UPDATE: The webcast has been moved to March 16 at 17:00 UTC (1 pm EDT) due to bad weather in Italy.
Has it been cloudy where you live and you haven’t yet been able to see Comet PANSTARRS? The Virtual Telescope Project will have a live webcast of this comet, C/2011 L4 PANSTARRS, from Italy, March 15, on March 16 at 17:00 UTC, 1 p.m. EDT. “We have been waiting for it for over one year, and now the waiting is over,” said astrophysicist Gianluca Masi, who will host the webcast, which you can see at this link. Masi said they are keeping an eye on the skies, and will keep us updated on if they need to change the time of the webcast.
If you’re waiting for the weekend to see it with your own eyes, check out our detailed guides on how to see it here and here. Both are filled with graphics and great info on how to see this comet.
This comet has been a challenge to see, and was actually closest to the Sun on March 10, meaning that is when it was at its brightest. However, while Comet PANSTARRS will fade over the next few weeks, it will also rise higher into a darker sky and become – for a time – easier to see. So keep looking!
That red comet is pretty, but perhaps not so realistic. Credit: Game of Thrones Wiki/HBO (Screencap)
A blood-red comet appears in the sky. People quake in its wake.
This phenomenon, which happens in the second season of the medieval fantasy Game of Thrones, had us all wondering — can you ever actually see a red comet?
We talked to Matthew Knight, an astronomer at the Lowell Observatory in Arizona who observes comets. He gave us some answers just in time for the third season of Game of Thrones, which begins March 31.
At first blush, he said, the comet’s red color wouldn’t be possible because the strongest emissions from comets are in the blue and green regions, mostly from neutral gases such as hydroxide and cyanide.
There is a type of emission that is close to red, called “forbidden oxygen”, which occurs when atoms make a rare energy transition between states of “excitement”. But it’s very faint and short-lived, Knight wrote.
The visible light from a comet comes from a combination of reflected solar continuum (sunlight reflecting off of dust grains) and cometary emission (neutral and/or ionized molecules of gas that emit photons at a particular wavelength). The sunlight reflecting off of dust grains basically looks like sunlight and since the Sun appears yellow/white, this component cannot look red.
A small caveat is that due to the physical properties of dust grains, comet dust often actually does “redden” sunlight slightly when measured with sensitive equipment. However, this reddening is at a very low level and is not enough to cause the reflected sunlight to appear a deep red like in Game of Thrones. The strongest comet emissions in the region where human eyes can see are in the blue and green regions.
Comet Hale-Bopp. which displays the usual cometary colors. Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria
So what ingredients does a comet need to look like the one in Game of Thrones? According to Knight, it would have to meet these criteria:
Be visible in daylight, which really only happens about once a century;
Be close to the sun (he supposes this one is, given how straight the tail is);
Have a “strange composition” that is different from anything we know in the solar system. The composition could be that forbidden oxygen he talked about, coming from a comet whose ices are carbon monoxide and carbon dioxide. But that would be hard, because those types of ices would not survive long when exposed to sunlight.
If we really want to think in a science fiction vein, Knight suggests that maybe the comet could be made up unpredictably:
Alternatively it could be something else entirely unknown in cometary chemistry or dust, with really weird properties causing a much stronger reddening than is normally seen. In any event, the composition would be so anomalous that this comet would almost certainly have originated in another solar system. That would make comet scientists very interested in studying it!
But don’t despair yet. Comet ISON might be bright enough for daylight viewing when it swings by Earth late in 2013. Comets are unpredictable beasts, but we’re pretty sure of one thing: no matter how bright it is, it won’t look red.
Halley's Comet, as seen by the European Giotto probe. Credit: Halley Multicolor Camera Team, Giotto Project, ESA
NASA missed the chance to visit Halley’s Comet in 1986 when the famed sentinel swung close to Earth, as it does every 76 years. Luckily for history, the Europeans flew Giotto past it on this day (March 13) in 1986, and some other nations sent their own probes.
