A meteor that exploded in the air near the Dead Sea 3,700 years ago may have wiped out communities, killed tens of thousands of people, and provided the kernel of truth to an old Bible story. The area is in modern-day Jordan, in a 25 km wide circular plain called Middle Ghor. Most of the evidence for this event comes from archaeological evidence excavated at the Bronze Age city of Tall el-Hammam located in that area, which some scholars say is the city of Sodom from the Bible.
After months parked in front of a computer, I’m thrilled to announce the publication of my new book. The full title is — get ready for this — Wonders of the Night Sky You Must See Before You Die: The Guide to Extraordinary Curiosities of Our Universe. In a nutshell, it’s a bucket list of cosmic things I think everyone should see sometime in their life.
I couldn’t live without the sky. The concerns of Earth absorb so much of our lives that the sky provides an essential relief valve. It’s a cosmos-sized wilderness that invites both deep exploration and reflection. Galileo would kill to come back for one more clear night if he could.
To me, the stars are irresistible, but my sense is that many people don’t look up as much as they’d like. We forget. Get busy. Bad weather intervenes. So I thought hard about the essential “must-sees” for any watcher of the skies. Some are obvious, like a total solar eclipse or Saturn through a telescope, but others are just as interesting — if sometimes off the beaten path.
For instance, we always hear about asteroids in the news. What does a real one look like from your own backyard? I give directions and a map for seeing the brightest of them, Vesta. And if you’ve ever looked up at the Big Dipper and wondered how to find the rest of the Great Bear, I’ll get you there. I love red stars, so you’re going to find out where the reddest one resides and how to see it yourself. There’s also a lunar Top 10 for small telescope users and chapters on the awesome Cygnus Star Cloud and how to see a supernova.
The 57 different sights are a mix of naked-eye objects plus ones you’ll need an ordinary pair of binoculars or small telescope to see. At the end of each chapter, I provide directions on how and when to find each wonder. Because we live in an online world with so many wonderful tools available for skywatchers, I make extensive use of mobile phone apps that allow anyone to stay in touch with nearly every aspect of the night sky.
For the things that need a telescope, the resources section has suggestions and websites where you can purchase a nice but inexpensive instrument. Of course, you may not want to buy a telescope. That’s OK. I’m certain you’ll still enjoy reading about each of these amazing sights to learn more about what’s been up there all your life.
While most of the nighttime sights are visible from your home or a suitable dark sky site, you’ll have to travel to see others. Who doesn’t like to get out of the house once in a while? If you travel north or south, new places mean new stars and constellations. I included chapters on choice southern treats like Alpha Centauri, the Southern Cross and the Magellanic Clouds, the closest and brightest galaxies to our own Milky Way.
One of my favorite parts of the book is the epilogue, where I share a lesson my dog taught me about the present moment and cosmic time. I like to joke that if nothing else, the ending’s worth the price of the book.
The staff at Page Street Publishing did a wonderful job with the layout and design, so “Wonders” is beautiful to look at. Everyone who’s flipped through it likes the feel, and several people have even commented on how good it smells! And for those who understandably complained that the typeface in my first book, Night Sky with the Naked Eye, made it difficult to read, I’ve got good news for you. The new book’s type is bigger and easy on the eyes.
“Wonders” is 224 pages long, printed in full color and the same size as my previous book. Unlike the few but longer chapters of the first book, the new one has many shorter chapters, and you can dip in anywhere. I think you’ll love it.
The publication date is April 24, but you can pre-order it right now at Amazon, BN and Indiebound. I want to thank Fraser Cain here at Universe Today for letting me tell you a little about my book, and I look forward to the opportunity to share my night-sky favorites with all of you.
Earth is no stranger to meteors. In fact, meteor showers are a regular occurrence, where small objects (meteoroids) enter the Earth’s atmosphere and radiate in the night sky. Since most of these objects are smaller than a grain of sand, they never reach the surface and simply burn up in the atmosphere. But every so often, a meteor of sufficient size will make it through and explode above the surface, where it can cause considerable damage.
A good example of this is the Chelyabinsk meteoroid, which exploded in the skies over Russia in February of 2013. This incident demonstrated just how much damage an air burst meteorite can do and highlighted the need for preparedness. Fortunately, a new study from Purdue University indicates that Earth’s atmosphere is actually a better shield against meteors than we gave it credit for.
