Impact strikes are a rare but dangerous threat to spacecraft. The highest threat is for spacecraft in near-Earth orbit, where decades of satellite debris have accumulated. Given the tremendous speeds of orbiting spacecraft, even a fleck of paint can pose a threat. Beyond Earth orbit the threat is less, but it isn’t zero, as demonstrated by a rare impact with the Lunar Reconnaissance Orbiter (LRO) currently orbiting the Moon. 

The impact was small, and the spacecraft survives. The effect was so subtle that it would likely not have been noticed if the orbiter wasn’t taking images at the time. You can see the effect of the impact in the image above. It was taken in 2014 by one of the LRO’s Narrow Angle Cameras. These cameras take high resolution black and white images of the lunar surface. To take these images, the camera scans the surface line by line, Taking a narrow line image of the surface one after the other to create a complete picture. As you can see in the image, the sharp resolution near the top shifts suddenly to a wiggly image. This means the camera was jostled suddenly as it gathered lines of images. The LRO has two Narrow Angle Cameras, as well as a wide angle one, and only one Narrow Angle Camera showed this wiggly effect. This means the satellite itself was not jostled in a significant way, but only one camera.

This can be explained by a meteoroid impact with the camera. Based upon computer recreations of the image, the meteoroid was only about 0.8 millimeters in diameter, or about the size of the ball at the tip of a ball point pen. But the meteoroid was moving at more than 15,000 miles per hour relative to the spacecraft, which is about ten times faster than a speeding bullet. This gave it enough energy to jostle the camera in a noticeable way.

It did not, however, have enough energy to seriously damage the spacecraft. The impact occurred in 2014, and since that time the Lunar Reconnaissance Orbiter has worked perfectly well. It continues to gather data on the lunar surface, and will help us determine future landing sites for crewed missions to the Moon.

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One of the questions I’m asked from time to time is whether a funny looking rock someone found is a meteorite. The short answer is no, since meteorites are extremely rare. The not-so-short answer is that it probably isn’t a meteorite, but there are some basic tests you can do yourself. 

Perhaps the simplest test is simply to put a magnet near your rock and see if the magnet is attracted to it. Most meteorites are iron rich, and are attractive to magnets. Only about 1% of meteorites aren’t attractive to magnets, and the vast majority of terrestrial rocks aren’t attracted to magnets, so if it isn’t magnetic, you can be pretty sure it isn’t a meteorite. Because meteorites are iron rich, they also tend to be heavier than a typical rock of the same size.

Some terrestrial rocks are attracted to magnets, such as magnetite and hematite. They are also heavier than typical rocks, so they will past the first test as well. Fortunately they can be ruled out by another simple test known as the scratch test. If you scratch your rock against an unglazed piece of ceramic, such as the underside of a bathroom tile or the bottom of an old coffee cup, the rock might leave a streak of color. If the streak is black or gray, then it’s probably magnetite. If the streak is red, then it’s hematite. Real meteorites will typically leave either no streak or a distinctly brownish streak.

So suppose your rock is heavy, magnetic, and doesn’t seem to be hematite or magnetite? This is where you have to get a bit more invasive, because you need to see what the interior of your rock is like. The easiest way to do this is to file down a small patch of the rock. It doesn’t have to be a large patch, just enough to get through the outer surface. If your patch has flakes of bright metal, and doesn’t have what looks like air bubbles, then you might actually have a meteorite.

I should point out that these tests aren’t foolproof. There are meteorites that don’t pass these tests, and there are terrestrial rocks that will. It helps to look at examples of real meteorites and imposters to get a feel for the differences. But by this point you’ve probably found that your rock isn’t magnetic, or it’s a piece of slag metal, or something similar.

But even if your rock isn’t a meteorite, you shouldn’t think any less of it. Your rock has been around for a long time, and in the course of your travels it caught your eye, encouraged you to pick it up, and say “Hello, Rock!”

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When you think about the risks of a meteor impact with Earth, you might think the bigger the rock the bigger the danger. It turns out that’s only part of the story. 

