In about 4 billion years, the Milky Way will collide with its nearest neighbor, the Andromeda Galaxy. The two are rushing toward each other at about 70 miles per second km per second. There is no chance that you'll be flung off to space right now, because the Earth's gravity is so strong compared to its spinning motion.
This latter motion is called centripetal acceleration. At its strongest point, which is at the equator, centripetal acceleration only counteracts Earth's gravity by about 0.
In other words, you don't even notice it, although you will weigh slightly less at the equator than at the poles. NASA says the probability for Earth stopping its spin is " practically zero " for the next few billion years. Theoretically, however, if the Earth did stop moving suddenly, there would be an awful effect.
The atmosphere would still be moving at the original speed of the Earth's rotation. This means that everything would be swept off of land, including people, buildings and even trees, topsoil and rocks, NASA added. What if the process was more gradual?
This is the more likely scenario over billions of years, NASA said, because the sun and the moon are tugging on Earth's spin. That would give plenty of time for humans, animals and plants to get used to the change. By the laws of physics, the slowest the Earth could slow its spin would be 1 rotation every days. That situation is called "sun synchronous" and would force one side of our planet to always face the sun, and the other side to permanently face away.
By comparison: Earth's moon is already in an Earth-synchronous rotation where one side of the moon always faces us, and the other side opposite to us. But back to the no-spin scenario for a second: There would be some other weird effects if the Earth stopped spinning completely, NASA said. For one, the magnetic field would presumably disappear because it is thought to be generated in part by a spin. We'd lose our colorful auroras, and the Van Allen radiation belts surrounding Earth would probably disappear, too.
Then Earth would be naked against the fury of the sun. Every time it sent a coronal mass ejection charged particles toward Earth, it would hit the surface and bathe everything in radiation. Gravity in low Earth orbit is almost as strong as gravity on the surface.
To avoid falling back into the atmosphere, you have to go sideways really, really fast. The speed you need to stay in orbit is about 8 kilometers per second. Only a fraction of a rocket's energy is used to lift up out of the atmosphere; the vast majority of it is used to gain orbital sideways speed. This leads us to the central problem of getting into orbit: Reaching orbital speed takes much more fuel than reaching orbital height.
Reaching orbital speed is hard enough; reaching to orbital speed while carrying enough fuel to slow back down would be completely impractical. If you want to slow all the way down to zero—and drop gently into the atmosphere—the fuel requirements multiply your weight by 15 again.
These outrageous fuel requirements are why every spacecraft entering an atmosphere has braked using a heat shield instead of rockets—slamming into the air is the most practical way to slow down. And to answer Brian's question, the Curiosity rover was no exception to this; although it used small rockets to hover when it was near the surface, it first used air-braking to shed the majority of its speed. I think the reason for a lot of confusion about these issues is that when astronauts are in orbit, it doesn't seem like they're moving that fast; they look like they're drifting slowly over a blue marble.
At the correct orbital velocity, gravity exactly balances the satellite's inertia, pulling down toward Earth's center just enough to keep the path of the satellite curving like Earth's curved surface, rather than flying off in a straight line. The orbital velocity of the satellite depends on its altitude above Earth. The nearer to Earth, the faster the required orbital velocity. At an altitude of miles kilometers , the required orbital velocity is a little more than 17, mph about 27, kph.
To maintain an orbit that is 22, miles 35, kilometers above Earth, the satellite must orbit at a speed of about 7, mph 11, kph. That orbital speed and distance permit the satellite to make one revolution in 24 hours. Since Earth also rotates once in 24 hours, a satellite at 22, miles altitude stays in a fixed position relative to a point on Earth's surface. Because the satellite stays right over the same spot all the time, this kind of orbit is called "geostationary.
In general, the higher the orbit, the longer the satellite can stay in orbit. At lower altitudes, a satellite runs into traces of Earth's atmosphere, which creates drag. The earth is moving toward Leo at the dizzying speed of kilometers per second. It is fortunate that we won't hit anything out there during any of our lifetimes! Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue.
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