The full story of NASA’s withdrawal is in Bruce Murray’s Journey Into Space: The First Three Decades of Space Exploration. Murray, the former director of the Jet Propulsion Laboratory, has chapters upon chapters on Halley, but here are some notable highlights.
First of all, there were at least three initiatives for NASA to send a mission to the famed comet. The missions below are in chronological order, and it appears it was only when the preceding one was killed that the next was envisioned:
– Solar sail. This mission would use the power of the solar wind — bits streaming from the sun — to bring a spacecraft within Halley’s gravitational influence. In fact, the spacecraft would stay with Halley as it whisked out of the solar system and would return (long dead) when Halley came back in 2061.
– A rendezvous with Comet Tempel 2. Another idea would see a spacecraft swing close to Comet Tempel 2 but also have a probe that would take a picture of Halley from a distance. NASA also considered splitting the mission in two to meet annual budgetary requirements, but the Comet Science Working Group was cool to the idea. There also was some thought about bringing the Europeans into this mission, but that never worked out.
– Galileo-type hardware. A third initiative had the Jet Propulsion Laboratory envisioning a distant flyby of Halley, basically using similar types of parts that flew in a spacecraft (called Galileo) to Jupiter.
All three of these initiatives fell to budget cuts during the 1970s and 1980s. What caused the budget cuts? In large part, the space shuttle program. To be sure, the shuttle was an impressive piece of hardware, and we are not doubting what it contributed to the construction of the International Space Station and to human spaceflight in general. But it was a large project and in those tight times, something had to give.
Perhaps the most interesting cancellation came in 1979, when NASA administrator Robert Frosch and his deputy went to President Jimmy Carter’s office to plead for the case of two projects: a solar electric propulsion system that would eventually power the Halley-Tempel 2 mission, and the Compton Gamma Ray Observatory (which flew into space, after many delays, in 1991).
Carter, according to Murray, was reading a book on black holes penned by Walter Sullivan of the New York Times. (We’re assuming it’s the 1979 book Black Holes: The Edge of Space, the End of Time.) When presented with the options, Carter said he was “partial to the gamma-ray thing because of this connection with the black-hole problem.”
That signaled the beginning of the end for NASA’s Halley-Tempel 2 mission.
Comet C/2011 L4 (PANSTARRS) and the crescent Moon with earthshine over the Sonoran Desert. Credit and copyright: Nic Leister.
Astrophotographers were out in force last night to try and capture Comet PANSTARRS (C/2011 L4 PANSTARRS) as it posed next to the setting crescent Moon. Those with clear skies were rewarded with great views, such as this very picturesque view from Arizona by Nic Leister. See more below:
Comet PANSTARRS and the Waxing Crescent Moon as seen over Castroville, Texas on March 12, 2013. Credit and copyright: Adrian New.
Adrian New wrote via email: “Here in historic Castroville, Texas we had an impressive view of the Comet PANSTARRS and the waxing crescent Moon. Both were easily visible close to the horizon and not affected by the light towers. Taken with a Nikon D800 at ISO 800 and a 2 second exposure at F/4. Lens was a Nikon 300mm F/4.”
Comet PANSTARRS and the lunar crescent in a colorful Arizona sunset, March 12, 2013. Credit and copyright: Chris Schur.
Chris Schur said, “The comet was an easy naked eye object with tail from Arizona, at our elevation of 5150 feet.” This image was taken March 12th around 7:15 MST.
Comet PANSTARRS and the very young Moon, seen in Salem, Missouri on March 12, 2013. Credit and copyright: Joe Shuster, Lake County Astronomical Society.
Joe Shuster from Missouri said he managed to outlast some clouds to get a shot of PANSTARRS and the very young Moon. He used a Canon T1i, Nikon 200mm AIS lens, ISO 800, 4s.
Crescent Moon and Comet PANSTARRS over Columbia, Missouri, March 12, 2013. Credit and copyright: Naghrenhel on Flickr.
