Godot

I'm not sure about the best way to articulate my question, so forgive me if I am somewhat obtuse.
Does our path around the sun essentially describe a spiral? I recall coming across something stating that our orbit decreases by some infinitessimally small amount anually such that in the far distant future we will fall into the sun. Any truth to this?
IIRC, this same source quite pithily went on to further state that we shouldn't worry about it too much; the sun would go supernovae long before we would be destined to fall into it, thereby rendering the debate purely academic.

Zadok001

Wouldn't it have to be? We have no source of additional propulsion, and space is not ENTIRELY empty - There are particles out there. No doubt we collide with some of 'em. Hence, if it's not a spiral now, it will be someday.

fando

I don't know about Earth, but moons tend to spiral out from their planets when the orbit is prograde (in the same direction as the planetary spin). Conversely, they spiral in when the orbit is retrograde. The explanation for outward prograde drift is simple: A tidal locked orbit (moon doesn't rotate with respect to the planet) is the lowest energy state for a prograde satellite and planet. To reach this state, the moon exchanges rotational energy for a larger orbit and slightly slower planetary rotation. However, the explanation for inward retrograde drift is more subtle. Drat, I forgot the details... something about a kind of rotational/orbital friction--a retrograde orbit goes against the 'grain' of the main planet's rotation, causing it to 'wind in'. Anyhow, a moon in retrograde orbit can be destroyed by tidal forces if it gets too close to the planet (Roche Limit). An example of a Moon in retrograde orbit is Triton, which orbits Neptune. The best known satellite in a prograde orbit is our moon, which is slowly moving away from us and will eventually be tidal locked with Earth. However, the timescale for this to occur is longer than the expected remaining lifetime of the Sun.

Based on that, I'd expect the Earth to spiral outwards because it is in a prograde orbit w.r.t the Sun. However, I haven't read or heard anything official on this.

Majestyk

Does solar wind contribute any to the orbital path? It's miniscule in comparison to planetary density but it's fairly constant.

SULPHUR

That would be something to keep you busy.Best of luck.

fando

The integrated solar radiation pressure on Earth is very minsicule compared to its mass, so no. However, solar radiation is likely a minute factor in the chaotic variations in our orbit. The gravitational influences of other planets are much more noteworthy.

Autonemesis

The orbits of small bodies can be significantly affected by solar radiation over time. If the body rotates, it is acted on by a force arising from something called the Yarkovsky Effect. A small asteroid that rotates has a morning and afternoon side. The afternoon side will be warmer than the morning side, because it has been exposed to the sunlight longer. It will radiate more infrared energy. The extra departing photons impart a tiny positive thrust to the afternoon side of the asteroid.

If the afternoon side faces backwards along the oribtal path, the asteroid's orbital velocity slowly increases. If the afternoon side faces the direction of travel, it slows down over time. This serves to increase or decrease the period and size of the orbit.

It's thought that small bodies less than a kilometer wide on a collision course with Earth, could be deflected by applying a reflective coating to the surface, thereby altering how much energy is absorbed and how much the Yarkovsky thrust contributes to the asteroid's motion. Given enough warning, a couple of tons of gypsum powder might be all we need to deflect a continent-destroying space rock.

Bag of Ass

Forgive me if I'm being too elementary, but I thought the orbits were ellipses with the sun at one focus.

Majestyk

On the face of it, the Yarkovsky effect sounds as though it would increase the likelihood of elliptical orbits. The release of energy affecting orbital speed would be greater as the distance from the sun was smaller. As the speed increased, the orbital path would expand. As the distance between objects increased, the effect would become less prominent and the orbit would contract. Creating an oscillating effect that would produce an elliptical orbit.

*edit* scratch that. There would have to be an acting for to cause decrease the speed other than distance to cause a contraction in the orbital path. Got ahead of myself, there. Sorry.

*edit 2* Although the trajectory established after the increase in speed would be elliptical. (One of these days I'm gonna have to learn calculus)

Tim Thompson

If anything, Earth is moving slightly outward in its orbit, as the sun loses mass in the form of solar wind & radiation. Of course, this is a trivially small effect that may not be currently observable. However, when the sun evolves off of the main sequence, and becomes a red giant star, it will lose much mass, and Earth will retreat from the sun as the sun grows into a giant. It will be a race to see if Earth can avoid being swallowed by the giant sun. For now, it's too close to call, but last I heard, theoreticians favored Earth surviving (barely).

The Yarkovsky effect is theoretical, not practical. There are no observations of it, though it has been suggested, how one might observe it (Yarkovsky effect on small near-earth asteroids: Mathematical formulation and examples, Vokrouhlicky D, Milani A, Chesley SR, Icarus 148(1): 118-138, November 2000). On smaller particles the effect of radiation is called the Poynting-Robertson effect. Particles arounf .06 to 0.6 microns in size will be blown oput of the solar system in roughly 1,000,000 years by radiation pressure. Smaller & larger particles tend to be pulled in as the force becomes a drag.