Orbits

Orbits are the pathways taken by objects under the influence of the gravity of another object. These trajectories are governed by the fundamental laws of gravity and the motion of the object. The ability to calculate the orbit of an object, be it a planet, moon, asteroid, or spacecraft, makes it possible to predict where it will be in the future. Both solar system objects and spacecraft can be found in a wide range of orbits, including specific types of Earth orbits that are particularly useful for some types of spacecraft.

All objects in space are attracted to all other objects by the force of gravity. In the case of an object orbiting a planet or other celestial body, where the mass of the object is much less than the mass of the planet, the object will fall towards the planet. However, if the object has some initial velocity , it will not fall straight towards the planet, as its trajectory will be altered by gravity. If the object is going fast enough, it will not hit the planet, because the planet's surface is curving away underneath it. Instead, it will keep "falling" around the planet in a trajectory known as an orbit. Orbits require a specific range of speeds. If the object slows down below a minimum orbital velocity, it will hit the planet; if it speeds up beyond a maximum escape velocity, it will move away from the planet permanently.

Orbital Elements

The shape of an orbit around a planet (or another body) can be defined by several key factors, known as orbital elements. One is the orbit's mean radius, also known as the semimajor axis. The second is the eccentricity of the orbit, or the degree by which the orbit differs from a circle. The third factor is the inclination of the orbit, or the angle between the plane of the orbit and the plane of Earth's orbit. An inclination of 0 degrees would mean the orbit is perfectly aligned with Earth's orbital plane. Three other factors, known as the right ascension of the ascending node, argument of periapsis, and true anomaly, further refine the orientation of the orbit as well as the position of the object in orbit at a given time.

Calculating these orbital elements requires a minimum of three measurements of the position of the object at different times. Additional observations help refine the calculation of the orbit and reduce errors. Once the orbital elements are known, other key parameters of the orbit can be computed, such as its period, and the closest and farthest the object is in its orbit (known respectively as periapsis and apoapsis). For objects orbiting Earth, periapsis and apoapsis are known as perigee and apogee; for objects orbiting the Sun, these locations are known as perihelion and aphelion .

Types of Orbits

Solar system objects travel in a wide variety of orbits. Most planets go around the Sun in nearly circular, low-inclination orbits. The exception is Pluto, which has an inclined orbit that is so eccentric that it is closer to the Sun than Neptune is for twenty years out of each 248-year orbit. Asteroid orbits can be more eccentric, particularly for those objects whose orbits have been altered by the gravity of Jupiter or another planet. Comet orbits, however, can be extremely eccentric, especially for long-period comets that pass through the inner solar system only once every hundreds, or thousands, of years.

Spacecraft orbiting Earth can be found in several different types of orbits based on their altitude and orientation. Many spacecraft, including the space shuttle and International Space Station, are in low Earth orbit, flying a few hundred kilometers above Earth and completing one orbit in about ninety minutes. These orbits are the easiest to get into and are particularly useful for spacecraft that observe Earth. Higher orbits, extending out to altitudes of tens of thousands of kilometers, are used by specific types of communications, navigation, and other spacecraft. At these higher orbits it can take many hours to complete a single orbit.

Special Classes of Orbits

There are several special classes of orbits of particular interest. The best-known special orbit is geostationary orbit, a circular orbit 36,000 kilometers (22,320 miles) above Earth. At this altitude it takes a satellite twenty-four hours to complete one orbit. To an observer on the ground a satellite in this orbit would appear motionless in the sky, hence the name geostationary. Geostationary orbit is also known as a Clarke orbit, after science fiction writer Arthur C. Clarke, who first proposed the concept in 1945. This orbit is used today by hundreds of communications and weather satellites.

Satellites in geostationary orbit do not work well for people in high latitudes, because the satellites appear near the horizon. To get around this limitation, the Soviet Union placed communications satellites in highly inclined, elliptical orbits, so that they appeared to hang nearly motionlessly high in the sky for hours at a time. Such orbits are known as Molniya orbits, after the class of spacecraft that launched them.

Another special type of orbit is Sun-synchronous orbit. This nearly polar orbit is designed such that the spacecraft's orbital path moves at the same apparent rate as the Sun. This allows the spacecraft to pass over different regions of Earth at the same local time. Sun-synchronous orbits are used primarily by remote sensing satellites that study Earth because these orbits make comparisons between different regions of Earth and different times of the year easier. The Mars Global Surveyor and Mars Odyssey spacecraft orbiting Mars also use versions of Sun-synchronous orbit.

see also Asteroids (volume 2); Comets (volume 2); Gravity (volume 2); Satellites, Types of (volume 1); Trajectories (volume 2).

Jeff Foust

Bibliography

Szebehely, Victor G., and Hans Mark. Adventures in Celestial Mechanics. New York:John Wiley & Sons, 1997.

Internet Resources

Braeunig, Robert A. "Orbital Mechanics." <http://users.commkey.net/Braeunig/space/orbmech.htm>. Graham, John F. "Orbital Mechanics." <http://www.space.edu/projects/book/chapter5.html>.

Important Satellite Orbits. University Corporation for Atmospheric Research. <http://www.windows.ucar.edu/spaceweather/types_orbits.html>. Satellite Orbits. <http://www.factmonster.com/ce6/sci/A0860928.html>.