Naghrenhel on Flickr shared the story of this image: “It was a very cloudy night and I’d almost given up locating the comet PanStarrs. Then I caught a glimpse of the moon, only 2% illuminated, and decided to take a picture. I was pleasantly surprised to see the moon’s companion appear. I still couldn’t see it with an unaided eye, probably due to city light pollution. But the right exposure of the camera caught the comet. Thanks to the Universe Today website informing me of their close proximity or I would have missed the comet completely.”
Comet PANSTARRS as seen from Gastonia, North Carolina on March 12, 2013. Credit and copyright: Jim Craig.Comet PANSTARRS from 3/12/2013 at about 7:50 pm. up on Mt. Wilson above Los Angeles. Credit: Tim Song Jones.Comet PANSTARRS as seen through the clouds in Indianapolis, Indiana. Credit: John Chumack.
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Comet PANSTARRS seen over Venice, California on March 11, 2013. Credit and copyright: Thad Szabo.
NASA scientist Fred Espenak captured this wonderful timelapse video of Comet PANSTARRS as it set over the Dos Cabezas Mountains in Arizona. The photos were taken from San Simon, AZ using a Nikon D90 and Nikkor 18-200 VR zoom lens at 200mm. All exposures were 2 seconds at F/5.6 (ISO 800).
I’m now seriously jealous, as my location has been socked in with clouds all week so far. If you’re in the same boat, enjoy some more images of Comet PANSTARRS from Universe Today readers:
Comet PANSTARRS as seen over Fountain Hills, Arizona. Credit and copyright: Nice Leister,Comet PANSTARRS from Tucson, Arizona on March 11, 2013. Credit and copyright: Rob Sparks.Comet PANSTARRS on March 11, 2013. Credit: Adam Block/Mount Lemmon Sky Center.Comet PANSTARRS over Alabama USA. Credit an copyright: Kristen Lyles..
Comet PANSTARRS as seen from Arizona on March 10, 2013. Credit and copyright: Chris Schur.
The first of three bright comets anticipated in 2013 became visible to North American observers this past weekend. Comet C/2011 L4 PanSTARRS is now currently visible low to the southwest at dusk, if you know exactly where to look for it.
Observers in the southern hemisphere have been enjoying this comet for the past few weeks as it reached naked eye visibility above 6th magnitude around late February and began its long trek northward. Comet PanSTARRS is on a 106,000+ year orbit with a high inclination of 84.2° with respect to the ecliptic. This also means that PanSTARRS is currently moving roughly parallel to the “0 Hour” line in Right Ascension (The same point occupied by the Sun during next week’s Vernal Equinox on March 20th) and is only slowly gaining elevation on successive evenings.
Observers in Hawaii and Mexico picked up PanSTARRS late last week, and scattered reports of sightings from the southern continental United States started trickling in Saturday night on the evening of March 9th. We managed to grab Comet PanSTARRS low to the southwest on Sunday evening on March 10th, about 30 minutes after local sunset.
Comet PanSTARRS seen from Hudson Florida on the evening of March 10th (Photo by Author).
We were surprised by the star-like appearance of the coma, about +1st to 2nd magnitude with a tiny fan-shaped tail. The comet was visible in binoculars only (I used our trusty pair of Canon 15×45 Image-Stabilized binocs for the task) and I couldn’t yet pick out the comet with the naked eye.
Several sightings westward followed. Clay Davis based in Santa Fe, New Mexico noted a visual magnitude of -0.5, saying that PanSTARRS was “Brighter than Mars” at magnitude +1 but “A challenge to keep in view.” Note that observer estimations of the brightness of comets can vary based on local sky conditions. Also, unlike a pinpoint star, the brightness of comets extends over its visible surface area, much like a faint nebula. The first sightings of the comet for many observers has been contingent on the weather, which can trend towards overcast for much of North America in early March. From our +28.5° northern latitude vantage point here just north of Tampa Bay Florida we had about a 10 minute window from when the sky was dark enough to spy PanSTARRS before it set below the local horizon.
