The night sky seldom offers a first—an event never before seen, even by the most grizzled observer. This past January featured one of those rare spectacles, when the asteroid-observing spacecraft near became the first interplanetary craft visible to the naked eye. As the craft zipped in for a close approach (using Earth’s gravity to change its trajectory), the sun glinted off its large solar panels, resulting in star-bright flashes in the northwestern sky.
Fail to spot near? Don’t despair. While there won’t be any more interplanetary probes to view, May offers a slew of other man-made objects slowly crisscrossing the heavens. They weren’t visible two months ago, and they won’t be around after August. But they’re here now during satellite season, when Earth’s northern axis tilts just 5 degrees from its maximum sunward extreme, allowing satellites as low as 200 miles to stand in full sunlight even an hour after sunset.
In the past four decades, some 7,000 satellites have been launched into orbit around Earth. They deliver weather images, monitor ground resources, provide telecommunications and navigational aid. More often they’re used by the military—to spy on us even as we try to spy them.
Most satellites are invisible, but a few hundred are large enough (typically school-bus size) and low enough (200 to 600 miles up) to catch our eye. The lowest satellites orbit Earth in 90 minutes and take 9 minutes to cross the sky. Satellites perched 600 miles up appear some ten times dimmer, take an extra 20 minutes to circle the globe, and require up to 18 minutes to cross from horizon to horizon.
How do you know you’ve spotted a satellite? Elementary, my dear Westar. If it’s a silent dot with no accompanying lights that moves in a straight line, and if it crosses the sky in minutes rather than seconds, you’ve spotted either an alien mother ship or a satellite. Satellites sometimes appear to zigzag, but that’s an optical illusion resulting from imperceptible eye muscle movements.
Satellites travel every which way, but never east to west. That’s because they’re usually launched in the same general direction as the rotation of Earth, which gives them a free 1,000-miles-per-hour boost. Most don’t circle the equator but are angled northward or southward so they can fly over much of our planet in just a few orbits.
Many television viewers aim backyard dishes at communications satellites stationed 22,300 miles up. Orbits at that distance are geosynchronous—the satellites orbit at the same speed as Earth and so appear motionless. Don’t even think of finding them; they are a million times fainter than their lower cousins and hopelessly invisible.
Satellites can shine fairly steadily or wink on and off if they’re tumbling. And if they’re tumbling—and most do—they’re no longer functioning.
Some of this space junk was always junk. Last October we passed the fortieth anniversary of the first shiny, useless, basketball-size artificial satellite, Sputnik. The first Sputnik was too small to be seen by the naked eye, and the awestruck world gazed unwittingly at the huge discarded third-stage booster that went with it into orbit. Today over 22,000 pieces of unrecyclable debris clutter the skies, garbage masquerading as fireflies.
If you need help tracking the major satellites, the Web site http://liftoff.msfc.nasa.gov/realtime/jtrack/spacecraft.html provides current passages. Or simply go out as evening twilight ends and look around. After only a minute or two you’ll strike orbiting pay dirt.