When it comes to really big impacts, such as the 10 kilometer wide Chicxulub asteroid that led to the extinction of dinosaurs, it is true that bigger is badder. But such impacts are exceedingly rare, occurring only once in 10 to 100 million years. It is exponentially more likely that a smaller meteor will strike the Earth. For smaller impacts factors such as speed, trajectory and location play a significant role.

Small asteroid impacts over the years. Credit: NASA/Planetary Science

The severity of a meteor impact is typically measured not in diameter but in kinetic energy. This is caused by a combination of mass and speed. A small meteor moving at great speed has more kinetic energy than a larger meteor moving slowly. In the same way, a bullet can do much more damage than a baseball, even though a baseball has much more mass. Since smaller meteors outnumber larger ones, speed is a greater risk factor. For example, the Chelyabinsk meteor that struck Russia in 2014 was only about 20 meters across. Meteors of this size are estimated to strike the Earth every 50 years or so. But this particular meteor struck the atmosphere with a speed of nearly 20 kilometers per second. It released about 500 kilotons of energy (about 25 times the energy of Hiroshima) resulting in damage to more than 7,000 buildings and injuring nearly 1,500 people.

The amount of damage is quite small for its energy, which was due to the fact that it had a fairly shallow trajectory. If it had entered our atmosphere at a much steeper angle, most of the energy would have been released near the Earth’s surface, causing much more damage. Instead most of the energy was released higher in the atmosphere, so only the resulting shockwave caused immediate damage. Trajectory matters. In 1972 a slightly smaller meteor struck our atmosphere with such a shallow angle that it entered and left our atmosphere. The only impact it had on us was to provide a daylight fireball.

The largest impact in recorded history was the 1908 Tunguska impact. This was caused by a 40-meter wide asteroid or comet striking our atmosphere at about 15 kilometers per second. The energy it released is estimated to be about 30 megatons, or 2,000 times that of Hiroshima. Observers 40 miles from the event were knocked to the ground by the shockwave and said the heat was so strong they felt as if they were on fire. The Tunguska impact knocked down trees over a 2,000 square kilometer area. Fortunately it occurred in a remote area of Siberia, so there was no fatalities.

The location of an impact matters. If the Tunguska event had occurred near a major city like London or New York, it would have killed millions. If it had occurred during the height of the cold war, it could have triggered World War III. While the Tunguska event was large, it isn’t geologically rare. It’s likely that an event of that size occurs every couple centuries or so. But the Earth has lots of ocean and remote lands, so the chance of such an impact striking a city is quite small.

While the risk of deadly impacts from medium-sized impacts isn’t zero, the risk is quite low. So there’s no reason to brace for impact.

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The Geminid meteor shower peaks tonight into Monday morning. If you have a chance, you should really check them out. It’s best to have a western view, and they tend to peak between midnight and 2:00 am.

Their name derives from the fact that they seem to come from the direction of Gemini. Like other meteor showers, the Geminids are the debris trail of a comet. In this case a rock comet known as Phaethon. Phaethon is more asteroid-like than comet-like in many ways, and as a result the Geminids are consistently good meteor showers.

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While watching the night sky, you will occasionally see a meteor streak across the sky. Commonly known as shooting stars, they can be seen at any time of the year. But there are times, such as this week, when a great number of them can be seen in a night. Sometimes meteors will occur at a rate of more than 100 an hour. But why do these showers of meteors occur?

Most of the meteors we observe are due to dust particles roughly the size of a grain of sand. As these particles strike Earth’s atmosphere at great speed, the air around it is heated to the point where it ionizes and glows. The meteor itself typically burns up very quickly and never reaches the ground. There is a small amount of dust spread throughout our solar system, and this is why they can occur at any time.

As the Earth passes through comet dust we can have a meteor shower. Credit: AstroBob

Meteor showers occur when there is a concentration of dust particles striking Earth’s atmosphere. This happens when the Earth passes near the orbit of a comet. As a comet passes through the inner solar system, light and heat from the Sun causes it’s surface to vent gas and dust. This gives a comet its tail, but it also means that the comet releases a trail of dust. Much of this dust continues to orbit the Sun in much the same path as the comet itself. As the Earth passes through such a region, lots of meteors can occur in a matter of hours.