Here are a few more images from Universe Today readers:
Comet PanSTARRS as imaged by Robert Sparks (@HalfAstro) on the night of March 10th from Tucson, Arizona. All Rights Reserved, part of the Universe Today photo gallery.First views of Comet PANSSTARRS from Tucson, Arizona. Credit and copyright: Adam Block/Mount Lemmon SkyCenter.Comet PANSTARRS from Puerto Rico on March 10, 2013. Credit and copyright: Efrain Morales.
To see the comet we suggest;
A clear uncluttered southwestern horizon;
A reasonably clear sky;
Binoculars.
First naked eye sightings of the comet for U.S. and European latitudes should be forthcoming over the next few evenings. PanSTARRS just passed perihelion yesterday on March 10th at 0.3 Astronomical Units from the Sun (or 46.5 million kilometres, just inside the orbit of Mercury).
Comet PanSTARRS looking west at 8PM EDT the evening of March 12th from latitude 30 degrees north. (Created by the author using Starry Night).
And Comet PanSTARRS may put on its best show over the next few nights. The Moon reaches New phase today at 3:51PM EDT/ 19:51 UT and starts lunation number 1116. On the next few evenings, the slim crescent Moon will slide by Comet PanSTARRS. Look for the 2% illuminated Moon 5° to the lower right of the comet on the evening of Tuesday March 12th. On the next evening, the 5% illuminated Moon will be 9° above Comet PanSTARRS on Wednesday, March 13th. The age of the Moon will be 28 hours old on Tuesday evening and 52 hours on Wednesday the 13th respectively, an easy catch. The Moonwatch website is a great place to check for those early lunar crescent sighting possibilities worldwide. Note that Comet PanSTARRS also passes less than 30’ from the planet Uranus (about the diameter of the Full Moon) on the evening of the 12th at 8 PM EDT/24UT. +6th magnitude Uranus may just be visible near the head of the comet using binoculars or a small telescope. Keep in mind, they just appear to be close as seen from our Earthly vantage point. PanSTARRS is currently 1.1 A.U.s from the Earth, while Uranus is on the other side of the solar system at 21 A.U.s distant!
Comet PanSTARRS looking west at 8PM EDT from latitude 30 degrees north on the evening of March 13th. (Created by the author using Starry Night).
PanSTARRS also crosses the Celestial Equator today on March 11th and the Ecliptic on March 13th. Observers from dark sky sites may get the added bonus of the zodiacal light, a true photographic opportunity!
Spacecraft studying the Sun are also giving us views of Comet PanSTARRS from a different perspective. NASA’s twin STEREO A & B spacecraft are positioned to monitor the Sun from different vantage points along the Earth’s orbit. Often, they see comets as an added bonus. Comet PanSTARRS has just moved into the field of view of STEREO-B’s Heliospheric Imager and has given us amazing views of the comet and the Earth in the distance over the past week.
The view of Comet PanSTARRS from NASA’s STEREO Behind observatory. (Credit: NASA/SECCHI).
From STEREO, the remarkable fan-shaped dust tail of PanSTARRS stands out in profile. The dust tail of a comet always points away from the Sun. Driven by the solar wind, a comet’s tail is actually in front of it as it heads back out of the solar system! An ultimate animation of Comet PanSTARRS just came to our attention today via @SungrazerComets on Twitter;
Animation of comet 2011 L4 PanSTARRS entering STEREO-B’s HI camera, note the twin ion/dust tail reminiscent of Hale-Bopp! (Credit: NASA/STEREO/NRL).
As of this writing, PanSTARRS seems to be performing as per predictions with an observed magnitude of around +1. The comet will continue on its northward trek, becoming a circumpolar object for observers based around latitude 50° north on April 2nd. Comet PanSTARRS should dip back below +6th magnitude around April 15th.
Comet PanSTARRS as imaged by Mike Weasner from Cassiopeia Observatory in southern Arizona on the night of March 10th. Used with permission.
