Bright meteor captured on a webcam in Portsmouth, New Hampshire on May 17, 2016. Via www.portsmouthwebcam.com
It came from outer space—literally! On Tuesday, May 17, 2016, the early morning sky briefly lit up with the brilliant flash of a fireball—that is, an extremely bright meteor—over much of eastern New England states and even parts of southeastern Canada.
The event, which occurred around 12:50 a.m. EDT (04:50 UTC), was reported by witnesses from Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut, New York, Ontario, and Québec, and captured on several automated cameras like a webcam in Portsmouth, NH (seen above) and a police dashcam in Plattsburgh, NY (below).
The fireball appeared to be moving from southwest to northeast and for some witnesses created an audible sonic boom, heard (and felt) several minutes later.
Meteors are the result of debris in space rapidly entering Earth’s upper atmosphere, compressing the air and causing it to quickly release energy in the form of heat and optical light. If the entering object is massive enough it may violently disintegrate during its fall, creating both light and sound. This particular meteor technically classifies as a bolide, due to its brightness, eruption, and visible fragmentation. Learn more about the various types of meteors here.
No reports of a meteorite impact at ground level have been made although I must assume there will be individuals who go on the hunt—meteorite fragments, especially those associated with witnessed events, can be quite valuable.
Full-frame image of the May 17, 2016 meteor from Portsmouth, NH (portsmouthwebcam.com)
Did you witness the event or capture it on camera? Report your sighting of this or any other fireballs on the AMS siteand be sure to send your fireball videos or images to the American Meteor Society here.
A brilliant Geminid flashes below Sirius and Orion over Mount Balang in China. Credit: NASA/Alvin Wu
2015 looks like a fantastic year for the Geminids. With the Moon just 3 days past new and setting at the end of evening twilight, conditions couldn’t be more ideal. Provided the weather cooperates! But even there we get a break. With a maximum of 120 meteors per hour, the shower is expected to peak around 18:00 UT (1 p.m. EST, 10 a.m. PST) December 14th, making for two nights of approximately equal activity: Sunday night Dec. 13-14 and Monday night Dec. 14-15. Continue reading “Viewing Guide to the 2015 Geminid Meteor Shower”
Multi-photo composite showing Perseid meteors shooting from their radiant point in the constellation Perseus. Earth crosses the orbit of comet 109P/Swift-Tuttle every year in mid-August. Debris left behind by the comet burns up as meteors when it strikes our upper atmosphere at 130,000 mph. Credit: NASA
Every year in mid-August, Earth plows headlong into the debris left behind by Comet 109P/Swift-Tuttle. Slamming into our atmosphere at 130,000 mph, the crumbles flash to light as the Perseid meteor shower. One of the world’s most beloved cosmic spectacles, this year’s show promises to be a real crowd pleaser.
The author takes in last year’s moon-drenched Perseids from a recliner. Credit: Bob King
Not only will the Moon be absent, but the shower maximum happens around 3 a.m. CDT (8 UT) August 13 — early morning hours across North America when the Perseid radiant is highest. How many meteors will you see? Somewhere in the neighborhood of 50-100 meteors per hour. As always, the darker and less light polluted your observing site, the more zips and zaps you’ll see.
Find a place where there’s as few stray lights as possible, the better to allow your eyes to dark-adapt. Comfort is also key. Meteor showers are best enjoyed in a reclining position with as little neck craning as possible. Lie back on a folding lawn chair with your favorite pillow and bring a blanket to stay warm. August nights can bring chill and dew; a light coat and hat will make your that much more comfortable especially if you’re out for an hour or more.
The Perseids appear to radiate from spot below the W of Cassiopeia in the constellation Perseus, hence the shower’s name. This map shows the sky facing northeast around 12:30 a.m. local time August 13. Source: Stellarium
I’m always asked what’s the best direction to face. Shower meteors will show up in every corner of the sky, but can all be traced backwards to a point in Perseus called the radiant. That’s the direction from which they all appear to stream out of like bats flying out of a cave.
Another way to picture the radiant it is to imagine driving through a snowstorm at night. As you accelerate, you’ll notice that the flakes appear to radiate from a point directly in front of you, while the snow off to the sides streams away in long trails. If you’re driving at a moderate rate of speed, the snow flies past on nearly parallel paths that appear to focus in the distance the same way parallel railroad tracks converge.
Meteors in a meteor shower appear to radiate from a point in the distance in identical fashion to driving a car in a snowstorm. The motion of the car (Earth) creates the illusion of meteors radiating from a point in the sky ahead of the observer. Credit: Bob King
Now replace your car with the moving Earth and comet debris for snow and you’ve got a radiant and a meteor shower. With two caveats. We’re traveling at 18 1/2 miles per second and our “windshield”, the atmosphere, is more porous. Snow bounces off a car windshield, but when a bit of cosmic debris strikes the atmosphere, it vaporizes in a flash. We often think friction causes the glow of meteors, but they’re heated more by ram pressure.
