Canadian Space Agency
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Table of Contents
Orbital Mechanics
Things That have Orbits
A Survey of the Universe
1. Satellites, Spacecraft and Space Stations
Image Courtesy of NASA
The International Space Station is in a low earth orbit (LEO) which is approximately circular. Its altitude is about 380 km and it has an orbital inclination of approximately 50 degrees, with an orbital period of 94 minutes.
2. Comets and Meteorites
The location of Comet Halley near the beginning of the 21st century
(heading outwards from the Sun), and an image of a typical comet as it nears the Sun.
Comets travel in orbits around the Sun. There are two types of orbits that comets are seen to follow:
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Elliptical Orbits
Comets in this group are in closed orbits around the Sun and therefore visit the inner solar system at periodic intervals ranging from a few years to centuries. Comet Halley is a member of this group.
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Hyperbolic Orbits
Comets in this group are one-time visitors to the inner solar system. Their orbits are hyperbolic. These comets range into the interstellar space of the galaxy.
3. Planets, Moons, and Rings
Saturn's rings from above the ecliptic.
Saturn's rings are made up of small particles of rock and ice which orbit the planet. At a very large distance the rings appear solid, but they are really made of small chucks of debris which slide easily past each other in their orbit around Saturn.
The chunks closer to the planet orbit more quickly than chunks further from the planet.
The particles follow precise Keplerian orbits around Saturn.
Gaps appear in Saturn's ring system at places where the orbital period of the particles becomes synchronized to the period of some of Saturn's small nearby moons. These moons exert a small periodic gravitational tug on these particles and eventually tug them out of their orbit, leaving a gap in Saturn's ring system.
4. The Earth and Its Moon
Looking back towards the Earth from the Moon's farside.
While the Moon orbits the Earth, the Earth is orbiting the Sun. In reality it is not the Earth that follows a Keplerian orbit around the Sun (white dashed line), but rather the centre of mass of the Earth-Moon system.
The centre of mass of the Earth-Moon system is called the barycentre and lies about 1000 km from the Earth's geometric centre on an imaginary line joining the centres of Earth and the Moon.
This causes a wobble in the Earth's motion as it swings back and forth during each lunar cycle (yellow solid line in the diagram to the left).
The mass ratio of the Earth-Moon system is 81:1, hence the centre of mass of the Earth-Moon system is 81 times further from the center of the Moon than from the centre of the Earth.
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5. Stars in Globular Clusters
Orbiting the Milky Way Galaxy are clusters of stars called globular clusters (because of their shape). These clusters contain from a few hundred to several million stars, all orbiting their collective centre of mass like a swarm of bees buzzing around a hive. Of course the stars are so far away and the size of the clusters is so large that it is impossible to actually see the stars moving.
These clusters are gravitationally bound, meaning that gravity prevents the individual stars within the cluster from heading off into intergalactic space. Although the stars appear close together, they are very far apart and make thousands of orbits over billions of years without ever colliding with another star.
6. Galaxies
All the stars, dust clouds, nebulae and gas (mostly hydrogen), which make up galaxies are in Keplerian orbits around the galactic centre. A typical galaxy contains more than a billion stars.
Galaxies also form gravitational bound clusters (of galaxies), orbiting a common centre of gravity.
These clusters of galaxies form even larger groups called superclusters.
