The Sun ,the Solar System and Halley’s Comet

I thought it would be a good time to talk today about the Sun, the solar system and Halley’s comet all in the one go. This is a sort of follow up to my article in Sydney Comet Watch  where Chris left a comment asking why Halley’s Comet does not just exit the solar system instead of heading back to the Sun as it does.

And the answer is simply….. gravity.

Sun Gravitational Effect

(Click to Enlarge)

The sun has an enormous amount of mass, in fact 333,000 more than that of Earth. And with such a great mass comes more gravity. The Earth’s gravitational pull will effect any object that comes within 1.5 million km of it. The Sun’s graviational pull reaches out much further than that.

This is where the solar system comes in which is defined as the distance which objects are influenced by the gravity from the Sun. So we know that the planets are all orbiting the Sun as they are all safely inside this field influenced by the gravity of the sun…. and this includes Pluto which today is 4,745 million km from the sun. That is a huge distance of space to have an object so far out held by the Sun’s gravity.

Now Halley’s comet eventually will reach a distance slightly beyond the distance of Pluto. After the comet slingshots around the sun, last happening back in 1986,  it is catapulted back out into the solar system at a high rate of motion. As Halley goes further out and begins to reach it’s furthest distance from the Sun, it’s motion will have diminshed to such an extent that it suddenly begins noticing the Sun’s attraction. The Sun’s gravity is tugging on it telling it to come back. So just beyond the distance of Pluto to the Sun, Halley’s comet begins swinging around in answer to the Sun’s gravitationsl pull and begins heading back towards the inner solar system.

Halley’s comet  is effected by all sorts of influences along the way and I believe it first came under the effect of Jupiter’s graviational pull back in 1608. Now due to these influences, it is pretty diffacult to accurately determine the exact orbit of Halley’s comet. For example if there is an unknown planet out beyond Pluto that we have yet to discover, and this planet exerts a slight gravitational pull on the comet, this will alter Halley’s orbit and most likelydelay it’s return and orbital path back into the inner solar system. This is why the comet does not always return every 75 years….. once it actually returned on the 79th year.

Finally, getting back to the Sun….. can we assume the Sun’s gravitational pull only extends out to the edge of the known solar system then? Well it is now believed that the graviational pull now extends out 2 light years away from the Sun at which the gravity from other stars begins to be stronger than that of the Sun. Because of this, it is also believed that when stars pass by us over millions of years, our sun’s gravity has been strong enough to pick up asteroid sized objects from other solar systems and other stars have actually stolen objects that once existed in our solar system. Sort of like trading one object for another.

The picture above shows the outer solar system as is stands today as well as showing where Halley’s Comet is currently located. The comet is still heading out towards the edge of the solar system because it’s speed is still great enough for it to be defying the sun’s gravitational field.


~ by sydneystargazers on November 16, 2009.

One Response to “The Sun ,the Solar System and Halley’s Comet”

  1. Sounds as though my theory was pretty much correct :). So I guess eventually the Voyager craft will lose momentum and head back towards the sun again, as the sun would be pulling on them too…

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