A bright fireball breaking to pieces near Yellow Springs, Ohio. Meteors are really tubes of ionized air energized by the passage of comet bits. Credit: John Chumack
The incoming bit of ice or rock rapidly compresses and heats the air in front of it, which causes the particle to vaporize around 3,000°F (1,650°C). The meteor or bright streak we see is really a hollow “tube” of glowing or ionized air molecules created by the tiny rock as its energy of motion is transferred to the surrounding air molecules. Just as quickly, the molecules return to their rest state and release that energy as a spear of light we call a meteor.
Imagine. All it takes is something the size of a grain of sand to make us look up and yell “Wow!”
Speaking of size, most meteor shower particles range in size from a small pebble to beach sand and generally weigh less than 1-2 grams or about what a paperclip weighs. Larger chunks light up as fireballs that shine as bright as Venus or better. Because of their swiftness, Perseids are generally white and often leave chalk-like trails called trains in their wakes.
Comet 109P/Swift-Tuttle seen during its last pass by Earth on Nov. 1, 1992. A filament of dust deposited by the comet in 1862 may cause a temporary spike in activity around 18:39 UT on August 12. Credit: Gerald Rhemann
This year’s shower is special in another way. According to Sky and Telescope magazine, meteor stream modeler Jeremie Vaubaillon predicts a bump in the number of Perseids around 1:39 p.m. (18:39 UT) as Earth encounters a debris trail shed by the Comet Swift-Tuttle back in 1862. The time favors observers in Asia where the sky will be dark. It should be interesting to see if the prediction holds.
How To Watch
Already the shower’s active. Go out any night through about the 15th and you’ll see at least at least a handful of Perseids an hour. At nightfall on the peak night of August 12-13, you may see only 20-30 meteors an hour because the radiant is still low in the sky. But these early hours give us the opportunity to catch an earthgrazer — a long, very slow-moving meteor that skims the atmosphere at a shallow angle, crossing half the sky or more before finally fading out.
I’ve only seen one good earthgrazer in my earthly tenure, but I’ll never forget the sight. Ambling from low in the northeastern sky all the way past the southern meridian, it remained visible long enough to catch it in my telescope AND set up a camera and capture at least part of its trail!
A Perseid meteor streaks across the northeastern sky two Augusts ago. Give the shower an hour’s worth of your time – you won’t be disappointed. Credit: Bob King
The later you stay up, the higher the radiant rises and the more meteors you’ll see. Peak activity of 50-100 meteors per hour will occur between about 2-4 a.m. No need to stare at the radiant to see meteors. You can look directly up at the darkest part of the sky or face east or southeast and look halfway up if you like. You’re going to see meteors everywhere. Some will arrive as singles, others in short burst of 2, 3, 4 or more. I like to face southeast with the radiant off to one side. That way I can see a mix of short-trailed meteors from near the radiant and longer, graceful streaks further away just like the snow photo shows.
If there’s a lull in activity, don’t think it’s over. Meteor showers have strange rhythms of their own. Five minutes of nothing can be followed by multiple hits or even a fireball. Get into the feel of the shower as you sense spaceship Earth speeding through the comet’s dusty orbit. Embrace the chill of the August night under the starry vacuum.
Various meteorites from 2008 TC3.
Credit: P. Jenniskens, et. al. Click image for full description
Asteroids, meteors, and meteorites … It might be fair to say these rocks from space inspire both wonder and fear among us Earthlings. But knowing a bit more about each of them and how they differ may eliminate some potential misgivings. While all these rocks originate from space, they have different names depending their location — i.e. whether they are hurtling through space or hurtling through the atmosphere and impacting Earth’s surface.
In simplest terms here are the definitions:
Asteroid: a large rocky body in space, in orbit around the Sun.
Meteoroid: much smaller rocks or particles in orbit around the Sun.
Meteor: If a meteoroid enters the Earth’s atmosphere and vaporizes, it becomes a meteor, which is often called a shooting star.
Meteorite: If a small asteroid or large meteoroid survives its fiery passage through the Earth’s atmosphere and lands on Earth’s surface, it is then called a meteorite.
Another related term is bolide, which is a very bright meteor that often explodes in the atmosphere. This can also be called a fireball.
Let’s look at each in more detail:
Asteroids An artists impression of an asteroid belt. Credit: NASA
Asteroids are found mainly in the asteroid belt, between Mars and Jupiter. Sometimes their orbits get perturbed or altered and some asteroids end up coming closer to the Sun, and therefore closer to Earth. In addition to the asteroid belt, however, there have been recent discussions among astronomers about the potential existence of large number asteroids in the Kuiper Belt and Oort Cloud. You can read a paper about this concept here, and a good article discussing the topic here.
