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0	5180	Beyond the outer limits of human adventure. Beyond the barrier of human credibility.
15870	20150	What orbits are most satellites in? There's a lot of room out there and there are actually an
20150	25150	infinite number of orbits that a satellite can be in when orbiting around Earth. But that doesn't
25150	30050	mean all those infinite orbits are actually useful. There are actually surprisingly few
30050	34930	orbits that are targeted for most satellites. This is because the primary purpose for almost
34930	39790	all satellites primarily has to do with the surface of the Earth. Orbital motion is completely
39790	45350	ignorant of Earth's rotation. But for satellites, Earth is all that matters. There are only a few
45350	50710	orbits that take advantage of both the rotation of the Earth and orbital motion. So naturally,
50710	56390	these orbits are most targeted and getting kind of crowded. You can see just how crowded it is
56390	62310	in space in general through this visualization. There are over 15,000 objects that we're
62310	67690	currently tracking. This includes all active satellites, all dead satellites that are still
67690	72910	orbiting, all rocket bodies that were used to get satellites into their targeted orbits, and
72910	79030	all debris from the few satellite collisions that have happened over the years. Keep in mind that
79030	84130	this visualization doesn't show the actual size of these objects, but there are actually several
84130	88310	orders of magnitude smaller than the dots represented. So even though it does look very
88310	93010	crowded, there's actually quite a bit of space left over. Now looking at just the active satellites
93010	98930	and ignoring all else, it suddenly looks much less crowded. There are only about 2,200 active
98930	104670	satellites in orbit. But even just glancing at them, you can already see natural groupings start
104670	110090	to arise. The first of these groupings I'll cover is the most valuable real estate in space. It's
110090	115850	the ring of satellites farther out than anything else, and it's called geostationary orbit, or GEO.
115850	120670	GEO is a very unique spot in Earth's orbit. It is an orbit that is exactly matched to Earth's
120670	126590	rotation, so when a satellite is in geostationary orbit, it doesn't move in relation to the Earth's
126590	132370	surface. So when I select one and speed up the simulator, you can see how Earth is rotating under
132370	137730	the ring of the geostationary orbit. Let's look at the two-dimensional view of Earth with the
137730	142610	satellite's orbital motion projected on it. This is typically referred to as the ground track of
142610	147670	the satellite. In the ground track, you can see it's not moving at all. The red circle, or satellite's
147670	153130	footprint, indicates what the satellite is able to view from its current position. In this case, since
153130	158310	it's a geostationary satellite that's so far out, it can see almost the entire western hemisphere
158310	163950	of the Earth. This is why this orbit is so valuable. It's the only place where you can put a satellite
163950	169990	in orbit and keep it still, and it can access a wide swath of the Earth's surface. For instance,
169990	175530	this satellite that we're showing right now is used to broadcast satellite radio to both North and
175530	180690	South America. Now it's only still in relation to the Earth's surface, so it's still orbiting, but
180690	185730	the orbital speed at that distance is exactly matched to the Earth's rotation. Geostationary
185730	190470	satellites have to be on the equator, otherwise they'll still move north and south. You can see
190470	195670	this with an old GOES weather satellite. Even though it's at the same orbital distance as GEO,
195670	200270	it's not exactly on the equator. It makes a figure eight on the Earth's surface, and at the top and
200270	204870	bottom of that figure eight, you can also see it covers more of the northern or more of the southern
204870	209650	hemisphere, depending on what position of the figure eight it's in. All the orbits within that ring of
209650	214430	geostationary satellites have an orbital velocity faster than the rotation of the Earth, so it's
214430	220550	much harder to maximize their usefulness over certain parts of the Earth's surface. At a glance,
220550	226230	you can tell that geostationary satellites are much farther away than low Earth orbit satellites,
226230	232710	but that distance is actually much farther than you might even realize. It's actually 22,000 miles
232710	238970	or 36,000 kilometers away from the Earth's surface. That distance is so significant, it actually takes
238970	244530	light almost a quarter of a second to travel the round trip between the surface of the Earth to
244530	249070	the satellite and back. This can naturally cause issues with the operations of many different
249070	253930	satellites. So now let's move a bit closer to Earth. The next major group of satellites is in
253930	259130	medium Earth orbit, or MEO. This is a relatively sparse region of space that is loosely defined as
259130	263850	below geostationary orbit and above low Earth orbit. There are far fewer satellites in this
263850	269870	regime than either GEO or LEO, but one group of satellites in MEO probably affects your life more
269870	276970	than any other. GPS satellites. Not only are U.S. GPS satellites in MEO, but also all the other major
276970	283250	GPS constellations are in this region as well. This includes Galileo for the EU, Baidu for China,
283250	289070	and GLONASS for Russia. Here you can see the U.S. GPS constellation. The crisscrossing pattern of
289070	294270	the orbits and having multiple satellites in each orbital plane allows them to completely blanket
294270	299890	the Earth's surface in GPS coverage. Medium Earth orbit ends up being the most optimal place to put
299890	305090	GPS satellites. By being in medium Earth orbit, each of the GPS satellites can still see
305090	310470	38% of the Earth's surface at any one time, but they're still moving fast enough to provide
310470	315430	complete coverage of the entire Earth's surface as they are moving through their orbit. Looking at
315430	319990	their associated ground tracks, you can easily see that GPS is completely blanketing the Earth.
