Types of Artificial Satellites and Their Orbits
Satellite Transmission.
We have to discuss about what we are doing today to better
understand the universe beyond our tiny little cosmic neighborhood. First, we must understand that although modern
astronomers and astrophysicists do lots of theoretical work involving tons of abstract
math, at its heart, astronomy is a field of inquiry that was born from an observation
of the night sky. As we have done for centuries, astronomers
continue to look out there, and try to make sense of all these phenomena we can see.
So how do we see them? With telescopes of course. These were invented around the time of Galileo,
and were about as powerful as a pair of binoculars you can get at the store. But over time, these instruments became more
powerful, with larger and larger mirrors to collect and focus light, so that we could
see objects that are more and more distant, until these instruments became as big as a house. But there was always one huge limitation. From earth’s surface, we can only see a
certain percentage of the space that surrounds us, and we have to peer through the atmosphere
to see it, so our view had always been obscured and hindered to some degree, especially for
instruments operating with wavelengths of light that tend to be absorbed by particles
in the atmosphere. If only we could put a telescope in space! Well in fact, we eventually ended up doing
exactly that. But that wasn’t the first thing we put up
into orbit. There are currently over a thousand active
artificial satellites in orbit around the earth. We say artificial to distinguish them from
natural objects, like the moon, which is earth’s natural satellite, and the artificial ones
orbit much more closely than the moon does. So what are these things? How did we get them up there, and what do
they do?
The first satellite launched into orbit was
the Russian Sputnik 1, in 1957, and it was pretty much just a small metal sphere with
some radio antennas. This triggered the Space Race, which was part
of the Cold War between Russia and the United States, based on a fear that Russia would
put weapons in space to attack America. Born out of this fear was a frenzy of aeronautical
innovation, which led to the moon landing, and thousands of satellites, both for spying
and for doing science. So how far away are these things? Well satellites can orbit at a variety of
distances from the Earth.There is low-earth orbit. This means around two thousand kilometers
above the surface. Objects here go around the earth very fast,
in about two hours or a little less. This is where the international space station
is, so except for the humans that have been to the moon, all the manned spaceflight that
has happened so far has been in low-earth orbit. So how do these objects stay up there instead
of falling to the earth? It’s not because there is no gravity. In truth, gravity is almost as strong in low-earth
orbit as it is on the surface of the earth.
The reality is that these objects are, in
fact, falling to the earth. It’s simply that they are moving so incredibly
fast that they fall to the earth at the same rate that earth’s curvature is produced,
always falling but never landing. This is why astronauts experience weightlessness,
because they are literally in freefall. Objects that are very close to the earth have
to orbit very fast, at least about eight kilometers per second. But the farther away objects get from earth,
the more slowly they can go and maintain orbit, so orbital velocity and orbital radius are
inversely proportional. If we go a little higher, there is medium-earth
orbit. This contains objects between two thousand
and about thirty-six thousand kilometers above the earth. Objects right in the middle of this region
go around the earth in about twelve hours, a little more slowly than their low-earth
orbit counterparts, closer to five or so kilometers per second. And lastly, many of the satellites are very
far away, in what we call geostationary orbit. These objects are about thirty-six thousand
kilometers above sea level, traveling around three kilometers per second, and they go around
the earth once a day. That’s why we call it a geostationary orbit. Objects go around the earth at the same rate
that the earth rotates, so they always point to the same spot on earth. So how are each of these orbital distances
useful? Low-earth orbits are cheapest, and provide
high bandwith for communication with very little time lag, since light can bounce there
and back in a tiny fraction of a second. Also, since they are so close, they can image
earth’s surface very effectively, so we can get some great pictures. But they go so fast that they are always over
a different part of the earth. For this reason, it is usually the case that
a network of satellites will be deployed to complete any given function. The low-earth orbit environment has become
rather congested with objects, which has resulted in a lot of space debris. This can be very dangerous, as the probability
of collisions becomes much greater, which given the incredible orbital velocities, can
be destructive to any structures and would certainly be deadly to any humans in the way. Medium-earth orbit satellites are typically
used for navigation, communication, and observations regarding geodynamical phenomena that can
be seen best from a little ways a way.And lastly, the advantages of a geostationary
orbit are very clear, because it allows for a satellite to be directly above the same
part of the earth at all times.
The satellite orbits with precisely the same
rotational velocity as the earth itself. The best part about this is that the antenna
on the earth that communicates with that satellite doesn’t have to do much of anything, it
can just remain pointed directly at the satellite and never move, receiving a constant transmission
of information. These satellites are usually for communication,
broadcast, or weather observation, and the possibility for such devices was first proposed
in great detail by science-fiction writer, Arthur C. Clarke. No more than three such satellites are required
to provide coverage of the entire world, so this application is useful indeed. And so with that, we have a better understanding
of one of the most astonishing feats of human technology.
We are able to place sophisticated instruments
in orbit around the Earth to collect data that is impossible to get here on the surface,
including a wide variety of telescopes. Out of all these space telescopes, which collectively
target electromagnetic radiation from all areas of the spectrum, the most famous is
probably the Hubble telescope, which free from the limitations of earth-bound instruments,
has collected some of the most astonishing images ever taken. These are images of distant galaxies and other
objects in what had previously appeared to be empty space.
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