The asteroid Vesta as seen by the Dawn spacecraft. Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA
Asteroids are sometimes referred to as minor planets or planetoids, but in general, they are rocky bodies that do not have an atmosphere. However, a few have their own moons. Our Solar System contains millions of asteroids, many of which are thought to be the shattered remnants of planetesimals – bodies within the young Sun’s solar nebula that never grew large enough to become planets.
The size of what classifies as an asteroid is not extremely well defined, as an asteroid can range from a few meters wide – like a boulder — to objects that are hundreds of kilometers in diameter. The largest asteroid is asteroid Ceres at about 952 km (592 miles) in diameter, and Ceres is so large that it is also categorized as a dwarf planet.
Most asteroids are made of rock, but as we explore and learn more about them we know that some are composed of metal, mostly nickel and iron. According to NASA, a small portion of the asteroid population may be burned-out comets whose ices have evaporated away and been blown off into space. Recently, astronomers have discovered some asteroids that mimic comets in that gas and dust are emanating from them, and as we mentioned earlier, there appears to be a large number of bodies with asteroid-like compositions but comet-like orbits.
How Often Do Asteroids Hit Earth?
Meteor Crater near Winslow, Arizona. Image credit: NASA.
While we know that some asteroids pass very close to Earth’s orbit around the Sun, we’ve been lucky in the history of humanity that we haven’t had a large asteroid hit Earth in the past several thousand years. It wasn’t until satellite imagery of Earth became widely available that scientists were able to see evidence of past asteroid impacts.
One of the more famous impact craters on Earth is Meteor Crater in Arizona in the US, which was made by an impact about 50,000 years ago. But there are about 175 known impact around the world – a few are quite large, like Vredefort Crater in South Africa which has an estimated radius of 190 kilometers (118 miles), making it the world’s largest known impact structure on Earth. Another notable impact site is off the coast of the Yucatan Peninsula in Mexico, and is believed to be a record of the event that led to the extinction of the dinosaurs 65 million years ago. You can see images of some of the most impressive Earth impact craters here.
These days, asteroid impacts are less of a threat. NASA estimates that about once a year an automobile-sized asteroid enters Earth’s atmosphere, creates an impressive fireball and disintegrates before ever reaching the surface. Studies of Earth’s history indicate that about once every 5,000 years or so on average an object the size of a football field hits Earth and causes significant damage. Once every few million years on average an object large enough to cause regional or global disaster impacts Earth. You can find more information about the frequency of impacts in this article from NASA.
Meteors, Meteoroids and Bolides
A bright meteor from September 21, 1994. Credit: John Chumack.
Space debris smaller than an asteroid are called meteoroids. A meteoroid is a piece of interplanetary matter that is smaller than an asteroid and frequently are only millimeters in size. Most meteoroids that enter the Earth’s atmosphere are so small that they vaporize completely and never reach the planet’s surface. When they burn up during their descent, they create a beautiful trail of light known as a meteor, sometimes called a shooting star.
Mostly these are harmless, but larger meteors that explode in the atmosphere – sometimes called bolides — can create shockwaves, which can cause problems. In February 2013 a meteor that exploded over Chelyabinsk, Russia shattered windows with its air blast. This meteoroid or bolide was estimated to be 18 meters (59 feet) in diameter. In 1908, a rocky meteoroid less than 100 meters in diameter is believed to have entered the atmosphere over the Tunguska region of Siberia in 1908 and the resulting shockwave knocked down trees for hundreds of square kilometers
How often is Earth hit by meteroids?
Chelyabinsk fireball recorded by a dashcam from Kamensk-Uralsky north of Chelyabinsk where it was still dawn.
Because of the Chelyabinsk meteor in 2013, astronomers have acquired more information about the frequency of larger meteors that hit Earth, and there is now a growing consensus that the Earth gets hit by bigger space rocks more often than we previously thought. You can read more about that concept here.
This video from the B612 Foundation shows a visualization of the location of 26 space rocks that hit Earth between 2000 and 2013, each releasing energy equivalent to some of our most powerful nuclear weapons. The B612 foundation says that a Hiroshima-scale asteroid explosion happens in our atmosphere on average once a year, but many are not detected because they explode high in the atmosphere, or because most of the Earth’s surface is water and even a large percentage of land is fairly uninhabited by humans.
Estimates vary of how much cosmic dust and meteors enter Earth’s atmosphere each day, but range anywhere from 5 to 300 metric tons. Satellite observations suggest that 100-300 metric tons of cosmic dust enter the atmosphere each day. This figure comes from the rate of accumulation in polar ice cores and deep-sea sediments of rare elements linked to cosmic dust, such as iridium and osmium.
