The snappily-named object A/2017 U1 may be more familiar to you as the interstellar visitor that zipped through our solar system at nearly 16 miles per second, discovered in mid-October. It has now been given a less alphanumeric name by the Minor Planet Center: ‘Oumuamua. That Hawai'ian name “reflects the way this object is like a scout or messenger sent from the distant past to reach out to us (ʻou means reach out for, and mua, with the second mua placing emphasis, means first, in advance of)”
At 400 meters (about a quarter mile) across, ‘Oumuamua is a relatively small visitor to our solar system. Though it passed through the innermost regions of the Solar system, closer to the Sun than Mercury, that’s not nearly close enough to be considered a sun-grazing comet, and well too far away to hit the Sun directly.
At 400 meters across A/2017 U1 is considerably larger than the vast majority of the comets spotted by the SOHO satellite, one of our Sun-monitoring satellites. SOHO’s main goal is to watch out for solar flares and other events on the surface of the Sun which could pose a hazard to the Earth, but its continual monitoring of the sun has also discovered a huge number of comets - in 2015, NASA celebrated SOHO’s 3,000th comet discovery. These comets are usually only a few tens of meters across, ten times smaller than our interstellar visitor. SOHO has also spotted objects which blur the boundaries between comets and asteroids, probably a fairer comparison to our interstellar wanderer. One such discovery, comet 322P, is estimated to be around 100m in diameter, not so far off of the estimated size of 'Oumuamua.
If the object had hit the Sun directly, it would have been astoundingly bad luck for our interstellar wanderer. Imagine travelling for billions of years, only to run smack into a star - that’s like skiing into the only tree on the entire mountain. If that had happened, though, that’s a straightforward end to this interstellar object. Plunging into the incredible heat of our Sun would have destroyed that object, however rocky it was.
Grazing the Sun involves swinging past the Sun at such a close distance that your object is traveling within a contour that’s less twice the size of the Sun. Generally, from observations by satellites like SOHO, it seems that only comets which are more than a few kilometers across will survive the intense environment that close to the Sun - comets smaller than that will evaporate entirely away, reaching the same fate as their plunge-diving cousins. Asteroids and other rocky objects are a little more durable than the ice of a comet, but the harshness of the space immediately surrounding the Sun will abrade away the surface of even very durable materials.
Would we have been able to spot this abrasion of a small rock? The more comet-like our visiting object were, the easier it would be, since SOHO easily spots comets a tenth the size of our visitor. Rocky objects are harder to spot because they tend not to form large tails, but they will still reflect light into any waiting cameras, and as the detection of 322P proves, intermediate objects are still readily detectable at the size of 'Oumuamua. If the object were 100% rock, it reflects so little light that it would be much more difficult to observe with SOHO unless the object were another factor of ten or so larger - kilometers instead of hundreds of meters across. However, since it seems that 'Oumuamua was one of these mysterious, rocky/icy objects like the objects in our own Kuiper belt, it might have been more analogous to the hybrid comets we've spotted so far. In that case, as long as it had gone within SOHO’s field of view, we might have had a good chance of seeing the reflected sunlight from its surface. SOHO can spot objects a little beyond the surface of the Sun out to 30 times the radius of the Sun (the very surface of the Sun is too bright, and so it’s blocked from view). It might have been harder, given the brief flash of observation time we would have had before it annihilated, to determine exactly where it had come from, and we certainly wouldn’t have had time to get more information on our first interstellar visitor, like its color (red)!
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