PDA

View Full Version : Planetary geology Q's

StormSeeker
2004-Mar-16, 02:06 AM
A few quick questions. Me and a few friends got into a thought experiment this afternoon, and brought up some interesting ideas... I need some intellegent brains to pick, however, before we meet again, and I think this is the place.

1. Is there a limiting upper size to a non-gas giant planet?

2. Given a planet with double the radius of the Earth, but the same mass, all other things being relativly equal, would this planet have the same gravity as Earth while having four times the surface area?

3. What are some other differences such a planet would have from Earth? One I can thing of might be a thinner atmosphere (Same ammount of air over a greater area?).

4. Let's say you wanted to bring down an asteroid to the surface of a planet, but not cause a lot of destruction in the process. Given today's technology -- +/- 5 years -- would it be feasable to strap a few rockets on it, bring it into orbit, then ease it down?

JohnOwens
2004-Mar-16, 02:14 AM
2. Given a planet with double the radius of the Earth, but the same mass, all other things being relativly equal, would this planet have the same gravity as Earth while having four times the surface area?

I'll take this one. No, it would only have 1/4 the Earth's gravity. If the idea is that you want a larger planet with the same gravity as Earth, you'll need it to be four times as massive, and it will have half the mean density of Earth.

roidspop
2004-Mar-16, 02:29 AM
Ok, I'll play:

1. The composition of the protostellar cloud would be one limiting factor...just how much non-icy material is available and how much can be swept up? Some clouds will be richer and some poorer. Then the events surrounding the ignition of the star may be very important...the T-Tauri wind may be able to scour away atmospheres from "hot Jupiters" and just generally arrest the growth of planetary bodies...if onset is later, you may get larger terrestrial bodies and if it's earlier, they may be smaller. Then there's the absolute limitation imposed by the ability of the atom to withstand crushing...beyond some limit, el cruncho! Do you have a planet or is it a collapsar?

2. Double the radius, g drops to 1/4

4. How big the asteroid is makes a lot of difference, of course. But why use rockets? Break the thing up into smaller chunks (a few tonnes apiece), shape them into re-entry bodies, de-orbit and let them fall into the ocean. It would put on a terrific light show into the bargain.

daver
2004-Mar-16, 02:40 AM
A few quick questions. Me and a few friends got into a thought experiment this afternoon, and brought up some interesting ideas... I need some intellegent brains to pick, however, before we meet again, and I think this is the place.

1. Is there a limiting upper size to a non-gas giant planet?

I suppose it depends on where it forms; there has been some suspicion that above a certain size a terrestrial starts sucking down everything within reach and becomes a gas giant.

3. What are some other differences such a planet would have from Earth? One I can thing of might be a thinner atmosphere (Same ammount of air over a greater area?).

This planet has 1/8 the density of earth, which makes it pretty anomalous. Its density would be less than Saturn (and less than that of water), its escape velocity around 8 km/sec. Maybe what you have here is a mini gas giant--mostly hydrogen, but very small.

4. Let's say you wanted to bring down an asteroid to the surface of a planet, but not cause a lot of destruction in the process. Given today's technology -- +/- 5 years -- would it be feasable to strap a few rockets on it, bring it into orbit, then ease it down?

It depends on how big the asteroid is. Probably anything small enough to bring down with rockets is small enough to shroud in some sort of thermal protection system and aerobrake in--figure in the 100 ton range. That is extremely small for an asteroid.

Cougar
2004-Mar-16, 03:14 AM
2. Given a planet with double the radius of the Earth, but the same mass, all other things being relativly equal, would this planet have the same gravity as Earth while having four times the surface area?
Yes, it would have 4 times the surface area. You may (or may not) be wondering, if the "new" earth has the same mass, why would its gravity be any different? Well, we earthlings usually consider the earth's gravity as we are stannding on the surface. If we go up in a hot air balloon, the gravity on us at the increased altitude is (slightly) less because we're farther from the center of the earth. If you double the radius and then stand on the surface of the "new" earth, you're a LOT farther from the center of the earth, and as others have correctly said, the force of gravity on you would then be 1/4 of what it was. From the standpoint of an orbiting satellite, the force of gravity would be the same for our earth and the supersized earth (assuming the orbit is large enough so you're not scraping the treetops when you double the earth's radius).

StormSeeker
2004-Mar-16, 05:01 PM
Ok, I'll play:

4. How big the asteroid is makes a lot of difference, of course. But why use rockets? Break the thing up into smaller chunks (a few tonnes apiece), shape them into re-entry bodies, de-orbit and let them fall into the ocean. It would put on a terrific light show into the bargain.

Let's say we wanted the thing down in one large chunk?

StormSeeker
2004-Mar-16, 05:05 PM
2. Given a planet with double the radius of the Earth, but the same mass, all other things being relativly equal, would this planet have the same gravity as Earth while having four times the surface area?

I'll take this one. No, it would only have 1/4 the Earth's gravity. If the idea is that you want a larger planet with the same gravity as Earth, you'll need it to be four times as massive, and it will have half the mean density of Earth.

Yes, that's exactly what I was aiming for. Thank you.

daver
2004-Mar-16, 07:39 PM
I'll take this one. No, it would only have 1/4 the Earth's gravity. If the idea is that you want a larger planet with the same gravity as Earth, you'll need it to be four times as massive, and it will have half the mean density of Earth.

Yes, that's exactly what I was aiming for. Thank you.

Hmm, we talked about this a while back--it reminds me a bit of the Jack Vance Big Planet series, or Riverworld.

daver
2004-Mar-16, 07:42 PM
Ok, I'll play:

4. How big the asteroid is makes a lot of difference, of course. But why use rockets? Break the thing up into smaller chunks (a few tonnes apiece), shape them into re-entry bodies, de-orbit and let them fall into the ocean. It would put on a terrific light show into the bargain.

Let's say we wanted the thing down in one large chunk?
It can't be done with current technology past the 100 ton or so range. You might be able to get 1000 tons (on the rough order of 10 meter radius) but not larger.