[00:00.000 --> 00:05.180]  Beyond the outer limits of human adventure. Beyond the barrier of human credibility.
[00:15.870 --> 00:20.150]  What orbits are most satellites in? There's a lot of room out there and there are actually an
[00:20.150 --> 00:25.150]  infinite number of orbits that a satellite can be in when orbiting around Earth. But that doesn't
[00:25.150 --> 00:30.050]  mean all those infinite orbits are actually useful. There are actually surprisingly few
[00:30.050 --> 00:34.930]  orbits that are targeted for most satellites. This is because the primary purpose for almost
[00:34.930 --> 00:39.790]  all satellites primarily has to do with the surface of the Earth. Orbital motion is completely
[00:39.790 --> 00:45.350]  ignorant of Earth's rotation. But for satellites, Earth is all that matters. There are only a few
[00:45.350 --> 00:50.710]  orbits that take advantage of both the rotation of the Earth and orbital motion. So naturally,
[00:50.710 --> 00:56.390]  these orbits are most targeted and getting kind of crowded. You can see just how crowded it is
[00:56.390 --> 01:02.310]  in space in general through this visualization. There are over 15,000 objects that we're
[01:02.310 --> 01:07.690]  currently tracking. This includes all active satellites, all dead satellites that are still
[01:07.690 --> 01:12.910]  orbiting, all rocket bodies that were used to get satellites into their targeted orbits, and
[01:12.910 --> 01:19.030]  all debris from the few satellite collisions that have happened over the years. Keep in mind that
[01:19.030 --> 01:24.130]  this visualization doesn't show the actual size of these objects, but there are actually several
[01:24.130 --> 01:28.310]  orders of magnitude smaller than the dots represented. So even though it does look very
[01:28.310 --> 01:33.010]  crowded, there's actually quite a bit of space left over. Now looking at just the active satellites
[01:33.010 --> 01:38.930]  and ignoring all else, it suddenly looks much less crowded. There are only about 2,200 active
[01:38.930 --> 01:44.670]  satellites in orbit. But even just glancing at them, you can already see natural groupings start
[01:44.670 --> 01:50.090]  to arise. The first of these groupings I'll cover is the most valuable real estate in space. It's
[01:50.090 --> 01:55.850]  the ring of satellites farther out than anything else, and it's called geostationary orbit, or GEO.
[01:55.850 --> 02:00.670]  GEO is a very unique spot in Earth's orbit. It is an orbit that is exactly matched to Earth's
[02:00.670 --> 02:06.590]  rotation, so when a satellite is in geostationary orbit, it doesn't move in relation to the Earth's
[02:06.590 --> 02:12.370]  surface. So when I select one and speed up the simulator, you can see how Earth is rotating under
[02:12.370 --> 02:17.730]  the ring of the geostationary orbit. Let's look at the two-dimensional view of Earth with the
[02:17.730 --> 02:22.610]  satellite's orbital motion projected on it. This is typically referred to as the ground track of
[02:22.610 --> 02:27.670]  the satellite. In the ground track, you can see it's not moving at all. The red circle, or satellite's
[02:27.670 --> 02:33.130]  footprint, indicates what the satellite is able to view from its current position. In this case, since
[02:33.130 --> 02:38.310]  it's a geostationary satellite that's so far out, it can see almost the entire western hemisphere
[02:38.310 --> 02:43.950]  of the Earth. This is why this orbit is so valuable. It's the only place where you can put a satellite
[02:43.950 --> 02:49.990]  in orbit and keep it still, and it can access a wide swath of the Earth's surface. For instance,
[02:49.990 --> 02:55.530]  this satellite that we're showing right now is used to broadcast satellite radio to both North and
[02:55.530 --> 03:00.690]  South America. Now it's only still in relation to the Earth's surface, so it's still orbiting, but
[03:00.690 --> 03:05.730]  the orbital speed at that distance is exactly matched to the Earth's rotation. Geostationary
