Plate 18 · Space
The Space Station Is Falling
Astronauts drift around the space station, but not because gravity has gone. The station and everyone in it are falling all the time — just moving sideways so fast that they never hit the ground.
Scroll
Gravity Is Still There
The International Space Station flies about 420 kilometers up. Gravity hasn’t vanished there; it is still nearly nine-tenths as strong as on the ground.
Astronauts float because they and the station are falling together, at the same rate. It is like riding an elevator that suddenly falls freely: your feet stop pressing on the floor, so you float.
Earth and the station’s orbit · the station drawn far larger than life
Newton’s Cannon
More than three centuries ago, Isaac Newton imagined a cannon on top of an enormously tall mountain, with no air to slow it, firing balls straight out sideways.
The faster the ball, the farther it lands. At 4 kilometers a second it comes down about 1,500 kilometers away; at 6, more than 3,000.
Newton’s cannon · the mountain impossibly tall
Falling Around the Earth
Fire it a little faster and something wonderful happens. As the ball falls, the round Earth curves away beneath it. However far it drops, the ground drops away just as far, and the ball never reaches it.
It keeps falling around the Earth forever: an orbit. At the station’s height that takes about 7.7 kilometers a second, and one lap takes about 93 minutes. From the mountaintop, a little higher, 7.6 is enough.
Getting Away
Faster still, and the orbit stretches into a long oval. From the ground it takes about 11.2 kilometers a second, from this mountaintop about 10.8, and then the ball never comes back: escape velocity.
The Moon is falling around the Earth too. It is far away, where gravity is weak, so it needs only about one kilometer a second. Newton checked: the Moon is 60 times farther from Earth’s center than an apple, so the pull is 3,600 times weaker — and in one minute the Moon falls toward Earth just as far as an apple falls in one second. The force that drops an apple also holds the Moon.
Where the Station Is Now
The station on this globe is placed from live data. It goes around the Earth every 93 minutes and sees about 16 sunrises a day.
There is still a whisper of air up there, which drags on the station and makes it sink, little by little. Every so often, engines push it back up.
Position: wheretheiss.at · orbit: CelesTrak
Your Turn
Set the cannonball’s speed and press “Fire.” Find the speed that just makes it around the Earth — and the one that sends it away for good.
Try thisHow fast must the cannonball go to fly away and never come back?
What the model leaves out
Newton’s cannon ignores air resistance and needs an impossibly tall mountain. The station’s orbit is worked out from published orbital elements, including the drift caused by the Earth’s bulging middle and the drag of thin air; the further from the date of those elements, the larger the error. The station is drawn thousands of times larger than life, or you could not see it. Except in “Where the Station Is Now,” the sunlight is placed for looks, not for the real time.
A common misreading
“There is no gravity in space” is wrong. Earth’s gravity is exactly what bends the paths of the space station and the Moon into orbits; without it, they would fly off in straight lines. Astronauts feel weightless because everything around them is falling together, not because gravity is missing.
Sources
- NASA: What is microgravity?
- University of Virginia physics lectures: Isaac Newton
- NASA: International Space Station facts and figures
- CelesTrak: Orbital elements
- wheretheiss.at: live station position
Data
Live station position: wheretheiss.at. Orbital elements: CelesTrak (GP data from the 18th Space Defense Squadron). Elevation and seafloor (globe colors): Terrain Tiles on AWS Open Data; ETOPO1, NOAA; GMTED2010 and SRTM, USGS.
Follow the question
Plate 24The Solar System TodayWhere are the eight planets right now? The farther from the Sun, the slower they go, and the longer a lap takes.
Plate 27What Pushes a RocketA rocket doesn’t push against the air. It throws a large part of itself backwards very fast, and the rest of it is pushed forwards.