Their study, which was conducted with the support of NASA’s Office of Planetary Defense, recently appeared in the scientific journal Meteoritics and Planetary Science – titled “Air Penetration Enhances Fragmentation of Entering Meteoroids“. The study team consisted of Marshall Tabetah and Jay Melosh, a postdoc research associate and a professor with the department of Earth, Atmospheric and Planetary Sciences (EAPS) at Purdue University, respectively.
In the past, researchers have understood that meteoroids often explode before reaching the surface, but they were at a loss when it came to explaining why. For the sake of their study, Tabetah and Melosh used the Chelyabinsk meteoroid as a case study to determine exactly how meteoroids break up when they hit our atmosphere. At the time, the explosion came as quite the a surprise, which was what allowed for such extensive damage.
When it entered the Earth’s atmosphere, the meteoroid created a bright fireball and exploded minutes later, generating the same amount of energy as a small nuclear weapon. The resulting shockwave blasted out windows, injuring almost 1500 people and causing millions of dollars in damages. It also sent fragments hurling towards the surface that were recovered, and some were even used to fashion medals for the 2014 Sochi Winter Games.
But what was also surprising was how much of the meteroid’s debris was recovered after the explosion. While the meteoroid itself weighed over 9000 metric tonnes (10,000 US tons), only about 1800 metric tonnes (2,000 US tons) of debris was ever recovered. This meant that something happened in the upper atmosphere that caused it to lose the majority of its mass.
Looking to solve this, Tabetah and Melosh began considering how high-air pressure in front of a meteor would seep into its pores and cracks, pushing the body of the meteor apart and causing it to explode. As Melosh explained in a Purdue University News press release:
“There’s a big gradient between high-pressure air in front of the meteor and the vacuum of air behind it. If the air can move through the passages in the meteorite, it can easily get inside and blow off pieces.”
To solve the mystery of where the meteoroid’s mass went, Tabetah and Melosh constructed models that characterized the entry process of the Chelyabinsk meteoroid that also took into account its original mass and how it broke up upon entry. They then developed a unique computer code that allowed both solid material from the meteoroid’s body and air to exist in any part of the calculation. As Melosh indicated:
“I’ve been looking for something like this for a while. Most of the computer codes we use for simulating impacts can tolerate multiple materials in a cell, but they average everything together. Different materials in the cell use their individual identity, which is not appropriate for this kind of calculation.”
This new code allowed them to fully simulate the exchange of energy and momentum between the entering meteoroid and the interacting atmospheric air. During the simulations, air that was pushed into the meteoroid was allowed to percolate inside, which lowered the strength of the meteoroid significantly. In essence, air was able to reach the insides of the meteoroid and caused it to explode from the inside out.
This not only solved the mystery of where the Chelyabinsk meteoroid’s missing mass went, it was also consistent with the air burst effect that was observed in 2013. The study also indicates that when it comes to smaller meteroids, Earth’s best defense is its atmosphere. Combined with early warning procedures, which were lacking during the Chelyabinsk meteroid event, injuries can be avoided in the future.
This is certainly good news for people concerned about planetary protection, at least where small meteroids are concerned. Larger ones, however, are not likely to be affected by Earth’s atmosphere. Luckily, NASA and other space agencies make it a point to monitor these regularly so that the public can be alerted well in advance if any stray too close to Earth. They are also busy developing counter-measures in the event of a possible collision.
Halley’s Comet may be at the far end of its orbit 3.2 billion miles (5.1 billion km) from Earth, but this week fragments of it will burn up as meteors in the pre-dawn sky as the Eta Aquarid meteor shower. The comet last passed our way in 1986, pivoted about the Sun and began the long return journey to the chilly depths of deep space.
Today, Halley’s a magnitude +25 speck in the constellation Hydra. Although utterly invisible in most telescopes, you can imagine it below tonight’s half-moon near the outermost point in its orbit four Earth-sun distances beyond Neptune. Literally cooling its jets, the comet mulls its next Earth flyby slated for summer 2061.
