karn (11/16/82)
I got some questions about the STS-5 orbital elements I posted over the weekend. Here is a narrated explanation of what each of the numbers mean. I hope this answers some questions. Numbers such as these are available for free from NASA for virtually any object in orbit. With some tracking programs (available in BASIC, Fortran and C) you can plug in these numbers and determine where the satellite is (or will be) at any given time and when it will pass above your horizon. Phil Karn --------------------------------------------------------------------- STS-5 Orbital elements (no longer valid due to maneuvers, unfortunately) Epoch time: 82318.08417663 This is the time at which all following numbers are valid. It is represented as year 82, day of year 318, fraction of day .08417663 (in Universal Time.) In English, this time was Sat Nov 13 21:01:12 1982 EST. Inclination: 28.4655 deg The angle of the orbit plane to the earth's equator. This is determined by the "launch azimuth", ie, the compass direction in which Columbia travels during launch. The lowest possible value, which gives maximum payload capacity, is obtained by launching due east and is the latitude of the launch site. Unfortunately, low values mean poor visibility from northern observers, since Columbia never travels farther than 28.4655 degrees north or south latitude. RA of node: 49.6153 deg The Right Ascension of the point in the orbit where the Columbia crosses the earth's equator going south to north. Right Ascension is the astronautical equivalent of longitude, except that it doesn't rotate with the earth. This value is the only one that varies with the launch time, and is one of the factors that sets a "launch window." Eccentricity: 0.0003574 A measure of the ellipticity of the orbit; the definition is the same as in analytic geometry. 0 = perfect circle, 1 = parabola, 0 < e < 1 is an ellipse, and > 1 is a hyperbola. The Voyager "orbits" past Jupiter and Saturn have eccentricities > 1. Arg of perigee: 266.3829 deg The angle in the orbit plane between the northbound equator crossing and perigee (closest approach to earth). A value of 0 means the perigee passage occurs as the shuttle crosses the equator going north; values between 0 and 180 mean perigee occurs in the northern hemisphere, etc. Argument of perigee is undefined for a perfectly circular orbit (which of course "never" happens.) Mean anomaly: 139.6376 deg The position of the shuttle along its orbit at the instant of the epoch. This value increases at a constant rate from 0 to 360 deg over an orbit, regardless of the orbit's eccentricity. Mean motion: 15.86846206 rev/day The number of orbits (perigee passages, actually) in one day. This sets the period (time to complete an orbit) of the satellite. Semi major axis: 6689.27 km Half the straight-line distance between the apogee and perigee points. Knowing the Mean Motion allows you to compute the semi-major axis, and vice versa. Period: 90.746034 min Time of one orbit from perigee to perigee. Equal to 1440/mean motion Apogee: 318.36 km Maximum altitude above the (oblate) earth. Perigee: 313.58 km Minimum altitude above the (oblate) earth.