When the LINEAR Telescope discovered a faint object of 18th magnitude, on January 13th 1999, nobody suspected that this would become one of the most commented objects for many years. Within days of its discovery 1999 AN10 was already being flagged as an object which could make particularly close approaches to the Earth.
In 1999 the closest approach was at the very safe distance of 0.1948AU (29.2 million kilometers), on the afternoon of February 11th. In the future, however, the distance may be very much less, with a potentially zero miss distance if an encounter happens in August. In fact, 1999 AN10 will cross the Earthís orbit for no less than 600 years - with a period of 1.76 years at present, this implies a total of some 350 encounters in this time. Even if the danger at a single encounter is very small indeed, the fact that there are so many possible encounters adds to the danger.
What makes this object uncomfortable as a neighbour is that fact that itís orbit is chaotic. Each close encounter with the Earth modifies it. At present, the orbit after 2027 is unpredictable, simply because we do not know how close the asteroid will come then. If it passes very close, its orbit will be considerably modified, if its approach is not so close, the modification will be small. One of the consequences is that an asteroid can be diverted into an Earth-crossing trajectory by a previous close encounter. The closer the asteroid comes, the more unpredictable the future orbit will be, as a small change in the distance of closest approach will convert into a large future uncertainty.
At present, it is not even certain if its orbital period will get longer, or shorter after the 2027 encounter. At present, a slightly longer period is most likely, but that may change as new observations are added.
While it is perhaps unfair to liken 1999 AN10 to a hand grenade with a dodgy pin which is hurtling around the inner solar system, it does need to be watched carefully for the future. For the encounter on August 7th 2027, which could get to as close as about 37 000km from the centre of the Earth (a height of just 31 000km above its surface), there are two possible knock-on encounters - one on August 6th 2044 and another on August 7th 2046. Both require the asteroid to pass at exactly the right (or the wrong) distance from the Earth at its 2027 encounter to be deflected into an orbit which will allow the asteroid to hit the Earth. Where things get complicated though, is that a miss, for example in 2044, may set up a new potential impact at a later date. The problem is one of a game of interplanetary billiards, with the Earth as the target, whereby each shot sets up a further possible shot.
At present, the impact probability is just 1 in 500 000 for the 2044 encounter - low, but much higher than for any other known object. This though is a factor of more than 1000 greater than the initial estimate of a 1 in 1 billion risk of an impact originally calculated supposing an impact in 2039 - which is now known to be impossible. Bear in mind that NASA estimates that the risk to the Earth each year from an unknown 1km asteroid is anything between 1 in 100 000 and 1 in 1 million. In other words, 1999 AN10 is still hovering around the noise level. Only if the probability goes significantly over 1 in 100 000 will it become a definite threat.
A lot of potential impact years have been discussed. Initially there was a slight possibility of an impact in 2027, although that was always unlikely. We know now (see below) that that is impossible. Then a suggestion was made that there might be an impact in 2039. That required the asteroid to pass close in 2027, then again in 2034 and finally be deflected into the Earthís path in 2039. Again, we know that that cannot now happend, although the asteroid will come quite close in 2034.
The 2027 encounter could though swing the asteroid on to an encounter in 2044 or 2046. The asteroid could, according to its minimum distance in 2027, pass close in one or other of these years (but never in both).
This is the million dollar question. We know very exactly the line along which the asteroid moves. This line has a thickness of just 1000km. In other words, when we say that the minimum distance from the Earth is 37 000km in 2027, that distance is hardly likely to reduce much more. What we know less well is where the asteroid is on that line. At present, the estimates of the asteroid's path have an uncertainty of +/-4 hours as where it will be on the line in 2027. The very latest measurements have moved the centre of that uncertainty somewhat further from the Earth. At present the best guess as to the minimum distance of approach in 2027 is 200 000km - half the distance to the Moon. If we are very unlucky with the errors the distance may be several times higher. Last week, the best estimate was much smaller. The exact value will probably change quite a lot still.
The NASA NEO Web page explains in more detail how this works.
So far 1999 AN10 has been observed for just 130 days and we are trying to extrapolate its movement nearly 30 years into the future (16 orbits of the Sun), on the basis of just one fifth of a single orbit. A 4 hour error in 28 years time is equivalent to estimating the orbital period with a precision of 15 minutes (an error of 0.0016%). The astonishing thing is not the 4 hour error, but rather that the error is only 4 hours.
In an extreme case, the asteroid could get closer than the Earthís geostationary orbit. This would be very interesting. A 1km asteroid would have zero effect on the Earth (it is far too small to affect a planet with just a close approach). Itís mass is also too small to have a significant effect on geostationary satellites, unless one suffered a very unlikely direct hit.
More interesting would be the observational effects. The asteroid would pass well within the Earthís magnetic field and could cause strong aurorae. There is also the possibility that dust could be lifted from the asteroidís surface by electrostatic forces (simply because the asteroidís gravitational field is so weak) and produce a significant dust coma. Even though the asteroid would probably only get to about magnitude +3, a dust coma could make it much brighter.
1999 AN10 has an absolute magnitude of 18.1. What this means is that its diameter is around 1km. If the asteroid is very bright (25% albedo), it could be as small as 640m in diameter. If it is very dark (5% albedo), the diameter could be as great as 1440m.
A 1km body, with a density 3 times that of water, impacting the Earth at 15km/s, would have a kinetic energy of 40 000 Megatons of TNT. This is similar to the combined nuclear arsenals of the USA and USSR at the height of the cold war. While it is ridiculous and alarmist to talk of the danger of an impact in the foreseeable future, that figure should be enough to show how silly and short-sighted it would be to ignore the danger of this asteroid completely.
In the end though, 1999 AN10 is a phantom menace. It is there, threatening, but in an intangible way.