What Would Have Happened To That Interstellar Object If It Had Hit The Sun?

What would have happened if A/2017 U1 had hit or grazed the sun? Would we have noticed?
This animation shows the path of A/2017 U1, which is an asteroid — or perhaps a comet — as it passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our solar system. Image credit:  NASA/JPL-Caltech

This animation shows the path of A/2017 U1, which is an asteroid — or perhaps a comet — as it passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our solar system. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

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.

A sun grazing comet as witnessed by the ESA/NASA Solar and Heliospheric Observatory, or SOHO, as it dived toward the sun on July 5 and July 6, 2011. SOHO is the overwhelming leader in spotting sungrazers, with almost 3000 spotted to date. SOHO can see the faint light of a comet, because the much brighter light of the sun is blocked by what's known as a coronograph. Image credit: ESA&NASA/SOHO

A sun grazing comet as witnessed by the ESA/NASA Solar and Heliospheric Observatory, or SOHO, as it dived toward the sun on July 5 and July 6, 2011. SOHO is the overwhelming leader in spotting sungrazers, with almost 3000 spotted to date. SOHO can see the faint light of a comet, because the much brighter light of the sun is blocked by what's known as a coronograph. Image credit: ESA&NASA/SOHO

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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How Come The Oort Cloud Isn't Torn Away From Our Sun By Nearby Stars?

If the Oort Cloud is three light years away from our Sun, then it’s closer to Alpha Centauri than our Sun, right? So how can it stay around our Sun if the mass of Alpha Centauri is 1.1 times the mass of our Sun - wouldn’t the gravity of Alpha Centauri rip it away?
An illustration of the Kuiper Belt and Oort Cloud in relation to our solar system. Image credit: NASA

An illustration of the Kuiper Belt and Oort Cloud in relation to our solar system. Image credit: NASA

Originally posted on Forbes!

The Oort cloud is an interesting feature of our solar system; a nebulous, spherical cloud of comets which marks the very outer limit of our solar system. The Oort cloud is also the source of our long period comets - those icy fragments of the early solar system which orbit our Sun very infrequently. To be classified as a long period comet, more than 200 years must pass between trips near the Sun. Hale-Bopp is probably the most well known of these, as it was visible to the naked eye for a long time in 1998. A more recent visitor was the Lovejoy comet, which swung near the Sun in 2011.

The Oort cloud is very far from the Sun. It is outside the bubble produced by our Sun’s solar wind and magnetic field by a considerable distance. While Voyager 1 has left this magnetic bubble, and entered what is called “interstellar space”, it has several hundred more years of traveling before it even reaches the inner edge of the Oort cloud. How is part of the solar system in interstellar space? Well, this means that the solar system at such a large distance from the Sun is not entirely ruled by our own star - the presence of other stars is mixing with the influence of our Sun.

This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. The inner edge of the main part of the Oort Cloud could be as close as 1,000 AU from our sun. The outer edge is estimated to be around 100,000 AU. Image credit: NASA/JPL-Caltech

This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. The inner edge of the main part of the Oort Cloud could be as close as 1,000 AU from our sun. The outer edge is estimated to be around 100,000 AU. Image credit: NASA/JPL-Caltech

The inner edge of the Oort cloud is typically quoted as beginning at somewhere between 1,000 and 5,000 au from the Sun. 5,000 au is about 0.08 light years away from the Sun, which is a little over four weeks of travel time for a beam of light, and considerably closer to our Sun than to Proxima Centauri, the closest star. These Oort cloud objects at the inner edge of their cloud are fairly reasonably more attached to our Sun than they are to anything else, and there are a lot of them here.

As we travel from the inner Oort cloud to the outer region, we should note that the Oort cloud is not an even assembly of objects, from some inner bound to a fixed outer bound. Instead, while there is something of an inner boundary, the outer boundary is more of a fizzling out, with objects getting fewer and farther between as you go farther and farther from the Sun. This means that the “outer boundary” is a very tricky thing to attach a number to. How many objects need to be out there to still count as part of the Oort cloud? Just one? Or do we need a higher density of objects before we’re dropping our delineation down? As a result of this fuzziness, plus the fact that it’s very hard to spot Oort cloud objects in the first place, estimates of the outer bound of the Oort cloud range from 50,000 to 200,000 au. It’s that 200,000 au that works out to 3.1 light years away from our Sun. NASA often quotes this outer edge as sitting at 100,000 au, which is about 1.6 light years, which means that this fuzzy “edge” is extending less than half the way out to Alpha Centauri.