But this is but Act One in a forecasted three act cometary saga for 2013. Comet C/2012 F6 Lemmon will grace early dawn skies in April for northern hemisphere observers, and then all eyes will be on Comet C/2012 S1 ISON for the hoped for grand finale later this year. Interestingly, ESA’s Solar Heliospheric Observatory will get a look at this sungrazing comet as it passes through its LASCO C3 camera’s field of view. Clear skies, and may 2013 go down as the Year of the Comet!
-Check out photos of Comet PanSTARRS and more being added daily to the Universe Today’s Flickr gallery.
Comet L4 PANSTARRS setting over Brindabella Ranges to the west of Canberra, Australia on March 5, 2013. The photo gives a good idea of the naked eye of the comet. Credit: Vello Tabur
This is the big week so many of us in the northern hemisphere have been waiting for. Comet C/2011 L4 PANSTARRS, which has put on a splendid show in the southern hemisphere, now finally comes to a sky near us northerners!
Sky watchers in Australia and southern South America report it looks like a fuzzy star a little brighter than those in the Big Dipper with a short stub of a tail visible to the naked eye. The comet should brighten further as it wings its way sunward. Closest approach to the sun happens on March 10 at a distance of 28 million miles. That’s about 8 million miles closer than the orbit of Mercury.
Though very low in the western sky after sundown, the comet should be visible across much of the U.S., southern Canada and Europe beginning tonight March 8.
Comet PANSTARRS will be visible through about March 19 for sky watchers living near the equator. Map is drawn for Singapore. All maps created with Chris Marriott’s SkyMap software
PANSTARRS’ low altitude presents a few challenges. Approaching clouds, general haziness and the extra thickness of the atmosphere near the horizon absorbs the comet’s light, causing it to appear fainter than you’d expect. A casual sky watcher may not even notice its presence. That’s why I recommend bringing along a pair of binoculars and using the map that best fits your latitude. Find a place with a wide open view to the west, focus your binoculars on the most distant object you can find (clouds are ideal) and then slowly sweep back and forth across the sky low above the western horizon
Comet PANSTARRS map for the southern U.S. March 6-21. Time shown is about 25 minutes after sunset facing west. Map is drawn for Phoenix, Ariz.
As the nights pass, PANSTARRS rises higher in the sky and becomes easier to spot for northern hemisphere observers while disappearing from view in the south. On the 12th, a thin lunar crescent will shine just to the right of the comet. Not only will it make finding this fuzzy visitor easy-peasy, but you’ll have the opportunity to make a beautiful photograph.
Comet PANSTARRS and the thin crescent Moon should make a striking sight together about a half hour to 45 minutes after sunset on March 12. Stellarium
The maps shows the arc of the comet across the western sky in the coming two weeks for three different latitudes. Along the bottom of each map is the comet’s altitude in degrees for the four labeled dates. The sun, which is below the horizon, but whose bright glow you’ll see above its setting point, will help you determine exactly in what direction to look.
One of your best observing tools and the one closest at hand (pun intended) is your hand. Photo: Bob King
A word about altitude. Astronomers measure it in degrees. One degree is the width of your little finger held at arm’s length against the sky. Believe it or not, this covers two full moon’s worth of sky. Three fingers at arm’s length equals 5 degrees or the separation between the two stars at the end of the bowl of the Big Dipper. A fist is 10 degrees. This weekend PANSTARRS will be 2-3 “fingers” high around 25 minutes after sunset when the sky is dark enough to go for it.
The northern U.S. is favored for this leg of the comet’s journey. Notice how the comet arcs up higher in the sky compared to the southern U.S. and especially the equator. Map drawn for Duluth, Minn. The comet will remain visible for many weeks. Earth is closest to PANSTARRS on March 5 at 102 million miles.