Some meteor showers have sharp peaks, others like the Eta Aquarids, a broad, plateau-like maximum. The shower’s been active since mid-April and will continue right up till the end of this month with the peak predicted Saturday morning May 6. Observers in tropical latitudes, where the constellation Aquarius rises higher than it does from my home in northern Minnesota, will spy 25-30 meteors an hour from a dark sky in the hour or two before dawn.
Skywatchers further north will see fewer meteors because the radiant will be lower in the sky; meteors that flash well below the radiant get cut off by the horizon, reducing the rate by about half ( about 10-15 meteors an hour). That’s still a decent show. I got up with the first robins a couple years back to see the shower and was pleasantly surprised with a handful of flaming Halley particles in under a half hour.
While a low radiant means fewer meteors, there’s an up side. You have a fair chance of seeing an earthgrazer, a meteor that skims tangent to the upper atmosphere, flaring for many seconds before either burning up or skipping back off into space.
The Eta Aquarids will be active all week. With the peak occurring Saturday morning, you should be able to see at least a few prior to dawn each morning. The quarter-to-waxing gibbous moon will set in plenty of time through Friday morning, leaving dark skies, but cuts it close Saturday when it sets about the same time the radiant rises in the east.
For best viewing, find as dark a place as possible with an open view to the east and south. I like to tote out a reclining lawn chair, face east and get comfy under a warm sleeping bag or wool blanket. Since twilight starts about an hour and three-quarters before your local sunrise, plan to be out watching an hour before that or around 3:30 a.m. I know, I know. That sounds harsh, but I’ve discovered that once you make the commitment, the act of watching a meteor shower becomes a relaxed pleasure punctuated by the occasional thrill of seeing a bright meteor.
You’ll be in magnificent company, too. The Milky Way rides high across the southeastern sky at that hour, and Saturn gleams due south in Sagittarius at the start of dawn. If you’d like to contribute observations of the shower to help meteor scientists better understand its behavior and evolution, check out the International Meteor Organization’s Eta Aquariids 2017 campaign for more information.
If one of your New Year’s resolutions is to spend more time under the stars in 2017, you’ll have motivation to do so as soon as Tuesday. That morning, the Quadrantid (kwah-DRAN-tid) meteor shower will peak between 4 to about 6 a.m. local time just before the start of dawn. This annual shower can be a rich one with up to 120 meteors flying by an hour — under perfect conditions.
Those include no moon, a light-pollution free sky and most importantly, for the time of maximum meteor activity to coincide with the time the radiant is highest in the pre-dawn sky. Timing is everything with the “Quads” because the shower is so brief. Meteor showers occur when Earth passes through either a stream of dusty debris left by a comet or asteroid. With the Quads, asteroid 2003 EH1 provides the raw material — bits of crumbled rock flaked off the 2-mile-wide (~3-4 km) object during its 5.5 year orbit around the sun.
Only thing is, the debris path is narrow and Earth tears through it perpendicularly, so we’re in and out in a hurry. Just a few hours, tops. This year’s peak happens around 14 hours UT or 8 a.m. Central time (9 a.m. Eastern, 7 a.m. Mountain and 6 a.m. Pacific), not bad for the U.S. and Canada. The timing is rather good for West Coast skywatchers and ideal if you live in Alaska. Alaska gets an additional boost because the radiant, located in the northeastern sky, is considerably higher up and better placed than it is from the southern U.S. states.
The Quads will appear to radiate from a point in the sky below the Big Dipper’s handle, which stands high in the northeastern sky at the time. This area was once home to the now defunct constellation Quadrans Muralis (mural quadrant), the origin of the shower’s name. As with all meteor showers, you’ll see meteors all over the sky, but all will appear to point back to the radiant. Meteors that point back to other directions don’t belong to the Quads are called sporadic or random meteors.
Off-peak observers can expect at least a decent shower with up to 25 meteors an hour visible from a reasonably dark sky. Peak observers could see at least 60 per hour. Tropical latitude skywatchers will miss most of the the show because the radiant is located at or below the horizon, but they should be on the lookout for Earthgrazers, meteors that climb up from below the horizon and make long trails as they skirt through the upper atmosphere.