[03:05.730 --> 03:10.470]  satellites have to be on the equator, otherwise they'll still move north and south. You can see
[03:10.470 --> 03:15.670]  this with an old GOES weather satellite. Even though it's at the same orbital distance as GEO,
[03:15.670 --> 03:20.270]  it's not exactly on the equator. It makes a figure eight on the Earth's surface, and at the top and
[03:20.270 --> 03:24.870]  bottom of that figure eight, you can also see it covers more of the northern or more of the southern
[03:24.870 --> 03:29.650]  hemisphere, depending on what position of the figure eight it's in. All the orbits within that ring of
[03:29.650 --> 03:34.430]  geostationary satellites have an orbital velocity faster than the rotation of the Earth, so it's
[03:34.430 --> 03:40.550]  much harder to maximize their usefulness over certain parts of the Earth's surface. At a glance,
[03:40.550 --> 03:46.230]  you can tell that geostationary satellites are much farther away than low Earth orbit satellites,
[03:46.230 --> 03:52.710]  but that distance is actually much farther than you might even realize. It's actually 22,000 miles
[03:52.710 --> 03:58.970]  or 36,000 kilometers away from the Earth's surface. That distance is so significant, it actually takes
[03:58.970 --> 04:04.530]  light almost a quarter of a second to travel the round trip between the surface of the Earth to
[04:04.530 --> 04:09.070]  the satellite and back. This can naturally cause issues with the operations of many different
[04:09.070 --> 04:13.930]  satellites. So now let's move a bit closer to Earth. The next major group of satellites is in
[04:13.930 --> 04:19.130]  medium Earth orbit, or MEO. This is a relatively sparse region of space that is loosely defined as
[04:19.130 --> 04:23.850]  below geostationary orbit and above low Earth orbit. There are far fewer satellites in this
[04:23.850 --> 04:29.870]  regime than either GEO or LEO, but one group of satellites in MEO probably affects your life more
[04:29.870 --> 04:36.970]  than any other. GPS satellites. Not only are U.S. GPS satellites in MEO, but also all the other major
[04:36.970 --> 04:43.250]  GPS constellations are in this region as well. This includes Galileo for the EU, Baidu for China,
[04:43.250 --> 04:49.070]  and GLONASS for Russia. Here you can see the U.S. GPS constellation. The crisscrossing pattern of
[04:49.070 --> 04:54.270]  the orbits and having multiple satellites in each orbital plane allows them to completely blanket
[04:54.270 --> 04:59.890]  the Earth's surface in GPS coverage. Medium Earth orbit ends up being the most optimal place to put
[04:59.890 --> 05:05.090]  GPS satellites. By being in medium Earth orbit, each of the GPS satellites can still see
[05:05.090 --> 05:10.470]  38% of the Earth's surface at any one time, but they're still moving fast enough to provide
[05:10.470 --> 05:15.430]  complete coverage of the entire Earth's surface as they are moving through their orbit. Looking at
[05:15.430 --> 05:19.990]  their associated ground tracks, you can easily see that GPS is completely blanketing the Earth.
[05:19.990 --> 05:24.950]  When we look at the footprint of just one of the GPS satellites, you can see it's still quite large
[05:24.950 --> 05:30.130]  and covers a vast portion of the Earth's surface, but it's still much, much smaller than geostationary
[05:30.130 --> 05:35.890]  satellites. MEO does have very real trade-offs, though. The GPS constellations are lonely for a
[05:35.890 --> 05:42.050]  reason. Let's go down closer to the Earth to the next orbit, low Earth orbit. Of the 2,200 satellites
[05:42.050 --> 05:48.010]  currently in orbit, about 1,600 of them are all in low Earth orbit, or LEO. LEO is very close to
[05:48.010 --> 05:53.970]  the Earth's surface and is defined as all orbits that are lower than 2,000 kilometers. Naturally,
[05:53.970 --> 05:57.830]  because it's so close to the Earth's surface, it requires the lowest amount of energy to reach.
