From the depths of space it came. Speeding towards Earth, it passed within 60 lunar distances before it was discovered by the Pan-STARRS telescope. It’s a small asteroid or comet, like many that pass near Earth, but this one began its journey in another star system. 

We know this not because it has unusual characteristics, but simply from the path of its orbit. Planets, asteroids and comets follow elliptical paths around the Sun, we can determine their orbital paths by measuring their speed and distance from the Sun over a period of time. Since their orbits are determined by gravity, we can use Newton’s gravitational law to figure out its path knowing its speed and position.

Because of the gravitational pull of the Sun, an object moves faster when it is closer to the Sun and slower when it is farther away. At its farthest distance from the Sun, an object doesn’t have enough speed to overcome the Sun’s gravitational pull, so its path curves toward the Sun to make another orbit. By measuring the speed of this particular object, named A/2017 U1, we can calculate how far it could travel from the Sun before turning back. What we found was that it’s moving so fast it will escape the Sun’s gravity. It will leave our solar system never to return.

This can sometimes happen to a comet or asteroid that formed in our solar system. An asteroid can make a close approach to a large planet like Jupiter, giving it a boost of speed that sends it hurtling out of the solar system. We use this effect with spacecraft such as Voyager and New Horizons, which are now on their way toward interstellar space. But that couldn’t have happened to A/2017 U1. The path of its orbit is almost perpendicular to the orbits of the planets, and it hasn’t passed near any of the large planets in our solar system. So it must have been moving fast when it entered our solar system.

We’ve long thought that interstellar comets and asteroids were possible. We’ve known that with the right gravitational boost comets and asteroids can leave our solar system on rare occasions, and it stands to reason that comets and asteroids in other star systems can occasionally leave theirs. Given time, some of them will eventually visit other stars. But this is the first confirmed example of an interstellar visitor.

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Yesterday the kilometer-wide asteroid 2014 JO25 passed within 1.8 million kilometers of Earth. Although that’s nearly 5 times farther than the Moon, it’s a “near miss” by astronomical standards. If such an asteroid were to strike Earth, it would make a crater more than 10 kilometers wide and spread debris more than 100 kilometers in all directions. That’s large enough to wipe my home city of Rochester NY off the map. Fortunately there was no risk of impact for Rochester or anywhere else on Earth, but what are the odds that a sizable asteroid would strike your home town? 

We know that large asteroid impacts are rare. Small asteroids are far more common than large ones, and they tend to vaporize before striking the ground. We also know that the potential damage from an asteroid depends not only on its size, but on the speed of impact as well as the angle and location of impact. A fast asteroid striking nearly perpendicular to the ground is far more dangerous than a slower one coming in at a shallow angle.

To study the threat of various impacts, a team simulated 50,000 impacts with asteroids ranging 15 – 400 meters in diameter. These are the most common size for potentially threatening impacts. They then looked at how many lives might be lost not just from the impact, but from secondary factors such as explosive air bursts and tsunami. What they found was that land impacts were an order of magnitude more dangerous than water impacts. They also found that the most dangerous aspects of an impact were not the actual impact location and resulting debris, but rather the heat and explosive pressure changes that occur in the surrounding area. This was exactly what happened with the meteor near Chelyabinsk in 2013, where the air blast caused most of the injuries.

Just to be clear, the risk of being injured or killed by an asteroid is extraordinarily low. You should worry more about your cholesterol and looking both ways before crossing a street. But studies like this help us define a defense strategy against possible impact strikes, which is worth the gamble.

Paper: Clemens M. Rumpf, et al. Asteroid impact effects and their immediate hazards for human populations. Geophys. Res. Lett., 44, doi:10.1002/2017GL073191 (2017)

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Traditionally the difference between comets and asteroids is simple. Comets form tails of gas and dust when they are near the Sun, and asteroids do not. But as we’ve learned more about asteroids and comets we’ve found that things aren’t as clear cut. 

