Ask Smithsonian: How Does a Satellite Stay Up?
Meet a Harvard-Smithsonian researcher who monitors all the satellites and explains why they rarely fall
What goes up must come down, right? That’s not necessarily true in space, where satellites swarm around the planet, locked in by speeds that help defeat the downward pull of gravity.
Although satellites do come down more often these days—mostly the result of a life of planned obsolescence—some have floated around for years, if not decades, without a pre-programmed fall-back-to-Earth date. And that’s cluttering the orbital space.
So what keeps them in orbit? Satellites—that is, artificial satellites, as opposed to natural satellites like the moon—are carried into space by rockets. The rocket must fly 100-to-200 kilometers above the earth to get outside the atmosphere. Once at a pre-determined orbit elevation, the rocket starts heading sideways at speeds of up to 18,000 miles per hour, says Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
The rocket switches off and drops its payload—the satellite—which is now in the same orbit, zooming along at those same speeds. The Earth is curving away while both the rocket and the satellite “fall” around the Earth. The satellite stays in that orbit as long as it keeps its speed to stay balanced by the headwinds.
At those heights, the atmosphere is just thin enough to prevent the satellite from burning up—as it will if it drops lower and encounters thicker air, which causes greater headwinds and thus greater friction.
Most satellites are dropped in a range of up to 2,000 km above the earth. The satellites in the very low end of that range typically only stay up for a few weeks to a few months. They run into that friction and will basically melt, says McDowell.
But at altitudes of 600 km—where the International Space Station orbits—satellites can stay up for decades. And that’s potentially a problem. They travel so fast—5 miles a second—that their “footprint” can be hundreds of miles long. “When you think of them as being that big, suddenly space doesn’t look as empty anymore,” McDowell says.
The first satellite was launched by the former U.S.S.R. in late 1957. The Sputnik-1 became an icon of modernity and prodded the U.S. into further accelerating its own space exploration plans. Just months after Sputnik, America launched Explorer-1. In the intervening decades, thousands of satellites have been carried up into space.
McDowell keeps close tabs on the action. By his reckoning, there are some 12,000 pieces of space debris and several thousand satellites in orbit, with a little over a thousand that are still active. However, the active count “is uncertain, as monitoring of radio transmissions from these satellites to their owners is not widely done—except perhaps by the National Security Agency—and sometimes the owners, especially military ones, don’t tell me when their satellites have been switched off,” says McDowell.
About a third of satellites are owned by various militaries, of which a third to a half are used for surveillance, he says. Another third are civilian-owned, and the final third are commercial. Russia, the U.S., China and Europe are the main players in the launch business, but many other countries have capabilities or are developing them. And dozens of countries have built their own satellites—launched by other nations or commercial space companies.
And the trend is to send up devices with long lifespans—10- to 20-years on average. On top of that, retired or dead satellites mostly stay in orbit, powered by solar panels.
Adding to the mix: the burgeoning “personal” satellite business. These micro satellites have largely been developed and used by universities, but at least one company is selling directly to the public and there are D.I.Y. sites, also.
The dissemination of satellite technology is driven in part by the same factors that have resulted in the spread of other formerly sophisticated technologies, like gene sequencing—more knowledge, faster computing, and less-expensive machinery. But also “there are more tickets to ride available”—more launch opportunities, says McDowell.
All of which makes for an ever-more-crowded orbital space.
There are lots of near-misses—with engineers playing the role of air traffic control from Earth, maneuvering the satellites out of harm’s way as needed. Satellite owners have been asked—by NASA, among other space agencies—to take steps to reduce the likelihood that today’s prized flying machine doesn’t become tomorrow’s floating bucket of junk. That’s being done by pushing low orbiters into the burnout zone or deliberately crashing large satellites into the South Pacific, McDowell says.
In the meantime, the Earth may be reaching its capacity for orbiting objects.
Just as humans have become more aware of the need for stewardship of the terrestrial environment, “we’re going to have to be serious about the ecology of near outer space,” says McDowell.
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