Q&A: Do probes / shuttles need velocity to re-enter Earth’s atmosphere?

Question by TheWritingWriter: Do probes / shuttles need velocity to re-enter Earth’s atmosphere?
I’m by no means an engineer / aerospace encyclopedia, as you can tell, but I thought there was a particular amount of “orbital force” or something which prevented objects in space from being dragged down into the atmosphere. Essentially, I’m talking about the polar opposite of escape velocity – is there a velocity required to re-enter a planet’s atmosphere without being caught in its upper orbital fields?

Sorry if this makes no sense, lol.

Best answer:

Answer by Stan Dalone
It’s more like the other way around. Objects need a certain amount of velocity (depending on altitude) to stay in orbit. If they slow down, they drop into a lower orbit or they drop far enough to be slowed by drag from the upper atmosphere. Then, of course, they suffer the heat of reentry, which incidentally isn’t caused by friction as most people think: it’s caused by supercompression of air in front of the reentering body.

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2 comments on “Q&A: Do probes / shuttles need velocity to re-enter Earth’s atmosphere?
  1. Larry454 says:

    There is no minimum velocity relative to the earth’s surface that is required to re-enter the atmosphere. It can be accomplished by simply falling inward from a location that is static relative to the spinning earth. The reason that most spacecraft re-enter with some significant velocity component is:

    1. They are already in orbit, moving at roughly 17,000 mph relative to the surface, and to slow down requires a lot of reverse thrust, which requires a lot of fuel, which is very heavy – so they did not take it along to begin with.

    2. They are already in a trajectory inbound from another body (like the Moon), moving at a comparable speed to item 1 above, and they cannot slow to a relatively low speed for the same reasons.

    3. They may be falling inward from a nearly “static” location above the earth’s surface, but they reach a relatively high speed before hitting the outer edges of the atmosphere. This would be true for – say – suborbital ballistic vehicles.

    There is a minimum velocity required to attain orbit. This speed varies according to your altitude above the earth. There is also a minimum speed required to escape the earth’s gravitational field. At the earth’s surface, this is roughly 7 miles per second, but it decreases as you start from higher locations. There is no minimum speed required to re-enter the atmosphere.

  2. rowlfe says:

    Well, yes and no…

    Orbital mechanics started with Isaac Newton. He proposed firing a gun with the barrel parallel to the ground. The shell falls to the ground at some distance. So then, what if you used more powder? The shell would fall to the ground further away. OK, so using more and more powder, the shell falls further and further away, until it falls to match the curve of the earth and comes all the way around to shoot yourself in the back, or lacking any other obstacle, continue around and around forever. He could not build such a gun, so this was purely a thought experiment, but you get the idea I think, that he discovered the concept of an artificial satellite. All you need do is throw something fast enough so it falls at the same rate as the earth below curves. Velocity is critical, too little and it falls to the ground, too much and it flies off into space, just right and it goes around forever. OK, so using this idea, in order to descend from orbit to return to the ground, you must slow down so your velocity is not enough to keep you in orbit. It is velocity that keeps you in orbit, so to land you must lose velocity. On space capsules they use “retro rockets” facing in the direction of travel to act as a brake to slow down. The shuttle turned over to face backwards and they used the auxiliary engines to burn and slow down. Once they completed the retro burn, they then turned over to face forward for the descent into the atmosphere. Once below the critical speed of Newton’s projectile, you descend to the ground, just as Newton’s projectiles did without enough powder. The problem you face on the return is air friction. If you don’t slow enough, you can skip back up and out like a stone skips on water, only to come down somewhere else until finally like the stone, you “sink” into the atmosphere. The problem with skipping is you do not have control over where you eventually come down. The shuttle was a flying brick with limited maneuverability. When it came down, all they could do was point it in the right direction and hope they timed it right so the landing strip was where they were when the wheels hit the ground. If you slow too much, your angle of descent is too steep and you burn up in spite of your heat shield. Hit it just right on speed and the time of your retro burn and you glide in under control to arrive at a landing location of your choice. It is sort of like driving with your eyes closed. You CAN do it if you have a map, instruments to tell you where you are, and time things exactly right. That, by the way, is how a cruise missile gets to a designated target, a “map” and GPS to tell it where it is so it can then navigate to the target. GPS was originally developed by and for the military for just that kind of thing, a cruise missile.

    By the way, “escape” velocity refers to a spacecraft leaving the gravitational field of the earth to reach another planet, such as the moon, or of the sun to reach another star. So far, two spacecraft, Voyager I and Voyager II, have left the solar system and are in deep space between stars. Both are about 12 billion miles away, traveling at about 35,000 MPH. They were launched in 1977 a couple of months apart. They gave us pictures in real time of Jupiter, Saturn, Neptune and Uranus as they flew by each. Voyager II was directed to fly through the rings of Saturn and thus did not fly by the outer planets as Voyager I did. Both craft are still in good health sending reports about the environment they are in. It takes over 11 hours for the radio signals to get to us. The transmitter is rated at 25 watts. Just look at a 25 watt light bulb and see how small that really is. They are powered by a radioactive decay heat source with a half life of 86 years, so if they started with 1000 watts of power, in 86 years that will have dropped to 500 watts. It only takes about 250 watts to keep the spacecraft alive. By the way, a trivia point, the moving parts are lubricated with genuine whale oil, because whale oil is the only lubricant we know of which can withstand the deep cold in the depths of space and still lubricate moving parts.

    I know, this is way off the beaten path that answers the question, but it IS interesting…

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