Orbital Mechanics · 10:51am Oct 28th, 2016
Not gonna lie. It's rocket science.
Always relevant XKCD
Your life is a whole lot better with satellites. They give us our weather, our communications, our navigation.
You probably don't think about satellites much. Or maybe you do, not judging. The overview of how satellites orbit is actually simpler than you might think. The details are a lot more complicated, though.
First, a basic question. How does stuff orbit? Well, you get it going really fast and then gravity keeps it going around Earth. The speed balances the force of gravity. Any faster, and the object would go flying off into space. Any slower, and it would fall out of orbit.
The speed required to keep the balance changes depending on the altitude at which the object orbits. The closer it gets to the surface of earth, the faster it has to go to keep equilibrium with gravity.
Orbits to scale
The International Space Station, orbiting at a distance of 210 miles (the dotted line inside the blue area of the above picture), travels at roughly five miles per second. The outside dashed line represents 22,236 miles from earth, and satellites out there travel at less than two miles per second.
These are just two of a few basic orbits. Low Earth Orbit (LEO) is by far the most common hangout for satellites. It extends up to 1,200 miles. Geostationary Orbit is at 22,236 miles, and it's the place where the satellite's relative speed matches the Earth's rotation, so it always stays above the exact same spot on Earth. This is useful for communications, because you can just keep your antennas pointed in the same direction and the satellite will always be there.
As long as your antenna is always there.
When choosing where to put your satellite, what should you pick? Geostationary Orbit certainly offers the advantage that the satellite will always be right there. However, being so far away could be a drawback. Geostationary Orbit also has a major disadvantage that they can't cover the poles very well.
Let me explain. A geostationary satellite has to rotate with the earth to stay in the same spot. That means it has to rotate in the same direction, which means that it can only orbit directly above the equator. This is why (if you're in North America) your satellite TV dish points south, towards the equator.
What if you want your satellite to move at a different speed than Earth? That makes things simpler. The ISS, at its lower orbit, takes about 90 minutes to make one revolution around the planet. You can also tilt the orbit at whatever inclination you wish. Inclination is usually measured in degrees above the equator.
As these blogs often do, let's take a step into the military side of things. Of course, the military uses communication, weather, and imagery satellites just like anyone else. They also have satellites to pick up signals and other things.
In the case of an imagery satellite, you want it in LEO. That way, not only is the distance shorter for better pictures, but the satellite also comes back around faster for another picture. You can also incline the orbit of a LEO satellite to pass over any part of the world.
The track of a satellite at a 55 degree inclination.
Of course, the disadvantage of a LEO satellite is that even if you incline the orbit, it won't be rotating in sync with the Earth and you'll have to figure out the timing to get it to orbit over the parts you want it to orbit over. In the case of an imagery satellite, you may also have to take into account timing the day/night cycle in order to take good pictures with sun illumination. LEO sats also zoom by, the ground beneath them constantly changing, only time enough to take a snapshot and move on.
This is where Highly Elliptical Orbits (HEO) come in.
Note how the satellite goes fast when it gets close to Earth, but slows down over most of the rest of its orbit. This gives the satellite the ability to hang out for a while at the top of its altitude. This is what you use if you want a somewhat-geostationary orbit closer to the poles. A lot of Russian communications satellites use this orbit, as do a lot of satellites that spy on Russia. Granted, most of the countries in the world that have the ability to launch satellites or are worth spying on are in the northern hemisphere.
With so many satellites (estimated 1,500) traveling at thousands of miles per hour, outer space gets cramped in a hurry.
Satellite map. You can clearly see the ring of geostationary sats, the LEO sats clustered close to the planet, and those at HEO that peak over the northern hemisphere.
Despite this, satellites do not collide often. Good thing, too. A huge cloud of debris could be produced, perhaps creating a chain reaction that takes out other satellites.
To keep space as clear as possible, US Strategic Command and its counterparts around the world monitor space objects with big radars and maneuver satellites accordingly. The UN suggests that old satellites that are no longer needed be disposed of. LEO satellites are typically deorbited in the South Pacific. Geostationary satellites are boosted a few hundred miles to a graveyard orbit. However, it takes fuel to move a satellite, and perhaps as many as half of all satellites aren't properly disposed of.
in tracking satellites, it's good that orbitology is fairly predictable, if complicated. However, in the case of spy satellites, that means foreign countries can predict when they're going to fly over and can hide.
What if instead of hiding you want to take out a satellite?
Missiles have been the most tested anti-satellite systems. The Chinese have successfully tested ASAT systems at altitudes over five hundred miles. The United States once destroyed a satellite at an altitude of 130 miles with a ship-fired missile that wasn't designed for satellite killing. In the 1980's, the US Air Force briefly had the capability to destroy satellites with an F-15 launched weapon that was good for 350+ miles but the program was cancelled. Russia has examined several designs for ASAT missiles.
Lasers are easier. Russia has demonstrated the capability to at least blind satellites if not destroy them outright.
A satellite designed to explode near another satellite was also a Russian fascination. They lost the space race and apparently were kind of insecure about it.
Like this, but less innuendo.
During the Cold War, Russia and the United States both operated anti-ballistic missiles, which presumably could have also been used against satellites in LEO. It sure would have messed them up either way. Many ABMs had nuclear warheads of their own, and the EMP pulse or even just debris could have wrecked lots of sats.