319990	324950	When we look at the footprint of just one of the GPS satellites, you can see it's still quite large
324950	330130	and covers a vast portion of the Earth's surface, but it's still much, much smaller than geostationary
330130	335890	satellites. MEO does have very real trade-offs, though. The GPS constellations are lonely for a
335890	342050	reason. Let's go down closer to the Earth to the next orbit, low Earth orbit. Of the 2,200 satellites
342050	348010	currently in orbit, about 1,600 of them are all in low Earth orbit, or LEO. LEO is very close to
348010	353970	the Earth's surface and is defined as all orbits that are lower than 2,000 kilometers. Naturally,
353970	357830	because it's so close to the Earth's surface, it requires the lowest amount of energy to reach.
357830	364570	For example, SpaceX's Falcon 9 rocket can lift over 8,000 kilograms to geostationary transfer
364570	369930	orbit, which is the orbit that puts satellites on their way to geostationary orbit. But it can lift
369930	377150	more than double that amount to low Earth orbit, or 22,900 kilograms. This is really the primary
377150	382550	reason why most of the satellites are in LEO. It's the default choice. Essentially, unless your
382550	388550	has some pressing reason to be in a different orbital plane, it goes to LEO. Since LEO satellites
388550	393590	are closest to the Earth, they also move the fastest, completing an orbit in only about 90
393590	398430	minutes. You can see that with the International Space Station, which is in low Earth orbit. 90
398430	404090	minutes is a lot faster than the 24 hours it takes for geostationary satellites. This also makes
404090	408910	communication a lot harder, as you can see from the satellite's ground track. Since the satellite
408910	414050	is so much closer to the Earth, its red circle is minuscule when compared to the geostationary
414050	418490	satellites, and even when compared to the MEO satellites. Combined with the fact that they're
418490	423210	orbiting so fast, they're actually only in view of any single point on the Earth's surface for
423210	427450	about 5 to 10 minutes at a time. That means all communications with these satellites need to
427450	432730	happen within that very short pass window. But it also means that they are able to view large
432730	437630	portions of the Earth as they race around it very quickly. Within LEO, there are orbits that have
437630	443290	additional value other than just being close to the Earth's surface. Sun Synchronous Orbit, or SSO,
443290	448370	aligns the orbital motion of the satellite with the Sun. The Sun provides power for all satellites
448370	453550	and provides illumination for satellites trying to image the Earth's surface. Sinking the orbit
453550	458530	with the Sun means that the satellite will pass over the same location at the same time every day,
458530	463450	which can be naturally useful, especially for imaging satellites. That predictability can also
463450	468790	be used to predict the power generation of a satellite and really optimize its usefulness.
468790	475030	Between them, LEO and GEO cover the vast majority of satellites, and MEO makes up the majority of
475030	481470	the remaining ones. But there is still a class of orbits that can be useful for unique cases.
481670	486450	This class is called Highly Elliptical Orbits. These are orbits that are not circular, but rather
486450	491710	pass very close to the Earth's surface on one side of the orbit and very far away on the other.
491710	496390	There are a large number of these orbits that are used for very interesting reasons, but the one that
496390	501890	I'm going to cover today is called Molnaya, and this is what it looks like. As you can see, this
501890	506370	orbit passes very close to the Earth's surface in the Southern Hemisphere, but then is very far away
506370	511190	in the Northern Hemisphere. Since satellites go faster when they are closer to the Earth's surface,
511190	516510	this means that those satellites have vastly more time over the Northern Hemisphere. It allows these
516510	521370	satellites to essentially stay over one location in the Northern Hemisphere for a very long time,
521370	525550	and then race around the Southern Hemisphere quickly to reduce the amount of uselessness it
525550	529830	has. One reason this was used was to keep a satellite over the high latitudes of Northern
529830	535470	Russia, which would normally be out of view for geostationary satellites. This is just one example
535470	540890	of how Highly Elliptical Orbits can be more useful than their circular counterparts. There are
540890	545830	numerous other orbits that I could cover that are useful for different reasons and in different ways,
545830	550310	but the ones that I've covered today are the vast majority of the ones that are used by satellites
550310	553670	currently in operation and currently orbiting our world.