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.

The post Lunar Reconnaissance Orbiter Hit By Meteoroid appeared first on One Universe at a Time.

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.

The post Brace For Impact appeared first on One Universe at a Time.

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.

The post Shattering Theory appeared first on One Universe at a Time.

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.

The post Dodgeball appeared first on One Universe at a Time.

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.

The post Impact Factor appeared first on One Universe at a Time.

Yesterday the B612 foundation released a video showing 26 meteor impacts with energies ranging from 1 kiloton to 600 kilotons.  They were detected by the global nuclear weapons test network.  The claim is that these meteor impacts are occurring much more frequently than originally thought, and they are in the energy range of atomic bombs.  By comparison, the nuclear weapons dropped on Japan during World War II were in the 15 – 20 kiloton range.  The message the video conveys is that we have vastly underestimated the very serious risk of death be meteor.  The reality isn’t nearly so sensational.

The B612 foundation is a U.S. nonprofit organization trying to raise money to launch a satellite to look for meteors that threaten Earth.  As B612’s founder Ed Lu states, “The fact that none of these asteroid impacts shown in the video was detected in advance is proof that the only thing preventing a catastrophe from a ‘city-killer’ sized asteroid is blind luck.” This is technically true, but it’s true in the same way that the fact you haven’t been struck by lightning is blind luck. There is a real and legitimate risk that a meteor could impact a city with enough energy to do grave damage, but the odds are not very high. The number of impacts recorded by the nuclear test network is about what we would expect given our understanding of meteor impact rates.  So no, this new video doesn’t mean a city impact is much more likely.

Part of what the video actually shows is just how remote much of the Earth really is.  From 2001 to 2013 there were 26 sizable impacts on the Earth, but of these only 1 (the Chelyabinsk meteor in 2013) was widely observed.  In our hyper-populated, hyper-connected world, the other 25 impacts were so remote or so small they were largely unnoticed. The Chelyabinsk meteor was an exception because it was not only in a populated region, it was particularly large.  The estimated energy of Chelyabinsk is 500 kilotons, equivalent to about 25 atomic bombs.

Barringer Crater in Arizona is clear evidence of the meteor impact threat. Credit: Shane Torgerson

Although these impact explosions have roughly the energy of a nuclear weapon, the comparison is not a very good one.  For one, a weapon is specifically detonated near the surface of the Earth for maximum destructive effect.  Meteor impacts tend to lose much of their energy high in the atmosphere, which produces a great deal of light and noise, but generally not much damage on the ground.  Even a large impact like Chelyabinsk produced only limited damage on the ground.  Meteor impacts also don’t produce large levels of radioactive material in the way nuclear weapons do.

Just to be clear, projects like the one proposed by B612 are a good idea.  There is a small risk of meteor collisions, and it would be good to be prepared.  But the idea that we are unexpectedly at a high impact risk is simply false.

The post Hit Me appeared first on One Universe at a Time.

If you look at Earth from a distance, two things stand out clearly.  The first is that Earth is a watery world.  Water covers the majority of its surface, and there is so much water vapor in its atmosphere that it collects into clouds.  The second is that it has a very, very large moon.  There are moons in our solar system larger than Earth’s, but no other planet has such a large moon in comparison to its size.  Both of these are likely due to the fact that Earth is a broken world.

None of the other rocky inner planets have sizable moons.  Mars has two small moons that are likely captured asteroids, and neither Mercury nor Venus have any moon at all.  Earth has a moon that rivals the moons of Jupiter in terms of size.

Just how the Earth got such a large moon is still debated, but the prevailing theory is that during the formation of the solar system proto-Earth collided with a Mars-sized planetoid sometimes named Theia.  Part of the mass of Theia was captured to become Earth, and the remains formed a debris ring around Earth, much of which coalesced to form the Moon.

The collision model for the origin of the Moon. Credit: Black Cat Studios

There is actually a lot of evidence to support this model.  Analysis of lunar rocks from the Apollo missions have found that the Earth and Moon have the same chemical composition.  Oxygen isotopes from Lunar and terrestrial rocks are basically identical, for example.  Other solar system bodies such as Mars, and other asteroids have different oxygen isotopes.  This means that the Earth and Moon must have a common origin.  The Moon didn’t, for example, form elsewhere in the solar system to be later captured by Earth.

Another aspect is that the Moon has a density about 60% that of Earth.  This is exactly what is predicted by the collision model, where the lighter outer layers of Theia and proto-Earth are scattered to the debris disk, while the heavy core of proto-Earth remains.  If the Earth and Moon had formed in orbit as separate bodies, their densities should be much more similar.

The water of our world is also due to collisions with other bodies.  The infant Earth was hot, and had little atmosphere, so any water formed with the planet would have boiled off into space.  The wetting of our planet must have occurred after it had cooled.  This was likely due to bombardment by either comets or meteors.  Surprisingly, the primary contributor was likely meteors, not comets.  Comparison of hydrogen isotopes from certain meteors are actually a better fit to Earth’s hydrogen than those of comets.  Water was also deposited on the Moon, but with less mass and no atmosphere it simply couldn’t develop lakes or oceans.

