We live — and stop me if I’m going too fast — on a planet.
I mean, sure, duh. But this isn’t the natural state of the Universe; or, at least, it’s
not the way things usually are. Most of the Universe is pretty empty — that’s why
we call it “space” — and if I were to magically transport you someplace randomly
in the cosmos, the chances are you’d be a million light years from the nearest substantial object.
Evolving on a planet has warped our sense of physics. If I throw an object away from
me, it comes back. That’s bizarre! It should just keep going, moving away from me at a
constant speed. Instead though it goes up, slows, stops, then falls back down toward me.
The difference between living on a planet and being in deep space is gravity. Gravity
from an object goes on forever, but it gets weaker rapidly with distance. A zillion light
years away, the Earth’s gravity is fantastically weak, but here on Earth it’s literally a force to be
reckoned with. And in some places it can be a lot stronger than what we experience right here.
For most of history, gravity was just a fact of life, neither understood nor examined terribly
closely. In the mid 1600s, scientists like Robert Hooke and Isaac Newton started investigating
it using math — in fact, the two men got into a bitter feud over who thought of what
first. But whoever it was who first got it right, now we have a much better understanding
of how gravity works.
One thing before we get to gravity. An important concept that comes up a lot is mass. It’s
a bit tricky to define, but you can think of it as how much stuff makes up an object.
I know, that’s not very scientific sounding, but it’s not a bad way to think about it.
Something with more mass has more stuff in it.
Size doesn’t really play into this; two objects can have the same mass but one can
be much larger than the other. In that case, the bigger object’s mass is more spread
out, so we say it has lower density, where density is how much mass is inside a given volume.
In science terms, mass tells us how much an object resists having its motion changed.
An object with more mass is harder to get moving than an object with less mass, which
is pretty obvious if you’ve ever tried pushing on a toy car versus a real truck. But mass
is also defined using gravity.
Everything that has mass also has gravity and can inflict this force on another object.
The amount of force you feel from the gravity of an object like a planet depends on three
things: How much mass it has, how much mass you have, and how far away you are from it.
In fact, distance dominates here; the force of gravity weakens with the square of the
distance. Double your distance from an object and the force of gravity drops by 2 x 2 = 4
times. Go 10 times farther away and the force drops by 10 x 10 = 100 times.
Gravity is also attractive: It can only draw things in, not repel them. But how it attracts
things is where it gets fun.
If I hold up a rock and let go of it, it falls to the ground. What might be hard to see is
that it gets faster the longer it drops. Forces accelerate objects, so the longer the force
acts, the more the object’s velocity changes – in this case getting faster. If I drop
a rock from higher up, it’ll move faster when it hits the ground. Other forces act
on moving objects, as well, like friction and air resistance, counteracting gravity,
making this acceleration hard to see. But in space, the force of gravity becomes very clear.
Two objects that have mass will attract each other. If there are no other forces acting
on them, they’ll accelerate toward each other until they meet. Remember, though, that
the force of gravity depends on those masses. If one is really massive, and the other not
so much, then in more practical terms the massive one will pull in the less massive
one. The more massive one does move, but much less than the other one.
When objects are free to move under the effects of gravity, we say they are in orbit. The
simplest kind of orbit may not be what you think: It’s actually just a line! When you
drop a rock, it’s very briefly in orbit. Ignoring things like the Earth’s rotation
(which adds a bit of sideways motion) it’s close enough to say the rock just falls straight
down, and is stopped because the Earth itself gets in the way.
That’s not a terribly interesting orbit! So what if, instead of dropping the rock,
we throw it? That gives it a little bit of sideways motion, so instead of hitting the
ground at my feet, it hits a bit farther away. If I throw it harder, it moves horizontally
even more before it hits.
What if I throw it really hard?
This is where Newton’s genius comes in. He realized that if you throw the ball hard
enough sideways, it will fall at the exact same rate the Earth would curve away underneath
it. As Douglas Adams said in “Hitchhiker's Guide to the Galaxy,” flying is just falling
and missing the ground. It turns out, that’s exactly what orbiting is, too.
A rock thrown hard enough sideways will fall toward the Earth, but always miss it, going
instead into a circular path around it, guided only by gravity. It will orbit the Earth in
a circle, taking about 90 minutes to go around the planet once.
Circles are simple orbits. The speed at which the orbiting satellite travels depends on
the mass of the object it’s orbiting, and its distance from it. The farther it is, the
weaker gravity is, so it doesn’t have to travel as quickly to maintain the orbit.
