We can send vehicles/rovers to space and have them travel at very high speeds for millions of miles to other planets. But vehicles on earth (jets/planes) have significantly shorter range and speed. Why is this? ELI5
In: Physics
If you need to explain this to a 5 year old have them wave their hand in the air then have them stick it in water and wave it. Tell them see how it was harder to move your hand in the water? That is because air is less dense than water. Space is even less dense then air so moving in space is easier than moving in air.
There is no air in space. The amount of resistance air gives at very high speed is absurdly huge. What they do with rockets is that they make them gain most of their speed higher up where the air is very thin or there isn’t any. The space shuttle is a good example. If you look at the chart I linked below, you will see that the shuttle gained most of it’s speed way above the altitude where commercial jets fly (that is about 10 km in metric).
[https://www.quora.com/Does-the-space-shuttle-heat-up-when-exiting-the-atmosphere-like-it-does-on-re-entry](https://www.quora.com/Does-the-space-shuttle-heat-up-when-exiting-the-atmosphere-like-it-does-on-re-entry)
The atmosphere. In space, depending on exactly where you are, there’s basically no resistance. You can go as fast as you want and there’s nothing pushing back on you. Have you ever stuck your hand out the window and had the air blow it back? Imagine that at 25,000 miles per hour or more. Imagine what that would do the structure of anything going that fast. And the heat would pretty much vaporize anything. You know how spacecraft need heat shields to reenter the atmosphere so they don’t burn up? That’s what would happen to anything going that fast. The Concorde, which flew at roughly mach 2, got so hot that the inside of the windows were warm to the touch, an this was at an altitude where the air was already super thin and the outside temperature was 100 degrees below zero. Spacecraft go more than 10 times that fast.
* Space is mostly empty.
* Earth’s atmosphere is packed with air molecules.
* Compared to literally nothing (space) the air molecules create a huge amount of friction against the things trying to move through it.
The atmosphere is stuff and space isn’t. The shuttle Columbia didn’t get destroyed in space, but in the atmosphere because it was moving so fast. Remember they reentry heat is caused by compressing the air and not friction and is an issue with super fast planes as well. The SR-71pumoed fuel through the hot parts before the going to the engine to keep things cool and it ‘only’ went Mach 3.2.
Back to Columbia, the damaged insulation didn’t keep that heat away from parts of the structure and they failed. Now that it was unbalanced it tumbled, and the air tore it apart. None of that would happen in space.
In space nothing is slowing you down.
It’s kinda like the difference between bicycle and walking. With bicycle, using the same amount of energy, you end up going much faster. That’s because air is basically the only thing slowing you down when biking, but when walking you gotta keep moving your legs which adds extra energy requirement constantly slowing you down.
In space, you don’t have air slowing you down. No matter how fast you go, nothing is pushing against you, slowing you down.
Rockets are pretty much perfectly inertial. Once they get up to speed, there are basically no moving parts or anything – they just keep going. The engines don’t even have to run for very long. Maybe a few weeks at most for an electric rocket engine, but usually on the order of minutes. Jets require turbine engines, which are under a lot of stress and have many moving parts at high temperature.
Because there is no air in space.
This is basically it.
Flying through air slows you down through wind resistance.
In space there isn’t really much to slow you down.
In space any little bit of extra engine use adds to your speed.
In the air you need to constantly run your engine just to keep moving at the same speed.
In space once you are in motion, you will stay that way. A spacecraft on some course will keeping going at that speed and course forever unless some outside force acts on it. Spacecraft can basically cost at a constant speed towards their destination without having to burn any fuel.
On earth you are constantly being slowed down by friction. Air resistance means that if you stop your engine for a moment you will slow down. You can do clever tricks with gliding to use the air resistance to extend your range somewhat, but you can’t just keep going forever.
Another problem with air is that it really gets in the way if you are starting to really fast.
As you go faster and faster the friction from the air becomes more and more and you don’t just have to work harder and harder to keep going at that speed, the friction with the air also starts heating up your craft. If you go at speeds many, many times the speed of sound the heat from the friction becomes a real technical problem, which is why spacecraft returning to earth and entering the atmosphere at extremely high speeds get heated up so much that they are in danger of burning up.
Going fast in the air is hard and dangerous.
On the other hand going fast in space is almost required.
Technically if you reach an altitude of 100km above the ground you have reached space. But if you want to to stay there instead of simply falling back down to earth you either have too keep running your engines the entire time or you have to enter into an orbit.
An orbit is basically going so fast sideways that you continue to miss the ground as you fall down. If you are in orbit you won’t have to run any engine or anything you just stay where you are at more or less the same altitude or speed you have (at least for circular orbits).
However orbits require orbital speeds and those are really fast.
To stay in orbit a speed is required that you wouldn’t be able to achieve while in the atmosphere.
The Space Station goes around the earth at what would be something like two dozen times the speed of sound in air near the ground.
Such speeds are very hard to achieve and sustain inside the air and necessary to stay in orbit.
Wave your arms in the air quickly. Do it for about a minute. Now fill a bathtub or go to a pool. Do the same thing for a minute with your arms underwater. Did you feel that you used a lot more energy in the pool or tub?
Why do you think you feel more tired after doing the same thing in the pool? Did the water “resist” your arm movements more?
Air has resistance too and we mostly don’t notice it at the speeds humans can move unaided. However compared to space, air resistance in our atmosphere makes a huge difference to how fast we can make vehicles or objects travel.
Try to run in the sea and then on a road.
In the sea you have water and air slowing you down.
On the road you only have air.
You already know that air exist because when it’s windy you can feel it pushing against you.
In space you have neither air or water so nothin’ stopping you:-)
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