Comet tails are produced when “volatiles” such as water ice on the comet’s surface are sublimed by the Sun’s heat. This means comets must have surface ice, leading to the idea that they are “dirty snowballs.” Asteroids, on the other hand, were seen as dry rocky bodies, hence their lack of tails. On a basic level that’s true, and comets do tend to be more icy than asteroids. But comets can be quite rocky in composition, and asteroids can have plenty of icy volatiles. So sometimes a comet can appear much like an asteroid and an asteroid can have a comet-like tail.

A good example of this can be seen in the asteroid P/2016 J1. It was discovered in 2016 by Pan-STARRS, and was found to have two separate components (called A and B). These two objects have extremely similar orbits, meaning that they were once a single asteroid that was broken in two by a collision or gravitational interaction. Computer simulations of their orbits show that the asteroid likely split apart about six years ago. That would make it the youngest fragmented asteroid we know.

P/2016 J1 is in the asteroid belt, taking about 5.5 years to orbit the Sun. At its closest approach (perihelion), the asteroid is about twice as far from the Sun as Earth. Last year when it was near perihelion, the two parts of the asteroid started to become active and produced long dusty tails. In other words they looked like comets. This was probably caused by ice that was exposed by the fragmentation. Once heated by the Sun, they started to sublime and created the tails we could observe.

This doesn’t mean comets are simply broken asteroids. What it does demonstrate is that the interiors of asteroids can have lots of icy volatiles. They are just hidden by an asteroid’s the rocky exterior. So comets and asteroids can’t easily be divided into dirty snowballs and dry rocks. Instead it is more a matter of degree. Asteroids and comets are far more similar than we once thought.

Paper: Fernando Moreno, et al. The splitting of double-component active asteroid P/2016 J1 (PANSTARRS). arXiv:1702.03665 [astro-ph.EP] (2017)

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Since matter is made of atoms, it’s fairly common for materials to form into crystal lattices. Table salt, quartz, diamonds, etc. are all crystal formations. The arrangement of atoms in such crystals are regular and periodic, but there are some materials where the atoms are arranged in a crystal-like structure, but their patterns are not periodic. These quasicrystals follow patterns similar to Penrose tiles, where there is some broad sense of symmetry, but not a rigid repeating arrangement. 

Crystals can be identified by their overall symmetry, and follows the way different shapes can tile on a flat surface. Since lines, triangles, squares and hexagons can all tile a plane, crystals must have an n-fold symmetry of 2, 3, 4, or 6. But in 1982 Dan Shechtman found that aluminium-manganese alloys could form a 5-fold symmetry, like some Penrose tiles, hence the origin of quasicrystals.

Most quasicrystals are manufactured in the lab. It’s tricky to get them to form, since the tendency for atoms to arrange in regular patterns is so strong. But there are a couple of cases where quasicrystals formed natural, and it’s a bit of a mystery as to how they occurred. New work suggests that they could have formed through the collision of rare asteroids.

There are only two examples of natural quasicrystals, both from the same meteorite. This particular meteorite also has evidence of shock fractures, indicating it had undergone a collision at some point in its history. This led a team to suspect that meteor collisions could produce quasicrystals through a rapid succession of compression, heating, and cooling. So they devised an experiment where a bullet-speed projectile was fired at small sample of the meteorite. Since the natural quasicrystals are formed of aluminum, copper, and iron, they used a sample that contained a copper-aluminum alloy. They found that impact with the projectile did indeed form quasicrystal structures.

So it seems that quasicrystals do form naturally through asteroid collisions, but are still likely to be quite rare.

Paper: Paul D. Asimow, et al. Shock synthesis of quasicrystals with implications for their origin in asteroid collisions. PNAS (2016) DOI: 10.1073/pnas.1600321113

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On Monday amateur astronomer John Mckeon captured an unusual flash on the limb of Jupiter while he was filming the planet through a telescope. He posted the video on Reddit, wondering if it might be some sort of meteor or comet impact. It was soon confirmed that the flash was also observed by Gerrit Kernbauer. It does indeed appear to have been an impact event. 

Such impact events have been recorded before. Similar impacts were recorded by amateurs in 2010 and 2012. Because such events are unexpected, we rely upon the luck of observers. While such impacts happen fairly regularly, the only time we had a heads-up on an impact was when comet Shoemaker-Levy 9 impacted Jupiter under the watchful eye of Hubble and other telescopes.