But other measurements – which includes meteor radar observations, laser observations and measurements by high altitude aircraft — indicate that the input could be as low as 5 metric ton per day. Read more about this here.
For a documented list of bolide events, you can check out this page from JPL.
Meteorite A stunning slice of the Glorieta pallasite meteorite cut thin enough to allow light to shine through its many olivine crystals. Credit: Mike Miller
If any part of a meteoroid survives the fall through the atmosphere and lands on Earth, it is called a meteorite. Although the vast majority of meteorites are very small, their size can range from about a fraction of a gram (the size of a pebble) to 100 kilograms (220 lbs) or more (the size of a huge, life-destroying boulder). Meteorites smaller than 2mm are classified as micrometeorites.
Meteorites have traditionally been divided into three broad categories, depending on their structure, chemical and isotopic composition and mineralogy. Stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites that are largely composed of metallic iron-nickel; and, stony-iron meteorites that contain large amounts of both metallic and rocky material.
Meteorites have also been found on the Moon and Mars and conversely, scientists have traced the origination of the meteorites found here on Earth to four other bodies: the Moon, Mars, the asteroid 4 Vesta, and the comet Wild 2. Meteorites are the source of a great deal of the knowledge that we have have about the composition of other celestial bodies.
How Often Do Meteorites Hit Earth?
On Feb. 28, 2009, Peter Jenniskens (SETI/NASA), finds his first 2008TC3 meteorite after an 18-mile long journey. “It was an incredible feeling,” Jenniskens said. The African Nubian Desert meteorite of Oct 7, 2008 was the first asteroid whose impact with Earth was predicted while still in space approaching Earth. 2008TC3 and Chelyabinsk are part of the released data set. (Credit: NASA/SETI/P.Jenniskens)
According to the Planetary Science Institute, it is estimated that probably 500 meteorites reach the surface of the Earth each year, but less than 10 are recovered. This is because most fall into water (oceans, seas or lakes) or land in remote areas of the Earth that are not accessible, or are just not seen to fall.
In short, the difference between asteroids and meteors all comes down to a question of location. Asteroids are always found in space. Once it enters an atmosphere, it becomes a meteor, and then a meteorite after it hits the ground. Each are made of the same basic materials – minerals and rock – and each originated in space. The main difference is where they are when they are being observed.
Time lapse-photo showing geminids over Pendleton, OR. Credit: Thomas W. Earle
Wouldn’t it be nice if a meteor shower peaked on a weekend instead of 3 a.m. Monday morning? Maybe even showed good activity in the evening hours, so we could get our fill and still get to bed at a decent hour. Wait a minute – this year’s Geminids will do exactly that!
Before moonrise this Saturday night December 13th, the Geminids should put on a sweet display. The radiant of the shower lies near the bright pair of stars, Castor and Pollux. Source: Stellarium
What’s more, since the return of this rich and reliable annual meteor shower occurs around 6 a.m. (CST) on Sunday December 14th, both Saturday and Sunday nights will be equally good for meteor watching. After the Perseids took a battering from the Moon last August, the Geminids will provide the best meteor display of 2014. They do anyway! The shower’s been strengthening in recent years and now surpasses every major shower of the year.
The official literature touts a rate of 120 meteors per hour visible from a dark sky site, but I’ve found 60-80 per hour a more realistic expectation. Either way, what’s to complain?
The third quarter Moon rises around midnight Saturday and 1 a.m. on Monday morning. Normally, moonlight would be cause for concern, but unlike many meteor showers the Geminids put on a decent show before midnight. The radiant, the location in the sky from which the meteors will appear to stream, will be well up in the east by 9:30 p.m. local time. That’s a good 2-3 hours of meteor awesomeness before moonrise.
The author takes in this year’s moon-drenched Perseids in comfort.
Shower watching is a total blast because it’s so simple. Your only task is to dress warmly and get comfortable in a reclining chair aware from the unholy glare of unshielded lighting. The rest is looking up. Geminid meteors will flash anywhere in the sky, so picking a direction to watch the shower isn’t critical. I usually face east or southeast for the bonus view of Orion lumbering up from the horizon.
Bring your camera, too. I use a moderately wide angle lens (24-35mm) at f/2.8 (widest setting), set my ISO to 800 or 1600 and make 30-second exposures. The more photos you take, the better chance of capturing a meteor. You can also automate the process by hooking up a relatively inexpensive intervalometer to your camera and have it take the pictures for you.
As you ease back and let the night pass, you’ll see other meteors unrelated to the shower, too. Called sporadics, they trickle in at the rate of 2-5 an hour. You can always tell a Geminid from an interloper because its path traces back to the radiant. Sporadics drop down from any direction.
