Sometimes good starwatching is as simple as stepping outside on a clear night and looking up at the sky. It’s exciting when there’s a particular object to see, like a Saturn-Jupiter conjunction or a lunar eclipse. But even when there’s no headliner celestial event, there are always things happening. Satellites, for one thing. It’s worth keeping an eye on them.
Satellites are easy to spot within an hour or two of sunset or sunrise. Look up and scan the sky slowly back and forth. And I do mean slowly. Our eyes are excellent at detecting movement against a stationary background — an important survival skill for our pre-human ancestors when hungry predators stalked them through forests or tall grass.
As you scan, you’ll see many stars and a few constellations, which I hope you’ll recognize. All of these appear motionless. Then, sooner or later and quite suddenly, you’ll see a moving star.
Unlike a shooting star, this one doesn’t streak and vanish in a second; rather it moves across the sky with an eerie steadiness. What you’re seeing is sunlight reflecting off a satellite. Its orbit is high enough that though you’re in darkness down below, the satellite is still in sunlight above you. (If the object you’re watching has flashing lights, it’s an airplane.)
The majority of satellites, some 80 percent according to the Orbiting Now website, which lists active satellite information from orbit data collected by the U.S. Space Force and other public domain resources, are in what’s called low Earth orbit (LEO) at an altitude of 150 to 1,500 miles. The thousands of satellites in LEO include communications, weather, and military reconnaissance satellites, the International Space Station, and the Hubble Space Telescope.
The GPS system in your smartphone listens for signals from satellites in medium Earth orbit (MEO) at an altitude of 12,600 miles. This system was originally developed in the 1970s by the United States for military use, then gradually made available for civilian applications. China and Russia have their own systems. To do its magic, your GPS device must have a line of sight to at least four GPS satellites in MEO, all of which have highly accurate onboard clocks synchronized to a master atomic clock at ground stations.
The satellites all continuously transmit a signal indicating this time. The key is that radio signals, even traveling at the speed of light, take a bit of time to reach your GPS device. The farther a given satellite is from you, the longer its signal takes to reach you. Your GPS device does some math involving the difference in the various satellites’ times of transmission and the actual time of arrival on your device (which correlates to distance) to accurately determine its own position in three-dimensional space.
Even higher than MEO is geosynchronous orbit (GSO) at 22,230 miles. In GSO, the speed necessary to maintain orbit exactly matches that of Earth’s rotation. So, observers on the ground will always see satellites in GSO in the same positions in the sky. The advantage of GSO is that you always know where these satellites are, which means you can point an antenna at them and never have to move it to send or receive data. Satellite TV services work this way.
While you’re out satellite hunting, you may see something very new. The company SpaceX is building a satellite internet system called Starlink. When a group of Starlink satellites passes by, you can’t miss it. You’ll see a string of 15 or more tightly spaced satellites moving together in a straight line. It’s a strange and unnatural sight. The goal is to provide fast and reliable internet service to anyone in the world using a network of tens of thousands of satellites. But the project is controversial.
That’s because when it’s completed Starlink will have 42,000 satellites in LEO — about six times as many satellites as now orbit Earth. So far, fewer than 5,000 are in orbit, and they’re already causing problems.
Astronomers use very large and powerful ground-based telescopes to study the cosmos. They carefully plan their observing runs around the orbits of satellites that they know will cross the telescope’s field of view on a given night. This is because the bright reflected sunlight from a satellite pass can ruin a night’s work. The giant telescopes are designed to gather and analyze data from the smallest and most incredibly faint objects.
The Starlink satellites in this early phase already make it difficult for some observatories to plan sessions. As more and more satellites are launched, many astronomers fear that ground-based space science may become impossible. This is to say nothing of amateur astronomers doing important backyard work, or astrophotographers, whose images often grace this column.
And then there is the physical hazard that Starlink presents. LEO has only so much useful real estate. Jamming in this many new satellites increases the chances of collisions with other orbiting objects. If you’ve seen the movie Gravity you’ve seen a highly dramatized version of what could happen in a runaway positive feedback loop of orbital collisions producing debris, which then produces even more collisions, producing even more debris.
This extreme outcome is unlikely. But the collision danger is real, as are the obstacles to science. And as designed, Starlink satellites can’t simply move to a higher and less problematic orbit because that would reduce their data bandwidth (radio transmissions disperse as they travel). Also, the increased time delay of higher orbits would cause problems with internet data traffic.
The most vexing part of this problem to scientists and concerned citizens is that SpaceX began this project without clearly communicating its full extent to any scientific or government body. Neither SpaceX nor Elon Musk, its owner, asked permission to do this.
I find it outrageous that a person or a corporation, no matter how wealthy, would be entitled to lay claim to so much of our collective night sky — science be damned. To bring my blood pressure down I’ll ignore Starlink fly-bys and instead look for Perseid meteors.
This annual meteor shower peaks on the night of Aug. 12. But it’s worth beginning to watch for meteors zipping across the sky (much faster than satellites) starting about now. NASA and space.com both have excellent online guides for when and how to see the Perseids. Clear skies!