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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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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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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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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The Rosetta spacecraft is on its way to an asteroid known as 67P/Churyumov–Gerasimenko. In August it will go into orbit around the asteroid, and then in November it will put a lander known as Philae on the rock.  The surface gravity of this asteroid is less than 1/20 that of Earth, so Philae will actually have harpoons to keep it attached to the asteroid.  It’s an ambitious mission, since unlike many landings we have no idea what the surface of the asteroid will be like. In fact until recently, we weren’t entirely sure what the shape of 67P/Churyumov–Gerasimenko actually is.

Credit: ESA/Rosetta/MPS

But as Rosetta approaches the asteroid we are starting to get an idea of its shape. What we’re finding is rather interesting. It turns out 67P/Churyumov–Gerasimenko is what’s known as a contact binary. These are thought to form when two asteroid or comet bodies come into contact at speeds low enough to remain in contact. Contact binaries are not particularly rare, but they are unusual, and we’re lucky to have the opportunity to study them close up. It will, however, make orbiting and landing on the asteroid a bit more of a challenge.

As we get higher resolution images, the necessary changes to Rosetta’s orbit can be made. Then it will just be 3 2 1 contact.

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We generally think of asteroids as looking like gray rocks. While that’s true to our limited eyes, more sensitive instruments find they have a variety of colors. You can see an example of this in the image above of the asteroid Vesta. This false color image was made by observing Vesta at various wavelengths in the visible and infrared spectrum. It shows that Vesta has variations in color too subtle for us to see with our eyes.

The color variation is caused by the varying composition of Vesta. The green regions, for example, indicate the presence of iron. While Vesta is the only asteroid with such detailed color observations, other asteroids have been observed at different wavelengths to determine their overall color. It turns out the color of an asteroid correlates well with the family it is a part of.

Asteroids aren’t just scattered randomly through the asteroid belt. Because of the gravitational interactions of Jupiter and the other planets, asteroids tend to be clumped into groups or families. In 2002, the Sloan Digital Sky Survey (SDSS) observed more than 10,000 asteroids at different wavelengths to determine their overall color. What was found was that there was less color variation within a particular family than their was between families.

Asteroid families are defined by the similarity of their orbits. What this study showed is that asteroid families also share similar coloring. Since the coloring of an asteroid is determined by its composition, this means asteroid families have similar compositions. Asteroid families are chemically similar.

This has important consequences for the history of our solar system. It means that asteroid families formed within their own family. They didn’t form first and then get pushed into groups by the gravitational tugs of the planets. This means the orbital dynamics of the solar system likely stabilized before the asteroids formed.

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In modern astronomy, large catalogs of data are vital to our understanding of the universe. But just what does it take to create these large catalogs? Scott Manley has a great video of one example, discovering and cataloging the asteroids in our solar system. It tracks asteroid discoveries from 1980 to the present.

We now have almost 600,000 asteroids in the catalog which includes not only their location, but also the size and shape of their orbits. If you want to track which asteroids might hit Earth, this is the catalog you need. If you want to mine asteroids for rare Earth metals, this catalog will come in handy.

As you can see, this is a decades old and on-going effort. It draws upon amateurs and professionals, ground-based and space-based observations. This is what modern astronomy looks like. It’s only by working together that such large catalogs are possible.

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Asteroids come in a range of sizes, from hundreds of kilometers in diameter down to a meter wide and smaller.  Determining just how many asteroids there are is a challenge, because the smaller an asteroid’s size, the more difficult it is to observe.  Additionally, smaller asteroids are far more numerous than larger ones.

The size distribution of asteroids roughly follows a power law distribution.  For every 1 kilometer asteroid, there might be a hundred 100-meter ones, ten thousand 10-meter ones, and a million 1-meter ones.  Of course this relationship isn’t exact, and variations in this distribution can significantly change the numbers, particularly for smaller asteroids.  To really get a handle on the size distribution of asteroids you need to make a large survey of mid to small-sized asteroids.

For smaller asteroids, the only way to efficiently determine their size is to estimate it from their brightness.  The idea is that larger asteroids reflect more light, therefore would appear brighter.  The problem with this trick is that some asteroids reflect much of sunlight that strikes it (have a high albedo) while others don’t reflect much light at all (low albedo).  If you imagine looking at a snowball versus a charcoal briquette, you can imagine the difference.  Of course there’s also every range in between as well.  The brightness of an asteroid depends on its albedo and its size.  So a small, high albedo asteroid can be as bright as a large, low albedo one.

