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View Full Version : (Slowly Spiralling In) This may be a dumb question...

Mike45
2007-Oct-23, 03:56 AM
..but shouldn't Earth, and all other satellites of the Sun be very slowly spiraling inward towards the Sun, or am I misunderstanding gravity?

pzkpfw
2007-Oct-23, 04:13 AM
In my laypersons words:

An orbit is where the speed around an object is just fast enough to balance against the fall towards that object.

e.g. a spacecraft leaves orbit and descends towards Earth by slowing down (firing rockets against the direction of orbit).

e.g. a spacecraft goes to a higher orbit by speeding up (firing rockets in the direction of orbit).

[Getting the shuttle to the ISS isn't as simple as it might seem.]

Earth hasn't fallen into the sun as it is going fast enough to keep falling.

I just thunked of something:

Get a ball and attached it to a long rubber band.
The ball will be (and stay) a certain distance from you.

Swing faster (or use a heavier ball) and the distance would be greater.
Swing slower (or use a lighter ball) and the distance would be less.

Jens
2007-Oct-23, 05:31 AM
..but shouldn't Earth, and all other satellites of the Sun be very slowly spiraling inward towards the Sun, or am I misunderstanding gravity?

I'm just curious, but why would you think that? I mean why not drift slowly away from the sun, or stay in the same orbit?

Neverfly
2007-Oct-23, 05:55 AM
I'm just curious, but why would you think that? I mean why not drift slowly away from the sun, or stay in the same orbit?

Gravity well. I can see why. I myself had wondered that same question at one point.

Tim Thompson
2007-Oct-23, 06:01 AM
... shouldn't Earth, and all other satellites of the Sun be very slowly spiraling inward towards the Sun ... ?
No. In fact, in a 2 body system, and assuming both masses are constant, one would expect the opposite. The planets should very slowly spiral outwards from the sun. As an example, our own moon spirals away from Earth at about 3.82±0.07 cm/year (= 1.50±0.03 inches/year). The basic explanation is that the tidal gravity of the moon raises a bulge on Earth, which in turn is pulled ahead of the moon by Earth's rotation, and that bulge then pulls the moon like a whip. That action transfers momentum from Earth (which consequently slows down its rotation by about 0.0015 seconds per day per century) to the moon. That additional momentum causes the moon to move outwards to a higher energy orbit. Eventually, in a 2 body system, after about 50 billion years, the moon would reach its maximum distance and stop moving away. At that time Earth & the moon would be tidally locked together, meaning that the same side of the moon would be always facing Earth and the same side of Earth would be facing the moon, and so it would remain in the absence of outside forces. In principle, the same thing should happen for the sun and its planets, but very much more slowly than it does for Earth and the moon.

But it's not a 2-body system. It's really a 3-body system: Earth, moon & sun. In about 50 billion years, when the moon reaches its limit, it will then spiral back towards Earth, becuase of the tidal gravity of the sun, and not because of Earth. Give it another 50 billion or so years after that, and the moon will indeed slam into Earth.

Mars' moon Phobos is already on the inbound leg of its tidal journey back to Mars. But I don't remember the timescale, how long it will take for Phobos to join Mars.

In the case of the sun & its planets & etc., there is a complication. The mass of the sun is not constant. Normal solar wind only costs the sun about 1.6x10-14 of its mass per year, but that will cause anything in orbit around the sun to very slowly spiral outwards as the sun loses mass. But later on, when the sun evolves into a red giant and AGB star, it will lose as much as 50% or 60% of its mass as an intense solar wind. That will cause anything that survives to spiral outwards significantly.

Tog
2007-Oct-23, 07:27 AM
My explanation is just a variation on those above, but I wanted to go into the speed issue a bit.

An object in orbit is trying to move in a straight line, like roller blading down a sidewalk. The gravity of the second object basically dips the sidewalk into a rounded funnel shape. This will cause the roller blader to turn toward the low part of that dip. This doesn't slow them down at all, however. If they are going too fast for the amount of "dip" they will change course and continue on (out into the street). If they are going too slow for the dip, they will spiral down into it like coins in those things at the zoos and malls that collect for charities. They won't fall straight in though, because they are still moving forwards just as fast as they were before (basically)*. What will happen is they will spiral closer and closer to the gravity source. If the speed is just right, the object will spin around the dip at about the same level, forever.

Here's the fun bit. First, we need to explain a tangent. This different than a thread jack. A tangent is a line that extends from the edge of a circle, parallel from the center of that circle to the contact point of the circle and line. Another way to look at it is by setting a ball on a table. The surface of the table is a tangent to the part of the ball it touches. Let's say you want to go to Mars, but getting there cheaply is more important than getting there fast. To go to Mars, you wouldn't aim at it. You would accelerate on the tangent of the Earth Orbit. As the speed of the craft increases, your orbital distance from the sun increases. Eventually you would extend the orbit out to Mars. To get there fast, you would aim slightly more toward it, at the right time in the orbit to get to the intersection about the same time it does.

*What Tim said about the speed is correct. The speed is changing slightly all the time, which has an effect on the orbits, but for a basic answer, the changes won't make a big difference.

Now I have a question:

Mars' moon Phobos is already on the inbound leg of its tidal journey back to Mars. But I don't remember the timescale, how long it will take for Phobos to join Mars.
Would it survive to impact, or would it be torn apart from tidal forces before impact?

grant hutchison
2007-Oct-23, 11:02 AM
Would it survive to impact, or would it be torn apart from tidal forces before impact?It'll depend on whether Phobos is a single "monolithic" chunk of rock, or a rubble pile held together by gravity. If it's the former, it should make it to the surface; if the latter, it'll be eased apart by tidal gravity on the way down.
(Its density is rather low for a single rock. So I guess it's probably a rubble pile.)

Grant Hutchison

Nereid
2007-Oct-23, 11:32 AM
There are a whole bunch of other effects, in this real solar system of ours, in addition to the dominant ones Tim Thompson has already covered, including:

* spiralling in due to loss of energy through the emission of gravitational wave radiation (a GR effect that dominates in binary pulsars, but is negligible for the Earth)

* spiralling in, or out, due to the net force vector from anisotropic emission of radiation (Yarkovsky effect)

* spiralling in due to the Poynting-Robertson effect - trivial compared with the Yarkovsky effect

* orbit changes due to collisions with large(ish) bodies (I don't know if there are orbit changes of any consequence due to the persistent rain of meteors and micro-meteors, whether from comet streams, zodiacal light particles, or random objects) - there will certainly be such collisions, but how many and what effects they will have is, I suspect, pretty close to unknowable today.

Mike45
2007-Oct-23, 03:48 PM
I'm just curious, but why would you think that? I mean why not drift slowly away from the sun, or stay in the same orbit?

I had that assumption because of the way a black hole works, and how it's gravity allows it to eat surrounding matter. The Sun obviously has a very significant hold on space for a great distance; Wikipedia says the outer edge of the Oort cloud is 1.6 light years out. So, I figured our solar system would eventually be doomed, and gravity would overcome all of the matter.

mugaliens
2007-Oct-23, 04:00 PM
I'm just curious, but why would you think that? I mean why not drift slowly away from the sun, or stay in the same orbit?

Tidal effects and frame dragging do sap some of the orbital energy, thus resorting in a slow inward spiral.

From what I understand, though, it's not clear whether the gravitational constant is all that constant. Thus, while most observed systems are indeed spiraling inward, if gravity isn't as constant as we think it is, some "lighter" systems might be spiraling outward.

Is there an astrophysicist in the house who knows the answer to this question?