The Solar Orbit

The Solar Orbit

In the case of the solar orbit it is the Sun’s gravity that is pulling on the satellite and making it go around and around the Sun. Of course, all the orbits are also solar orbits, because the spacecraft that is orbiting the Earth or even sitting on the Earth before it is launched, is already in a solar orbit – along with the Earth and everything on it, or going around it.

So when we want to place a spacecraft into a solar orbit, what we really want to do is place it in a solar orbit that is different from the one it is already in – namely, the Earth’s orbit. Usually we want to do this to make the spacecraft travel through the solar system to another planet, such as Mars or Venus, but sometimes (as in the case of Pioneer spacecraft) we just want to find out what is out there in interplanetary space.

The principles involved are just the same that are involved when satellites change from one orbit to another. We are already in one solar orbit (the Earth’s) and we simply want to change to another. If we want to change to a lower orbit, which means one nearer the Sun, we have to slow the spacecraft down from the Earth’s orbital speed. We do this by firing a rocket engine in the opposite direction to the Earth’s motion around the Sun. This slows down the spacecraft from the Earth’s speed to a lower speed, and it drops to a lower orbit, inside the Earth’s. It is still going in the same direction, remember, but closer to the Sun. If we had a powerful enough rocket to cancel out all of the Earth’s speed, our spacecraft would be stopped still and would simply fall right into the Sun.

Neptune and Its Great Dark Spot

If we want to go into an orbit that is outside the Earth’s orbit – which is what we have to do to go to Mars, for example, we fire the rocket engine in the same direction that the Earth is going. This gives it a push (the rocket fires for only a few minutes) ahead of the Earth, and it costs out of the higher (father from the Sun) orbit.

In either case, whether we want to go to a lower solar orbit or a higher one, the spacecraft has to escape from the Earth’s gravitational force. And to do this, it must reach escape speed, which is a little over 25,000 per hour. Of course, the farther out (higher) or in (lower) we want the spacecraft to go, the more we have to speed it up beyond this escape speed.

And remember, just as with Earth orbits, even though you slow a spacecraft down from Earth’s orbital speed around the Sun in order to make the spacecraft fall into a smaller solar orbit, by the time it falls to that orbit it is going faster than when it left the Earth. And even though you speed a spacecraft ahead of the Earth in order to push it into a larger solar orbit, by the time it gets there it is going more slowly than before. That is why satellites that are launched “behind” the Earth into inward path eventually overtake and pass the Earth, and satellites that are launched ahead of the Earth into outward paths eventually fall behind.

The Moon Covers the Pleiades

The Moon Covers the Pleiades

The slender crescent Moon plays a game of astronomical tag with a cluster of bright young stars, the Pleiades, on the evening of March, 20. In fact, the Moon appears to “catch” the Pleiades, hiding the stars from our view, before it sets.

Of course, the Moon only appears close to the Pleiades. The cluster of several hundred stars is really about 400 light-years from Earth, some 10 billion times more distant than our Moon. Still, the view should be impressive, particularly through a good pair of binoculars.

The Moon Passes the Pleiades Star Cluster. Image credit: The University of Manchester / Derekscope, with the Moon from a 2009 conjunction.

The Pleiades is one of the loveliest naked-eye objects in the night sky. Six tightly grouped stars in the shape of a small dipper are normally visible to the naked eye and dozens more can be seen with binoculars. The brightest star in the Pleiades is Alcyone, a blue-white giant several hundred times more luminous than our Sun.

Look for the Moon and the Pleiades fairly high in the west as twilight deepens on March 20. Depending on what part of North America you live in, the Moon already may appear in front of the cluster. As the evening progresses the Moon keeps moving relative to the cluster and many of the cluster’s stars will be occulted, or hidden from view, by the Moon. Because the Moon has no atmosphere and the stars appear as pinpoints of light, the individual Pleiades disappear instantly when the dark limb of the Moon passes in front of them.

The Pleiades cluster, named for the mythological Seven Sisters of the Pleiades (the daughters of Atlas), contains several hundred stars. The Pleiades became known as the Sailors’ Stars during classical times when the cluster ascended into the eastern sky at the beginning of the Mediterranean Sea’s calm-weather season.