To find PANSTARRS, locate it on the map for a particular date, note its approximate altitude and relation to where the sun set and look in that direction. Assuming your sky to the west is wide open and clear, you should see a comet staring back. If you don’t find it one night, don’t give up. Go out the next clear night and try again. While Comet PANSTARRS will fade over the next few weeks, it will also rise higher into a darker sky and become – for a time – easier to see. I also encourage you to take out your telescope for a look. You’ll see more color in the comet’s head, details in its tail and an intensely bright nucleus (center of the comet), a sign of how fiercely sunlight and solar heating are beating up on this tender object.
Impactors strike during the reign of the dinosaurs (image credit: MasPix/devianart)
The recent meteor explosion over Chelyabinsk brought to the forefront a topic that has worried astronomers for years, namely that an impactor from space could cause widespread human fatalities. Indeed, the thousand+ injured recently in Russia was a wake-up call. Should humanity be worried about impactors? “Hell yes!” replied astronomer Neil deGrasse Tyson to CNN’s F. Zakharia .
The geological and biological records attest to the fact that some impactors have played a major role in altering the evolution of life on Earth, particularly when the underlying terrestrial material at the impact site contains large amounts of carbonates and sulphates. The dating of certain large impact craters (50 km and greater) found on Earth have matched events such as the extinction of the Dinosaurs (Hildebrand 1993, however see also G. Keller’s alternative hypothesis). Ironically, one could argue that humanity owes its emergence in part to the impactor that killed the Dinosaurs.
More than a dozen known impactors created 50 km sized craters (and larger) on Earth. One such example is the Manicouagan crater in Quebec, Canada. The crater is 215 million years old, and exhibits an 85 km diameter (image credit: NASA).
Only rather recently did scientists begin to widely acknowledge that sizable impactors from space strike Earth.
“It was extremely important in that first intellectual step to recognize that, yes, indeed, very large objects do fall out of the sky and make holes in the ground,” said Eugene Shoemaker. Shoemaker was a co-discoverer of Shoemaker-Levy 9, which was a fragmented comet that hit Jupiter in 1994 (see video below).
Hildebrand 1993 likewise noted that, “the hypothesis that catastrophic impacts cause mass extinctions has been unpopular with many geologists … some geologists still regard the existence of ~140 known impact craters on the Earth as unproven despite compelling evidence to the contrary.”
Beyond the asteroid that struck Mexico 65 million years ago and helped end the reign of the dinosaurs, there are numerous lesser-known terrestrial impactors that also appear destructive given their size. For example, at least three sizable impactors struck Earth ~35 million years ago, one of which left a 90 km crater in Siberia (Popigai). At least two large impactors occurred near the Jurassic-Cretaceous boundary (Morokweng and Mjolnir), and the latter may have been the catalyst for a tsunami that dwarfed the recent event in Japan (see also the simulation for the tsunami generated by the Chicxulub impactor below).
Glimsdal et al. 2007 note, “it is clear that both the geological consequences and the tsunami of an impact of a large asteroid are orders off magnitude larger than those of even the largest earthquakes recorded.”
However, in the CNN interview Neil deGrasse Tyson remarked that we’ll presumably identify the larger impactors ahead of time, giving humanity the opportunity to enact a plan to (hopefully) deal with the matter. Yet he added that often we’re unable to identify smaller objects in advance, and that is problematic. The meteor that exploded over the Urals a few weeks ago is an example.
In recent human history the Tunguska event, and the asteroid that recently exploded over Chelyabinsk, are reminders of the havoc that even smaller-sized objects can cause. The Tunguska event is presumed to be a meteor that exploded in 1908 over a remote forested area in Siberia, and was sufficiently powerful to topple millions of trees (see image below). Had the event occurred over a city it may have caused numerous fatalities.
Mark Boslough, a scientist who studied Tunguska noted, “That such a small object can do this kind of destruction suggests that smaller asteroids are something to consider … such collisions are not as improbable as we believed. We should be making more efforts at detecting the smaller ones than we have till now.”