Set your clock for 4 or 5 a.m. Tuesday, put on a few layers of clothing, tuck hand warmers in your boots and gloves, face east and have at it! The Quads are known for their fireballs, brilliant meteors famous for taking one’s breath away. Each time you see one chalk its way across the sky, you’re witnessing the fiery end of an asteroid shard. As the crumble burns out, you might be fulfilling another resolution: burning away those calories while huddling outside to see the show.
Children ice skating in Khakassia, Russia react to the fall of a bright fireball two nights ago on Dec.6
In 1908 it was Tunguska event, a meteorite exploded in mid-air, flattening 770 square miles of forest. 39 years later in 1947, 70 tons of iron meteorites pummeled the Sikhote-Alin Mountains, leaving more than 30 craters. Then a day before Valentine’s Day in 2013, hundreds of dashcams recorded the fiery and explosive entry of the Chelyabinsk meteoroid, which created a shock wave strong enough to blow out thousands of glass windows and litter the snowy fields and lakes with countless fusion-crusted space rocks.
Documentary footage from 1947 of the Sikhote-Alin fall and how a team of scientists trekked into the wilderness to find the craters and meteorite fragments
Now on Dec. 6, another fireball blazed across Siberian skies, briefly illuminated the land like a sunny day before breaking apart with a boom over the town of Sayanogorsk. Given its brilliance and the explosions heard, there’s a fair chance that meteorites may have landed on the ground. Hopefully, a team will attempt a search soon. As long as it doesn’t snow too soon after a fall, black stones and the holes they make in snow are relatively easy to spot.
OK, maybe Siberia doesn’t get ALL the cool fireballs and meteorites, but it’s done well in the past century or so. Given the dimensions of the region — it covers 10% of the Earth’s surface and 57% of Russia — I suppose it’s inevitable that over so vast an area, regular fireball sightings and occasional monster meteorite falls would be the norm. For comparison, the United States covers only 1.9% of the Earth. So there’s at least a partial answer. Siberia’s just big.
Every day about 100 tons of meteoroids, which are fragments of dust and gravel from comets and asteroids, enter the Earth’s atmosphere. Much of it gets singed into fine dust, but the tougher stuff — mostly rocky, asteroid material — occasionally makes it to the ground as meteorites. Every day then our planet gains about a blue whale’s weight in cosmic debris. We’re practically swimming in the stuff!
Most of this mass is in the form of dust but a study done in 1996 and published in the Monthly Notices of the Royal Astronomical Society further broke down that number. In the 10 gram (weight of a paperclip or stick of gum) to 1 kilogram (2.2 lbs) size range, 6,400 to 16,000 lbs. (2900-7300 kilograms) of meteorites strike the Earth each year. Yet because the Earth is so vast and largely uninhabited, appearances to the contrary, only about 10 are witnessed falls later recovered by enterprising hunters.
A couple more videos of the Dec. 6, 2016 fireball over Khakassia and Sayanogorsk, Russia
Meteorites fall in a pattern from smallest first to biggest last to form what astronomers call a strewnfield, an elongated stretch of ground several miles long shaped something like an almond. If you can identify the meteor’s ground track, the land over which it streaked, that’s where to start your search for potential meteorites.
Meteorites indeed fall everywhere and have for as long as Earth’s been rolling around the sun. So why couldn’t just one fall in my neighborhood or on the way to work? Maybe if I moved to Siberia …
One of the best yearly meteor showers contends with the nearly Full Moon this year, but don’t despair; you may yet catch the Geminids.
The Geminid meteor shower peaks next week on the evening of Tuesday night into Wednesday morning, December 13th/14th. The Geminids are always worth keeping an eye on in early through mid-December. As an added bonus, the radiant also clears the northeastern horizon in the late evening as seen from mid-northern latitudes. The Geminids are therefore also exceptional among meteor showers for displaying early evening activity.
First, though, here is the low down of the specifics for the 2016 Geminids: the Geminid meteors are expected to peak on December 13th/14th at midnight Universal Time (UT), favoring Western Europe. The shower is active for a two week period from December 4th to December 17th and can vary with a Zenithal Hourly Rate (ZHR) of 50 to 80 meteors per hour, to short outbursts briefly topping 200 per hour. In 2016, the Geminids are expected to produce a maximum ideal ZHR of 120 meteors per hour. The radiant of the Geminids is located at right ascension 7 hours 48 minutes, declination 32 degrees north at the time of the peak, in the constellation of Gemini.
