This blog has moved to

I have switched to a new Pelican-powered blog at

This WordPress-powered blog will disappear from the interweb soon.

Please update your feeds!


Watch out for the 2013 gamma-Delphinids

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.

Visibility of the gamma-Delphinids 2013

Is Java freezing your system? Tune the garbage collector!

I regularly open huge images and tables (>1GB) in interactive Java-based (astronomy) software such as Aladin and TopCat. Because of the way memory allocation works in Java, the area where objects reside in memory (called the heap) needs to be reserved up front using the “Xmx” switch. Hence I tend to run memory-intensive applications using:

java -Xmx4000m

If you don’t use this flag you will get an OutOfMemoryError exception as soon as your application exceeds the default heap size, which is typically set at only a few hundred megabytes.

However, I frequently found myself faced with a horrible performance experience when using a large heap. Java applications would freeze my entire (64-bit) Linux system for anywhere between 2 and 60 seconds! This happened regardless of the JVM used (I tried Oracle Java 1.7, Sun Java 1.6, GCJ 1.5). I verified that my system had plenty of memory available and was not swapping, hence a lack of memory was not to blame. A profiler revealed that these freezes were instead caused by an insane number of interrupts which ate 100% of all CPU cores in so-called “system” cycles.

The cause of these system freezes is Java’s garbage collection mechanism; a built-in automated memory management system which reclaims memory occupied by objects that are no longer in use. Whilst this feature makes programming in Java a bit easier than, say, C++; it comes with the disadvantage that garbage collection in a large heap can introduce a considerable overhead. Some collection algorithms deal less effectively with large heaps than others, and unfortunately in my case, Java appeared to be using a collection strategy which paused the application during the whole duration of each garbage collection run, hence resulting in frequent freezing.

The trick to avoid these freezes is to tell Java to use a collection strategy which runs concurrently to the application, hence avoiding lengthy interruptions of the entire process. This can be achieved using the “XX:+UseConcMarkSweepGC” flag, i.e.:

java -Xmx4000m -XX:+UseConcMarkSweepGC

There are in fact many more tuning parameters which can influence the behaviour of the garbage collection, but “UseConcMarkSweepGC” looks like the first obvious thing to try if you are experiencing annoying freezes in memory-intensive Java applications.

Comet PanSTARRS attracts 2x more men than women?

Two weeks ago, I posted an animation on YouTube showing where Comet PanSTARRS would be visible. The video attracted more than 15 000 hits, and although this is not a proper statistical analysis, I would like to draw attention to an interesting result in the demographic analytics provided by YouTube: 75% of the viewers were male.

Demographic statistics for the visibility animation of comet PanSTARRS.

YouTube demographic statistics for the animation on comet PanSTARRS’s visibility.

Although the numbers are only based on the ~20% of viewers which were logged into a YouTube account while watching, statistics like these may reveal broad trends about the public interest in astronomy. If we were to assume that all people interested in astronomy are equally likely to have watched the animation, and if in addition we assume that these people are all equally likely to have a YouTube account regardless of their age/gender, then one might conclude that (middle-aged) men are twice more likely to seek for comet information than women. Interestingly, this is broadly consistent with the (unfortunate) trend of large male majorities in astronomy clubs and university departments.

There is no doubt that the above assumptions are wrong to some degree, and that the YouTube statistics are hence biased. It is not clear how severe the bias is however. I tried Googling for demographic statistics of YouTube users in general, but could not find consistent information. (Does anyone know a reliable source? Are 75% of YouTube users male anyway?!)

If the biases can be accounted for using a proper statistical analysis, then the analytics offered by science-themed YouTube videos would provide a way to measure the public interest as a function of age, gender and topic.

Map: when can you see comet PanSTARRS?

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.

The movie was created using my Python visibility-maps module. The original frames that went into making the movie can be found here and may be used freely.

How frequently do large meteoroids hit us?

The internet is buzzing about a fireball which caused a powerful sonic boom over Chelyabinsk in Russia, injuring hundreds. A question asked by many is: how common is such event?

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!

From where can you spot 2012 DA14?

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.

Visibility of 2012 DA14

How do you organise $HOME?

Home directories often turn into spooky graveyards of random files, temporary directories and images of lolcats. It takes courage to delete the mess, because there may be one or two important files hiding amongst the pr0n. As a result, many scientists have grown afraid to run “ls ~” in public, fearing that the output of said command will expose them as file-hoarding maniacs. (By the way, the fear of running “ls ~” should be called domusindexophobia in Latin.)

For years I have employed the popular strategy of sticking random files on the desktop, and whenever it becomes a mess, create a folder called “oldstuff” and move everything into it. The strength of this strategy is that it can be repeated indefinitely (oldstuff2, oldstuff3, oldstuff4…), the weakness is that you remain a file-hoarding, domusindexophobic maniac.

In the past few months I decided to adopted a more sensible strategy. Read the rest of this entry

Footprint of the IPHAS Galactic Plane survey

As part of my post-doc at the University of Hertfordshire, I’m helping to calibrate, release and exploit data obtained by the INT Photometric H-Alpha Survey (IPHAS). This is a 1800 deg2 optical survey of the Northern Galactic Plane, carried out in the narrow-band Hα and broad-band Sloan r’/i’ filters. I’ll be talking more about the survey in future posts, but as a warm-up I produced this plot of the survey’s 15270 telescope pointings:

Footprint of the IPHAS survey.

Let me explain this footprint briefly. The survey uses the Wide-Field Camera (WFC) at the 2.5-meter Isaac Newton Telescope (INT) in La Palma. This camera consists of four thinned 2048 × 4096 pixel CCDs arranged in an L shape like this: Read the rest of this entry

Visual & video observations of the 2012 Draconids outburst

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):

Visual activity curve of the Draconids 2012

Read the rest of this entry