Since comet and meteor impacts occur on Jupiter so regularly, it’s a commonly held idea that Jupiter actually protects Earth from comet collisions by deflecting them, or even colliding with them. But in fact Jupiter can deflect a comet toward us just as easily as it can away from us. So the role of Jupiter as Earth’s protector is still unclear, though it’s suspected that it did play a role in the early stages of our solar system.

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Today the object 2015 TB145 will make a “close” flyby of Earth. It’s been nicknamed Spooky due to its Halloween arrival date, and we’re already starting to learn a few things about it.

Spooky was presumed to be an asteroid when it was discovered, but was unusual because of its high relative speed (35 km/s). Its orbit is also unusual, being more inclined away from the plane of the solar system than most asteroids. Its trajectory is more in line with comets, though Spooky has no indication of a coma or tail. But we’re now starting to get infrared data from NASA’s Infrared Telescope Facility (IRTF) in Hawaii and radio observations from Arecibo, which supports the idea that Spooky is a dead comet.

In particular, the infrared observations allow us to determine the brightness (albedo) of the comet. It’s albedo is about 0.06, which, though about as dark as asphalt, is actually brighter than most comets.  So it is likely an old comet that has lost all its volatiles (surface ice and such) and now looks more asteroid-like than comet-like at first glance.

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Euphrosyne is the 5th most massive asteroid in the solar system. It has the highest density of any asteroid, so it’s only the 12th largest in terms of diameter. Despite its size, however, we actually don’t know that much about Euphrosyne.

One of the reasons for this is the fact that Euphrosyne is quite dark. It’s about the color of asphalt, which makes it difficult to observe in visible light. But like many objects, the asteroid is much brighter in the infrared. That’s because objects give off heat that can be seen in the infrared. That’s what makes the NEOWISE spacecraft so useful. It scans the sky at infrared wavelengths, so it’s able to see dark objects like Euphrosyne.

From the NEOWISE data a team was able to locate and track about 1,400 smaller asteroids that follow a similar orbit to Euphrosyne. From their orbits and characteristics, these are part of the same asteroid family, and likely originated from a large impact with Euphrosyne. Because of the asteroid’s unique orbit (being rather inclined relative to other asteroids) it is easier to trace the orbits of these asteroids back to the original collision.

Studying a family of asteroids such as this is important, because gravitational interactions with planets can cause the orbits of some of the smaller asteroids to shift so that they cross the orbit of Earth. Such “near Earth objects” or NEOs could pose an impact threat to our planet. By studying Euphrosyne and its family, we can get a better understanding of the orbital dynamics that can make an asteroid a potential threat.

There’s still a great deal to learn about Euphrosyne, but with infrared observations we’re making progress.

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Our home planet is exceptional for being a warm rocky planet with plenty of water on its surface. There have been several proposed origins for Earth’s water, such as that it originated within the primordial materials of our planet, or that it was brought by cometary or meteor impacts. We know from measuring isotopic ratios in Earth’s water that much of it was formed before even our solar system, and that it seems to match water found in certain asteroids. So the most popular model for Earth’s water is that it was brought to our planet by asteroid-like meteors. But did asteroids in our young planetary system really have enough water to produce our oceans?

It would seem the answer is yes, and new research tends to support that conclusion. In this work, the authors looked at the spectra of a white dwarf and found evidence of hydrogen at oxygen in its atmosphere. A white dwarf is a sun-like star that has reached the end of its life. After is has consumed much of its hydrogen, fusing it to heavier elements like helium and carbon) it collapses under its own weight until it is roughly the size of Earth (but still the mass of the Sun). Because a star will enter a red giant stage before collapsing to a white dwarf, most of the lighter elements like hydrogen would be cast off. Likewise, heavier elements like oxygen would tend to settle into the core of the star. So one would think we wouldn’t see much of either hydrogen or oxygen in the spectra of a white dwarf.