Roughly 400 years ago, the astronomer Johannes Kepler realized that there can be other shapes
of orbits as well. He discovered the planets orbit the Sun on ellipses, when previously
it was thought they orbited in perfect circles. An elliptical orbit happens when you throw
the rock sideways even harder than it takes for a circular orbit; it goes up higher on
one end of the orbit than on the other.
In fact, the harder you throw the rock, the more elongated the orbit gets. An orbit like
this is still closed; that is, the orbit repeats itself and the rock is still bound to the
Earth by gravity. At some point, though if you throw the rock hard enough, an amazing
thing happens: It can escape.
Remember, gravity gets weaker with distance. If you throw a rock hard enough, while gravity
can slow it down, the gravity gets weaker the farther away the rock is. If the rock
has enough velocity, gravity weakens too quickly to stop it. The rock can escape, moving away
forever, so we call this the escape velocity.
The escape velocity of an object like a planet or star depends on how much mass it has and
how big it is. For the Earth, that turns out to be about 11 kilometers per second — for
Jupiter, it’s about 58 kilometers per second, and for the Sun it’s a whopping 600 kilometers
per second. Whatever the particular escape velocity for your cosmic location is, if you
fling a rock away from it faster than that, I hope you kissed it goodbye first, ‘cause
it ain’t coming back. One way to think of it is that the rock is always slowing, getting
ever closer to stopping, but it never actually stops. If it could travel infinitely far away,
it would stop, but that’s kind of a long trip.
This works in reverse, too. If I go way far away from the Earth and drop a rock, it’ll
accelerate. When it hits the planet it’ll be moving at escape velocity, that same 11
kilometers per second. And if I give it a little sideways kick, it’ll miss the Earth
but still pass us at escape velocity. An escape orbit is open — it doesn’t come back — and
is shaped like a parabola.
What if you throw the rock even harder than that? The rock doesn’t come back, and moves
away even faster. The orbit is now a hyperbola, which is similar to a parabola, but is even
more open. The rock never stops, even at infinity. It just keeps movin’ on.
Like all forces, gravity gets weaker with distance. But its force never quite drops
to zero; it just gets smaller and smaller as you get farther and farther away.
So why then are astronauts on the space station “weightless”?
Gravity is still pulling on the astronauts! In fact, at the height of the station, Earth’s
gravity has only decreased by a little bit; it’s still about 90% as strong as it is
on the Earth’s surface. If they were in a tower 320 kilometers high they’d weigh
90% of what they do on the Earth’s surface. But the big difference is that the astronauts
are in orbit, falling around the Earth. Weight is actually not just the force of gravity
on a mass, but how hard a surface pushes back on that mass. For example, when you stand
on the ground, the ground pushes back. Otherwise you’d fall through! The force of the ground
back on you is what causes you to have weight.
In free fall, there’s nothing pushing back. You’re falling freely, and so you have no
weight. NASA likes to call this condition “microgravity,” since there are subtle
forces acting on you.
This actually highlights the difference between mass and weight. In space you have the same
mass as you do on Earth, but no weight. If another astronaut pushed on you they’d have
to exert a force, but if you stood on a scale in space it wouldn’t register anything.
Space is weird. Well, compared to Earth.
One more thing, and this is truly weird: Photons, particles of light, have no mass, yet they
can be affected by gravity, too, bending their direction of flight as they pass a massive
object! It turns out gravity can actually warp space! Light travels along the fabric
of space like a truck on the road, and if the road curves, so does the truck. I know
this is an odd concept, and we’ll be dealing with it later in more detail when we push
escape velocity to its limits… with black holes.
Today you learned that gravity is a force, and everything with mass has gravity. Gravity
accelerates object, changing their speed and/or direction. An object moving along a path controlled
by gravity is said to be in orbit, and there are many different kinds: straight lines,
circles, ellipses, parabolae, and hyperbolae. You can’t ever escape gravity, but if you
travel faster than escape velocity for an object you’ll get away from it without falling
back. And if you’re in orbit, in freefall, you have no weight, but you still have mass.
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Crash Course Astronomy is produced in association with PBS Digital Studios, and you can head
over to their channel and find more awesome videos. This episode was written by me, Phil
Plait. The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.
It was co-directed by Nicholas Jenkins, and Michael Aranda, edited by Nicole Sweeney,
and the graphics team is Thought Café.