Captured by an all-sky camera, a Geminid fireball brighter than Venus streaks across the sky above New Mexico on Dec. 14, 2011. Before disintegrating in the atmosphere the meteoroid was about 1/2 inch across. Credit: Marshall Space Flight Center, Meteoroid Environments Office, Bill Cooke
Geminid meteors immolate in Earth’s atmosphere at a moderate speed compared to some showers – 22 miles per second (35 km/sec) – and often flare brightly. Green, red, blue, white and yellow colors have been recorded, making the shower one of the more colorful. Most interesting, the meteoroid stream appears to be sorted according to size with faint, telescopic meteors maxing out a day before the naked eye peak. Larger particles continue to produce unusually bright meteors up to a few days after maximum.
Most meteor showers are the offspring of comets. Dust liberated from vaporizing ice gets pushed back by the pressure of sunlight to form a tail and fans out over the comet’s orbital path. When Earth’s orbit intersects a ribbon of this debris, sand and gravel-sized bits of rock crash into our atmosphere at high speed and burn up in multiple flashes of meteoric light.
Phaethon sprouts a tail (points southeast or to lower left) when close to the Sun in this image taken by NASA’s STEREO Sun-observing spacecraft in 2012. Credit: Credit: Jewitt, Li, Agarwal /NASA/STEREO
But the Geminids are a peculiar lot. Every year in mid-December, Earth crosses not a comet’s path but that of 3200 Phaethon (FAY-eh-thon), a 3.2 mile diameter (5.1 km) asteroid. Phaethon’s elongated orbit brings it scorchingly close (13 million miles) to the Sun every 1.4 years. Normally a quiet, well-behaved asteroid, Phaethon brightened by a factor of two and was caught spewing jets of dustwhen nearest the Sun in 2009, 2010 and 2012. Apparently the intense heat solar heating either fractured the surface or heated rocks to the point of desiccation, creating enough dust to form temporary tails like a comet.
While it looks like an asteroid most of the time, Phaethon may really be a comet that’s still occasionally active. Periodic eruptions provide the fuel for the annual December show.
Most of us will head out Saturday or Sunday night and take in the shower for pure enjoyment, but if you’d like to share your observations and contribute a bit of knowledge to our understanding of the Geminids, consider reporting your meteor sightings to the International Meteor Organization. Here’s the link to get started.
And this just in … Italian astronomer Gianluca Masi will webcast the shower starting at 8 p.m. CST December 13th (2 a.m. UT Dec. 14) on his Virtual Telescope Project site.
This diagram maps data gathered from 1994-2013 on small asteroids impacting Earth's atmosphere to create very bright meteors (bolides). The location of impacts from objects ranging from 1 meter (3 feet) to nearly 20 meters (60 feet) in size such as Chelyabinsk asteroid are shown globally. (Credit: Planetary Science, NASA)
How hazardous are the thousands and millions of asteroids that surround the third rock from the Sun – Earth? Since an asteroid impact represents a real risk to life and property, this is a question that has been begging for answers for decades. But now, scientists at NASA’s Jet Propulsion Laboratory have received data from a variety of US Department of Defense assets and plotted a startling set of data spanning 20 years.
This latest compilation of data underscores how frequent some of these larger fireballs are, with the largest being the Chelyabinsk event on February 15, 2013 which injured thousands in Russia. The new data will improve our understanding of the frequency and presence of small and large asteroids that are hazards to populated areas anywhere on Earth.
On Feb. 28, 2009, Peter Jenniskens (SETI/NASA), finds his first 2008TC3 meteorite after an 18-mile long journey. “It was an incredible feeling,” Jenniskens said. The meteorite which impacted in the Nubian Desert of Africa on Oct 7, 2008 was the first asteroid whose impact with Earth was predicted while still in space approaching Earth. Meteorite 2008TC3 and Chelyabinsk’s are part of the released data set. (Credit: NASA/SETI/P.Jenniskens)
The data from “government sensors” – meaning “early warning” satellites to monitor missile launches (from potential enemies) as well as infrasound ground monitors – shows the distribution of bolide (fireball) events. The data first shows how uniformly distributed the events are around the world. This data is now released to the public and researchers for more detailed analysis.
The newest data released by the US government shows both how frequent bolides are and also how effectively the Earth’s atmosphere protects the surface. A subset of this data had been analyzed and reported by Dr. Peter Brown from the University of Western Ontario, Canada and his team in 2013 but included only 58 events. This new data set holds 556 events.
The newly released data also shows how the Earth’s atmosphere is a superior barrier that prevents small asteroids’ penetration and impact onto the Earth’s surface. Even the 20 meter (65 ft) Chelyabinsk asteroid exploded mid-air, dissipating the power of a nuclear blast 29.7 km (18.4 miles, 97,400 feet) above the surface. Otherwise, this asteroid could have obliterated much of a modern city; Chelyabinsk was also saved due to sheer luck – the asteroid entered at a shallow angle leading to its demise; more steeply, and it would have exploded much closer to the surface. While many do explode in the upper atmosphere, a broad strewn field of small fragments often occurs. In historical times, towns and villages have reported being pelted by such sprays of stones from the sky.
