Water is very useful for rocket fuel and life support in space. But does water, in abundance and form which conceivably could be usefully mined this century, really exist in NEA's? Or do we have to go all the way out to the frost line, the Asteroid Belt, to find asteroids with available water? (Lunar polar water ice is not topic here, I think, but low delta-v asteroidal water).
No and yes.
I'll address the "no" answer first. The answer is no if you truly mean "water ice". Ceres is close to the limit of where water can exist as water, as opposed to in the form of hydrated rock. Those intriguing bright spots on Ceres may well be water ice that has been exposed to vacuum (but they might just be salts). Ceres most likely is inside the frost line, but just slightly. That's part of what makes Ceres so intriguing. Ceres orbits at 2.7+ AU. It is not a Near Earth Asteroid. Contrary to mainstream thinking, perhaps the current frost line is at 2.7 AU (as opposed to 5 AU). There's no way that it is at 1 AU.
The answer is yes if you look beyond ice as a source of water. Some carbonaceous asteroids contain water in the form of hydrated minerals. Some of them contain quite a bit of water, up to 22%. We'll find out more when OSIRIS-REx launches next year and returns a sample from 101955 Bennu in 2023.
$\begingroup$ Okay, "ice" does not include hydrated minerals, that explains some of my confusion reading about this. Is there really a debate about where the frost line is? 2.7 or 5 AU is a huge difference, and I would think that it is pretty straight forward physics with insolation at different distances. Can't Dawn figure out where it is relative to the frost line? $\endgroup$ Jul 6, 2015 at 12:07
2$\begingroup$ @LocalFluff - Where the frost line is now and where it was when the solar system formed are two different things. Even during the formation of the solar system, the frost line was not at one fixed location. It instead migrated inward and outward with the growth of the proto-Sun and with changes in the composition and density of the protoplanetary disk. $\endgroup$ Jul 6, 2015 at 12:29
$\begingroup$ But the young sun was fainter, and while forming with maybe violent eruptions, the disc was protected by material. How could the Sun ever have sublimated most water in asteroids 5 AU away? $\endgroup$ Jul 6, 2015 at 14:28
$\begingroup$ That's backwards, @LocalFluff. The proto-Sun was more luminous than the star it would become. Protostars than eventually form a one solar mass star follow the Hayashi track and then the Henyey track during the course of their pre-stellar evolution. Perhaps you should frame this as another question. $\endgroup$ Jul 6, 2015 at 22:16
Plugging numbers into the Stefan–Boltzmann law gives us a temperature near 273°K (0°centigrade) for bodies near earth's orbit. The exact answer for atmospherless bodies depends on albedo. Any water on nearby asteroids will thus boil until it freezes, and then sublimate. That's why the search for nearby ice is focused on lightless, cold regions of craters near the moon's south pole.
Space probe Rosetta and its attendant comet are still 200 million km from the sun, and already outgassing water. IIRC, that started back in January, when the comet was 390 million km from the sun, well beyond Mars. Black body temp. out there would be around -100°C.
Looks like I was wrong about when jets first appeared: More jets from Rosetta's comet! September 19, 2014. On that day, the comet was 500 million km from the sun. That's outer belt. No spectra that I know of, so possibly not water. Water seems most likely though.
Likely we'll have to go out at least that far to find ice on small bodies.
1$\begingroup$ But are NEA's believed to contain some substantial fraction of water (ISRU-wise) under their hard outgassed dark crusts? Recent encounters with 67P and Ceres has given me the impression that the surface and the interior could be very different. But those objects aren't NEA. Boltzman, I think, talks about molecules exposed to the Sun in naked space. Not so much about the interior of asteroids. $\endgroup$ Jul 5, 2015 at 14:35
1$\begingroup$ I figure that if an object is near the sun for any length of time, even its interior is going to reach thermal equilibrium. We haven't seen a lot of evidence that asteroid interiors are hermetically sealed so as to prevent outgassing. They tend to be more rubble pile-ish, so no hidden water reserves. -Don't mean to be snarky here, just short, as comment space is limited. $\endgroup$ Jul 5, 2015 at 14:50
$\begingroup$ They could have water locked away in minerals: sservi.nasa.gov/articles/… That might be worth mining. $\endgroup$ Jul 5, 2015 at 16:00
1$\begingroup$ @WayfaringStranger An NEA could have a high aphelion. If an NEA had a 1 x 5.2 AU orbit, surface temperatures would vary from 120 to 270K. And more time would be spent in the colder parts of its orbit. However such an asteroid wouldn't fit LocalFluff's "low delta-V" criteria. $\endgroup$– HopDavidJul 9, 2015 at 20:28
Likely. Very likely.
Snodgrass Agarwal Combi Fitzsimmons Guilbert-Lepoutre Hsieh et al. 2017 The main belt comets and ice in the solar system. The Astronomy and Astrophysics Review 25, 5
Schorghofer Hsieh 2018 Journal of Geophysical Research: Planets 123 9 2322-2335
…and follow-on works in the past ~2 years.