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At the core of astronomy, is gravity.

And the theory of gravity first came to us from the genius of physics and

astronomy of the 17th century, Isaac Newton.

Isaac Newton was an extraordinary figure in the history of science.

Early in his life,

he came up with the theory of gravity that has stood us in good said for centuries.

Until it got its final adjustment in general relativity from Albert Einstein.

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Newton came up with his theory of gravity using the telescope but

almost entirely based on mathematics and physical thinking.

Even though he developed a very successful theory of gravity,

there were things about gravity that Newton didn't understand.

When he was asked about this force of nature that apparently operated over

the vacuum of space instantaneously, and what that meant, he said,

I frame no hypothesis.

In other words even Newton, as brilliant as he was,

could not explain all the subtleties of gravity.

However Newton's theory of gravity applies in most of the situations of the universe,

which involve relatively weak gravity.

It perfectly describes the objects of the solar system, the stars within the galaxy,

and most of the motions between galaxies in the larger universe.

When Apollo 8 was heading towards the moon, Ed Anders, one of the astronauts,

was patched through to his son.

And his son asked, Daddy, who's driving the spacecraft?

Anders said, Isaac Newton's driving, son.

And so, even though general relativity is a superior and more all encompassing

theory of gravity, Newton's theory works for almost all the work that NASA does.

Sending spacecraft around the solar system, or to the moon or

an orbit of the earth.

Newton's universal law of gravity has a very simple mathematical form.

It says that the force between any two objects equals a constant of nature,

the gravitational constant, times the mass of

the two objects divided by the square of the distance between them.

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And that's why the deterministic view of Newton's theory is simply wrong.

Newton's law of gravity is not deterministic in any complex situation.

Notice also the extraordinary nature of gravity with it's infinite reach.

The force of gravity between two objects diminishes with the square of

the distance.

But the inverse square of a very large number never goes to

zero until the number is infinite.

In other words gravity has infinite reach.

That has profound consequences for how we deal with gravity,

how we do the calculations and also how we understand the universe.

The key attribute of a new tone in gravity is that it's an inverse square law.

This is what allows us to calculate the force between objects at

different distances.

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Also we can see that Newton's theory unifies things that

happen on the earth with things that happen in space.

It turns out that the apple probably didn't fall on Newton's head,

that's just a story.

It is nice, however, that his childhood home in Lincolnshire does have

an apple orchard in the backyard.

Perhaps he was watching an apple fall when he got his insight.

And if you work this mathematically, it turns out that the motion of an object or

an apple dropping is similar to the motion of the moon in the orbit of the earth,

for example.

If you do the math it turns out that the moon is 60 times

further away than the earth's surface is from the center of the earth,

where all the gravity seems to act.

Do the math it turns out that the apple does drop in one second, 3,600 times, or

60 squared,

further than the moon deviates from a straight line in its orbit of the earth.

Newton's theory really does unify the motions of

all objects operating under gravity.

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The other thing that was part of his early calculations was how the gravity of

an extended object operates.

For a spherical object like the earth,

the earth acts as if all the mass was concentrated at the center.

So we can replace the extended mass of the earth by the sum of

its mass located at its center.

Newton's simple equation applies to point sources of mass, and for

an extended object like a planet or a person the calculation is actually quite

complicated, because you need to work out the force of gravity between all parts of

the earth on a person, and vice verse, to calculate the total gravity.

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Given a massive object like the earth, you can use Newton's law to

work out how fast an object has to go before being liberated from that object.

That was the example we looked at earlier, where Newton imagined a cannon

firing horizontally off a tall mountain and what speed would be required

before the falling of the cannonball in a parabolic trajectory matched the rate at

which the falling surface of the earth fell away from that object.

An orbit.

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That's the orbital velocity.

He also calculated how much more energy would be

required to liberate the object entirely from the gravity of the earth.

Essentially send it to an infinite distance from the earth.

And for any object, the escape velocity is the square root of 2 or about 40%

more than the circular velocity or the velocity required to put it in orbit.

These are the fundamental relationships that underlie the entire space

telecommunications and space travel business.

The energy requirement to create orbital, or

escape velocities, are the basis of almost everything that NASA does.

In terms of exploring the solar system, or other gravity situations, we can think of

terms of the gravity as a potential well, where a certain amount of energy or

velocity or kinetic energy is required to be liberated from that potential well.

Being liberated from the earth's gravity, of course does not imply being

liberated from the solar system because the earth is in orbit around the sun.

So a separate calculation is involved, in understanding how much velocity or

kinetic energy,

is required to liberate an object, like a satellite, from the solar system itself.

Quite important in space travel, and in sending satellites around

the solar system, are particular situations where gravity balances.

These are called the Lagrange Points.

They were first theorized by a mathematical physicist in

France 200 years ago.

The Lagrange Points are valuable in space exploration.

They're places where gravity balances so

very little energy is required to keep a spacecraft or a probe in these situations.

Some of the Lagrange Points are unstable, in which case small retro-rockets, or

ballistics, are required to keep a satellite in its position.

Only one of them is stable.

These are valuable locations and many large space missions of the recent past or

future, are destined for the Lagrange Points.

In particular, the second Lagrange Point, L2, is a favored location for

many NASA and ESA missions, such as the Wilkinson Microwave Anisotropy Probe,

WMAP, and Herschel and Planck, two current satellites.

The James Webb space telescope is also destined to be launched there in 2016.

Space travel also uses other tricks of gravity, such as gravitational assist.

Gravitational assist is a nice idea.

If you bring a fast-moving object up behind a larger,

more massive object, then even without them colliding, their

gravitational interaction can transfer kinetic energy to the smaller object.

A space probe.

It's a gravity assist.

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Gravity as assist is used routinely to get space probes into the outer solar system.

Often these probes have to do close pass-bys of

inner solar system objects like Venus or the Earth to be able to

push themselves fast into the outer solar system and reach their targets.

This is a particular and complex pattern of gravity assists enjoyed by

the Cassini probe as it headed towards the Saturn system.

Another important feature of gravity is the tidal force.

For an extended object like a planet or a moon, the gravity on the near side

of the object from a second object, is larger because the inverse square law,

then the gravity of the far side of the object.

This difference between the gravity force and

the near and far side amounts to a stretching force, or tidal force.

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Tides also operate to cause tides in the oceans on the earth and

actually land tides.

Earthquakes are more frequent slightly at new moon and

full moon because in that situation the earth, sun and

moon align to create a slightly larger tidal force on the earth.

The dominant force in the universe is gravity.

Even though it's the weakest of the four forces of nature.

Its infinite range and universal positive attraction means that it governs how

structure happens in the universe.

The theory behind gravity was first put in place by Issac Newton.

And even though Einstein embellished the theory, with a theory that acts better

in situations of strong gravity, Newtonian gravity still works perfectly well, and

is quite precise for most situations in the solar system and the galaxy.