Earth orbit isn't so much "high" as "fast.

Originally shared by Yonatan Zunger

Earth orbit isn't so much "high" as "fast." The ISS routinely passes closer to my house than Los Angeles; it wouldn't be a terribly far commute. The only problem is that, if I were to travel in a straight line those 249 miles up to touch it, I would then simply be 249 miles above the ground, and what would happen next is technically known as "falling."

Things stay in orbit not by being high up, but by moving fast enough that they continually fall towards the ground and miss. Draw a line between yourself and the center of the Earth; gravity is pulling you along that line. Point your nose perpendicular to that line, and go: your normal straight-line motion is moving you away from the Earth. The art of orbiting is simply the art of keeping those two things in balance, so that you're moving so quickly through space that you're losing altitude through falling at the same speed that you're gaining it through hurtling.

Of course, you have to be going kind of fast for this to work. The ISS travels at a steady speed of 7.6km (4.76 miles) per second. 

This is why spacecraft don't simply fly straight up; they fly up about 26,000' to get out of the thickest part of the air, then turn 90° and thrust for speed. (This post talks more about why that makes more sense than taking off horizontally like an airplane: https://plus.google.com/+YonatanZunger/posts/VsYyUDxFUDr) 

It probably won't surprise you that when you're flying at this speed, running into things is not a good idea. The picture below is from a test run by the ESA (the European Space Administration) of a "hypervelocity impact." The block is made of solid Aluminum, and was cut in half after the test to see what happened. The pellet is not the one that was used in the test; you can see parts of the pellet used in the test in the form of those smears along the inside of the crater. At 6.8km/s, the impact blew the crater you see into the block of metal, and the shock wave in front of it opened up that second cavity at the bottom.

Note that the speed here was only 6.8km/s. Oribtal speed is a function of altitude alone; anything flying at the ISS' altitude will be going at 7.6km/s. But it might be going the other way, which means that collisions with random debris in orbit could happen at speeds as high as 15km/s. Meteoroids coming in from elsewhere in the solar system could be flying as fast as 72km/s.

The ESA's page (http://www.esa.int/Our_Activities/Operations/Space_Debris/Hypervelocity_impacts_and_protecting_spacecraft) about these hypervelocity impact tests is full of wonderful understatements. An impact of any 10cm object against any spacecraft would "most likely entail a catastrophic disintegration of the target." (I should say that space travel includes phrases like "hard start" for what happens when fuel and oxidizer accumulate in a rocket engine's chamber before the engine ignites, and "spontaneous disassembly" for what happens if the airframe is separated into multiple pieces on an unscheduled basis. For those outside the field, those translate as "the engine explodes" and "the spacecraft explodes," respectively)

The thing I keep thinking about when I see this picture is imagining being aboard a spacecraft – especially something big, like the ISS – and hearing a loud "bang" resonating throughout the ship. That's all you would know at first: something, somewhere aboard, just caused the entire ship to shake.

Space travel is not for the faint of heart.