Types of Orbits

Types of Orbits

There are generally two types of orbits employed by human-built satellites. Closest to the Earth is the elliptical polar orbit (the very first satellite to orbit the planet was Orbiting Geophysical Observatory 4, with an apogee of 564 miles and a perigee of 256 miles. The extremely elliptical orbit represented the path of OGO 5, whose apogee was nearly 92,000 miles and whose perigee was less than 200 miles.

Looking like huge dragon flies because of their array of solar panels, antennas and experiment booms, but nicknamed “streetcar” satellites because each of them can carry about 26 different scientific experiments, the first OGO’s returned a wealth of information about the Sun and interplanetary space as well as the Earth itself.

Intelsat-2. Three or four Intelsat III’s – each having the capacity of four color-television or 1200 high-quality telephone channels – were placed into geostationary orbit at equally spaced intervals around the Earth to provide world-wide coverage. By 1970 there were 70 communication stations in 45 countries able to communicate via Intelsat III. For many of these countries, it was their first modern communication with the rest of the world. TRW built Intelsat III for a Communications Satellite Corporation, the manager for the International Telecommunications Satellite Consortium (Intelsat). Image in public domain

For example, solar flares, those huge tongues of fiery gases that leap millions of miles out into space, give off radiation – some of which is dangerous enough to harm an astronaut on the way to the Moon. Earth’s atmosphere protects us from much of this radiation. Nevertheless, practically every phenomenon on Earth is affected by solar flares. They interrupt radio, video and television transmission, cause difficulties in aircraft navigation and other communications – and they may also help to create the Auroras, those spectacular displays of light at each pole which have puzzled and fascinated men for centuries.

The first OGO also told us much about the solar wind, the vast streams of ionized particles which flow at a million miles per hour outward in all directions from the Sun, compressing Earth’s Van Allen radiation belt on the “windward” side and following it out on the “leeward” side.

Since the early times of the space exploration, humans used random, near-synchronous equatorial orbits for defense Comsats (communication satellites). Military communication satellites formed a slowly rotating belt which encircled the equator at about 21,000 miles altitude. Launched in groups containing up to eight individual spacecraft by a Titan IIIC and injected into orbit over a 3-minute period by an accelerating Titan III Transtage, each of these spacecraft had a slightly different orbital velocity, , causing them to drift past one another and eventually spread out around the Earth. With orbital periods (the time it takes to make one revolution around the Earth) ranging around 22-1/2 hours, they all drifted eastward about 30 degrees each day relative to the surface of the Earth.

The random orbital arrangement provides two advantages. First, it assures that if one satellite malfunctions, another satellite will eventually be in position to replace it. Secondly, the satellite can operate without stationary controls – complicated sensors and propulsion systems employed to maintain a spacecraft in one position at all times. Since the satellites drift past one another, any given ground station will generally have more than one satellite in view at any one time.

In the 1960s, as the major subcontractor to Philco-Ford for this program (known as the Initial Defense Satellite Communication System), TRW designed and built the spacecraft frame and housing, the power system, and the test units. TRW also built the separation and spin-up equipment – the equipment which separated each individual spacecraft from the structure atop the Titan IIIC Transtage, then imparted a spin to the spacecraft in order to stabilize it in orbit.

Standing still at 6875 miles per hour

In synchronous equatorial orbits, the TRW-manufactured Intelsat III spacecraft seem to be standing still as they hover over specified points on Earth, providing the world’s first truly commercial communication satellite system.

Intelsat III spacecraft are inserted into geostationary orbit by first being launched into a highly elliptical transfer orbit whose apogee matches the altitude of a geostationary orbit (about 22, 235 miles). Since the launches occur at Cape Canaveral, the initial orbit is inclined 33 degrees to the equator. The spacecraft must then be manoeuvred so that it will not only follow the equator (0 degrees inclination), but that it will do so in a circular path – and in an altitude pf around 22, 235 miles). The transfer orbit is planned so that its apogee occurs over the equator. After four to twelve revolutions, the 3140-pound-thrust solid-propellant apogee motor is fired, simultaneously circularizing the orbit and removing the plane inclination.