The natural space environment comprises different natural macroscopic particles. A summation is given below:
- Asteroid; A relative small body, composed or rock, carbon or metal, which is orbiting the Sun.
- Comet; A relative small body, which is composed of dirt and ice. Comets are characterized by tails of dust and gas, when the comet is near the Sun. Far away from the Sun, comets look similar to asteroids.
- Meteoroid; A small particle, originating from an asteroid or comet, usually orbiting the Sun.
- Meteor; The light trail of a meteoroid, which is burning up in the atmosphere of Earth. This burning is visible as a shooting star.
- Meteorite; A meteoroid which survives the burning process, and impacts on the surface of Earth.
Micro-meteoroids are small meteoroids, usually with a diameter below a few mm, which are not detectable with ground observations methods. Natural particles have high velocities, relative to Earth or spacecraft. They can reach up to 70 km/s, with an average around 20 km/s. The velocity is depending on the origin of the micro-meteoroid (an asteroid or comet). In general, meteoroids from asteroids are primarily prograde objects with a velocity of 10 up to 40 km/s. Originating from comets, micro-meteoroids can reach a velocity of 70 km/s, because they can be retrograde. In the Solar system, all major planets orbit the Sun counterclockwise as seen from the north ecliptic pole. Most planets spin in the same sense, including Earth. These motions are called prograde. Rotation in the opposite sense is called retrograde.
For Low Earth Orbit (LEO), micro-meteoroids are a minor part of the micro-particle environment. In geosynchronous orbits, micro-meteoroids are more likely to be encountered than space debris and for interplanetary missions, natural particles are the only hazard with respect to micro-particles.
Special attention must be paid to meteoroid showers, such as the Perseids and Leonids. These showers, which occur annually, can result in an increased risk for spacecraft by an increased meteoroid impact flux. Showers can be predicted, and action can be undertaken to minimize the impact risk for spacecraft.
Meteoroid showers occur when the Earth and an asteroid (or comet) orbit intersect. The Earth will enter the stream of the comet for a few days, and will be confronted with the individual particles. A summation of the most important meteor showers is given in the table below.
Meteoroid showers are named after a fixed point in the background star constellation. When a shower becomes visible on Earth, due to the burn-up in the Earth's atmosphere, the trails seem to originate from one fixed point, called the radiant. On the right, the definition of a radiant is presented in graphical form.
On the left, the Leonids radiant is visible as observed by a ground based telescope.
The combination of the star constellation and the radiant will result in the nomenclature. For example, the Perseid meteoroid shower will appear to radiate from the constellation of Perseus and the Leonid meteoroid shower from the constellation Leo. Showers vary in strength with respect to each other, depending upon factors such as age, body composition, shower particle density and distribution and how close Earth approaches the shower core.
Last update: 6 May 2014