april 2011 is the 50th anniversary of
the first man in space um and the first
complete orbit of the earth by a manned
spacecraft but also was the first
example where
we were actually able to use the sort of
theory of orbits to actually
for a practical application to actually
put somebody into space
the person in question was uh
soviet i should about to call him a
russian but of course at the time and
there were soviets a guy called yuri
gagarin so he
launched from the baikonur cosmodrome um
in kazakhstan
and headed to the east and actually
almost all satellites and spacecraft get
launched to the east because that's the
way the earth's rotating so you actually
kind of get a kick from the earth's
rotation which helps you on your way
and then flew all the way around the
earth came back across the pacific round
and then landed fairly close to where he
launched in the first place you look at
it on a map he doesn't actually quite
get back to where he started and so you
might think he hadn't completed a
complete orbit of the earth but of
course in the meantime while he was
flying around the earth had been
rotating and so actually where he landed
was sort of directly where he took off
in the first place it's just the earth
had me moved a bit in the meantime so
it's a bit difficult to see these things
on a map because of course what he was
really doing was flying above the
circular globe of the earth and was
following more or less a sort of a a
circular orbit around the earth but of
course as soon as you unwrap that and
have to project it onto a flat piece of
paper to make a map you always end up
with these rather strange looking bendy
paths but actually his orbit was
reasonably simple it was quite close to
a circle around the earth so in fact
although this was the first time that
anyone had actually put this theory into
practice the ideas go back on a very
long way and really the first person who
had figured out what orbits were about
was isaac newton it was actually part of
his principia where he figured out the
nature of gravity and so on along the
way in that he figured out how orbits
actually work and it was actually him
thinking about the way that gravity
works and on the orbit of the moon which
actually led to his full understanding
of gravity in the first place
and he had a rather neat sort of analogy
he used to sort of thought experiment he
did to explain how orbits actually work
which is his thought experiment was if
you took a cannon and hauled it up to
the top of a very high mountain and
fired the cannonball out horizontally
what he knew is it would go forward and
it would sort of fall slowly to earth
okay if you then put more gunpowder into
the cannon and crank things up a bit and
fire the cannonball it would obviously
go a bit further before it fell to earth
but the other thing he realized that
actually it would even go a little bit
further than you expecting because as
the cannonball falls towards the earth
of course there is a curvature to the
earth as well that's kind of falling
away so the cannonball actually goes
even further before it finally catches
up with where the ground is
and his great leap was to say okay so
let's put even more gunpowder into this
cannon and if i fire it with enough
force then the cannonball will indeed be
falling to earth but the curvature of
the earth will guarantee that the
cannonball falls kind of towards the
earth at exactly the same rate that the
earth is falling away and so although
the cannonball is always falling towards
the earth it never gets any closer to
the earth because the earth's curvature
is taking it away from where it might
land at the same rate what happens if
you put even more gunpowder in if you
put a little bit more gunpowder in
you'll end up on the elliptical orbit
originally initially you'll kind of head
away from the earth and then eventually
you'll come back around again so you'll
follow an ellipse rather than a circle
and if you put even more gunpowder in
the map then you actually escape from
the earth entirely and eventually go
into deep space
the reason why this was only kind of a
thought experiment and rather rather
than an actual practical experiment
is because it turns out well there's two
things firstly if you actually did this
at the top of a mountain it wouldn't
work because air resistance would slow
down your cannonball more than anything
else and so the cannibal would heat up
and slow down and fall to earth so you
can't put things into orbit that close
to the ground just because the earth's
atmosphere kind of heats up whatever's
flying through them and stops it staying
in orbit but the other problem is sort
of a more practical one that if you
actually figure out how fast you'd
actually need your cannonball to emerge
from the gun in order to go around on
this orbit in this sort of low earth
orbit where you're very close to the
earth it's about six kilometers per
second it needs to be going at and in
fact a modern bullet rifle bullet goes
at about one kilometer per second so
this is much faster than anything you
could actually practically do with a
cannon then but even with a gun now it
really wouldn't work so the trick is
obviously to get away from the
atmosphere which is causing all this
friction um so that's the first part of
the trick and then the other part of the
trick is to have something that that
gets you up to these ridiculously high
speeds of six or seven kilometers per
second if that needed the invention of
the rocket engine which really provides
rather than with a cannon you get this
single pulse that boosts you to a
certain speed but a rocket really allows
you to kind of drive up to high enough
speeds to really go into orbit it was a
truly astounding
achievement um but obviously one of the
things that people were kind of excited
about was whether this whether he really
had completed an orbit but it turns out
they're actually rules it's bizarre and
i found out about this that even in 1961
somebody had made up rules as to what
would constitute this achievement and
one of the rules for whatever reason
they come up with was the fact that the
astronaut had to stay in their
spacecraft the entire journey and land
in their spacecraft so take off in it
and land in it and one of the things
that actually happened it was you know
part of the design of the mission was
that gagarin actually parachuted out he
completed re-entry and then before the
thing hit the ground he actually
parachuted out because it was deemed
safer to land on a parachute than to
land in the space probe
and of course officially this means that
he wouldn't have broken the record he
wouldn't have achieved the first orbit
of the earth and the soviets actually
lied about this for a long time for
about a decade afterwards they pretended
that he'd actually landed in the
spacecraft and he was made to say that
he'd landed in the spacecraft and so on
so they got the record officially given
to them and only about in in the 1970s
did they actually admit well actually he
parachuted out by which point nobody
really cared very much so gagarin was on
what was known as a low earth orbit
because really at the time all you could
do was get a rocket just above the
atmosphere and that gets you into an
orbit that's kind of the lowest one that
you can do around the earth a couple of
hundred kilometers up
and when you're that couple hundred
kilometers up it only it's actually not
that far around the earth and you're
going very fast
so it works out that it takes about 90
minutes to go around the earth but
obviously you can go into orbit in
principle at any distance away from the
earth and so there's a whole series of
different orbits that get used nowadays
perhaps the most famous is a thing
called the geostationary orbit where if
you get up to about 36 000 kilometers
above the earth the speed that which you
need to go around gets slower as you get
further away because the pull of gravity
is getting smaller and actually you've
got further to go so actually it takes
longer to complete the orbit as well
and at this magic distance of 36 000
kilometers an orbit takes exactly 24
hours or 23 hours and 57 minutes or so
basically exactly a day which means that
you're staying above the same point so
as you go around you're going around the
earth at exactly the same speed that the
earth is rotating which means you stay
above the same point on the earth the
entire way around
and that's very useful for things like
communication satellites where you want
them to stay above their ground station
or maybe weather satellites where you
want to be monitoring a particular part
of the earth and so that's a
particularly useful orbit and then of
course if you take it even further if
you go out to 400 000
kilometers then it actually takes about
28 days to go around entirely and of
course that's the distance the moon is
away from us and the reason why a month
is around 28 days is that's how long it
takes the moon to complete one complete
orbit of the earth and that was really
what newton's great achievement and all
this was was tying all this stuff for
thinking about gravity at the surface of
the earth and then applying it to the
gravity at the distance of the moon and
realizing there were really different
aspects of the same thing all the things
like the international space station uh
the hubble space telescope and those
kinds of things they're all in low earth
orbits really the only orbit the space
shuttle can get up to is this low earth
orbit it can't go any higher than that
so anything that's been delivered into
the space by the space shuttle and
hasn't had a booster rocket to take it
up to a higher orbit will end up in one
of these low earth orbits