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  • Print publication year: 2006
  • Online publication date: July 2015

5 - Comet and meteoroid orbits

from Part I - Introduction


The forecasting of meteor storms is all about knowing the orbit of the meteoroids and their parent body in the past, present, and future. When I first learned about meteor orbits in space, I found it very hard to imagine the orbit in three dimensions. Modern computer games and 3D software tools have made it easier to visualize a comet orbit in space, but there remains a need to express the shape and orientation of an orbit with numbers, the so-called orbital elements. The astronomical language of orbital elements is the qaeωΩi-system.

5.1 Orbital elements

Theoretical astronomers still like to give position and motion by three positional coordinates X, Y, Z, which give the location of a comet or meteoroid at a given time, and three velocity coordinates Vx, Vy, Vz, which describe the direction of its motion. They tell us, for example, that the comet on January 1, 2005, is at −1.223 15, +0.352 52, +0.025 33 AU (1 AU = the Earth–Sun distance) and moves at a speed of +22.5251 km/s towards the X-direction, at +12.1523 km/s towards Y, and at +2.1523 km/s towards Z. However, from that it is hard to imagine what type of orbit the comet is in, or how it will move in the future.

Everyone else uses the fact that comets and their offspring move in elliptical orbits around the Sun. The orbits are more elongated than those of the planets and tilted out of the plane of Earth's orbit called the “ecliptic plane”. The orbital elements describe the shape and orientation of the ellipse (Fig. 5.1) by a system of six numbers (and a variety of derivatives): the size and shape of the orbit (two numbers: q and a), the orientation of the orbital plane (three numbers: ω, Ω, i), and the position in the orbit (one number: Tp).