The view of Earth from space often evokes thoughts of an Eden.  That pale blue dot that cradles humanity.  But our planet’s fragile beauty was born from violent collisions large and small.

Up next: Mars

The post Broken World appeared first on One Universe at a Time.

Venus is the second planet from the Sun, and the most like Earth in terms of its mass (80% Earth’s) and size (95% Earth’s).  In almost every other aspect it is radically different.  It has a thick, carbon dioxide atmosphere, little water, a weak magnetic field, and a surface temperature of 740 K (860 F, 460 C). Despite this radical difference, early Venus was a wet world much like early Earth. We know from the levels of hydrogen and deuterium in Venus’ atmosphere that it too had a wet past. But somehow Venus and Earth diverged.

Surface of Venus taken by Venera 13.
Credit: Soviet Space Agency.

Venus was a watery world for its first hundred million years or so.  Then, because of its proximity to the Sun, warmer temperatures caused more water vapor to enter the atmosphere.  Because Venus’ weak magnetic field couldn’t protect the atmosphere from the solar wind, the water vapor was gradually  photodissociated into hydrogen and oxygen.  The hydrogen escaped the atmosphere, leaving the oxygen to bond with carbon to form carbon dioxide.  This led to a runaway greenhouse effect, where rising carbon dioxide trapped more heat, causing more water vapor to enter the upper atmosphere, in turn leading to more carbon dioxide.

Perhaps the most unusual aspect of Venus is the fact that it rotates backwards on its axis compared to the other planets (what we call retrograde rotation).  On Venus you would see the Sun rise in the west and set in the East.  It’s rotation is also very slow, taking about 243 Earth days to make a complete turn. However because of its retrograde rotation a day on Venus is only about 117 Earth days long.  That means a year on Venus is a bit less than two days long.

We’re not exactly sure why Venus rotates in a retrograde fashion, but one idea is that the planet was impacted by two large bodies about 10,000 years apart.  Such a double impact could have given the planet its backward rotation while ensuring that any moons either escaped the planet or spiraled into it.

The runaway greenhouse effect of early Venus is sometimes held up as a cautionary tale for the global warming of our own planet.  Even Carl Sagan made such a comparison in his Cosmos series.  But it is important to recognize that early Venus and modern Earth are very different.  Global warming is a very real effect on Earth, and the evidence for anthropogenic greenhouse gases as a driving force of climate change is well documented.  But there’s no indication of such a runaway effect occurring here on Earth.

Up next: Earth

The post Cautionary Tale appeared first on One Universe at a Time.

Back in September of 2013 a bright flash was observed on the surface of the Moon.  It was soon found that this was due to a meteor impact on the lunar surface.  Now an analysis of this impact has been published in the Monthly Notices of the Royal Astronomical Society.

Captured frames of the impact flash. Credit: Madiedo, et al.

By analyzing the brightness and duration of the impact flash, the authors were able to get a good measure of the energy produced in the collision, which was equivalent to about 16 tons of TNT.  Determining the size of the meteor proved a bit more difficult.  Without an atmosphere to determine the path of the meteor, the team has no clear way to determine whether it was due to a smaller meteor traveling at high speed, or a much larger one traveling at a slower speed.  Based on the location of the impact, it was consistent with the September ε-Perseids shower.  If that is the case, then it was likely about 46 kilograms in mass.  If it was a sporadic meteor, it could have been much larger, about 450 kg.

A graph of cumulative impact risk. Credit: Madiedo, et al.

One of the things this impact shows, along with the Chelyabinsk meteor last February, is impacts with the Moon and Earth may be a bit more common than previously suspected.  One way to look at the odds of an impact is through an estimation of the cumulative number of impacts at a given size.  In a typical year, how many 10 cm meteors strike the earth, how many 1 meter in diameter, how many 10 meters wide, etc. Meteoroids generally have a power law size distribution.  For example, with a power law of 1, for every 100 meter wide meteoroid there would be 10 with a diameter of 10 meters and 100 with a diameter of 1 meter.  This is why the figure here uses a log scale.  The blue dashed line gives the general estimated risk, while the red line is the risk fit to the Chelyabinsk and lunar impacts.

Just to be clear, the risk of you and yours being injured by a meteor impact is extremely low, but this new data further supports the idea that keeping an eye out for possible Earth impacts is probably a good idea.

Paper: José M. Madiedo, José L. Ortiz, Nicolás Morales, and Jesús Cabrera-Caño,  A large lunar impact blast on 2013 September 11 MNRAS   first published online February 23, 2014 (doi:10.1093/mnras/stu083)

The post Rocking the Moon appeared first on One Universe at a Time.