The Moon is a 98% illuminated waning gibbous just 20 degrees from the radiant at the peak of the Geminids, making 2016 an unfavorable year for this shower. In previous years, the Geminids produced short outbursts topping 200 per hour, as last occurred in 2014.
The Geminid meteors strike the Earth at a relatively slow velocity of 35 kilometers per second, and produce many fireballs with an r vaule of 2.6. The source of the Geminid meteors is actually an asteroid: 3200 Phaethon.
A moderate shower in the late 20th century, the Geminids have increased in intensity during the opening decade and a half of the 21st century, surpassing the Perseids for the title of the top annual meteor shower.
The Geminid shower seems to have breached the background sporadic rate around the mid-19th century. Astronomers A.C. Twining and R.P. Greg observing from either side of the pond in the United States and the United Kingdom both first independently noted the shower in 1862.
Orbiting the Sun once every 524 days, 3200 Phaethon wasn’t identified as the source of the Geminids until 1983. The asteroid is still a bit of a mystery; reaching perihelion just 0.14 astronomical units (AU) from the Sun, (interior to Mercury’s orbit) 3200 Phaethon is routinely baked by the Sun. Is it an inactive comet nucleus? Or a ‘rock comet’ in a transitional state?
Observing meteors is as simple as setting out in a lawn chair, laying back and watching with nothing more technical than a good ole’ Mk-1 pair of human eyeballs. Our advice for 2016 is to start watching early, like say this weekend, before the Moon reaches Full on Wednesday, December 14th. This will enable you to watch for the Geminids after morning moonset under dark skies pre-peak, and before moonrise on evenings post-peak.
Two other minor showers are also active next week: the Coma Bernicids peaking on December 15th, and the Leo Minorids peaking on December 19th. If you can trace a suspect meteor back to the vicinity of the Gemini ‘twin’ stars of Castor and Pollux, then you’ve most likely spied a Geminid and not an impostor.
And speaking of the Moon, next week’s Full Moon is not only known as the Full Cold Moon (For obvious reasons) from Algonquin native American lore, but is also the closest Full Moon to the December 21st, northward solstice. This means that next week’s Full Moon rides highest in the sky for 2016, passing straight overhead for locales sited along latitude 17 degrees north, including Guatemala City and Mumbai, India.
Photographing the Geminids is also as simple as setting a camera on a tripod and taking wide-field exposures of the sky. We like to use an intervalometer to take automated sequences about 30 seconds to 3 minutes in length. Said Full Moon will most likely necessitate shorter exposures in 2016. Keep a fresh set of backup batteries handy in a warm pocket, as the cold December night will drain camera batteries in a pinch.
Looking to contribute some meaningful scientific observations? Report those meteor counts to the International Meteor Organization.
And although the Geminids might be a bust in 2016, another moderate shower, the Ursids has much better prospects right around the solstice… more on that next week!
A flash of light recently reminded us of the most stunning sight we ever saw.
We managed to catch an early Leonid meteor this past Saturday morning while waiting for the new Chinese space station Tiangong-2 to pass over southern Spain. The Leonids are active this week, and although the light-polluting just past Super Moon lurks nearby, we’ve learned to never ignore this shower, even on an off year.
First though, here’s a rundown on what’s up with the Leonids in 2016:
The Leonid meteors are expected to peak on the night of Thursday, November 17th into the morning of Friday, November 18th. The shower is active for a 25 day span from November 5th to November 30th and though the Leonids can vary with an Zenithal Hourly Rate (ZHR) of thousands of meteors per hour, and short outbursts briefly topping hundreds of thousands per hour, in 2016, the Leonids are expected to produce a maximum ideal ZHR of only 10 to 15 meteors per hour. The radiant of the Leonids is located at right ascension 10 hours 8 minutes, declination 21.6 degrees north at the time of the peak, in the Sickle or backwards Question Mark asterism of the astronomical constellation of Leo the Lion.
The source of the Leonids is periodic Comet 55P/Tempel-Tuttle.