For many meteor showers, we actually know the comet that causes it. For example, the Perseid meteor shower occurring this week comes from a comet known as Swift-Tuttle, which last passed near the Sun in 1992 and won’t return until 2126. The Orionid meteor shower coming up in October has its origins from Halley’s Comet.

Train tracks appear to emanate from a single point.

Since we know that meteor showers come from comets, why don’t we name them after their comets? Meteor showers were known long before they were connected to their comets of origin, and they were identified by the region of sky from which they appear to emanate. The Perseids mostly come from a region near the constellation Perseus, hence their name. The reason meteor showers appear to come from the same region of the sky is that most of them strike Earth’s atmosphere from the same general direction (the direction of the Comet’s orbit). Because of perspective, meteors coming from the same direction appear to emanate from a single region of sky, just as parallel train tracks appear to meet at a single point on the horizon.

If you have the chance, you should definitely check out the Perseids this week. All you need is a reasonably dark and clear view of the night sky. Spend a few hours watching the stars, and you should be able to see quite a few grains of comet dust streaking across Earth’s atmosphere.

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One of the big unsolved questions in chemistry and biology concerns the origin of life on Earth. One of the more speculative ideas on the matter is known as panspermia, or idea that life arose elsewhere and was seeded (either intentionally or through comets or meteors) on Earth. Thus, we came from outer space. It’s an idea that has been approached with both serious research and wild “ancient aliens” pseudoscience, so it doesn’t always get much respect in the field of astronomy. 

Despite the occasional claims, such as the red rain incident that was claimed to contain extraterrestrial organisms, or the application of Moore’s law to the complexity of organisms to argue that life is older than Earth, there is no solid evidence that extraterrestrial organisms seeded life on Earth. And although abiogenesis is an unsolved problem, there is no indication that life couldn’t have originated on Earth.

That said, we also know that the building blocks of life, such as amino acids, sugars and fatty acids, have formed in space, and could have been brought to Earth. Perhaps the most famous example comes from a meteorite that fell near Murchison, Victoria, in Australia. Commonly known as the Murchison meteorite, it was observed to fall in 1969, and its fragments total more than 100 kilograms. It is a carbonaceous chondrite meteorite, which means it formed in the early solar system when dust grains began coalescing into small asteroids, and was never heated to its melting point.

Because samples were gathered soon after impact, the amount of possible contamination from terrestrial organics is minimal, so we can be confident that building block materials aren’t due to contamination. We’ve also found, for example that the sugars and amino acids from the asteroid are a mix of left and right handed molecules. Terrestrial organisms mainly use left-handed proteins (of which amino acids are the building blocks) and right-handed sugars. This would imply that the Murchison organics have a non-biological origin.

By 2010, more than 70 amino acids and 14,000 other molecular compounds have been detected in the meteorite. We also now know that complex molecules can form in interstellar clouds, and these molecules could have survived through the formation of the solar system. So while life probably didn’t begin “out there,” it’s possible that organic material brought to Earth by meteors similar to Murchison could have helped jump start the rise of life by providing useful raw materials.

Paper: Kvenvolden, Keith A., et al. Evidence for extraterrestrial amino-acids and hydrocarbons in the Murchison meteorite. Nature 228 (5275): 923–926. (1970)

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There’s been news of the discovery of what could be the largest known meteor impact. The results have been published in Tectonophysics, but the results aren’t without controversy.

The layered structure of quartz crystals could be due to a meteor impact. Credit: Glikson, et al.

Currently the largest confirmed impact basin is Vredefort crater in South Africa, with a diameter of about 300 km. Slightly smaller is Sudbury basin, at about 250 km across. This new impact in Australia, if confirmed, would be about 400 km across. The evidence comes from quartz crystals found in core samples taken from a region known as Warburton Basin. These crystals have a layered fracture structure that could be caused by the impact of a large meteor. Based upon the samples, there would seem to be two impact regions each about 200 km across. It could have been caused by a meteor that split in two before impacting the Earth about 350 – 400 million years ago.