Inclination of the Orbit Plane

Inclination of the Orbit Plane

The satellite’s path is always in a plane; that means you could draw it on a piece of paper. And whatever kind of orbit we’re talking about, its plane always goes through the center of the Earth.

An orbit plane can be inclined at any angle between equatorial and polar, and we choose the inclination to suit whatever purposes we have for the satellite. Sometimes we choose the inclination that is the easier – that is so we can put the most payload into orbit with a given rocket. Many US satellites and manned spacecraft are put in orbit inclined at or near 33 degrees to the equator, because that is the easiest inclination for launches from Cape Canaveral (which is at 33 degrees latitude). Many Russian satellites are in orbits inclined in the neighbourhood of 57 degrees to the equator because that is the easiest for them; their principal launch site is at 57 degrees latitude. In both cases that is the easiest inclination because it takes advantage of the rotation of the Earth; you launch the satellite in the same direction that the Earth is already rotating (east), and that gives you some extra push.

Loops of Emission Nebulosity in NGC 3576. Photo in public domain

Putting the same satellite into a polar orbit takes a more powerful rocket, because now you get no benefit at all from the spin of the Earth. And unless you can launch from the Equator, putting it into an equatorial orbit takes some extra thrust because you have to change the plane of the orbit by firing a rocket engine in the right direction just as the satellite is crossing the equator.

Polar orbits are useful for weather satellites, because while the satellite is going around and around over the north and south poles, the Earth is turning below, inside the satellite’s orbit, and each north-south (or south-north) pass of the satellite scans a new part of the planet’s surface until it has all been scanned. If you make the orbit almost polar but not quite, you can make the orbit plane itself rotate at the same rate the Earth is going around the Sun (about 1 degree per day). This near-polar plane is also useful for weather satellites because if you start at the right time you can keep the Sun always behind the satellite. This will allow the satellite’s cameras to take pictures in daylight during the whole year.

For communication satellites like TRW’s Intelsat III, it is best to use a so-called geostationary or synchronous orbit. We already saw that there is a particular altitude for each speed, and therefore for each period (time it takes for one revolution around the Earth), from the hour-and-a-half period of Mercury spacecraft to the 28-day period of the Moon. Somewhere in between there must be an altitude that will give you a period of exactly 24 hours, and this turns out to be a little over 22,000 miles. If you put a satellite in circular orbit in the plane of the equator at that altitude, it will be going around the Earth at the same rate the Earth is going around its own axis. That means that the satellite will remain over the same place on the Earth at all times, and to someone on Earth it would appear to be standing still.

Unveiling the Universe

Unveiling the Universe

Galileo Galilei, the great Italian astronomer and mathematician, was born on February 15, 1564. Although his ideas were condemned as heresy by the Roman Catholic Church, Galileo revolutionized man’s concept of Earth’s place in the heavens. He did so by providing strong observational evidence supporting the views of Copernicus, who said that the Sun, not Earth, is the center of the solar system.

Galileo was the first person to use an extraordinary new invention – the telescope – to examine the heavens. With his telescope Galileo discovered four bright points of light circling Jupiter. The objects were Jupiter’s four largest moons: Io, Europa, Ganymede, and Callisto.

Galileo Galilei. By Giusto Sustermans, 1636

This finding was a key piece of evidence in support of the Copernican view of the heavens. Others had argued that if Earth really orbited the Sun our Moon could not keep up. By demonstrating that other moons could orbit another planet that everyone agreed was in motion itself, Galileo disproved this notion.

Galileo also turned his instruments on Venus, our nearest planetary neighbor. Again his telescope revealed crucial evidence in defense of the Copernican view of the solar system: Venus, like Earth’s Moon, exhibits phases. If the Sun and planets orbited Earth, Venus would always appear as a crescent. But Galileo found that Venus displays both crescent and gibbous phases, meaning that it must be in orbit around the Sun.

Galileo’s work is honored and respected today, but in the seventeenth century it created major problems for the astronomer. After an appearance before the Roman Inquisition he was forced to deny his own findings and was placed under house arrest for the last decade of his life.

Flowers are part of our universe. Illustration by Elena.

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.