How low can you orbit? (w/ Video)

How low can you orbit?
Earth’s atmosphere. Credit: NASA

The Earth's atmosphere is a total drag, especially if you're trying to orbit our planet. It's a drag. Get it? Atmospheric drag. Drag. Drag.

Hi, my name is Fraser Cain. I'm the publisher of Universe Today, and sometimes my team lets me write my own jokes.

I could have started off this episode with a reference to the "Adama Drop" in- viper deployment from BSG, but instead I went with a Dad joke. My punishment is drawing attention to it.

So how low can you go? And if you go low enough, will Ludacris appear in the mirror?

We all appreciate the Earth's atmosphere and everything it does for you. With all the breathing, and the staying warm and the not having horrible bruises all over your body from teeny space rocks pummeling us.

I've got an alternative view. The Earth's atmosphere is your gilded pressurized oxygenated cage, and it's the one thing keeping you from flying in space.And as we all know, this is your destiny.

Without the atmosphere, you could easily the Earth, a few kilometers over its surface. Traveling around and around the planet like a person sized moon. Wouldn't that be great?

Well, it's not going to happen. As you walk through the atmosphere, you bonk into all the air molecules. You don't feel it when you're moving at walking speed, but go faster, like an airplane, and it'll rock you like a hurricane.

The Earth’s atmosphere is a total drag, especially if you’re trying to orbit our planet. So how low can you go?

Without constant thrust pushing against the atmosphere, you'll keep slowing down, and when you're trying to orbit the planet, it's a killer. Our atmosphere is like someone is constantly pushing the brakes on the fly in space party.

If you've played Kerbal Space Program, you know the faster you're traveling, the higher you orbit. Conversely, the slower you travel, the lower you orbit. Travel slow enough and you'll eat it, and by it, I meant as much planet as you can co-exist with after a high speed impact.

Being more massive means more momentum to push against the . But with a , it acts like a parachute, slowing you down.

Hey, I know something that's super massive with a huge surface area. The International Space Station orbits the planet at an altitude between 330 km and 435 km.

Why such a big range? The atmosphere is constantly pushing against the ISS as it orbits the planet. This slows down the 's speed and lowers its orbit. It wouldn't last more than a couple of years if it wasn't able to counteract the atmospheric drag.

How low can you orbit?

Fortunately, the station has rockets to increase its speed, and a faster speed means a higher orbit. It can even get assistance from docked spacecraft. If the space station were to go any lower, it would require higher and higher amounts of thrust to prevent re-entry into the Earth's atmosphere.

So what are the limits? Anything below 160 km altitude will essentially re-enter almost immediately, as it's buffeted by the thicker atmosphere. You really wouldn't last more than a few hours at that altitude, but above 800 km you could orbit for more than 100 years.

How low can you orbit?
The International Space Station, photographed by the crew of STS-132 as they disembarked. Credit: NASA

Geosynchronous satellites that orbit the Earth and transmit our television signals are at an altitude of about 42,000 km. Satellites that high are never coming back down. Well, maybe not never.

Want to enjoy your orbital experience? Make sure you get yourself to an altitude of at least 300 km, 400 km just to be safe. You should shoot for more like 800 km if you just don't want to worry about things for a while.

Explore further

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Source: Universe Today
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User comments

Jun 12, 2015
The text does not answer the question of the title.
So how low can you orbit a planet? Suppose it is perfectly round and has no atmosphere. Can you orbit almost touching it? (My guess is yes.)

Jun 12, 2015
It did answer the question. The answer isn't a single number, it's a conditional number.

At 160 km you can orbit for a short period of time unless you have a sufficient amount of thrust to keep you in orbit. Presumably you'd also be doing damage to the exterior of the ship at some rate as well.

They state 300 km is a good minimum but 400 km is better, and 800 km sufficient for "long" term orbits.

As for a perfectly round planet with no atmosphere, I'll add two extensions to that, the planet is equally massive (i.e. you feel the same pull of gravity) at all points when felt from the surface. Also you'd want there to be no moons! Depending on how close you get to the surface, these changes in local gravity will change your orbital height and you may smash into the surface because your orbit will not be a perfect circle but some irregular elliptical orbit.

Jun 12, 2015
If you've played Kerbal Space Program, you know the faster you're traveling, the higher you orbit. Conversely, the slower you travel, the lower you orbit.

That's completely wrong. It's the exact opposite way around. Lower orbits have faster orbital velocity, meaning you go around the planet quicker, whereas higher orbits have slower orbital velocity. You speed up when you want to go up, but the change takes effect half an orbit later, and by then you're actually travelling slower.

Its like throwing a rock. The faster you throw, the higher it goes, but it's fastest at the bottom of the throw - not at the top.

Since the centrifugal force that effectively keeps you in orbit is mass x radius x angular velocity squared, the higher up you go, the lower the angular velocity necessary to keep you there. This is also the reason why comets in highly eccentric orbits travel fast when they're near the sun and slow when they're far away from it.

Jun 12, 2015
you bonk into all the air molecules.

Really? Give me a break.

Jun 12, 2015
When you're in orbit, the route you take around the planets acts as if it were a spinning disc. Any force applied on it results in gyroscope-like behaviour - the effect of the force is deflected 90 degrees from the direction of the impulse along the plane of the disc.

If we fire the engines up, the orbit goes up - but 90 degrees or quarter turn later - and then come down twice as much. If we point the rocket north and fire the engines, we don't immediately go anywhere, but instead we'll find that the orbital inclination chances such that we end up at a more northern latitude 90 degrees later. It's exactly like trying to turn a spinning gyroscope - you try to tilt it one way, but it flips the other way.

(And if you resist it tilting the other way, you're fighting against your own hands, which makes it feel like the gyroscope doesn't want to turn. If instead you remember the 90 degree rule, you can make it turn any way you want with ease.)

Jun 12, 2015
Several points...

IIRC, Skylab came down because NASA forgot that a SolarMax 'fluffs' the atmosphere so there was more, higher than expected, with consequent increase of drag....

Didn't that gravity mapping probe use a continuous but variable thrust to offset drag ? Hence modest system life before fuel ran out...

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