[05:57.830 --> 06:04.570]  For example, SpaceX's Falcon 9 rocket can lift over 8,000 kilograms to geostationary transfer
[06:04.570 --> 06:09.930]  orbit, which is the orbit that puts satellites on their way to geostationary orbit. But it can lift
[06:09.930 --> 06:17.150]  more than double that amount to low Earth orbit, or 22,900 kilograms. This is really the primary
[06:17.150 --> 06:22.550]  reason why most of the satellites are in LEO. It's the default choice. Essentially, unless your
[06:22.550 --> 06:28.550]  has some pressing reason to be in a different orbital plane, it goes to LEO. Since LEO satellites
[06:28.550 --> 06:33.590]  are closest to the Earth, they also move the fastest, completing an orbit in only about 90
[06:33.590 --> 06:38.430]  minutes. You can see that with the International Space Station, which is in low Earth orbit. 90
[06:38.430 --> 06:44.090]  minutes is a lot faster than the 24 hours it takes for geostationary satellites. This also makes
[06:44.090 --> 06:48.910]  communication a lot harder, as you can see from the satellite's ground track. Since the satellite
[06:48.910 --> 06:54.050]  is so much closer to the Earth, its red circle is minuscule when compared to the geostationary
[06:54.050 --> 06:58.490]  satellites, and even when compared to the MEO satellites. Combined with the fact that they're
[06:58.490 --> 07:03.210]  orbiting so fast, they're actually only in view of any single point on the Earth's surface for
[07:03.210 --> 07:07.450]  about 5 to 10 minutes at a time. That means all communications with these satellites need to
[07:07.450 --> 07:12.730]  happen within that very short pass window. But it also means that they are able to view large
[07:12.730 --> 07:17.630]  portions of the Earth as they race around it very quickly. Within LEO, there are orbits that have
[07:17.630 --> 07:23.290]  additional value other than just being close to the Earth's surface. Sun Synchronous Orbit, or SSO,
[07:23.290 --> 07:28.370]  aligns the orbital motion of the satellite with the Sun. The Sun provides power for all satellites
[07:28.370 --> 07:33.550]  and provides illumination for satellites trying to image the Earth's surface. Sinking the orbit
[07:33.550 --> 07:38.530]  with the Sun means that the satellite will pass over the same location at the same time every day,
[07:38.530 --> 07:43.450]  which can be naturally useful, especially for imaging satellites. That predictability can also
[07:43.450 --> 07:48.790]  be used to predict the power generation of a satellite and really optimize its usefulness.
[07:48.790 --> 07:55.030]  Between them, LEO and GEO cover the vast majority of satellites, and MEO makes up the majority of
[07:55.030 --> 08:01.470]  the remaining ones. But there is still a class of orbits that can be useful for unique cases.
[08:01.670 --> 08:06.450]  This class is called Highly Elliptical Orbits. These are orbits that are not circular, but rather
[08:06.450 --> 08:11.710]  pass very close to the Earth's surface on one side of the orbit and very far away on the other.
[08:11.710 --> 08:16.390]  There are a large number of these orbits that are used for very interesting reasons, but the one that
[08:16.390 --> 08:21.890]  I'm going to cover today is called Molnaya, and this is what it looks like. As you can see, this
[08:21.890 --> 08:26.370]  orbit passes very close to the Earth's surface in the Southern Hemisphere, but then is very far away
[08:26.370 --> 08:31.190]  in the Northern Hemisphere. Since satellites go faster when they are closer to the Earth's surface,
[08:31.190 --> 08:36.510]  this means that those satellites have vastly more time over the Northern Hemisphere. It allows these
[08:36.510 --> 08:41.370]  satellites to essentially stay over one location in the Northern Hemisphere for a very long time,
[08:41.370 --> 08:45.550]  and then race around the Southern Hemisphere quickly to reduce the amount of uselessness it
[08:45.550 --> 08:49.830]  has. One reason this was used was to keep a satellite over the high latitudes of Northern
[08:49.830 --> 08:55.470]  Russia, which would normally be out of view for geostationary satellites. This is just one example
[08:55.470 --> 09:00.890]  of how Highly Elliptical Orbits can be more useful than their circular counterparts. There are
[09:00.890 --> 09:05.830]  numerous other orbits that I could cover that are useful for different reasons and in different ways,
[09:05.830 --> 09:10.310]  but the ones that I've covered today are the vast majority of the ones that are used by satellites
[09:10.310 --> 09:13.670]  currently in operation and currently orbiting our world.