We see this a lot in nature, including in snowflakes. I know that sounds weird, but this video will help: These are branching structures that aren't regularly spaced like crystals instead they have a structure that's called self-similar: It looks the same no matter how much you zoom in or out. Out far from the star, where temperatures in the disk are cold, teeny tiny grains of dust and water ice can stick together in funny shapes, creating fractals. When stars are very young, they have a huge disk of material swirling around them it's from this material that planets form. Like Styrofoam or Swiss cheese, but much much holey-er. I'm not saying it's hollow, but maybe it's really porous. That's 100 times less dense than air! No solid object could have a density that low! That's incredibly low, and at first it seems ridiculously so. Instead of assuming a density to find the area, let's assume the size determined using normal methods is correct, use that to get an area, and from there get the density needed to match the observations.Īssuming a size for 'Oumuamua of 50 – 130 meters, what they get is a very low density: About 0.00005 grams per cc. So they assumed it had some normal density like 1 – 3 grams per cubic centimeter (roughly somewhere between the density of water to rock). That's because the amount of pressure sunlight exerts is very small, so if an object is massive it has to be spread out very thin and big to catch enough sunlight to accelerate it enough to match the observations. When the astronomers speculated it might be a thin and flat, giving it a large area like a sail, they had to assume a density for it.
Maybe it's fluffy.įractal structures can look solid but actually be mostly empty space this is called a Koch Curve, and snowflakes can have structures similar to this. So, I ask again: What the frak is 'Oumuamua?Ī new paper has come out that might have a solution, and it's really clever. Not much has changed with that hypothesis since I wrote that, and while I wouldn't dismiss it being an alien probe out of hand, the evidence doesn't support that conclusion, and in fact points against it. As in, a spaceship.īesides the obvious (it seems like a big leap!), I have my problems with this idea. But that, in turn, meant that 'Oumuamua was artificial. Like, really flat: So thin that it looked more like a solar sail, a very thin sheet of material designed to catch sunlight and accelerate. That makes some sense, but for the math to work out with the acceleration seen, 'Oumuamua had to be flat. That's when a couple of astronomers posited something interesting: Maybe this force was radiation pressure, literally the force of sunlight hitting it and giving it a tiny push. 'Oumuamua, the first object ever seen passing through our solar system from interstellar space, was thought to be emitting gas like a comet to explain its weird motion, but a new idea is that the comet is just very, very porous. Credit: ESA/Hubble, NASA, ESO, M. But again, if it is like our local comets, it would take so much water that we'd have noticed. So maybe it was some other kind of ice, like water. If it were like comets in our solar system, you'd expect to see lots of carbon monoxide (CO) and carbon dioxide (CO 2) coming from it, but none was seen. The problem with this is that no such venting was detected. Comets are made of rock and ice, so maybe the ice was turning into gas, and as this was blown off it acted like a very gentle rocket. Some force was acting on it, accelerating it very slightly. As it moves away, the Sun's gravity pulls on it, slowing it down … but it wasn't slowing down enough.
Then another very weird thing happened: More observations allowed a better determination of its trajectory, and it was found that it wasn't slowing down fast enough. It was hard to tell what it was it was too small, faint, and far away to get good observations, and worse, it was only seen on its way out, so it was farther from us literally every day. We'd never seen something from interstellar space pass through the solar system before! But what was it? At first it was classified as a comet, then an asteroid, and then maybe a comet again (this confusion is reflected in its provisional designations at first it was A/2017 U1, for "asteroid", then C/2017 U1, for "comet", then finally I/2017 U1, for "interstellar"). That was certainly enough to make it the object of intense scrutiny.
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