NASA and JPL emphasized that investment in early detection of asteroids has increased 10 fold in the last 5 years. Researchers such as Dr. Jenniskens at the SETI Institute has developed a network of all-sky cameras that have determined the orbits of over 175,000 meteors that burned up in the atmosphere. And the B612 Foundation has been the strongest advocate of discovering of all hazardous asteroids. B612, led by former astronauts Ed Lu and Rusty Schweikert has designed a space telescope called Sentinel which would find hazardous asteroids and help safeguard Earth for centuries into the future.
Speed is everything. While Chelyabinsk had just 1/10th the mass of Nimitz-class super carrier, it traveled 1000 times faster. Its kinetic energy on account of its speed was 20 to 30 times that released by the nuclear weapons used to end the war against Japan – about 320 to 480 kilotons of TNT. Briefly, asteroids are considered to be any space rock larger than 1 meter and those smaller are called meteoroids.
Two earlier surveys can be compared to this new data. One by Eugene Shoemaker in the 1960s and another by Dr. Brown. The initial work by Shoemaker using lunar crater counts and the more recent work of Dr. Brown’s group, utilizing sensors of the Department of Defense, determined estimates of the frequency of asteroid impacts (bolide) rates versus the size of the small bodies. Those two surveys differ by a factor of ten, that is, where Shoemaker’s shows frequencies on the order of 10s or 100s years, Brown’s is on the order of 100s and 1000s of years. The most recent data, which has adjusted Brown’s earlier work is now raising the frequency of hazardous events to that of the work of Shoemaker.
The work of Dr. Brown and co-investigators led to the following graph showing the frequency of collisions with the Earth of asteroids of various sizes. This plot from a Letter to Nature by P. Brown et al. used 58 bolides from data accumulated from 1994 to 2014 from government sensors. Brown and others will improve their analysis with this more detailed dataset. The plot shows that a Chelyabinsk type event can be expected approximately every 30 years though the uncertainty is high. The new data may reduce this uncertainty. Tungunska events which could destroy a metropolitan area the size of Washington DC occur less frequently – about once a century.
The estimated cumulative flux of impactors at the Earth. The bolide impactor flux at Earth (Bolide flux 1994-2013 – black circles) based on ~20 years of global observations from US Government sensors and infrasound airwave data. Global coverage averages 80% among a total of 58 observed bolides with E > 1 kt and includes the Chelyabinsk Chelyabinsk bolide (far right black circle). This coverage correction is approximate and the bolide flux curve is likely a lower limit. The full caption is at bottom. (Credit: P. Brown, Letter to Nature, 2013, Figure 3)
Asteroids come in all sizes. Smaller asteroids are much more common, larger ones less so. A common distribution seen in nature is represented by a bell curve or “normal” distribution. Fortunately the bigger asteroids number in the hundreds while the small “city busters” count in the 100s of thousands, if not millions. And fortunately, the Earth is small in proportion to the volume of space even just the space occupied by our Solar System. Additionally, 69% of the Earth’s surface is covered by Oceans. Humans huddle on only about 10% of the surface area of the Earth. This reduces the chances of any asteroid impact effecting a populated area by a factor of ten.
Altogether the risk from asteroids is very real as the Chelyabinsk event underscored. Since the time of Tugunska impact in Siberia in 1908, the human population has quadrupled. The number of cities of over 1 million has increased from 12 to 400. Realizing how many and how frequent these asteroid impacts occur plus the growth of the human population in the last one hundred years raises the urgency for a near-Earth asteroid discovery telescope such as B612’s Sentinel which could find all hazardous objects in less than 10 years whereas ground-based observations will take 100 years or more.