Now, for the bad news. The Moon is an 82% illuminated, waning gibbous phase at the peak of the Leonids, making 2016 an unfavorable year for this shower. In fact, the Moon is located just 42 degrees from the shower’s radiant in the nearby constellation of Gemini at the shower’s peak on Friday morning. In previous years, the Leonids produced a ZHR numbering in the 15-20 per hour. The estimated ZHR last topped 100 in 2008.
The Leonid meteors strike the Earth at a moderate/fast velocity of 71 km/s, and produce many fireballs with an r value of 2.5.
The Leonids are notorious for producing storms of epic proportions every 33 years. This last occurred in years surrounding 1999, and isn’t expected to occur again until around 2032. Some older observers still remember the great Leonid meteor storm over the southwestern United States in 1966, and the U.S. East Coast witnessed a massive storm in 1833.
We can attest to what the Leonids are capable of. We saw an amazing display from the shower in 1998 from Al Jaber Air Base in Kuwait, with an estimated rate of around 900 per hour towards dawn. When a shower edges towards a zenithal hourly rate of 1,000, you’re seeing meteors every few seconds, with fireballs lighting up the desert night.
And it is possible to defeat the waning gibbous Moon. Though the Moon is near the zenith as seen from the mid-northern latitudes in the early AM hours (the best time to watch the shower,) its almost always possible to view the shower with the Moon blocked behind a house or hill… unless you have the bad luck of viewing from latitude 20 degrees north, where the Moon crosses directly through the zenith on Friday morning.
But take heart, as we’re past the halfway mark in 2014, headed to the Leonid ‘storm years’ of the early 2030s.
Don’t miss the 2016 Leonids… if for no other reason, to catch a flash of storms to come.
Mars’ natural satellites – Phobos and Deimos – have been a mystery since they were first discovered. While it is widely believed that they are former asteroids that were captured by Mars’ gravity, this remains unproven. And while some of Phobos’ surface features are known to be the result of Mars’ gravity, the origin of its linear grooves and crater chains (catenae) have remained unknown.
But thanks to a new study by Erik Asphaug of Arizona State University and Michael Nayak from the University of California, we may be closer to understanding how Phobos’ got its “groovy” surface. In short, they believe that reaccretion is the answer, where all the material that was ejected when meteors impacted the moon eventually returned to strike the surface again.
Naturally, Phobos’ mysteries extend beyond its origin and surface features. For instance, despite being much more massive than its counterpart Deimos, it orbits Mars at a much closer distance (9,300 km compared to over 23,000 km). It’s density measurements have also indicated that the moon is not composed of solid rock, and it is known to be significantly porous.
Because of this proximity, it is subject to a great deal of tidal forces exerted by Mars. This causes its interior, a large portion of which is believed to consist of ice, to flex and stretch. This action, it has been theorized, is what is responsible for the stress fields that have been observed on the moon’s surface.
However, this action cannot account for another common feature on Phobos, which are the striation patterns (aka. grooves) that run perpendicular to the stress fields. These patterns are essentially chains of craters that typically measure 2o km (12 mi) in length, 100 – 200 meters (330 – 660 ft) in width, and usually 30 m (98 ft) in depth.
In the past, it was assumed that these craters were the result of the same impact that created Stickney, the largest impact crater on Phobos. However, analysis from the Mars Express mission revealed that the grooves are not related to Stickney. Instead, they are centered on Phobos’ leading edge and fade away the closer one gets to its trailing edge.
For the sake of their study, which was recently published in Nature Communications, Asphaug and Nayak used computer modelling to simulate how other meteoric impacts could have created these crater patterns, which they theorized were formed when the resulting ejecta circled back and impacted the surface in other locations.
As Dr. Asphaug told Universe Today via email, their work was the result of a meeting of minds that spawned an interesting theory:
“Dr. Nayak had been studying with Prof. Francis Nimmo (of UCSC), the idea that ejecta could swap between the Martian moons. So Mikey and I met up to talk about that, and the possibility that Phobos could sweep up its own ejecta. Originally I had been thinking that seismic events (triggered by impacts) might cause Phobos to shed material tidally, since it’s inside the Roche limit, and that this material would thin out into rings that would be reaccreted by Phobos. That still might happen, but for the prominent catenae the answer turned out to be much simpler (after a lot of painstaking computations) – that crater ejecta is faster than Phobos’ escape velocity, but much slower than Mars orbital velocity, and much of it gets swept up after several co-orbits about Mars, forming these patterns.”