It turns out we do tend to see hydrogen, which could be an indication that some of that light element didn’t get thrown off during the red giant stage. But the presence of oxygen would seem to indicate this material was accreted by the star relatively recently on a cosmic scale. The authors found evidence of other elements such as silicon and iron, which are common in asteroids. One explanation for this is that the white dwarf accreted an asteroid, and its remnants are seen in the star’s spectra.

So the authors calculated the size of such an asteroid from the spectra, and found that it would have been about the size of Ceres. If that’s the case, then the strength of the oxygen and hydrogen spectra would imply that the asteroid was about 38% water when it was accreted. That’s a sizable amount. We know that asteroids in our solar system such as Ceres and Vesta contain water, but the amount is still under investigation. If this roughly 1/3 fraction is common, then there was plenty of water for a young Earth to gain by meteor impacts. The fact that this was seen around a white dwarf is also an indication that water-bearing asteroids may be common in planetary systems.

This isn’t conclusive proof that Earth’s water did in fact come from meteor impacts, but it adds to a growing pool of evidence supporting that model.

Paper: R. Raddi, et al. Likely detection of water-rich asteroid debris in a metal-polluted white dwarf. MNRAS 450 (2): 2083-2093 (2015)

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The Japanese spacecraft Hayabusa 2 has launched successfully, and is on its way to the asteroid 1999 JU3. The mission is the successor to the first Hayabusa mission, which landed on the asteroid 25143 Itokawa, and returned dust particles from the asteroid to Earth. This new mission will also strive to return samples to Earth, but it is also more ambitious. The probe has a shape charge that will detonate on the asteroid’s surface to eject material, it has a lander, and three mini rovers.

Hayabusa 2 won’t reach it’s destination until 2018, and then it will be 2020 before it returns with samples, so it will be a while before we start getting data from it. If you are interested, you can check out a video on the mission (complete with epic background music).

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All large asteroids have been bombarded over the ages, and as a result smaller chunks have been cast adrift in the solar system. Some of these smaller bits fall to Earth as meteorites. One of the things we notice about meteorites is that many of them have certain similarities of composition and chemical signature. As a result they can be identified into groups. This would imply that these groups have a common origin, likely a particular asteroid.

For example, the Howardite-Eucrite-Diogenite (HED) group is a type of meteorite that have long thought to originate from the asteroid Vesta. The connection was first made by Guy Consolmagno, who with Michael Drake demonstrated that the composition of HED meteorites matched the spectra of Vesta back in the 1970s. There are other smaller asteroids that have similar spectra, but Consolmagno noted that of all the HED meteorites found on Earth, none contain a mineral known as olivine, which is found in the mantle of asteroids and planets. This means the HED must have come from a large, intact HED body, which points to Vesta.

But when the Dawn mission reached Vesta, it found something unexpected. Vesta is more than 500 kilometers in diameter, which is large enough for it to differentiate. That is, during its formation one would expect iron and other heavy elements to sink to its core, surrounded by a mantle (where you would find olivine among other things) and an outer crust. But one thing Dawn noticed was two large impact craters near the south pole of Vesta. These craters were large enough that they exposed the mantle in that area. But what Dawn didn’t find was exposed olivine.

That means there’s something odd about Vesta. The impact craters exposed material as deep as 80 kilometers, which is quite deep for an asteroid. The lack of exposed mantle could mean that Vesta just has a really thick crust, but that shouldn’t be the case given its size.  But it would be the case if Vesta isn’t an intact world. Basically a proto-Vesta could have been shattered by a collision with another planetoid when the solar system was young. The stripped iron core of proto-Vesta could then re-accrete what material it could.

Of course, if Vesta was shattered early on, then the HED meteorites couldn’t have originated from Vesta. So this week Consolmagno presented a talk at the AAS Division for Planetary Sciences meeting arguing against his original theory. The HED meteorites could indeed be material chipped off Vesta from smaller impacts, but the HED material didn’t originally form as a part of Vesta.

I should point out that this work hasn’t been peer reviewed, though it has been submitted for publication. Even the idea that Vesta is a shattered body is a bit controversial, so Consolmagno’s conclusions should be considered a bit tentative. But it’s an interesting idea, and it’s a good example of how science works. If you follow the evidence, you might find that even your long standing model turns out to be shattered by new evidence. So you dust yourself off and push forward with a new idea.

