Category Archives: Meteor science
Update (2013-06-11): it looks like no outburst was seen. The meteor shower will remain elusive for another while…
On 11 June 1930, three meteor observers in Maryland (USA) witnessed a flurry of shooting stars originating from the constellation of Delphinus. The mysterious meteor shower, called the gamma-Delphinids, lasted less than 30 minutes. The shower had never been seen before, and it has never returned since… until this year?
The gamma-Delphinids are one of a dozen rare meteor showers for which only anecdotal evidence exists. Such showers are thought to be caused by the dust trails of unknown long-period comets.
Meteoroid streams from long-period comets are thought to be very narrow. In fact the dust trails are so compact that our planet only encounters them when the gravitational pull from Jupiter and Saturn steers the stream exactly into Earth’s path. In contrast, famous meteor showers such as the Perseids and the Leonids originate from known short-period comets. Such streams are more widely dispersed due to their frequent exposure to planetary perturbations and solar radiation in the Solar System, and hence Earth encounters those short-period streams every year.
On 11 June 2013 near 8:30 UT, Earth is predicted to encounter the gamma-Delphinids for the first time since 1930. By measuring the time of the outburst, or its absence, we’ll be able to establish whether the shower is real, and learn about its origin. This is important because it teaches us about a large, Earth-crossing comet which we haven’t discovered yet.
Observers in North and South America are best placed to observe the event. Green and yellow areas in the map below indicate parts of the world where the sky will be dark, and the radiant above the horizon, near the predicted time of the meteor shower.
Comet C/2011 L4 (PANSTARRS) has brightened dramatically over the past week and is now visible with the naked eye from the Southern Hemisphere. Pan-STARRS is moving north rapidly and will become visible across Europe, North America and Asia from Thursday 7 March onward. The comet is expected to reach its peak brightness around the time of its closest approach to the Sun (called the perihelion) on Sunday 10 March. It may or may not lose brightness quickly afterwards, so you want to catch this comet as soon as possible!
I plotted the visibility of Pan-STARRS in the video below. Green/yellow areas in the animation indicate parts of the world where the comet will be above the horizon (and the Sun at least six degrees below the horizon). The movie shows that Pan-STARRS is only visible shortly after sunset, when it is located low above the Western horizon.
Answer: more common than you might think!
The population of Solar System bodies which cross Earth’s orbit range from mm-sized dust (producing meteors) to km-sized asteroids (producing mass extinctions). The frequency of such objects is constrained by meteor observations on one hand (there are ~1000 visible meteors per second across the planet), and asteroid surveys on the other hand (a 10km body will hit us every ~100 million years).
The object that struck Russia falls somewhere in the middle of this range. We know that a meteoroid needs to be larger than 1 meter in diameter to penetrate deep enough into the atmosphere to cause a significant airburst (though velocity, entry angle and composition are important too). At the same time, the scale of destruction is a lot smaller than the famous Tunguska event in 1908, which is thought to have been caused by a 50m-sized body. Hence a first guess for the size of the Russian meteoroid would be “between 1 and 20 meter”.
We don’t know the frequency of asteroids in this size range very well. There are not enough meteor observing cameras to detect these rare events, yet the objects are too small and faint to be detected by telescopes ahead of their impact (apart from one notable exception in 2008).
There is an industry that is very successful in detecting these impacts however; military space surveillance. The US Defense and Energy departments operate satellites to detect the heat signatures from nuclear weapons and rocket launches from space. In 2002, a team led by Professor Peter Brown obtained access to classified data on 300 large fireballs detected between 1994 and 2002 by the military. The authors combined this information with ground-based observations to estimate the relationship between the size of meteoroids and their impact frequency:
The work by Brown et al. may roughly be summarized as follows:
- a 10cm-body hits us every few minutes;
- a 1m-body every few months;
- a 10m-body every few years to decades;
- a 100m-body every few millenia.