The estimated cumulative flux of impactors at the Earth. The bolide impactor flux at Earth (Bolide flux 1994-2013 – black circles) based on ~20 years of global observations from US Government sensors and infrasound airwave data. Global coverage averages 80% among a total of 58 observed bolides with E > 1 kt and includes the Chelyabinsk Chelyabinsk bolide (far right black circle). This coverage correction is approximate and the bolide flux curve is likely a lower limit. The brown-coloured line represents an earlier powerlaw fit from a smaller dataset for bolides between 1 – 8 m in diameter15. Error bars represent counting statistics only. For comparison, we plot de-biased estimates of the near-Earth asteroid impact frequency based on all asteroid survey telescopic search data through mid- 2012 (green squares)8 and other earlier independently analysed telescopic datasets including NEAT discoveries (pink squares) and finally from the Spacewatch (blue squares) survey, where diameters are determined assuming an albedo of 0.1. Energy for telescopic data is computed assuming a mean bulk density of 3000 kgm-3 and average impact velocity of 20.3 kms-1. The intrinsic impact frequency for these telescopic data was found using an average probability of impact for NEAs as 2×10-9 per year for the entire population. Lunar crater counts converted to equivalent impactor flux and assuming a geometric albedo of 0.25 (grey solid line) are shown for comparison9, though we note that contamination by secondary craters and modern estimates of the NEA population which suggest lower albedos will tend to shift this curve to the right and down. Finally, we show estimated influx from global airwave measurements conducted from 1960-1974 which detected larger (5-20m) bolide impactors (upward red triangles) using an improved method for energy estimation compared to earlier interpretations of these same data.
Ron Cottrell captured the sunspot in all its swirly magnetic goodness in hydrogen-alpha light on October 19. To appreciate its size, he included the Earth (lower left) for reference. Credit: Ron Cottrell
That’s one big, black blemish on the Sun today! Rarely have we been witness to such an enormous sunspot. Lifting the #14 welder’s glass to my eyes this morning I about jumped back and bumped into the garage.
Properly shielded, it was very easy to see with the naked eye. Unlike some other naked eye sunspots, this one showed structure. The eastern end was darker, the western half grayer and more extended.
Watch the giant spot rotate into view and grow right before your eyes in this 72-hour time-lapse video taken by SOHO’s HMI imager Oct. 18-20, 2014
Through a small telescope, the mix of dark umbras scattered amid weirdly sculpted penumbral “islands” was incredible to see. Photographs like the one above are wonderful documents, but witnessing this beautiful complex magnetic mess with your own eyes is another experience altogether. Region 2192 continues to grow and size and complexity and is now the largest sunspot group of solar cycle 24which began in 2009 – more than five years ago!
Active region 2192 is now the largest sunspot group to appear in over five years. Compare to Jupiter and the Earth. Credit: SDO/HMI/Alex Young
Every sunspot marks a region on the Sun’s shiny outer skin called the photosphere where magnetic energy is concentrated. Strong magnetic fields within a sunspot group quell the turbulent churning of the photosphere, chilling the region by several thousand degrees. Sunspots appear dark against the Sun’s blazing disk because they’re cooler. Cooler meaning 8,000 F instead of 11,000 F, so yes, they’re still VERY hot.
Watch as Region 2192 crackles with energy and flares as seen in far ultraviolet light with NASA’s Solar Dynamics Observatory.
Energy stored in sunspots’ twisted magnetic fields can suddenly be released in violent, explosions called solar flares. Billions of tons of solar plasma – the sizzling mix of protons and electrons that composes the Sun – are heated to millions of degrees during the explosion and rapidly accelerated into space. Radiation from radio waves to X-rays and gamma rays fans out at the speed of light. Fortunately for us, our atmosphere and planetary magnetic field protect us from most of what flares can fling our way.
NASA’s Solar Dynamics Observatory took this photo of the Sun and Jupiter-sized sunspot region 2192 this morning Oct. 22 at 8:45 a.m. CDT. The view in a small telescope equipped with a safe solar filter is even better! Credit: NASA
But as the Sun rotates this monster into our line of sight, possibilities for Earth-directed flares and coronal mass ejections increase as do geomagnetic storms, the bringer of auroras. Already in the past 48 hours, the spot has dished out seven M-class flares and a powerful X-1 flare even before it has fully come into view. There’s more to come – Region 2192 harbors an unstable beta-gamma-delta magnetic field ripe for additional flaring including more of the X-classvariety.
The Sun on October 21 showing smaller sunspot regions along with our featured group. Credit: Sarah and Simon Fisher
There’s no doubt now that this behemoth will stick around to add a whole new dimension to tomorrow’s partial solar eclipse. I can’t wait to see the Moon’s black curve approach and at least partially occult the group from view. If you’re interested in getting some one-of-a-kind pictures of the scene, please see our own Dave Dickinson’s excellent guide on photographing the partial eclipse.
A sliver of a Moon rises in morning twilight today October 22 just a day away from its appointment with the Sun. Credit: Bob King
While we’re on the Moon, early morning risers had the pleasure of its company just one day before New Moon and solar eclipse. I was out watching the Orionid meteor shower. While not rich like the Perseids or Geminids I managed to catch a few including a few lucky shots with the camera.
An Orionid meteor slashes across the top of the frame directly above the constellation Orion early this morning October 22, 2014. Details: 24mm lens, f/2.8, 30-seconds at ISO 1600. Credit: Bob King
The shower has peaked but will still be active the remainder of the week if you’re inclined to take a look. And I can’t resist. How about one last sweet close-up photo of sunspot group 2192? I have a feeling you won’t mind.