Neil deGrasse Tyson hinted that humanity was rather lucky that the recent Russian fireball exploded about 20 miles up in the atmosphere, as its energy content was about 30 times larger than the Hiroshima explosion. It should be noted that the potential negative outcome from smaller impactors increases in concert with an increasing human population.
In 1908 the Tunguska impactor toppled millions of trees in a rather remote part of Siberia (image credit: Kulik). Had the object exploded over a city, the effects may have been catastrophic.
So how often do large bodies strike Earth, and is the next catastrophic impactor eminent? Do such events happen on a periodic basis? Scientists have been debating those questions and no consensus has emerged. Certain researchers advocate that large impactors (leaving craters greater than 35 km) strike Earth with a period of approximately 26-35 million years.
The putative periodicity (i.e., the Shiva hypothesis) is often linked to the Sun’s vertical oscillations through the plane of the Milky Way as it revolves around the Galaxy, although that scenario is likewise debated (as is many of the assertions put forth in this article). The Sun’s motion through the denser part of the Galactic plane is believed to trigger a comet shower from the Oort Cloud. The Oort Cloud is theorized to be a halo of loosely-bound comets that encompasses the periphery of the Solar System. Essentially, there exists a main belt of asteroids between Mars and Jupiter, a belt of comets and icy bodies located beyond Neptune called the Kuiper belt, and then the Oort Cloud. A lower-mass companion to the Sun was likewise considered as a perturbing source of Oort Cloud comets (“The Nemesis Affair” by D. Raup).
A halo of comets designated the Oort Cloud is theorized to encircle the periphery of the Solar System, and reputedly acts as a reservoir for objects that may become terrestrial impactors (image credit: NASA/JPL).
The aforementioned theory pertains principally to periodic comets showers, however, what mechanism can explain how asteroids exit their otherwise benign orbits in the belt and enter the inner solar system as Earth-crossers? One potential (stochastic) scenario is that asteroids are ejected from the belt via interactions with the planets through orbital resonances. Evidence for that scenario is present in the image below, which shows that regions in the belt coincident with certain resonances are nearly depleted of asteroids. A similar trend is seen in the distribution of icy bodies in the Kuiper belt, where Neptune (rather than say Mars or Jupiter) may be the principal scattering body. Note that even asteroids/comets not initially near a resonance can migrate into one by various means (e.g., the Yarkovsky effect).
Indeed, if an asteroid in the belt were to breakup (e.g., collision) near a resonance, it would send numerous projectiles streaming into the inner solar system. That may help partly explain the potential presence of asteroid showers (e.g., the Boltysh and Chicxulub craters both date to near 65 million years ago). In 2007, a team argued that the asteroid which helped end the reign of the Dinosaurs 65 million years ago entered an Earth-crossing orbit via resonances. Furthermore, they noted that asteroid 298 Baptistina is a fragment of that Dinosaur exterminator, and it can be viewed in the present orbiting ~2 AU from the Sun. The team’s specific assertions are being debated, however perhaps more importantly: the underlying transport mechanism that delivers asteroids from the belt into Earth-crossing orbits appears well-supported by the evidence.
A histogram featuring the number of asteroids as a function of their average distance from the Sun. Regions depleted of asteroids are often coincident with orbital resonances, the latter being a mechanism by which objects in the belt can be scattered into enter Earth-crossing orbits (image credit: Alan Chamberlain, JPL/Caltech).
Thus it appears that the terrestrial impact record may be tied to periodic and random phenomena, and comet/asteroid showers can stem from both. However, reconstructing that terrestrial impact record is rather difficult as Earth is geologically active (by comparison to the present Moon where craters from the past are typically well preserved). Thus smaller and older impactors are undersampled. The impact record is also incomplete since a sizable fraction of impactors strike the ocean. Nevertheless, an estimated frequency curve for terrestrial impacts as deduced by Rampino and Haggerty 1996 is reproduced below. Note that there is considerable uncertainty in such determinations, and the y-axis in the figure highlights the “Typical Impact Interval”.