Hence, a fireball like the one in Russia is likely to occur somewhere between every few years and every few decades, but the uncertainty is large. Shifting the trend slightly upwards or downwards can change the estimated frequencies by a factor of several, and so we should be careful to pin down any numbers with large confidence.
Whilst the Russian meteor appears to be the largest recorded event for a century, it would be imprudent to infer that it hence only occurs once a century. It is difficult to rely on historical records of the past, because less than 20% of the surface of our planet is inhabited, and so many impacts might have gone unnoticed. In fact there is some evidence for a possible ‘Brazilian Tunguska’ on 13 August 1930 and a ‘Guyana Tunguska’ on 11 December 1935, but events like these may have been undocumented or forgotten.
Moreover, there are reasons to believe that the impact frequencies are not constant, but may be elevated during certain periods (see my recent talk on this topic). Unfortunately, the US military announced in 2009 that they will no longer share fireball observations with scientists, so we’ll have to come up with other ways to pin down the exact danger coming from small asteroids!
On Friday 15 February, a 50-meter asteroid named 2012 DA14 will approach Earth to within a distance of just ~28 000 km. The internet is buzzing about this near-miss because the object is expected to become brighter than 9th magnitude for approximately 3 hours (18h00-21h30 UTC), peaking at a brightness of 7th magnitude near 19h45 UTC. Although this is just below the brightness limit of the unaided eye, it is within reach of good binoculars.
While there are plenty of maps online showing where in the sky you may find 2012 DA14, I could not find any maps showing where on Earth you have to be to get a good view. So I made a few maps myself. Green areas in the animated gif below indicate parts of the world where the asteroid will be above (and the Sun below) the horizon as it sweeps past. The maps were generated using a Python class which I pushed to my GitHub repository.
As reported in my previous post, the Draconids meteor shower showed an exceptional peak on the evening of 8 October 2012 near 17h UT. The peak was very pronounced in data from the Canadian Meteor Orbit Radar (CMOR), which reported rates up to 2300 meteors/hour. In fact, I am told that this radar system recorded more meteor orbits that day than on any other day in its 13-year history.
While the peak was clearly exceptional in terms of radio observations, it remains unclear how many meteors could be seen with the naked eye or video cameras. The time of the peak mainly favoured sparsely populated areas of our planet, and visual observations have so far only been reported by amateur-astronomers Alexandr Maidik in Ukraine (who recorded 55 Draconids between 16h00-18h00 UT) and Jakub Koukal in the Czech Republic (who recorded 60 Draconids between 17h00-19h10 UT).
Using their data, I posted this graph of the Zenithal Hourly Rate (ZHR) on the website of the International Meteor Organization (IMO):
For the second year in a row, reports are emerging about a major peak in the activity of the Draconids meteor shower. Radar observing stations in Canada and in Germany reported up to 1000-2000 meteors/hour between 16h and 18h UT on 8 October 2012. Evidence for an outburst is also apparent in data from some forward-scatter radio stations operated by amateur astronomers.
Radio observing systems are only sensitive to the smallest meteoroids, which produce very faint shooting stars. It is not yet clear how many meteors were bright enough to be seen with the naked eye or video cameras. However, independent video observers in the Czech Republic and Germany told me they recorded more Draconids than usual in the early evening hours (~10 to 40), which does indicate a significant enhancement (albeit no exceptional storm). Careful analysis will follow.
Note that the time of the peak did not favour most parts of Europe, which were still experiencing evening twilight. Observers across Asia were best placed to observe the event. Green and yellow areas in the map below indicate parts of the world where the sky was dark, and the radiant above the horizon, at 17h00 UT tonight:
This outburst may have been caused by the narrow trail of dust and debris left behind by the parent comet in 1959. Our planet had been predicted to encounter this specific dust trail in 2012 by meteor modeler Mikhail Maslov, but the level of activity appears to have been somewhat higher than expected. This is not a first for the Draconids: similar surprises occured in 1985, 1998, 1999, 2005, and MAJOR storms were seen in 1933 and 1946.