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I understand that there is no resistance in space, so how come you cant just launch a space ship towards Mars and when its out of Earths gravitational pull, give it a blast on its rockets and then turn them off, the idea that the rocket will simply travel forever?

If only "How" was still on TV, at least they had a bloke with a beard and leather patches on his elbows, who knew ABSOLUTELY EVERYTHING!

Because as it left the gravitational pull of earth it would be affected by other bodies. However satellites are often set on a course that allows the planets to give it a boost on its way, though in a different direction to the one it approached.

jimslip wrote:

> I understand that there is no resistance in space, so how come
> you cant just launch a space ship towards Mars and when its out
> of Earths gravitational pull, give it a blast on its rockets
> and then turn them off, the idea that the rocket will simply
> travel forever?

You can in theory, the projectile would maintain it's original trajectory until an external force acted on it, such as the gravity of another planet, or it ran into a star. But in theory one blast on the engines pointed in the right direction, and it would go forever.

You can't just 'point and shoot'. As andy at handiwork and Peter have both explained well, you may be out of Earth's gravitational pull but there are other bodies out there with gravity, which means that your space ship is never going to travel in a straight line.

However, having said that, NASA space craft sent to Mars have always simply 'coasted' most of the journey there. The trick is to fire the engines every now and then, as this effectively forces course corrections on the craft to enable it to arrive at its destination.

Or these?

william wrote:

> Jim Inter planetary physics my dear fellow-
>
> Basically the earth is falling to the sun all the time but the
> pesky sun moves and it continues to fall pulled by the gravity
> of the sun - the sun moving keeps it all moving and it a
> headache trying to work it all out like what makes the sun move
> ? all about gravitational forces
>
I can't understand how the Earth could be, 'Falling' towards the Sun, I thought there was no up or down in space and if we are falling towards the Sun, what is stopping us from crashing into it? Surely its gravitationl pull must be far greater than any object sitting near us?

jimslip wrote:

> Not quite the end apparently:
>
>
>


Thank you for that link jimslip. It's good to know that the asteroid will definitely miss us in Feb next year and the world is safe until 2020 at least.

In space terms its passing VERY FUCKING CLOSE indeed to Earth for my liking. 17,000 miles is like the width of an atom in space terms! Lol

jimslip wrote:

> I can't understand how the Earth could be, 'Falling' towards
> the Sun, I thought there was no up or down in space and if we
> are falling towards the Sun, what is stopping us from crashing
> into it? Surely its gravitationl pull must be far greater than
> any object sitting near us?


It's the speed of the Earth that's stopping us from crashing into the Sun.

Think of a roulette wheel with the ball circling around the outside of the wheel. The ball spins around for a while but is being slowed by friction with the wheel and also by air resistance. Eventually it loses too much speed to continue spinning, because when it slows down enough gravity becomes the stronger force and makes it fall towards the centre of the wheel. As it falls it gets caught by one of the numbered pockets/slots on the wheel.

Earth is the same as the roulette ball - its speed keeps it from falling into the Sun, just as the speed of the roulette ball keeps it from falling into a pocket. The difference is that there's nothing in space to slow the Earth down, so unlike the roulette ball it just keeps on circling and keeps on circling.

Some stupid punter will probably place a bet that this will indeed result in the end of the world. I'll give anyone infinity to one on that.

alicia_fan_uk