So you do you determine their size by brightness, if it also depends on albedo?  You make observations in the infrared instead of visible.  Asteroids don’t reflect all the sunlight that strikes them.  Some of that light is absorbed, which warms the asteroid a bit.  The asteroid radiates this heat as infrared light.  Just how much it radiates depends on its temperature a bit, but depends on its size even more.  A larger asteroid has more surface area, and therefore gives off more infrared light.  So in the infrared, the brighter asteroids are larger, and the dimmer asteroids are smaller.  You can see this effect in the image below.

Credit: NASA/JPL-Caltech

Given that our atmosphere absorbs much of the infrared light, as I discussed yesterday, an survey of asteroids really needs to be done from space.  Sending a satellite up just to survey asteroids is cost prohibitive, but fortunately satellite data can be used for multiple purposes.

In 2009, NASA launched the Wide-field Infrared Survey Explorer (WISE), with a mission to make a complete infrared survey of the sky.  Since it observes the sky in infrared, it can see small dim stars, galaxy clusters, black holes, and a range of other interesting things.  It also allows us to search the entire sky for smaller asteroids.

What we found was that there are actually less small asteroids than we originally thought.  From earlier observations of larger asteroids it was estimated that there were about 35,000 mid-sized asteroids (100 meters – 1 kilometer in size) in our solar system.  What we found was there are less than 20,000.  So the asteroid belt is considerably less crowded than we had thought.

Of course we still don’t know that much about the number even smaller asteroids.  We can use the power law model to estimate their numbers, but more data is needed to be sure.  Some of that information could come from NASA’s capture of a 25-meter asteroid, if the rumors are true.  But that’s a whole other story.

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Astronomers have found an asteroid with a ring system, and the results have been published Nature. While this is the first discovery of a ring system about an asteroid, such a thing isn’t entirely unexpected.  We have known for a while that asteroids can have moons, for example.  But what’s particularly interesting is how they discovered these rings.

The asteroid in question is known as  Chariklo, a 130 kilometer wide object that orbits between the orbits of Saturn and Uranus.  It is the largest example of the Centaur group, which are likely Kuiper belt objects that were captured by the gravitational pull of the outer planets.  In the Summer of 2013, Chariklo passed in front of a dim, 12th-magnitude star as seen from parts of South America.  It is a phenomenon known as an occultation.

Brightness of the background star during occultation.
Credit: F. Braga-Ribas, B. Sicardy, et al.

Such an event is a great opportunity to better determine the size and shape of an asteroid, so a team of astronomers dispersed through Brazil, Argentina, Uruguay and Chile made observations of the occultation.  Since each observation is made from a different vantage point, Chariklo’s occultation of the background star occurs at a slightly different orientation.  By combining these different observations the team could map the overall shape of the asteroid.

But what the team found was that from some vantage points the background star dimmed a couple times before being occulted by Chariklo, then dimmed a couple times after the occultation as well. Analysis of these pre and post dips makes it pretty clear that there are at least two rings around the asteroid.

The orientation of Saturn’s rings changes over time. Credit: NASA/HST

Knowing that Chariklo has a ring system, the team was also able to determine a bit about the composition of the rings.  As Chariklo (or any other ringed object) orbits the Sun, there are times when the rings are seen more edge on, and times when we see more of the rings.  The team compared spectral observations of the asteroid between 1997 and 2008, and found that the asteroid gradually dimmed during that period.  The observation of water ice in the spectrum also faded during that period.  This would be consistent with a ring system containing ice (which is very reflective) gradually shifting to an edge-on orientation.

One of the more surprising aspects of the discovery is that the rings are so sharp and dense.  Typically a ring system would diffuse over time unless there are shepherd moons to keep the rings in order.  So it may be that Chariklo has not only a ring system, but a moon system as well.

To determine whether that is the case, it will likely take some high resolution imaging.  But that remains to be seen.

Paper:  F. Braga-Ribas, B. Sicardy, et al. A ring system detected around the Centaur (10199) Chariklo. Nature (2014) doi:10.1038/nature13155.

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