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For an inner planet, Earth is bountiful with water. The origin of that water has been a matter of some debate. One idea is that a combination of Earth’s strong magnetic field and distance from the Sun allowed Earth to retain much of the water emitted from rocks as the planet cooled. Another is that water came to Earth through cometary or asteroid bombardment. But now it seems the origin of Earth’s water is more complex and more interesting that we’ve thought.

Last month an article in Science showed that much of Earth’s water existed before the formation of the solar system. The authors demonstrated this by looking a levels of deuterium in terrestrial water. Deuterium is an isotope of hydrogen that has a proton and neutron in its nucleus, rather than just a proton. As a result, it’s almost twice as heavy as regular hydrogen, and this means the way it chemically reacts is slightly different from regular hydrogen.

Deuterium isn’t very common compared to hydrogen, and exists at about 26 parts per million. When the team measured levels of deuterium in the water of Earth and other solar system bodies, they found the water contained deuterium at about 150 parts per million. This is interesting, because deuterium water is more likely to form in interstellar space. Water formed in the heat of a young solar system isn’t likely to produce much deuterium water. Given measured deuterium levels, the authors calculate that about half of Earth’s water was produced in the depths of space, before the solar system was formed.

This month another paper in Science found that water arrived on Earth earlier than expected. In this paper the team compared chondrite minerals on Earth with chondrite asteroids, specifically ones that likely originated from Vesta. Chondrite asteroids have a high quantity of water chemically bound to them, and one idea is that they could have been the source of Earth’s water. When they looked at the chemical makeup of terrestrial chondrites, they found them to be remarkably similar. This likely means terrestrial chondrites were themselves the source of Earth’s water. If that’s the case, then Earth was likely a water world a hundred million years earlier than the bombardment model predicts.

So it seems that Earth’s seas are more ancient both in origin and composition than we once thought.

Paper: Cleeves, L. I., et al. The ancient heritage of water ice in the solar system. Science, 345 (6204), p. 1590 – 1593 (2014)

Paper: Sarafian et al. Early accretion of water in the inner solar system from a carbonaceous chondrite–like source. Science, 346 (6209) p. 623-626 (2014)

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We generally think of comets and asteroids as two distinct types of bodies. Comets are “dirty snowballs” of mostly ice, which vaporizes to form long tails when they approach the Sun, while asteroids are dry, rocky bodies that typically live in the asteroid belt. It is generally true that comets tend to have an icy surface of volatiles that can evaporate off its surface, and asteroids generally don’t. But it also turns out that the two are far more similar than they are different.The idea of comets as dirty snowballs isn’t very accurate. For one thing, asteroids and comets are both rocky bodies, although asteroids can also contain large amounts of metals. Long period comets, originating from the Oort cloud, can have significant ice, and are closer to the traditional view of comets. Short period comets often have much of their ice evaporated away, so that they look more like asteroids. Asteroids can also have pockets of ice beneath their surface. As these pockets are exposed to sunlight they can create comet-like streams.

Another way to distinguish comets is by their orbits.  Comets tend to have more elliptical orbits, while asteroids tend to have more circular ones. This can be summarized in a quantity known as the Tisserand parameter, which is a measure of a body’s orbital size and eccentricity compared to the orbit of Jupiter. A Tisserand parameter between 2 -3 usually means an object is a comet, while a value greater than 3 tends to be an asteroid.

But there are also object that seem to cross the line between comet and asteroid. For example, an object known as P/2013 P5 has an asteroid like orbit and composition, but was observed to have six comet-like tails. Or consider the object known as Ceres. It is now considered a dwarf planet, but was once considered to be an asteroid. It has an asteroid object, and is both rocky and metallic. It also has water vapor plumes, and even a faint atmosphere derived from the water vapor. Ceres is a large asteroid with faint comet-like plumes.

As we learn more of both asteroids and comets, we find they are just two types of a range of small solar system bodies.

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