# Orbital simulation

1. Apr 26, 2006

### DaveC426913

(unrelated to my other post about an asteroid)

1]
I would like to check a postulation with someone who has an orbital simulator, or a good head for orbital mechanics.

Given an object such as a comet with orbital period of about 150 years, could it have a perihelion at a distance between Mercury and Venus?

If no, what does work? Change period? Change perihelion?

2] Could we tell anything about this object's mass from its orbit and/or visual observations if we do not know its density or its albedo factor?

Last edited: Apr 26, 2006
2. Apr 26, 2006

### SpaceTiger

Staff Emeritus
I don't see why not. The period determines the semimajor axis of the orbit, but it doesn't say anything about the eccentricity (which, combined with semimajor axis, could give you the perigee).

In a case where the orbiting mass is much smaller than the central one, the orbit is independent of the orbiting mass, so I would say no.

3. Apr 26, 2006

### DaveC426913

I guess all I have to do is find a table of comets and see if any of them have a perhelion and period similar. That'd be enough to show it's plausible.

Yes, but do astronomers use any other techniques? If we spotted a mystery object right now, and knew its orbit and size, but not its density, could we deduce anything abojut its mass?

4. Apr 26, 2006

### SpaceTiger

Staff Emeritus
If it were a very small object, we would assume it was rocky...and that narrows the range of possible densities. It doesn't directly tell you about the mass, though.

5. Apr 26, 2006

### DaveC426913

Ach! I can't find any info online that will demo the plausibility of this orbit.

I don't know (or care about) the semi-major axis or the eccentricity, I only know (and care about) the period and the perihelion.

Last edited: Apr 26, 2006
6. Apr 26, 2006

### SpaceTiger

Staff Emeritus
There are comets with periods less than 200 years (over a hundred of them) and perihelions in the inner solar system, so I don't see why you think this would be implausible.

7. Apr 26, 2006

### Janus

Staff Emeritus
I also am at a loss as to why you would seem to think this orbit might not be plausible.

Such an orbit would have a semi-major axis of about 4.2 billion Km (just inside Neptunes orbit), and an aphelion of between 8.3 & 8.35 billion Km, or just about 14% further than Pluto at its furthest.

8. Apr 26, 2006

### DaveC426913

Thank you. I just wasn't sure.

This is for a story for a friend (a different friend), and I wanted to check the numbers.

I know that you can't just pick numbers that are convenient. Orbits ahve some constraints.

9. Apr 26, 2006

### franznietzsche

But comets that pass into the inner solar system will need to have resonant orbits with the outer planets, to keep from being thrown off course, wouldn't they? It places restrictions on possible long-term stable orbits. Of course, if you ignore the planets and just look at the sun and the comet, then sure its perfectly possible.

10. Apr 26, 2006

### DaveC426913

Polar orbit.

11. Apr 26, 2006

### SpaceTiger

Staff Emeritus
Stability constraints are more of a concern when you need your object to remain in orbit for a long time. In many cases, comets don't make it more than a few orbits into the solar system anyway, so finding one on an "unstable" orbit wouldn't be a big surprise, nor a problem. If, for some reason, you need long-term stability for your comet, that's a different story entirely and will depend in a complicated way on the many-body interactions in the solar system. Do you need it to be stable? If so, for how long?

12. Apr 27, 2006

### DaveC426913

"Do you need it to be stable? If so, for how long?"

I don't know yet actually. I'll let you know when I get farther into the story!

The situation at this point in the story is that they don't know how long it's been there. They only found it on its latest dive into the inner system.

In a polar orbit I presume it would be more stable.