The Solar System recently welcomed another visitor from another star: 3I/ATLAS. We don't know much about it yet, but it seems considerably larger than ‘Oumuamua (which I wrote about previously) and 2I/Borisov - maybe 5 kilometers 3 miles) across, although considerably larger diameters - up to 12 km - have also been suggested, based on the hypothesis that it reflects only a small fraction of the incident light. It is thought to be comet-like, with the outside made up of grains of water ice and silicates. As the surface is heated by the sun, it is likely to form a coma - a long visible tail of gas and dust released as the ice melts. Watching this evolution will tell us much about the nature of 3I/ATLAS.
When it was spotted, it was roughly as far away as Jupiter. It is approaching rapidly and should reach 1.3-1.4 astronomical units (AU) from the Sun (1.3-1.4 times as far away from the Sun as the Earth is) in late November. Unfortunately its closest approach to the Earth will only be at 1.8 AU. Worse, when that happens, it will be on the far side of the Sun from us. Depending on how much it brightens, it should be visible with relatively small telescopes, though. So far, it appears to have a reddish color, perhaps a sign that it contains methane. Interestingly, its trajectory is near the ecliptic plane of the Solar System - the plane that contains all of the planets orbits.
It is also moving fast - faster than ‘Oumuamua - and will reach 150,000 miles/hour during its closest approach to the Sun. This simplified the detection, as can be seen from the gif below, combining images taken with the Asteroid Terrestrial-Impact Last Alert System:
The moving dot shows how 3I/ATLAS stands out from the stationary background stars. It is coming from the general direction of the center of our galaxy, which is also the direction of the highest density of nearby stars. It was likely ejected from some other solar system by a close encounter with a planet which left it with a velocity high enough to escape from that sun.
The three alien visitors that we have discovered are all quite different. ‘Oumuamua is an oddball, while 2I/Borisov and 3I/ATLAS seem, except for their much higher velocities, comet-like, not too different from the other comets that we see in the solar system. More data is needed to draw any conclusions, but it does seem that these interstellar objects are relatively common, with three discovered in 8 years.
Because of their large velocities and short notice of the arrival times, we probably won't be able to get a sample from one of these objects any time soon. But, if we are patient, it may be possible to do a fly-by and get some high-resolution photos.
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