Is ice stable in space?

Sublimation takes additional energy. Nevertheless, ice can still sublimate at 40 K, at the level of a few molecules per hour. One scientist has found that if a small ice sample, about 4 nanograms, were heated to 150 K ( -123 C, -190 F)), it could exist for over two hours without sublimating significantly.

Interstellar ice forms at typical temperatures below 40 K. Also, dust micro-grains can act as a substrate and catalyst for ice grains.

http://link.springer.com/article/10.1023%2FA%3A100...

Is ice stable is space? yes, in general.

However, as comets regularly show us, once the temperature goes up, ice becomes... another state (gas, in the case of comets).

On Triton, the energy released by tidal forces during its capture by Neptune, likely turned its ice mantle to liquid water, at temperatures up to 200 C (kept liquid because of the pressure, being trapped by the still frozen crust).

Is there a minimum pressure for sublimation? Apparently no. For some reason, "solid helium-3" appears impossible to keep stable.

However, for other substances, ice CAN be stable (different temperature for each substance).

When the Solar system was forming, the "ice line" was around where the asteroid belt was today. Beyond that, whatever water molecules existed were crystalized and acted like "Velcro". This explains why the gas giants formed so rapidly.

When the Sun's nuclear furnace finally turned on, the still-free molecules closer in (water, dust, etc.) were pushed outwards by the pressure of the new light, and formed the Oort cloud. Bodies of mostly ices that formed at a distance such that ices were stable.

When Pluto reaches perihelion (closest to the Sun) as it did in the 1980-1990s, its "atmosphere" becomes gaseous (mostly nitrogen). The rest of the time, this nitrogen falls back as nitrogen "snow", which explains why Pluto's crust looks so bright (this is what tricked the astronomers, back in the 1930s, into believing that Pluto was a much larger body).

Comets begin to sublimate somewhere outside the orbit of Mars. There is no real difference between ice and rock except that ice is made of lighter elements that typically melt or sublimate at earth-like temperatures. Out in the vicinity of the outer planets, however, normal water-ice behaves no differently than any other rock. It gets extremely hard, like rock.

Yes, ice is stable in space.

If heated it will sublimate directly to the gaseous state.

I don't know how they coalesced.

Theory of comet is what you are looking for.

Yes, such a phase transition is pressure related.

Yes. They are stable in space, untill they come close to hot bodies such as stars.

Do comets melt ? Not unless they careen towards a Sun. They last for millions of years.