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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Are There Sungrazing Asteroids?

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Where did the nickel-iron asteroids come from?

Where did the nickel-iron asteroids in the Astroid Belt come from? Nickel-iron, as I understand it, is created as a planet differentiates itself while cooling, after finishing with its planet-forming agglomeration period, as exampled by Earth’s nickel-iron core. If, as commonly presented, the Asteroid Belt is composed of left-over debris that never coalesced into a planet, because of Jupiter’s gravitational effects, then how did the nickel-iron come about?
A composite image of the Crab Nebula showing the X-ray (blue), and optical (red) images superimposed. The size of the X-ray image is smaller because the higher energy X-ray emitting electrons radiate away their energy more quickly than the lower energy optically emitting electrons as they move.  Credits for X-ray Image:   NASA  /CXC/ASU/J. Hester et al.; Credits for Optical Image:   NASA  /HST/ASU/J. Hester et al.

A composite image of the Crab Nebula showing the X-ray (blue), and optical (red) images superimposed. The size of the X-ray image is smaller because the higher energy X-ray emitting electrons radiate away their energy more quickly than the lower energy optically emitting electrons as they move. Credits for X-ray Image: NASA/CXC/ASU/J. Hester et al.;
Credits for Optical Image:
NASA/HST/ASU/J. Hester et al.

Originally posted at Forbes!

According to our current models of the universe, just after the Big Bang, only a few elements were in existence; hydrogen, helium, and a little bit of lithium. These are the three lightest elements in the periodic table, and it’s the main reason why astronomers can get away with calling everything that’s not hydrogen and helium a ‘metal’. (Don’t tell the chemists. Or the physicists.) Effectively, we can think of everything heavier than those two as contaminants on the primordial batch of gas that we started with.

These heavier elements are really useful tracers of a different astrophysical process; stars. Heavier elements are exclusively made either in the cores of stars, or in the end-of-life supernova explosions of very large stars, if the element is heavier than nickel. As a fun corollary, it means that all the lead and gold we find on our planet is the direct byproduct of a star exploding somewhere, many billions of years in our past.

Iron and nickel are both formed in the very centers of large stars, at the very end of their lifetime. Nickel can be formed through nuclear fusion (the same process that fuels our sun), but the form of nickel that is produced is unstable, and decays into iron relatively quickly – iron is often quoted as the heaviest element that a star can produce. All of the stars which are capable of creating nickel and iron go through a supernova as their dramatic end to life as a star – the supernova is triggered when nothing remains in the core of the star that can be burned. The burn that creates nickel is the very last one possible, so a supernova is imminent once this process starts.

Conveniently for the spread of metals, supernovae are pretty sizable explosions, and will spread some of the elements they’ve created into interstellar space. (Some fraction of these hard-won elements will stay stuck in the remnant of the star – a neutron star or black hole, at the stellar masses which can make iron in the first place.) Over many generations of stars, the small amounts of metals that have been created and dispersed build up, to the point that any given gas cloud has probably been filtered through a star at some point in the past. There’s some metal content already in the gas before anything else happens to it, just because it’s already gone through a few rounds of stars. So it was with the cloud of gas which became our solar system – the gas already held a good amount of nickel and iron, and all the other elements we now spot on our planet.

As the planets began to form, heat from impacts and other sources melted the existing rock, and, since iron and nickel are very dense materials, it is the iron and nickel which sank to the cores of the rocky inner planets during differentiation. (Differentiation simply refers to the process by which planets ordered their interiors, as the most dense materials sank to the center.) So it’s not that the nickel and iron are produced in this settling process – the settling just collects it together. Those nickel-iron asteroids are, as you say, either material which didn’t collect properly, or was in an object which was trying to become a planet, but then broke apart. But the nickel and iron itself was forged in the very core of an ancient star.

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