Comet to a place near you

For your "out of this world" discussions.

Comet to a place near you

Postby Enzo » Wed Aug 06, 2014 8:15 pm

So they launched Rosetta ten years and some ago, and apparently it has reached the comet it will study and has entered orbit around it. I wouldn;t have thought a small comet would have enough gravity to make anything orbit,k but what do I know?

Now the lander it carries won;t set down on the comet surface for a few months yet.

Far out.
E Pluribus Condom
User avatar
Enzo
Enlightened One
Enlightened One
Chortling with glee!
 
Posts: 11956
Joined: Thu Feb 23, 2006 5:30 am
Location: Lansing, Michigan

Re: Comet to a place near you

Postby Lance » Wed Aug 06, 2014 10:09 pm

I think it's relative. The comet doesn't have much gravity but it has more that the surroundings. So if you get the velocity just right you can orbit.
No trees were killed in the posting of this message.
However, a large number of electrons were terribly inconvenienced.

==========================================

Build a man a fire and he will be warm for a few hours.
Set a man on fire and he will be warm for the rest of his life.
User avatar
Lance
Administrator
Administrator
Cheeseburger Swilling Lard-Ass who needs to put down the remote and get off the couch.
 
Posts: 91394
Joined: Thu May 12, 2005 5:51 pm
Location: Oswego, IL

Re: Comet to a place near you

Postby Мастер » Wed Aug 06, 2014 11:09 pm

The maths are not too bad for circular orbits, much nastier for elliptical orbits. Here's what I'm getting.

Image
where Image is the radius of the orbit, Image is a constant, m is the mass of the comet, and Image is the time of one orbit.

So how big is this thing? If it is one millionth the size of the earth (I'm guessing it's much smaller than that?) then you can maintain the same time of orbit by reducing the radius of the orbit by 100 times. So that would viable - if it is the same density and same shape as earth, then it would have one millionth the mass by having a radius 100 times smaller, so you could orbit 100 times closer.

Well that's interesting - if something is round and as dense as earth, the fastest possible orbit (by whipping around just above the surface) always takes the same time, regardless of how big the thing is? Can that be right? So you could put something into orbit around a piece of rock the size/shape of an association football, it would take maybe an hour or two to go around? (This strikes me as so bizarre, I'd really like someone to double-check my analysis!)

You could also maintain the same radius of orbit by taking 1000 times longer to go around.

This is probably pretty robust to the presence of other objects - even if the gravity from the sun is strong (and it's strong enough that the comet is in orbit around the sun), it won't mess up the orbit around the comet, if that orbit is small - then the sun's gravity when the craft is behind the comet is almost exactly the same strength as when it is in front of the comet, so the craft will happily orbit the comet, while both orbit the sun together. (Witness the moon, in a nearly circular orbit around the earth, even though the earth/moon together are orbiting the sun.)

So I guess you could orbit just about anything, as long as it's dense. If it's just a big cloud of dust, then you have a problem, because to get close enough to have a reasonable orbit, you'll be inside it.
They call me Mr Celsius!
User avatar
Мастер
Moderator
Moderator
Злой Мудак
Mauerspecht
 
Posts: 23929
Joined: Tue Aug 02, 2005 2:56 pm
Location: Far from Damascus

Re: Comet to a place near you

Postby tubeswell » Wed Aug 06, 2014 11:15 pm

A bus station is where a bus stops. A train station is where a train stops. On my desk, I have a work station.

If you are seeing an apparent paradox, that means you are missing something.
User avatar
tubeswell
Enlightened One
Enlightened One
 
Posts: 324861
Joined: Sun Sep 19, 2010 11:51 am
Location: 129th in-line to the Llama Throne (after the last purge)

Re: Comet to a place near you

Postby Мастер » Wed Aug 06, 2014 11:17 pm

Something that highly irregular, I guess either you have to be (i) very far away, so the fact that it's irregular doesn't matter very much, or (ii) making constant course corrections, which will burn a lot of fuel.
They call me Mr Celsius!
User avatar
Мастер
Moderator
Moderator
Злой Мудак
Mauerspecht
 
Posts: 23929
Joined: Tue Aug 02, 2005 2:56 pm
Location: Far from Damascus

Re: Comet to a place near you

Postby Arneb » Thu Aug 07, 2014 10:20 am

Мастер wrote:Well that's interesting - if something is round and as dense as earth, the fastest possible orbit (by whipping around just above the surface) always takes the same time, regardless of how big the thing is? Can that be right? So you could put something into orbit around a piece of rock the size/shape of an association football, it would take maybe an hour or two to go around? (This strikes me as so bizarre, I'd really like someone to double-check my analysis!)


I can't check your analysis, but I seem to remember that the Apollo misssions took 1 1/2 hours to orbit the Moon, which is roughly the same time as our LEO satellites need to orbit the Earth.
Non sunt multiplicanda entia praeter necessitatem
User avatar
Arneb
Moderator
Moderator
German Medical Dude
God of All Things IT
 
Posts: 70004
Joined: Thu Nov 23, 2006 8:22 pm
Location: Potsdam, Germany

Re: Comet to a place near you

Postby Мастер » Fri Aug 08, 2014 8:50 am

Arneb wrote:
Мастер wrote:Well that's interesting - if something is round and as dense as earth, the fastest possible orbit (by whipping around just above the surface) always takes the same time, regardless of how big the thing is? Can that be right? So you could put something into orbit around a piece of rock the size/shape of an association football, it would take maybe an hour or two to go around? (This strikes me as so bizarre, I'd really like someone to double-check my analysis!)


I can't check your analysis, but I seem to remember that the Apollo misssions took 1 1/2 hours to orbit the Moon, which is roughly the same time as our LEO satellites need to orbit the Earth.


The idea that you could be in orbit around a bowling ball, and it would take roughly this amount of time (I don't know how the density of a bowling ball compares the density of the earth, but I think they're probably not dramatically different) strikes me as really bizarre. But that's what I'm getting from my maths.
They call me Mr Celsius!
User avatar
Мастер
Moderator
Moderator
Злой Мудак
Mauerspecht
 
Posts: 23929
Joined: Tue Aug 02, 2005 2:56 pm
Location: Far from Damascus

Re: Comet to a place near you

Postby Lance » Fri Aug 08, 2014 2:11 pm

Мастер wrote:The idea that you could be in orbit around a bowling ball, and it would take roughly this amount of time (I don't know how the density of a bowling ball compares the density of the earth, but I think they're probably not dramatically different) strikes me as really bizarre. But that's what I'm getting from my maths.

Well, if you went any faster you'd leave the orbit and fly off down the alley.
No trees were killed in the posting of this message.
However, a large number of electrons were terribly inconvenienced.

==========================================

Build a man a fire and he will be warm for a few hours.
Set a man on fire and he will be warm for the rest of his life.
User avatar
Lance
Administrator
Administrator
Cheeseburger Swilling Lard-Ass who needs to put down the remote and get off the couch.
 
Posts: 91394
Joined: Thu May 12, 2005 5:51 pm
Location: Oswego, IL


Return to Astronomy / Cosmology / Physics

Who is online

Users browsing this forum: No registered users and 1 guest

cron