Parent of the Perseids

Around this time every year, the Earth, on her year long trundle around the Sun, passes through the Perseid cloud of cometary debris. The resulting month long encounter produces arguably the most prolific and spectacular meteor shower for northern observers – the Perseids.  As many as 100 “shooting stars” an hour may be visible at its peak in mid-August and the shower is eagerly awaited by sky-gazers for it’s dazzling and reliable display of colourful meteors and fireballs.

The source of the Perseids is dust and debris contained in a relatively dense ‘cloud’ impacting the upper atmosphere of the planet and burning up due to rapid deceleration due to increased aerodynamic drag. The shower has been observed for millennia,  the first recorded sighting was in 69 BC, and most of the dust and debris responsible for the shower was pulled off a comet a thousand years ago. The particles that produce this astronomical light-show are generally tiny, on the order of centimetres, and pose little threat to the Earth below.  However,  the same cannot be said for their parent, comet Swift-Tuttle.

Composite Image of The Perseid Meteor Shower from Mount Hood (Gary Randall, 2012)

Comet Swift Tuttle (designation: 109P/Swift–Tuttle) is a typical Halley-like long period comet. It tears through the inner solar system when nearing the closest approach of its 133 year orbit around the Sun; an orbit that takes it out 12 AU past Pluto to 51 AU, and all the way back again.  Its last close encounter with Earth was in 1992, and it won’t return until 2126.

For a while following its rediscovery in 1992, almost 10 years away from its expected position, the orbital evolution of the comet was not well constrained and there was considerable cause for alarm when it was estimated to be on a collision course with Earth in 2126. Concern was justified:  its nucleus is 26km in diameter, considerably larger than the 10 km impactor that is thought to have caused the Cretaceous-Paleogene (K-T) mass extinction event 65 million years ago. However, reanalysis of ancient records of observations and improved calculations that included the effects of nucleus evaporation confirmed that the comet is on a very stable orbit and poses little threat to Earth for the next 2000 years.

That said, in a 1997 book by South African/American radio astronomer Gerrit Verschuur, comet Swift-Tuttle was described as the most dangerous object known to man for it’s ability to cause catastrophic damage if it was to impact the Earth. An exceptionally close encounter is expected in 4479, bringing Swift Tuttle to within 0.03 AU (approximately 4 million km) of the Earth – roughly 10 times the mean Earth-Moon distance. Travelling at a relative velocity of 60 km per second, Swift-Tuttle would unleash the equivalent of a devastating 3.2×1015 tons of TNT upon impact – 27 times the energy of the K-T impactor. For comparison, the largest nuclear weapon ever detonated was a ‘mere’ 50 megatons (106). It would very likely cause huge loss of life across the planet and result in a mass extinction unlike any known previously, whilst placing unbridled pressure on the capacity for human civilisation to recover. If the initial impact was survived, tsunamis, wildfires, earthquakes, years of darkness and a toxic atmosphere would follow. Harvard astrophysicist John Chambers estimates the chance of collision in 4479 to be 1 in 1,000,000. Best of luck to our descendants 2467 years from now!

It is worth bearing this in mind when you gaze up over the next few nights to witness the magnificent sight of the ancient dust of this comet burning up in our atmosphere, for one day their parent may put on a somewhat more spectacular, if devastating, show.

Men and Machines

Imagination will often carry us to worlds that never were. But without it we go nowhere.

– Carl Sagan (Cosmos, 1980)

Since the dawn of civilisation, humans have gazed up at the stars and planets overhead. Even now, separated from our forebears by an expansive gulf of time, technology and knowledge, the stars remain distant, esoteric but evocative targets. Our curiosity and thirst for understanding drives us on, pushing the limits of human endurance, engineering and science to the point where 528 humans from 38 nations have flown beyond the tenuous envelope of gases clinging to the surface of the Earth into wilderness of space. A first, unsteady and cautious step into the vast unknown that surrounds our tiny globe. Of these, only 12 have stepped foot on the surface of the Moon. At over 385,000 km away, reaching the desolate face of our lunar companion remains the pinnacle of manned spaceflight capability, yet it is a mere stone’s throw from Earth in astronomical terms. We peer out from the relative safety of our home, edge into the abyss that surrounds us and tentatively contemplate its content.

The delicate squishiness of the human form is not conducive to the hostile environment of space. Fleshy bags of meat and fluids don’t travel well in a vacuum, the near absolute-zero temperatures dessicate skin and lung and our fragile bones snap and break easily under undue strain. Bombarded by radiation, and far from the protective effect of the ozone layer, our cells mutate and die.  Ingenuity and engineering have surmounted these problems in the short term by wrapping our bodies in spacecraft and suits, but the frailties of our terrestrial form remain.

As with many aspects of our lives, we have increasingly outsourced the monumental task of space exploration to robotic envoys. Obedient, unfaltering and better able to withstand the hardships of space travel, these metallic pioneers are our eyes and ears in the depths of space, straddling the boundary of the known and unknown to help us elucidate the mysteries of our near and distant planetary neighbours. Beacons in the fog, they light the way out into space.

Moreover, these scientific emissaries are more than merely (very expensive) collections of navigational equipment, cameras, sensors and propulsion. They are more than laboratories, more than the experiments they conduct, or the raw data they return. More too than the images they record, most never seen by the eyes of a human. These magnificent machines, representative of the peak of human exploratory technology are much greater than the sum of their parts. Often the result of years of international collaboration, teamwork, anguish and joy, these are the ambassadors of our knowledge, the manifestations of the spirit of human curiosity and the first steps of a lonely species wandering out into the darkness. Whilst they wander space in isolation, they have the dreams and imagination of many people behind them.

This is why, when a launch fails or an unmanned probe goes missing, the loss is felt by us all. The cost can be counted in dollars or euros, but the real price is the setback to the campaign for understanding that our failed or lost probe was spearheading. A scout lost to the enemy. I’ve heard stories of folks who cried at the loss of Beagle II (the British-built Mars lander lost to the Martian atmosphere in 2003/4), and who amongst us are not moved by xkcd‘s wonderful homage to the late (but very successful) MER Spirit rover?

On the eve of the landing of MSL Curiositythe most complex rover ever designed, it is worth bearing in mind the hard work and dedication that it took for the latest generation of scientists and engineers to push the limits of our understanding and put a car-sized robot on Mars. I wish all those involved in the construction and operation of this wonderful machine the best of luck. Earth is rooting for you!


Follow Curiosity’s landing live at JPL’s site here