’s described as the size of the Eiffel Tower and will zip past us pretty close, coming within 3.9 million kilometres.  Are we all doomed?  Of course not.  Here’s why.

Asteroid Nereus (or to give it its full designation, asteroid 4660 Nereus) is a lump of rock about 330 metres in diameter.  It was discovered in 1982, about a month after it passed within 4.1 million kilometres of the Earth.  It’s not spherical, more egg shaped, according to radar observations.  In late 2001/early 2002, astronomers used a radio telescope at Goldstone to send radio waves at the asteroid during one of its previous close approaches, using the reflected signals to measure its size and shape.  They found that it has dimensions of  510 by 330  by 241 metres, and it rotates on its own axis every 15 hours or so.

There are actually many thousands of lumps of rock this size in the inner solar system.  None 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.  The current count of known asteroids is 1,113,527 (according to this NASA page), and this number is increasing all the time as we find more of them.  Some of these space rocks are large and easy to spot from Earth, but most are much smaller and are quite hard to find.

The problem with hunting for asteroids is that they are mostly (a) small, and (b) made of rock.  Small 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.  You’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.

Nereus is quite reflective for a rocky asteroid, but still tricky to spot because of its small size.  One estimate puts its maximum apparent magnitude (how bright it will appear to an observer on the Earth) at 12.6 which is pretty faint.  If you are lucky enough to live somewhere with dark skies, you can probably see stars an faint as about magnitude 6.  [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.  The scale is also logarithmic as well, which  means that a difference of one magnitude is actually a factor of 2.5 in brightness.]  At 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.

Now, most of these floating space rocks are never likely to cause us a problem.  They orbit the Sun in the asteroid belt, a region between Mars and Jupiter where there is a concentration of asteroids.  Not 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.   (The trick is to spot them coming!)

Nereus has an elliptical orbit within the inner solar system, taking 1.8 years to complete each orbit.  This orbit causes it to pass close to the Earth from time to time, but it also comes pretty close to Mars as well.  This actually makes it an excellent candidate for a sample return mission, and it was originally one of the candidates for the Hayabusa mission – but a delay meant that probe ended up visiting asteroid Itokawa instead.

OK, that’s the science background.  Should we worry about it coming so close?

Well, first some perspective.  At 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.  Just on that count, we don’t really have anything to worry about.  This distance may be small when you compare it to the size of the entire solar system, but it’s still a long way.  Nereus will actually come even closer in the future; in 2060 its orbital path will bring it within 1.2 million kilometres of Earth.  That’s three times the Earth-Moon distance, so still not a threat.  If you are interested, here are the predictions for future encounters (including a close approach to Mars in 2089.

So no, we don’t need to worry.

I’ve talked about other asteroids though, and how the inner solar system contains an awful lot of them.  If 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?  Well this is a risk.

In planetary terms, an asteroid impact could potentially do a lot of damage.  How 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.  There are lots of impact simulators around, but this one 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.  Their estimate is that an impact on this scale occurs on Earth (statistically) every 18,000 years.

There are plenty of efforts ongoing to find these asteroids and determine their orbits.  Many telescopes do this sort of work, and it’s something the Vera C. Rubin Observatory 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).

If we do find an asteroid that could impact the Earth, we need to have some way of dealing with it.  We can’t rely on Bruce Willis, so just last month NASA launched the DART mission to the binary asteroid system Didymos and its smaller companion Dimorphos.  The 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.

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.  It’s not the first time humans have visited as asteroid, and it’s not the first time we’ve deliberately bashed into one, but it is the first time we’ve actively tried to alter the trajectory of a solar system object.  If we do spot something dangerous headed our way, it makes sense to know how to make it less of a threat!

So, should we worry about Nereus?  No.  Categorically no.  It’s not a threat.  Should we worry about something we haven’t spotted yet hitting us in the future?  Well, it’s unlikely, but we really ought to be expending some resources to look.  It’s in our own interest as a species, after all.

Don’t have nightmares.