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## First ever asteroid from another solar system detected

In late October 2017, astronomers announced the first ever discovery of an asteroid (or comet?) coming into our Solar System from another stellar system. The object was first spotted on 19 October by the University of Hawaii’s Pan-STARRS telescope, during its nightly search for near-earth objects. Based on its extreme orbit and its rapid speed, it was soon determined that the object has come into our Solar System from somewhere else, and this makes it the first ever asteroid/comet with an extra-solar origin to have been discovered. Originally given the designation A/2017 U1, the International Astronomical Union (IAU) have now renamed it 1I/2017 U1, with the I standing for “interstellar”.

The object, given the designation A/2017 U1, was deemed to be extra-solar in origin from an analysis of its motion.

In addition to its strange trajectory, observations suggest that the object also has quite an unusual shape. It is very elongated, being ten times longer than it is wide. It is thought to be at least 400 metres long but only about 40 metres wide. This was determined by the rapid and dramatic changes in its brightness, which can only be explained by an elongated object tumbling rapidly.

The object has also been given the name Oumuamua (pronounced oh MOO-uh MOO-uh), although this is not its official name (yet).  This means “a messenger from afar arriving first” in Hawaiian. In other respects, it seems to be very much like asteroids found in our own Solar System, and is the confirmation of what astronomers have long suspected, that small objects which formed around other stars can end up wandering through space, not attached to any particular stellar system.

## Has Armageddon arrived?

Yesterday an extraordinary coincidence happened. As I blogged about here, a 50m long lump of rock (asteroid 2012 DA14) was scheduled to zip past Earth yesterday evening (15th of February) at some 12 kilometres per second at a distance of “only” 28,000 km. Although this sounds like a lot, it is closer than any asteroid that size has come to Earth to our knowledge for many decades.

But yesterday I awoke to the news that another lump of rock had exploded in the air over an area in the Ural mountains in southern Russia near Kazakhstan. Initial reports were that about 500 people were injured, this figure had risen to closer to 1,000 by the end of the afternoon.

Of course some people wondered whether these two events were connected. Well, I can assure you they were not. Earth is constantly being hit by particles from space, as we whizz around the Sun at 30 kilometres per second. Every day some 100 metric tonnes (a tonne is 1,000 kgs) of material enters the Earth’s atmosphere. Most of these are sand-grain sized particles, with the number of larger and larger objects becoming fewer and fewer. It probably follows what we call a power law size distribution, in much the same way as the interstellar dust grains I studied for my PhD do. In fact, at the very small level the particles are interstellar dust grains, it’s only when they become larger that we start referring to them as asteroids or lumps of rock.

What this power-law size distribution means is that larger lumps are rarer and rarer. NASA has calculated the size of the lump which exploded over Russia yesterday to be about 15m, and the damage gives an indication of what something this size entering the Earth’s atmosphere can do. To my memory, this is the first time I know of a meteor explosion causing human casualties. The 50m sized asteroid which thankfully missed Earth last night would have caused far more damage, enough to wipe out a large city. We believe 50m sized asteroids hit the Earth about once every 1,200 years, larger ones even less often. And, asteorids a few kilometres in size, large enough to cause widespread devastation and even lead to global catastrophies seem to happen as rarely as once every few hundred millon years.

The Earth will be hit by an asteorid large enough to cause global devastation, of that I have little doubt. But the chances of it happening when humanity still exists is very remote in my opinion. And, as long as we are vigilant whilst we still exist as a species, there is much that can be done to deflect such large asteorids which are on a collision course for Earth. Unlike the portrayals in movies, probably what we do not want to do is send a missle to destroy a threatening asteroid. Rather, we want to gently nudge its orbit into one which will miss the Earth.

Gently nudging an asteroid so it will miss Earth can be done in a number of different ways, but possibly the most elegant is just to paint one side of it white. This will cause the Sun’s light to reflect off of that side more than the rest of the asteroid, and we can use the fact that sunlight has momentum to push the asteroid using sunlight!

## The MRN size distribution

The most popular interstellar dust size-distribution used by astrophysicists is one due to Mathis, Rumpl and Nordsieck (MRN), which they published in 1977. It suggests that the number of dust grains of various sizes follows the following distribution – $n(a) = n_{0}a^{-3.5}$ where $n_{0}$ is the number at some size $a_{0}$. So, for example, if you want to compare the number of dust grains which have a size of 1mm compared to the number which have a size of 5mm then the ratio is $\frac{1}{5}^{-3.5} = 280$ or that 1mm sized dust grains are 280 times more numerous than 5mm sized dust grains. It is unlikely that the distribution of lumps of rock follows exactly the same size distribution, but if it did the number of 15m lumps compared to 50m lumps would be $\frac{15}{50}^{-3.5} = 68$, so 68 times more 15m lumps than 50m lumps.