Falling Leaves And The Perpetually Falling Space Station

Maple leaves come to rest on a sheaf of paper birch bark on the forest floor. Credit: Bob King
Maple leaves come to rest on a sheaf of paper birch bark on the forest floor. Credit: Bob King

October. Leaves are turning and falling. Autumn winds pluck them from the trees and send them bouncing down downtown sidewalks. Gravity is the reason leaves fall down rather than up. Like a leaf forever falling, the International Space Station (ISS) spends every second descending toward Earth. With leaves, the ground gets in the way and they come to rest on lawns, parking lots, lakes and forest floors. With the ISS, the ground keeps getting out of the way, making it impossible for the space station to settle down and come to a rest. We’re happy it doesn’t!

Instead of a ball, this example uses a cannon. If you shoot a ball with ever greater speed, it falls farther and farther away from the cannon until it describes an orbit around the Earth. Credit: Brian Brondel / CC BY-SA 3.0
Instead of a ball, this example uses a cannon. If you shoot a ball with ever greater speed, it falls farther and farther away from the cannon until it describes an orbit around the Earth. Credit: Brian Brondel / CC BY-SA 3.0

The reason our favorite flying lab never falls to the Earth is because it’s speeding around the planet at 17,150 miles an hour. It’s falling alright, but because its forward speed is so great, the curve of Earth drops away beneath it all the while, so the ISS never hits the ground. That’s true for all the other satellites as well. As long as they maintain their orbits.

Atmospheric drag — there’s still a bit of air at 250 miles up — does cause the station to gradually slow and drop closer to the surface, the reason that Russian resupply rockets are needed to occasionally boost the ISS to a higher orbit.

Another way to picture the space station is to imagine throwing a ball. A thrown ball arcs into the air and then drops back to the ground. The harder you throw, the longer the arc and the more time it stays in the air before hitting the ground. If you could throw a ball at 17,150 mph the arc would circle the Earth at the altitude of the space station and never fall back to Earth. Throw it really, really fast and it would escape Earth’s gravity altogether.

 Expedition 49 Flight Engineer Kate Rubins of NASA wears a hand-painted spacesuit decorated by patients recovering at the MD Anderson Cancer Center to raise awareness about the benefits of pairing art with medicine. Credit: NASA
Expedition 49 Flight Engineer Kate Rubins of NASA wears a hand-painted spacesuit decorated by patients recovering at the MD Anderson Cancer Center to raise awareness about the benefits of pairing art with medicine. Credit: NASA

If you enjoy watching the space station pass over at night, you’re in luck. For many locations in the northern hemisphere, it’s visible during convenient evening viewing hours for the next three weeks. Many nights will offer two passes, one in early twilight and one at nightfall. I refer you to Heavens Above and Spaceweather Flybys to find out when it’s visible over your city or region. At Heavens Above you select a location and click on the ISS link to get a table of passes for the coming 10 nights. Clicking on the time will produce a helpful map of the night sky at pass time with the satellite’s path shown.

You can also download an ISS app for your phone to alert you with flyby times for your location. Here are links to free Android and iPhone programs.

The International Space Station flies approximately 250 miles over thunderstorms visible during a nighttime pass. Lightning can be seen flashing brightly inside the clouds below. A Russian Soyuz spacecraft (left) and Progress spacecraft (right) are seen in the foreground.
The International Space Station flies approximately 250 miles over thunderstorms visible during a nighttime pass on Sept. 18. Lightning can be seen flashing brightly inside the clouds below. A Russian Soyuz spacecraft (left) and Progress spacecraft (right) are seen in the foreground. Credit: NASA

The space station like many satellites takes advantage of speed imparted it by Earth’s west to east rotation, rising in the western sky and traveling east. The football field-sized satellite shines anywhere from magnitude 0 (bright as Vega) to –3.5 or just one magnitude shy of Venus. You can’t miss it! A typical pass lasts about 5 minutes.

Next time it’s clear, find out when the station’s falling your way. Maybe you’ll even catch sight of a silhouetted leaf whirling down at the same time in gravity’s sweet clutch.