Hey it’s Professor Dave, coming to you via satellite transmission.
Now that we have wrapped up a survey of the solar system, let’s get back to Earth and
talk 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.
What do these images tell us, and who was Edwin Hubble, for whom the telescope is named?
To have this discussion is to learn about the dramatic changes to the fields of astronomy
and cosmology in the 20th and 21st centuries, so let’s move forward begin talking about this now.