Expert: Jayendra Upadhye - 8/5/2008

Question
QUESTION: You are just perfect for this question. I think you are as curious as I am concerning the genesis of this Earth. My question is as follows:  Assume that a satellite, similar to our Moon, is orbiting the Sun at a distance of 4 million miles from Earth, will each orbit that that satellite makes around the Sun eventually bring it closer to the Earth? I understand that it may take perhaps 100 million years to actually bring it into a position where the Earth may eventually capture the satellite and it would become another moon to our system. The satellite would be orbiting at a speed slightly above the Earth's speed, probably around 68,000 mph. In you studied opinion, could this be possible?
P.S. I don't believe the current impact theory of an asteroid hitting the Earth and therby creating the Moon.

ANSWER: Dan!
You have asked a strange question! :)
You trash what scuientists / mathematicians/physicists have arrived at after years of clinical dispassionate study, and yet you immediately present "your theory" as a possible explanation!!
You indeed have guts man!

But first things first.
1 - A "satellite" orbiting the sun would be a PLANET itself!
2 - At 4 million miles, it would be zipping past the sun every few days!!
   Mercury has an average orbital speed of 47.87 km/sec.
   Its period is 88 days and semimajor axis is 57,909,100 km.
   By the famous thumb rule square of the orbital period varies a the cube of orbital radius,
   at 4 million miles the orbtal period would be:- (1.6km to the mile)..
   T in days = sqrt ((88*88/(57909100*57909100*57909100))*(4000000*1.6*4000000*1.6*4000000*1.6)) = 3.23 days
3 - Once established so close to the BOSS of all, the chances of it spiralling away from the sun are ZERO.
4 - Its orbital speed assuming a circular orbit would be (2*3.1416*4000000*1.6)/(3.23*24*3600) = 144 km/sec.
   = 144 km/sec / 1.6 km/mile * 3600 sec/hr = 324000 mph!!

So there you have your answer!

Now i am a trifle worried this may become a fire fight! :)

So take it and try to understand the basics, or leave it in the dust bin.

It is NOT rocket science, but atleast simple science!
(though cross-check my figures, one can always blunder...being human!

regards
Jayen

---------- FOLLOW-UP ----------

QUESTION: Thanks for responding to my original query. I guess I may have inserted a confusing bit of information.  I meant that the planetary body circling the Sun would be 4 million miles from the Earth's orbit making it 89 million miles from the Sun. So, in effect, every orbit that such a body would make would bring it into the gravitational sphere of the Earth and the larger body(Earth)would exert perturbations which would, in time, capture the smaller body.  I believe that this probably happened with some of the satellites that are orbiting Jupiter, Saturn, and maybe even Phobos and Demos.
Einstein once said: "Never stop questioning things." So, with the new information I've provided, could it be that the  moon was captured by Earth?

Answer
Hi Dan,
The earth's orbital speed is 21 km/sec.
The moon's orbital speed is 1 km /sec with respect to the earth!

Meaning, it is almost at same velocity as that of earth.

For planets, "closer to the sun" converts to "higher speed".

To be a planet for some time, orbiting at 89 million miles, it would have had a PERIOD of
SQRT{((365*365)/(93*93*93))*(89*89*89)}=341.7 days.

speed would be (2*3.1416*89000000)/(341.7*24*3600)= 18.9 km/sec

You know what, you could be right!!
There would be significant orbital perturbations of THAT orbit by the earth,s mass.

Capture could be possible.

However, the early solar system was significantly different from what it is now.
Before the late bombardment phase, the Gas giants were also not where they are now.

The moon is almost as old as the solar system.
Possibilities exist about what you say.

On the surface atleast.

In my college days, i used to wonder about the possibility of a "sun synchronous satellite".
Imagine a satellite of an earth like planet somewhere in the milky way.

It orbits the earth every 1 year!
Such a satellite would appear to be always "inside" or always on the "outside" of earth's orbit,
sort of "phase locked" with the earth/sun system. (angle wise).

If the earth's orbit is almost a circle, then viewing from atop, such a system would show the
satellite to always have a fixed angle between line joining satellite-earth centers and earth-sun centers!!

But i gave up as i could not evolve the mathematics of such an orbit.

regards
Jayen