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At the core of astronomy, is gravity.

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

Â astronomy of the 17th century, Isaac Newton.

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

Â Early in his life,

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

Â 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

Â almost entirely based on mathematics and physical thinking.

Â Even though he developed a very successful theory of gravity,

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

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

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

Â I frame no hypothesis.

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

Â could not explain all the subtleties of gravity.

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

Â which involve relatively weak gravity.

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

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

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

Â was patched through to his son.

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

Â Anders said, Isaac Newton's driving, son.

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

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

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

Â an orbit of the earth.

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

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

Â the gravitational constant, times the mass of

Â 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.

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

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

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

Â the distance.

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

Â zero until the number is infinite.

Â In other words gravity has infinite reach.

Â That has profound consequences for how we deal with gravity,

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

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

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

Â different distances.

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

Â happen on the earth with things that happen in space.

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

Â that's just a story.

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

Â an apple orchard in the backyard.

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

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

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

Â for example.

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

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

Â where all the gravity seems to act.

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

Â 60 squared,

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

Â Newton's theory really does unify the motions of

Â all objects operating under gravity.

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

Â an extended object operates.

Â For a spherical object like the earth,

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

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

Â its mass located at its center.

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

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

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

Â 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

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

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

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

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

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

Â An orbit.

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That's the orbital velocity.

Â He also calculated how much more energy would be

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

Â Essentially send it to an infinite distance from the earth.

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

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

Â These are the fundamental relationships that underlie the entire space

Â telecommunications and space travel business.

Â The energy requirement to create orbital, or

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

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

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

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

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

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

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

Â kinetic energy,

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

Â Quite important in space travel, and in sending satellites around

Â the solar system, are particular situations where gravity balances.

Â These are called the Lagrange Points.

Â They were first theorized by a mathematical physicist in

Â France 200 years ago.

Â The Lagrange Points are valuable in space exploration.

Â They're places where gravity balances so

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

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

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

Â Only one of them is stable.

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

Â future, are destined for the Lagrange Points.

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

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

Â WMAP, and Herschel and Planck, two current satellites.

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

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

Â Gravitational assist is a nice idea.

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

Â more massive object, then even without them colliding, their

Â gravitational interaction can transfer kinetic energy to the smaller object.

Â A space probe.

Â It's a gravity assist.

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

Â Often these probes have to do close pass-bys of

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

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

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

Â the Cassini probe as it headed towards the Saturn system.

Â Another important feature of gravity is the tidal force.

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

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

Â then the gravity of the far side of the object.

Â This difference between the gravity force and

Â 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

Â actually land tides.

Â Earthquakes are more frequent slightly at new moon and

Â full moon because in that situation the earth, sun and

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

Â The dominant force in the universe is gravity.

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

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

Â structure happens in the universe.

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

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

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

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

Â