Comet Lovejoy is visible near Earth's horizon in this nighttime image photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on Dec. 22, 2011. Image credit: NASA

Comet Lovejoy is visible near Earth's horizon in this nighttime image photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on Dec. 22, 2011. Image credit: NASA

All these numbers are for a sense of scale. In actual fact, the Oort cloud is incredibly sensitive to gravitational forces from objects other than our Sun. One of these is a very large-scale gravitational inequality; our solar system is not at the center of the Milky Way galaxy. The gravitational pull from our Galaxy is therefore stronger on one side of the solar system than it is on the other, and this galactic tide is enough to gradually jostle the Oort cloud. This kind of perturbation is part of how we think we get the long period comets, which can come blazing into the inner solar system, and, if they are unlucky, sometimes completely evaporated by the Sun.

The Oort cloud is also sensitive to the motions of other stars nearby in the Galaxy, and other extrasolar objects, like clouds of gas. As stars pass nearby (or through) the outer reaches of the Oort cloud, they will disturb the delicate gravitational balance that keeps these objects in their long, distant orbits. Stars aren’t likely to smash directly into a comet out there, but they might jostle it out of its orbit, and send it down into the inner solar system - another way of getting comets into the rest of the solar system.

Comet Hale-Bopp. Alex Krainov shot this image at Zabriskie Point in Death Valley in April 1997. Image credit: Alex Krainov, CC BY-SA 3.0

Comet Hale-Bopp. Alex Krainov shot this image at Zabriskie Point in Death Valley in April 1997. Image credit: Alex Krainov, CC BY-SA 3.0

But these perturbing stars are in motion too, and they will pass through relatively quickly, on an astronomical timescale. Alpha Centauri is still arriving into the solar neighborhood, and isn't yet close enough to do much influencing. With the combination of the fading density of objects, the short time frame with which a star will be close enough to really dramatically pull on the objects sitting out there, and the length of time between stellar close passes being quite long, we don't expect the Oort cloud to have been stripped away from our star. But it is absolutely influenced by the presence of those stars, and by the Galaxy at large, and our long, once-a-millennia comets like Hale-Bopp are the result.

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Are There Sungrazing Asteroids?

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Can you transport life on meteorites?

The short answer to this question is that we don’t know - although there is a whole field of study dedicated to trying to figure it out. What I’m about to describe are not truly theories - they’re suggestions. If you could transport life around a solar system, how could you do it? We don’t have any evidence to show that these are the ways in which it happens, or even if nature needs the help.

The hypothesis that suggests that either bacterial life or the building blocks for it could be transported via comets or meteorites is called panspermia. It proposes that once life were to arise somewhere in the universe, a large impact upon that planet could create such an explosion that pieces of rock carrying bacteria were flung into space, where they cruise about until crash-landing on another life-friendly planet.

We know that meteorites blasted off other planets can make it to our own planet, since we’ve collected a few meteorites that originated on Mars (as discussed in a previous post), so the real question is whether or not bacterial life is hardy enough to survive this whole process, including the blasting off the original planet and the landing on the new planet. If it can, then this might be one way to get bacterial life from Planet A to Planet B within a solar system, however unlikely it might be for those pieces of rock to be flung in exactly the right directions. We have been testing how resilient some bacteria can be to exposure to outer space, which is a pretty unforgiving environment. Some microbes from the English town of Beer managed to partially survive a 533 day stint on the outside of the ISS, which is pretty impressive, but there’s no guarantee that a rock blasted off the surface of another planet will be covered with a particularly space-hardy breed of microbe, nor is there a guarantee that the meteoroids will arrive at another planet in a short period of time.

In order to transport entire bacteria, a lot has to go exactly right. But it might be a little easier to transport not the live bacteria, but the the organic molecules that are required to build them. Amino acids are one such set of complex organic molecules, and are often called the building blocks of life. If a comet or meteorite can bring amino acids to a planet, it might help to jump-start the development of life on that planet by skipping the steps required to build those molecules from scratch. These molecules don’t need to have formed on the surface of another life-bearing planet - surprisingly complex molecules can form in the gas between stars in our galaxy. Because these simpler building blocks may be more widely available than planets with life that are also being bombarded with impacts, it may be easier to distribute these molecules than it is to distribute entire bacteria.

Until we have more concrete proof of either of these processes happening out there, they will remain suggestions, but we’re working towards testing each of the steps individually to get a sense of how plausible all of the steps together might be.

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