# Homework Help: Problem involving Kepler's Law

1. Sep 30, 2008

### pinkerpikachu

1. Halley's comet orbits the sun roughly once every 76 years. It comes very close to the surface of the sun on its closest approach. Roughly how far our from the sun is it at its closest? Is it still 'in' the solar system? What planet's orbit is nearest when it is out there? [Hint: the mean distance in Kepler's third law is 1/2 the sum of the nearest and farthest distance from the sun.

2. (T1/T2)^2 = (R1/R2)^3

Any help is appreciated.

2. Sep 30, 2008

### Kurdt

Staff Emeritus
Can you work out the semi-major axis of the orbit? You may want to compare it with the known values of something else that orbits the sun.

Were you given any other info?

3. Sep 30, 2008

### HallsofIvy

Did you really mean "at its closest"? Yes, of course Halley's closest approach to the sun is inside the solar system. Otherwise we wouldn't be able to see it! Surely you must mean "farthest".

4. Sep 30, 2008

### pinkerpikachu

That was the only information given, and yes, i mean the closest.

The mean different is the farthest point in our solar system (relative to the sun), and the closest which IS the sun.

So pluto is the farthest being 5900 X 10^6 km from the sun.

(5900 X 10^6 +0)/2 = 2.95 X 10^6

76 days may or may not have to be converted into seconds

and....that is all I know