While its an interesting idea, quartz fractures such as these could also have been caused by other seismic events such as earthquakes, so quartz fractures alone is not particularly compelling evidence for a meteor impact. There is some evidence of overall basin geology that could be caused by an impact, but it is very different from other impact regions. In particular, the work argues that the impact features are now about 3 km below the surface, which makes it particularly difficult to study. It also isn’t clear that the two regions would be due to the same impact, or from two separate impacts in a similar era. The latter might seem unlikely, but we can’t rule it out.

It’s certainly possible that a large impact occurred there. We know that large impacts have occurred in Earth’s history, and the size of this new impact is perfectly plausible. But it’s important to keep in mind that finding a possible impact isn’t the same as confirming an impact basin. There’s still plenty of work to be done before Warburton Basin can be added to the list of large impact events.

Paper: A.Y. Glikson, et al. Geophysical anomalies and quartz deformation of the Warburton West structure, central Australia. Tectonophysics, Volume 643, 7, Pages 55–72 (2015)

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There’s a new video showing water droplets striking a bed of small glass beads. The result appears similar to impact craters from meteorites. In some ways this might seem rather childish and unscientific. There’s a common science demonstration where marbles are dropped into sand or mud to show how impact craters form. But in fact more advanced studies of impact craters often use similar experiments, just with more precision.

In the case of this work the team found that the water droplet impacts scaled with energy in much the same way that real meteorite impacts do. This is useful because it helps us frame the broad aspect of impact craters. Meteorite impacts are incredibly complex physical phenomena, so broad approximate models are necessary to get a handle on them. With these broad models we can look at things such as the lunar surface to estimate impact rates and the ages of craters. From this we can get an idea of the age and geologic activity of various planets and moons.

We’ve actually learned a great deal from studies such as these. Not just in terms of the specific aspects of craters, but also about periods of increased impact rates such as the late heavy bombardment. Combined with other evidence it helps us understand the history of our solar system.

So it might seem a little silly to drop water onto sand, but its actually serious science.

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In all my years studying astronomy, I’ve never owned a meteorite. That might not seem surprising given that meteorites are rare, but they actually aren’t. Some, such as pallasites are fairly rare and can be quite expensive, but small iron fragments can be purchased for under $50 depending on the size.

Nickel-iron fragments are by far the most commonly found meteorites, but they aren’t the most common to strike the Earth. About 92% of all observed meteor strikes are stony chondrites or achondrites. But these are fragile, and can be difficult to distinguish from terrestrial rocks. Only about 5% of strikes are iron-types, but because they are durable and have distinct magnetic properties they are much easier to find.

This week a small box arrived at the house as you can see above. These particular meteorites are fragments from the Diablo meteor that hit Arizona about 50,000 years ago. The meteor produced a kilometer-wide crater now known as Barringer Crater. They aren’t big or flashy, but they did come from space.

Of course technically everything on Earth came from space. These meteor fragments just arrived several billion years later than most of our planet.

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Meteorites provide one of the best sources of evidence for the history of our solar system. We have a wide range of them, with origins at different epochs of the solar system, from which we can analyze chemical composition and even the geology of other planets. The oldest meteorites are a stony type known as primitives. These look somewhat like rusty sandstone, and were formed in the earliest period of the solar system. They are called primitive because they were never reheated or combined with other material after their initial formation about 4.5 billion years ago. So they provide clues about the formation of solar system.

Credit: Roger R. Fu, et al.

Recently a paper in Science looked at the magnetic properties of a primitive meteorite known as Semarkona, and discovered a surprising fact about the early solar system. The authors studied small iron particles within small olivine chunks (chondrules) within the meteorite. These chondrules formed when regions of the early solar system were heated to a temperature hot enough to melt iron. The region then cooled and the iron droplets were flash frozen. This meant that any magnetic alignment due to an external magnetic field was locked in to the iron grains.

What the authors found was that within the chondrules the iron grains were aligned along a similar direction. This meant that they formed together within a magnetic field. By measuring the magnetic strength of these iron grains, they found the external magnetic field was likely around 50 microtesla, which is about the same strength as Earth’s magnetic field today.

