Taking things further – trail analysis

Studying a trail image can reveal a meteor’s speed, altitude and chemical composition

A surprising amount of data can be gleaned from an image of a meteor trail. For example, the trail’s speed can be determined photographically by fitting a device known as a rotating shutter to the camera. This is a large disc with slots spinning at a precisely controlled rate.

The camera points though the shutter so any trail photographed appears chopped. The number of chops can be used to help determine the velocity of the meteor. Modern video cameras used for meteor work are also sensitive enough to record trails at a high enough frame rate to determine the incoming speed without the need for a rotating shutter.

It’s also possible to determine the chemical make-up of a meteor by anaylsing its spectrum. The best way to get a spectrum

from a trail is to fit a diffraction grating over the front of the camera, arranged so that the gratings are aligned at right angles to the trail (something which can be achieved by considering the location of the shower radiant). A grating of 600 or 830 grooves per millimetre should produce a well dispersed, bright spectrum. Plastic diffraction grating sheet is a cheap option, although for serious work more expensive glass gratings are best.

Working with other stations is a big bonus. With two images of the same trail it’s possible to triangulate its position and determine the trail’s height. Software like UFOAnalyzer and UFOOrbit can be used to analyse images of the same trail from multiple stations to determine the original orbit of the meteoroid.

Bright meteors often come with a glowing, ionised column of gas known as a meteor train. Photographs can help determine something called the trail brightness threshold, above which trains appear for different showers. High cadence imaging in the range 20- to 30-second exposures, on continuous repeat, often provides enough detail to trace the evolution of bright trains as they expand and become distorted by high-altitude atmospheric winds. FIND OUT MORE SPA guide to meteor spectroscopy: www.popastro.com/meteor/ observingmeteors/spectra/index.php IMO guide to meteor spectroscopy: www.imo.net/docs/03spectra.pdf

A bright Quadrantid meteor trail passes through the first frame, followed by its train, which is distorting by high atmospheric winds over time

A rotating shutter chops the meteor trail into sections, the length of which can be used to determine the speed of the incoming meteoroid

Two images of the same meteor from stations 3.2km apart; the parallax shift allows us to determine trail height