Most people have heard of meteor showers, like the Perseids in August. These meteors come from a single parent body, usually a comet, and occur at a certain time of year.
There are also meteors which are not associated with showers, which occur throughout the year as the Earth sweeps up dust. These objects are meteoroids which originally belonged to a shower, but have, over time, lost that association. They may have been separated from their parent body centuries or even millenia in the past. The vast majority of meteoroids in the millimeter size range are sporadic, which means that knowing how many we encounter and from which directions is critical to estimating the hazard to spacecraft. These objects also represent the remnants of the population of comets in the inner solar system long ago, and so can be used to probe the history of the solar system.
Both optical and radar observations can provide information about the flux and directionality of sporadic meteors, but radar observations can sample meteoroids coming from all directions, while optical observations are limited to the night side of the Earth. Radar observations suffer from observing biases which are more difficult to account for that optical, so a combination of the two observing techniques is the best way to research.
Sporadic meteors don't come from completely random directions, which their name implies. Most sporadic meteors come from the six sporadic sources, which are radiant concentrations which are at constant positions in sun centred coordinates. The six sources are the north and south apex (the apex is the direction of the Earth's motion, so apex meteors collide head-on with the Earth), the helion and antihelion (the helion source is close to the direction of the sun, while the antihelion is close to the opposite direction) and the north and south toroidal (which are 60 degrees out of the plane of the ecliptic, above and below the apex). Six years of sporadic meteor radiants from the CMOR meteor radar are shown below, with five of the six sources visible (the sixth is not visible from CMOR's location). Meteor radars have the advantage of operating day and night, regardless of the weather; however, they do not typically allow analysis of the ablation of meteors, since they scatter from a single point on the meteor trail, and there are observing biases which are poorly constrained.
Video meteor observations
Observations of meteors can be made with image-intensified video, also known as electro-optical (EO) video. Although optical observations are confined to hours of darkness and clear skies, they can be used to look at phenomena like meteoroid fragmentation and wake. Optical observations also tend to have higher spatial resolution than transverse-scatter radar, and so produce more accurate orbits and trajectories.
The standard cameras consist of a ccd camera, operating at video rates, coupled to an image intensifier. The image intensifier multiplies each photon striking it, increasing the sensitivity of the camera by orders of magnitude. This allows observations with a wide field of view (often 30 degrees) and faint limiting magnitude (about +7th magnitude for a wide field system).
Cameras are normally operated in pairs, about 50 km apart. Triangulating then allows the height and trajectory of the meteor to be calculated, along with the speed and orbit.
New optical techniques
Advances in computer technology have recently allowed the introduction of new optical meteor systems. The Western Meteor Physics Group is in the process of constructing two new optical systems with funding from the Canadian Foundation for Innovation (CFI), the Ontario Research Fund (ORF), and NASA.
The first system consists of a pair of high resolution intensified cameras (currently running at 1024 x 1024 pixels, 20 frames per second). These cameras have much higher spatial resolution than standard video cameras, and also have 14-bit pixels instead of 8-bit, making much more accurate measurements of meteor brightness. The spatial resolution is comparable to photographic film.
The second system also runs at two stations. Each station has a wide field camera (25 degrees) with a standard video resolution (640 x 480 pixels), but operating at 80 frames per second. Meteors are detected on the wide field in real time, and a pair of mirrors mounted on high-speed galvanometers direct the light from the meteor into a small telescope. The light is captured by a narrow field camera (2 degrees), also operating at 80 fps. The narrow field camera has a pixel resolution of a few meters, and can therefore observe small fragmentation events, wake and even trail radius. A spectral system may be added in the future. This system will allow studies of meteoroid composition on an unprecedented scale.
CAMO cameras inside their enclosure. The wide field video camera is the round lens at the top right. The narrow field system uses the mirrors visible in the square aperture towards the bottom left.
The mirror system under construction: the telescope is mounted with the two mirrors on a mount which can rotate horizontally and vertically.