Basically, they theorized that if a meteorite stuck Phobos in just the right place, the resulting debris could have been thrown off into space and swept up later as Phobos swung back around mars. Thought Phobos does not have sufficient gravity to reaccrete ejecta on its own, Mars’ gravitational pull ensures that anything thrown off by the moon will be pulled into orbit around it.
Once this debris is pulled into orbit around Mars, it will circle the planet a few times until it eventually falls into Phobos’ orbital path. When that happens, Phobos will collide with it, triggering another impact that throws off more ejecta, thus causing the whole process to repeat itself.
In the end, Asphaug and Nayak concluded that if an impact hit Phobos at a certain point, the subsequent collisions with the resulting debris would form a chain of craters in discernible patterns – possibly within days. Testing this theory required some computer modelling on an actual crater.
Using Grildrig (a 2.6 km crater near Phobos’ north pole) as an reference point, their model showed that the resulting string of craters was consistent with the chains that have been observed on Phobos’ surface. And while this remains a theory, this initial confirmation does provide a basis for further testing.
“The initial main test of the theory is that the patterns match up, ejecta from Grildrig for example.” said Asphaug. “But it’s still a theory. It has some testable implications that we’re now working on.”
In addition to offering a plausible explanation of Phobos’ surface features, their study is also significant in that it is the first time that sesquinary craters (i.e. craters caused by ejecta that went into orbit around the central planet) were traced back to their primary impacts.
In the future, this kind of process could prove to be a novel way to assess the surface characteristics of planets and other bodies – such as the heavily cratered moons of Jupiter and Saturn. These findings will also help us to learn more about Phobos history, which in turn will help shed light on the history of Mars.
“[It] expands our ability to make cross-cutting relationships on Phobos that will reveal the sequence of geologic history,” Asphaug added. “Since Phobos’ geologic history is slaved to the tidal dissipation of Mars, in learning the timescale of Phobos geology we learn about the interior structure of Mars”
And all of this information is likely to come in handy when it comes time for NASA to mount crewed missions to the Red Planet. One of the key steps in the proposed “Journey to Mars” is a mission to Phobos, where the crew, a Mars habitat, and the mission’s vehicles will all be deployed in advance of a mission to the Martian surface.
Learning more about the interior structure of Mars is a goal shared by many of NASA’s future missions to the planet, which includes NASA’s InSight Lander (schedules for launch in 2018). Shedding light on Mars geology is expected to go a long way towards explaining how the planet lost its magnetosphere, and hence its atmosphere and surface water, billions of years ago.
Further Reading: Nature Communications
The Perseid meteor shower must have looked fantastic from 10,000 feet. That’s how high you would have had to go to get past the pervasive fog and overcast skies at my home last night. Tonight looks a little better for weather, so I’ll do what all hopeful amateurs astronomers do. Set the alarm for 2 a.m. and peek out the shade looking for those glimmers of starlight that indicate clear skies.
From observations reported as of mid-afternoon to the International Meteor Observers 2016 Perseids Quick-Look site, it appears the greatest activity or highest meteor counts happened over Europe and points east in two outbursts: a brief but intense display around 23:15 Universal Time (6:15 p.m. CDT in daylight) August 11 when some observers briefly saw up to 15 Perseids a minute (!) with many bright ones, and a second peak starting around 2:00 UT (9 p.m. CDT) and lasting till 5:00 UT (midnight CDT).
90+ Perseid meteors captured on video August 11-12, 2016 by Ohio amateur John Chumack
While Europeans clearly hit the jackpot — some observers calling it the best since the 2002 Leonid storm — U.S. observers varied in their meteor counts. A few thought the shower was a bust, others reported numbers more typical of an “average year” shower. It appears that Earth passed through a dense filament of comet dust while it was night in Europe but late afternoon in the Americas. C’est la vie météore!
We should be past peak by today, but experience shows that tonight should still be a very good time for Perseid watching. Indeed, the next few nights will reward skywatchers with at least a dozen an hour. I’ll be out watching and hopefully not imagining what’s happening 10,000 feet over my head. Good luck to you too!