BAA / RMetS Joint Meeting, 2004 November 27
Meteors
Dr Mason opened expressing his thanks to Messrs Bob Marriott and Gordon Rogers for filling in at such short notice at the previous BAA meeting, since train delays had prevented him from presenting his scheduled talk. He remarked that the words 'meteor' and 'meteorology' had a common root in the ancient Greek adjective meteoros, meaning 'up in the air', which had referred to all phenomena in the atmosphere, from rain and snow to clouds and meteors. The first known scientific study of meteors was contained within Aristotle's Meteorologica (350 BC), which essentially summarised humanity's complete knowledge of weather and climate of the time. However, despite the subject's long history, the reaction of astronomers to meteor showers had changed considerably in the space of only the past thirty years. Whereas in the past they had been thought fully understood – they started, a few days later they peaked, and then they stopped – it was now realised that there were often surprise changes in rate, multiple peaks, and a whole host of other phenomena to be explained.
The speaker first introduced meteoroids: the progenitor interplanetary particles behind meteor storms, which created the observed streaks of light when they collided with the Earth's atmosphere. These were typically found to weigh between 10-9-103kg, and, in total, an average of around 16,000 tonnes of such material entered the atmosphere per year. In terms of physical composition, it was important to realise the low physical density of meteoroids, typically between that of water (1g/cm3) and one-tenth that density, averaging around 0.3g/cm3. This unusually low density could be explained by their structure: silicate rods with large spaces between them, originally occupied by water ice, which would over the course of time have sublimated.
Moving onto the trails themselves, Dr Mason showed an image of Taurid meteors, remarking that each varied: there were often multiple brightenings, and whilst some ended in a terminal burst, others simply faded away. The speaker also added that the height at which they were at their brightest was found to correlate with the collision velocity of the meteoroid, typically in the range 70-120km, most commonly at 95-100km. Those which peaked highest were the fastest moving, whilst slower meteors penetrated deeper into the atmosphere. The distribution of velocities was itself found to be very large – it had to be remembered that the Earth orbited the Sun at a speed of 30km/s, and so there was a difference of 60km/s between the observed speed of meteors colliding head-on as compared to those which the Earth had just caught up with from behind. Typically, velocities between 11-72 km/s were observed. There was some variation between showers, for example Leonid meteors were faster than most, and typically very close to 72km/s.
Upon collision with the atmosphere, the speaker explained that meteoroids transferred their kinetic energy to the atoms and molecules around them in a process called ablation. The result was that the surrounding molecules became excited, and often ionised. They would subsequently de-excite with the emission of light, giving rise to the observed glow. The speaker also remarked that ionised material reflected radio waves, and so meteor trails could be observed not just in visible light, but also from the radio waves they scattered. In the past, this had been quite popular among amateurs – a way to observe meteors even in cloudy conditions.
At optical wavelengths, meteors could appear at brightnesses of up to mag –16 or –18 – one-hundred times the brightness of a Full Moon. Some observers chose to use a wide-angle lens to photograph the whole sky when looking for meteors: this ensured no part of the sky was left unobserved, but did come with the downside that only the very brightest meteors were generally detected. The speaker felt this technique was only really useful for recording bright fireballs, from progenitor particles the size of grapes or oranges. By contrast, for fainter meteors, the use of intensified video cameras was recommended. Video photography with such instruments had been pioneered by Andrew Elliot and Steve Evans, who had used it to obtain orbits and estimate the heights of meteor trails. In the past, such calculations had also been done by installing a rotating blinking shutter of known speed on a camera, chopping up the observed trails, or by triangulation from observations at different locations.
The speaker explained that the trails could remain visible for several seconds, or up to 30-minutes in extreme cases. However, as they hung in the atmosphere, they were observed to distort from their initial straight line, as they were carried in the wind. It was found that the dispersion rate depended upon altitude – the shortest lived trails were at 90km, whilst those higher or lower decayed more slowly. This was particularly useful to radio observers, who could use the measured dispersion rate of their radio signal to estimate the altitude of each meteor they observed.
Dr Mason went on to explain that there were two distributions of observed meteors: showers and sporadics. The latter group arose from randomly distributed progenitors, and could be seen at any time of year, in any part of the sky. By contrast, showers resulted when objects, often comets, left streams of debris lying behind then, strewn across particular parts of the solar system through which the Earth passed at various times of year. Most of the very brightest meteors were sporadics, including most fireballs, defined as those brighter than mag –5, and bolides, meteors so brilliant that they generated an audible sound. In the most extreme cases, very massive meteors might make it all the way through the atmosphere to leave rocky debris on the ground, meteorites.
The number of meteors seen was found to vary both with the time of day and time of year. The speaker first explained the daily variation, which was due to the direction of the Earth's spin with respect to its orbit around the Sun. At local dawn at any point on the Earth, the zenith pointed close to the direction in which the Earth was travelling through space as it orbited the Sun. Thus, any meteors above the horizon at this time made head-on collisions with the Earth, and as a result there were many of them and they had higher than average impact velocities. By contrast, at sunset, the zenith pointed away from the direction in which the Earth travelled: any meteors observed would be just catching up with it from behind, and so there would be few of them and they would have slower-than-average velocities with respect to the Earth. As a result, four times more meteors were seen just before dawn than just after dusk.
With regard to seasonal variations, the speaker returned to his division of meteors into two distributions. As comets approached the Sun, their surfaces were thought to crack, spewing fountains of dust grains into the interplanetary medium, leaving behind them streams of such material stretching through the solar system. As the Earth cut through these streams in its orbit around the Sun, brief showers of meteors were observed on the ground. All the meteors in the shower would appear to radiate from a common point in the celestial sphere, because all the dust grains were in near-identical orbits, similar to that of the parent comet. Some showers had a fairly steady rate from year to year, such as the Orionids, implying the dust stream to be fairly uniform. Others, such as the Bootids, had more erratic rates, indicating some bunching of the dust. Perhaps one of the most closely studied examples was the November Leonid shower, which Asher and McNaught had modelled as a series of fine filamentary clouds of particles, from which they had been able to accurately predict when there would be good Leonid displays, often correctly expecting several successive peaks in rate, each only around an hour in duration.
To close, the speaker recalled a story illustrating his earlier assertion that small-field cameras were often better than wide-field lenses for meteor observation. He explained that the Very Large Telescope (VLT) in Chile had been taking the spectrum of a supernova event, when a mag –8 meteor had passed through its field of view. This serendipitous accident probably seemed intensely frustrating at the time, but the result had been the highest resolution spectrum of a meteor trail ever recorded.
Following the applause, a member asked whether the increased use of Scramjets by the US military would make it more difficult to observe meteors by their radio signatures. Dr Mason replied that it was already very difficult to select frequencies which were not affected by a number of sources of noise, and he could only see this problem getting worse. Mr Boles then welcomed Dr Pat Espy of the British Antarctic Survey to present the second talk.