Yeah, WWIII is gonna be bad.
But let's think more awesome thoughts. One thing that has always fascinated me about space is the incredible numbers involved. Earlier, I quoted the ISS's speed at five miles per second - 17,150 miles per hour. When the US Navy shot down that satellite, the SM-3 missile intercepted it at a combined speed of 22,000 miles per hour. The warhead didn't have any explosives, it literally hit the target. At that speed, it didn't need any.
Vanguard I, launched in 1958, is the oldest surviving man-made space object still in orbit, and is expected to stay up for another two centuries. Lots of other satellites will be up there for much longer. We think of space vehicles being delicate and intolerant of mistakes, but still managed to get it right thousands of times. Some will still be there when you and I are gone.
I just wish I could see the stars from where I live.
I'm friends with the CEO of CubeCab (launch services for 5kg satellites), so I probably talk about satellites more than most people. He's also Winged Cat, so y'know... brony.
As Douglas Adams said, "space is big".
Which Gravity tried to do, even though they had the debris cloud traveling in the wrong direction... and hitting things at wildly differing orbits.
4274765 That is pretty darned cool. The people you meet around here, huh?
As they say, Sir Issac Newton is the deadliest sonofabitch in space.
I drove (I'm a Lyft driver) a guy a few months back who did a lot of work with GPS and he went into detail on how all that worked and all the stuff the military does that benefits civilians. Fascinating stuff. I asked if he had a blog or something I could follow, but he said he was far too busy for that.
I guess this will do.
4274835
Mass Effect much?
4274745
When that one pact by the UN that states no country is allowed to build that kind of stuff is repealed, which will be probably never
Or if some country builds it, thereby ignoring that pact...
That picture at the top. Describes me learning orbital mechanics from KSP perfectly xD
4274856
Yep, but the point still stand. Sir Issac is the deadliest SOB in space and Inertia is his loyal minion.
I'm not sure why you take the time to educate us ignorant bastards instead of writing for some Science Daily, but this is interesting stuff regardless. Much appreciated.
4274858
The Outer Space treaty prohibits WMDs placed in orbit, but smaller-scale weapons aren't violations of the treaty. Kinetic bombardment is still an option, for example. The US and Russia both designed orbital weapons during the Cold War, and Germany worked on orbital weapon plans during WW2, but there's no known orbital weapons right now. (Although Russia has had at least one space station with space-to-space weapons to prevent hostile boarding parties, and they had a failed launch of a weapons platform designed to take out other satellites.)
In 2006, a treaty was proposed for banning space weapons, which the US voted against. The only other country that didn't vote in favor of the proposition was Israel, who abstained.
Then in 2008, the US opposed a draft of the proposed treaty presented by Russia and China.
Then in 2014 the UN passed two resolutions against space weapons (note: UN resolutions don't really mean much). The US and Israel abstained from voting on the first (preventing an arms race in outer space), while nobody who did vote said "no". The US and Israel were joined by Georgia and Ukraine as the only 4 votes against the second resolution (no placing weapons in space).
So, while there are no publicly known weapons currently in orbit, the behavior of the US and Israel at the UN suggests to the tiny conspiracy theorist in my brain that there may be unknown orbital weapons. The more rational part says we just want to make sure the option stays on the table. Unlike almost every other country on the planet.
4275198
Ah, thanks for clearing that up
Uh, why do you have- Well, I guess you look this up on Wikipedia?
Fun story, Canada co-funded the WGS Satellite constellation (an american military comms sat), and we barely get any bandwidth! I am so sad, when I have to run networks off of a 2 Mb (little 'b') throughput connection...
Two minor points from someone who spends far too much time thinking about this kind of stuff.
First, it is interesting to note that you can actually tell where the oceans are in that satellite map without looking at the terrain since the geosynchronous satellites are mostly placed over the continents for coverage of the places people actually live.
Second, on the subject of warheads, at orbital velocities objects actually pack so much kinetic energy than chemical explosives don't actually add a significant amount of energy to the weapon. That means the only possible reason to have chemical explosives in an orbital weapon is to fragment it to improve the odds of a hit.
Also, this is relevant:
4274745 When launch costs come down. The old rods from god concept is technically sound and relatively easy to design and build, but was never implemented because getting it into orbit was considered to be prohibitively expensive for a conventional weapon.
4275198 It's highly unlikely that there are unknown weapons in orbit. It just costs too much to get anything up there for weapon deployment to be practical unless they are packing nuclear warheads which is exactly why those were banned. The only possible exception is an anti-satalite weapon since the targets are similarly expensive, but to be perfectly honest the US has plenty of missiles with proven anti-satalite capabilities so this seems highly unlikely to me.
4275585
I did say it was a tiny conspiracy theorist.
I heard some rumours of a Soviet anti-satellite 20mm cannon that was planned to be deployed at a similar time as the Star Wars program, any truth to it?
Images OK? SATCOMM antennas can be used by individuals as well, as seen here.
http://i.imgur.com/cQo00zM.jpg
4276060 https://en.wikipedia.org/wiki/Almaz#Defense_measures
4276396 ...uh, those two pages are word-for-word identical.