Monster Sunspot AR12192 taken by Karzaman Ahmad on October 21, 2014, from Langkawi Nagtional Observatory, Malaysia. Credit: Karzaman Ahmad and shared at spaceweather.com. Click the image to see additional animations and photos on Alex Young’s site
Thanks to the ubiquitousness of dashboard-mounted video cameras in Russia yet another bright object has been spotted lighting up the sky over Siberia, this time a “meteor-like object” seen on the evening of Saturday, Sept. 27.
The annual Perseid meteor shower radiates from a point in the constellation Perseus just below the W of Cassiopeia. Rates are usually about 100-120 meteors per hour from a dark, moonless sky at peak but will be cut in half due to moonlight this time around. This map shows the sky facing east around midnight Aug. 12-13. Source: Stellarium
Get ready for the darling of meteor showers this week — the Perseids. Who can deny their appeal? Not only is the shower rich with fiery flashes of meteoric light, but the meteors come in August when the weather’s couldn’t be more ideal. Peak activity is expected Tuesday night, Aug. 12-13, when up to 100 meteors an hour might be seen.
Ah, but there’s a rub. This year the moon will be only two days past full and radiant enough to drown out the fainter shower members. We’re more likely to see something like 30 meteors an hour, maybe fewer. But all it takes is one bright meteoric flash to make it all worthwhile. Nothing gets the heart pumping like a bright Perseid and the anticipation of the next.
While more meteors are surely more exciting, it’s not a number thing, but the experience of the raw event that makes all the difference. Sure beats sitting in front of a computer screen or watching the latest rerun of The Big Bang Theory, right?
A fine Perseid flashes straight out of the radiant on August 12 last year. The two bright dots above the start of the trail form the well-known Perseus Double Cluster. Credit: Bob King
Find a place away from glaring lights to allow your eyes to adapt to the darkness. That way you’ll see more meteors. While the Perseids spit out the occasional fireball, most shower members are going to be closer in brightness to the stars of the Big Dipper. Some leave “smoke” trails called meteor trains. They’re actually tubes of glowing air molecules created as the meteoroid particles speed through the atmosphere at 130,000 mph. Though ‘shooting stars’ can look surprisingly close by, they typically burn up 60-70 miles overhead.
Perseid meteors radiate from the constellation Perseus (hence the name) located a short distance below the “W” of Cassiopeia in the northeastern sky. To know for sure if you’ve seen the genuine item and not a random meteor, follow the trail backward — if it points toward the northeast, you’ve got a ringer!
A remarkable orbital view of a Perseid (right, center) burning up in Earth’s atmosphere photographed by astronaut Ron Garan on Aug. 13, 2011. The star Arcturus is directly above the bright trail. Credit: Ron Garan / ISS Expedition 28 crew / NASA
You can watch for Perseids all week long, but peak activity begins Tuesday evening and continues through dawn Wednesday. The later you stay up, the more meteors you’ll spot because the radiant or point in the sky from which the meteors appear to radiate rises higher with every hour. The higher the radiant, the fewer meteors that get cut off by the horizon.
Composite of bright Perseid meteors recorded by NASA all-sky cameras in 2011. Each is a grain of rock shed from the tail of comet 109P/Swift-Tuttle. Every year in mid-August, Earth passes through the comet’s debris trail as it orbits around the sun. Credit: NASA
The observing equipment you were born with and a comfortable chair are all you need to make the most of the event. OK, it’s nice to have a friend along, too, to share the ‘wow’ moments and keep from falling asleep. Sometimes I’m too lazy to haul out a chair and instead sprawl out on the deck or grass. Others prefer their Perseids from a steaming hot tub.
A 2010 Perseid meteor streaks over the European Southern Observatory’s Very Large Telescope (VLT). Credit: ESO
Left-behind sand, seed and pebble-sized particles from comet 109P/Swift-Tuttleare responsible for all the fun. Discovered in 1862, the comet circles the sun every 120 years. Over millennia, 109P has left a stream of debris along its orbit, which the Earth passes through every year in mid-August. Comet grit hits our atmosphere like bugs smacking a car windshield and vaporize in a flashes of light or meteors.
Normally I’d recommend facing east or southeast to watch the shower, but with the moon dominating that direction, look off to the northeast, north or southwest to keep from getting zapped by that old devil moonlight. Even a little dark adaption will help boost your Perseid count. Once situated, sit back, look up and enjoy each and every sparkler that drops from the sky.
And don’t forget to take in the big picture show rolling by. The sky’s a giant calendar that begins with the mid-summer constellations at nightfall and advances through the fall stars to the onset of winter with the rising of Orion at dawn. Let the months fall away as the Earth turns you toward the sun.