This is much higher than modern magnetic field strengths in the interplanetary regions of the solar system, and it actually supports our model of the early solar system. One of the mysteries of protoplanetary formation is just how they could form out of the gas and dust surrounding a young star. Computer simulations relying simply on gas dynamics find that protoplanets take a long time to form. But the interactions of ionized gas in a moderate magnetic field leads to early protoplanet formation. It’s generally been thought that protoplanets form early, and thus our solar system should have had a stronger magnetic field in its past. This new work shows that in fact there was a stronger magnetic field.

What gave rise to that stronger magnetic field is still an unanswered question. But now we have one more piece in the puzzle of our solar system’s origin.

Paper: Roger R. Fu, et al. Solar nebula magnetic fields recorded in the Semarkona meteorite. Science, 13 November 2014

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Suppose you were playing a game of dodgeball. Not the typical game of dodgeball where you are part of a team trying to dodge throws from your opponents while trying to hit them out in return. This game involves you standing motionless against a wall while the other team tries to throw balls at your head. You are not allowed to move out of the way.

The good news is your opponents are generally lousy shots. Most of what’s thrown doesn’t even come close to your head. The bad news is your opponents have a large number of balls to throw your way. You figure eventually you will get hit in the face, but you’d like to avoid getting hit if you can.

As the game progresses you start to get pretty good at determining where balls are heading. If you measure things in terms of the width of your head, then early on you can tell where a particular ball might hit to within a dozen head widths. As an object gets closer you can narrow that down to the width of your head, and eventually to within the width of a finger or less.

That being the case, you aren’t likely to worry much about a ball heading for a point 50 head widths from you. A ball heading for a point 5 head widths away might make you a bit nervous, but that’s still several feet from your face, which in dodgeball isn’t that close. You’re concerned about a ball coming within a head width, because that one might actually hit you.

While this is a pretty ridiculous game, it’s similar to the way we track objects that might hit Earth. We can’t move Earth out of the way, all we can do is track asteroids to see if any might be heading our way. Just as we measured distances in the game in terms of head-widths, we can measure the distances of close approaching asteroids in terms of Earth widths.

Credit: RiaNovosti

You can see a figure of known asteroids and their predicted distances of closest approach above. None of them seem to be heading toward us, and even the closest ones, such as Apophis, will only get within 3 Earth-widths of us. That’s a bit close, but doesn’t have any real chance of hitting us. Every now and then you hear about a small asteroid that comes within the Moon’s distance of Earth, which seems close until you realize the Moon is about 20 Earth-widths away from us. An asteroid at that distance is close on cosmic scales, but not dangerous.

This doesn’t mean there is zero chance of an asteroid hitting us. Large asteroids have hit Earth in the past, and they will hit Earth in the future. But the risk on the scale of human lifetimes is incredibly small.

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The moon is known for its cratered surface. Its lack of atmosphere and proximity to Earth make it easy to observe impact craters from past collisions. Since the Earth and Moon are in the same general region of the solar system, they were likely bombarded at similar rates. While we do find craters on Earth (such as Barringer crater in Arizona), they are typically not as obvious due to wind and water erosion.

Most of the craters we see on the Moon occurred during the late heavy bombardment period, nearly 4 billion years ago. According to a model of the early solar system known as the Nice (pronounced neese) model, Jupiter was roughly at its current distance when it entered a 1:2 resonance with Saturn. The resulting resonance drove Neptune (initially closer than Uranus) to the outer edge of the solar system, pushed Uranus and Saturn outward, and scattered much of the remaining protoplanetary material out to the farthest reaches of the solar system. Some of that material was also strewn through the inner solar system, causing the heavy bombardment. There is some evidence to support this idea, as Mercury appears to have gone through a heavy cratering period at the same time.

Today our solar system is largely cleared of protoplanetary debris, so sizable crater impacts are not as common. This doesn’t mean they never occur. The Chelyabinsk meteor last year comes to mind, though that one didn’t create an impact crater. However an impact on the Moon last year did. You can see the before and after in the image here. It’s not a particularly big change, but it was the largest lunar impact that was observed in real time.

The Lunar Reconnaissance Orbiter has found several impact craters not seen in earlier images from the Apollo missions. So we know the Moon is still being bombarded from time to time. It’s all a part of how the lunar surface changes over time.

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