Orbit determination

Diagram showing how orbit-determination data was handled in a 1962 NASA mission. (Of historical interest only.)

Orbit determination is the use of a set of techniques for estimating the orbits of objects such as moons, planets, and spacecraft. Determining the orbits of newly observed asteroids is a common usage of these techniques, both so the asteroid can be followed up with future observations, and also to verify that it has not been previously discovered.

Observations are the raw data fed into orbit determination algorithms. Observations made by a ground-based observer typically consist of time-tagged azimuth, elevation, range, and/or range-rate values. Telescopes or radar apparatus are used, because naked-eye observations are inadequate for precise orbit determination.

After orbits are determined, mathematical propagation techniques can be used to predict the future positions of orbiting objects. As time goes by, the actual path of an orbiting object tends to diverge from the predicted path (especially if the object is subject to difficult-to-predict perturbations such as atmospheric drag), and a new orbit determination using new observations serves to re-calibrate knowledge of the orbit.

For the US and partner countries, to the extent that optical and radar resources allow, the Joint Space Operations Center gathers observations of all objects in Earth orbit. The observations are used in new orbit determination calculations that maintain the overall accuracy of the satellite catalog. Collision avoidance calculations may use this data to calculate the probability that one orbiting object will collide with another. A satellite's operator may decide to adjust the orbit, if the risk of collision in the present orbit is unacceptable. (It is not possible to adjust the orbit every time a very-low-probability situation is encountered; doing so would cause the satellite to quickly run out of propellant.) When the quantity or quality of observations improves, the accuracy of the orbit determination process also improves, and fewer "false alarms" are brought to the attention of satellite operators. Other countries, including Russia and China, have similar tracking assets.

History

Orbit determination has a long history, beginning with the prehistoric discovery of the planets and subsequent attempts to predict their motions. Johannes Kepler used Tycho Brahe's careful observations of Mars to deduce the elliptical shape of its orbit and its orientation in space, deriving his three laws of planetary motion in the process.

The beginning of modern understanding of orbit determination is considered to be Anders Johan Lexell's work on computing the orbit of the comet discovered in 1770 that later was named Lexell's Comet,^[1] in which Lexell computed the interaction of comet with Jupiter that first made the comet fly close to Earth and then would have expelled it from the Solar system.^[2]

Another milestone in orbit determination was Carl Friedrich Gauss' assistance in the "recovery" of the dwarf planet Ceres in 1801. He introduced a method which, when given three observations (in the form of pairs of right ascension and declination), would result in the six orbital elements that completely describe an orbit. The theory of orbit determination has subsequently been developed to the point where today it is applied in GPS receivers as well as the tracking and cataloguing of newly observed minor planets.

In 2019, a new US asset is expected to become operational. The Space Fence—currently being built—will utilize S-band radar and will track a larger number of small objects than previous space radars: "about 200,000 objects and make 1.5 million observations per day, about 10 times the number" made by existing or recently-retired US assets.^[3]

Observational data

In order to determine the unknown orbit of a body, some observations of its motion with time are required. In early modern astronomy, the only available observational data for celestial objects were the right ascension and declination, obtained by observing the body as it moved in its observation arc, relative to the fixed stars. This corresponds to knowing the object's relative direction in space, measured from the observer, but without knowledge of the distance of the object, i.e. the resultant measurement contains only direction information, like a unit vector.

With radar, relative distance measurements (by timing of the radar echo) and relative velocity measurements (by measuring the doppler effect of the radar echo) are possible. However, the returned signal strength from radar decreases rapidly, as the inverse fourth power of the range to the object. This limits radar observations to objects relatively near the Earth, such as artificial satellites and Near-Earth objects.

Methods

Orbit determination must take into account that the apparent celestial motion of the body is influenced by the observer's own motion. For instance, an observer on Earth tracking an asteroid must take into account both the motion of the Earth around the Sun, the rotation of the Earth, and the observer's local latitude and longitude, as these affect the apparent position of the body.

A key observation is that (to a close approximation) all objects move in orbits that are conic sections, with the attracting body (such as the Sun or the Earth) in the prime focus, and that the orbit lies in a fixed plane. Vectors drawn from the attracting body to the body at different points in time will all lie in the orbital plane.

Lambert's method

If the position and velocity relative to the observer are available (as is the case with radar observations), these observational data can be adjusted by the known position and velocity of the observer relative to the attracting body at the times of observation. This yields the position and velocity with respect to the attracting body. If two such observations are available, along with the time difference between them, the orbit can be determined using Lambert's method. See Lambert's problem for details.

Gauss' method

Even if no distance information is available, an orbit can still be determined if three or more observations of the body's right ascension and declination have been made. A method, made famous by Gauss in his "recovery" of the dwarf planet Ceres, has been subsequently polished.

One use of this method is in the determination of asteroid masses via the dynamic method. In this procedure Gauss' method is used twice, both before and after a close interaction between two asteroids. After both orbits have been determined the mass of one or both of the asteroids can be worked out.

References

↑ Valsecchi, G. '236 Years Ago...' in Near Earth Objects, Our Celestial Neighbors: Opportunity and Risk : Proceedings of the 236th Symposium of the International Astronomical Union, Cambridge University Press, 2006, xvii-xviii
↑ J. A. Lexell (1779). "Disquisitio De Tempore Periodico Cometae Anno 1770 Observati". Philosophical Transactions of the Royal Society of London 69: 68–85. Bibcode:1779RSPT...69...68L. doi:10.1098/rstl.1779.0009.
↑ Gruss, Mike (2014-11-21). "Haney: U.S. Partners To Have Indirect Access to Space Fence Data". Space News. Retrieved 2014-12-01.