Estimated frequency of impactors as a function of diameter, energy yield, and typical impact interval. Results assume an impact speed of 20 km/s and density of 3 g/cm^3 (image credit: Fig. 2 from Rampino and Haggerty 1996, NASA ADS/Springer).
In sum, as noted by Eugene Shoemaker, large objects do indeed fall out of the sky and cause damage. It is unclear when in the near or distant future humanity will be forced to rise to the challenge and counter an incoming larger impactor, or again deal with the consequences of a smaller impactor that went undetected and caused human injuries (the estimated probabilities aren’t reassuring given their uncertainty and what’s in jeopardy). Humanity’s technological progress and scientific research must continue unabated (and even accelerated), thereby affording us the tools to better tackle the described situation when it arises.
Is discussion of this topic fear mongering and alarmist in nature? The answer should be obvious given the fireball explosion that happened recently over the Ural mountains, the Tunguska event, and past impactors. Given the stakes excessive vigilance is warranted.
Fareed Zakharia’s discussion with Neil deGrasse Tyson is below.
Comets Pan-STARRS and Lemmon over Bariloche.Argentina on March 4, 2013. Credit and copyright: Guillermo Abramson.
While those of us in the northern hemisphere are impatiently waiting to see Comet PANSTARRS (tonight, March 7 it should be visible in the southern parts of the US and Europe just after twilight), southern hemisphere observers have been dazzled by not one but TWO comets. Here, astrophotographer Guillermo Abramson captures both PANSTARRS and Comet Lemmon in one shot on March 4, 2013!
Below is a great shot Abramson took of Comet PANSTARRS on March 3:
Comet PANSTARRS sets behind Mt. Cathedral, in Bariloche, Argentina. Credit and copyright: Guillermo Abramson.
With this being the Year of the Comets make sure to submit all your comet astrophotos to our Flickr page. We’ll be posting more images from comet-watchers soon!
Want to get your astrophoto featured on Universe Today? Join our Flickr group or send us your images by email (this means you’re giving us permission to post them). Please explain what’s in the picture, when you took it, the equipment you used, etc.
Simulation of the close approach of C/2013 A1 to Mars in Celestia using info from the Minor Planet Center. Credit: Ian Musgrave/Astroblog.
The latest trajectory of comet 2013 A1 (Siding Spring) generated by the Near-Earth Object Program Office at the Jet Propulsion Laboratory indicates the comet will pass within 186,000 miles (300,000 kilometers) of Mars in October of 2014, and there is a strong possibility that it might pass much closer. The NEO Program Office’s current estimate based on observations through March 1, 2013, has it passing about 31,000 miles (50,000 kilometers) from the Red Planet’s surface. That distance is about two-and-a-half times that of the orbit of outermost moon, Deimos.
This video, above, is based on comet’s orbit calculated by Leonid Elenin, which has it is coming within 58,000 km, and visualized by SpaceEngine software.
The trajectory for comet Siding Spring is being refined as more observations are made. Rob McNaught discovered this comet on Jan. 3, 2013, at Siding Spring Observatory in Australia, and looking back at archival observations has unearthed more images of the comet, extending the observation interval back to Oct. 4, 2012. Further refinement to its orbit is expected as more observational data is obtained.
“At present, Mars lies within the range of possible paths for the comet and the possibility of an impact cannot be excluded,” said an update today from JPL. “However, since the impact probability is currently less than one in 600, future observations are expected to provide data that will completely rule out a Mars impact.”
JPL’s update also outlined how during the close Mars approach, the comet will likely achieve a total visual magnitude of zero or brighter, as seen from Mars-based spacecraft. From Earth, the comet is not expected to reach naked eye brightness, but it may become bright enough (about magnitude 8) that it could be viewed from the southern hemisphere in mid-September 2014, using binoculars, or small telescopes.
Siding Spring likely originated from the Oort cloud. Amateur and professional astronomers will be keeping an eye on this comet’s trajectory to determine if it will end up hitting Mars or not.
