EPOXI Mission News Options

[nprev]

I wonder what brave putative PI will someday propose a mission to catch & return snowballs from a comet...?

[fredk]

I wonder if some of those "boulders" on the ends are large, fluffy moons that have fallen to the surface.

Then you'd have to explain why just the ends, when the "waist" should be the gravitational potential minimum...

[centsworth_II]

I don't really think that any visible lumps on the surface are fallen snowballs. If I were to guess, maybe as the comet shrinks its dessicated surface breaks up into the clumps we see on the ends. I envision the "boulders" to be composed of freeze-dried masses of dust and organics. Maybe similar in texture to "astronaut ice cream".

[vjkane]

I wonder what brave putative PI will someday propose a mission to catch & return snowballs from a comet...?

Already proposed, in a fashion, by the comet coma rendezvous sample return mission (summary here and in detail here ). This mission proposed to return warm samples, though.

The challenge is to know which comets spit snowballs as opposed to just gently outgasing. If Rosetta finds good evidence for snowballs, I expect proposals to follow up with a sample return that doesn't require landing will be made.

Of course, the challenge is to keep those ices cold. The Enceladus sample return analysis from the Decadal Survey (brief as it was) didn't raise keeping temperatures below 0 C as a major issue.

[nprev]

The challenge is to know which comets spit snowballs as opposed to just gently outgasing.

Yeah, I was thinking that too, and I really don't see any way to do it other than via in situ examination. The only way around that might be to figure out a sample acquisition method that could cope with either chunks or dust with equal effectiveness, but nothing springs to mind.

The cryo storage problem might not be too bad; a Dewar flask of some sort might be enough for the flight hardware, but obviously post-landing recovery would have to be pretty expeditious and include a cold-storage facility.

[vjkane]

The cryo storage problem might not be too bad; a Dewar flask of some sort might be enough for the flight hardware, but obviously post-landing recovery would have to be pretty expeditious and include a cold-storage facility.

From the Enceladus Decadal Survey sample return mission concept study: "A desire for science benefit is to maintain the sample below 250 K at all times (to keep water below freezing), and it is expected that this could be achieved passively through the orientation of the Earth Return Vehicle (ERV) in the anti-sun direction during cruise, along with use of phase change material surrounding the sample to withstand re-entry heating until the sample is recovered. Also, qualification of aerogel would be required for the collection of larger particles at lower velocities than Stardust, along with preservation of ice in the collection event." p13

So keeping the sample moderately cool (in terms of cometary volatiles; some would evaporate at 250 K), doesn't appear to be the big problem. For the Enceladus concept, the big problem was ensuring planetary protection so any Enceladian life forms don't get loose here on Earth. I don't know if that would be considered a problem for a comet sample return; never have heard it brought up.

[Phil Stooke]

Earth-based radar might let us see which comets are surrounded by "snowballs" - not resolving them of course but revealing a diffuse target around the nucleus.

Phil

[nprev]

Interesting thought. It would have to be a very close approach to Earth by a target comet, though, and I wonder if anyone has any idea what the dielectric constant might be for a "typical" (is there such a thing?) comet nucleus, to say nothing of fluffy stuff like a snowball halo?

Definitely worth trying by Arecibo if there's ever an opportunity.

[djellison]

They did use Arecibo with Hartley 2. It'll be interesting to see, if with the benefit of hindsight, they can say 'Yup - we saw the golf balls'

[nprev]

Argh...I forgot that! Thanks, Doug.

Looks like it didn't penetrate the 'regolith' as much as I would have thought; good solid skin painting. Sure hope that they take a hard look now for any scattering around the nucleus. Could tell us something about the composition of the balls if they get any sort of returns since the radar frequency is constant.

[centsworth_II]

Are there any better Hartley radar images than this? It doesn't look like the snow storm rises above the noise.

Arecibo provides a taste of Comet Hartley 2


On the other hand, decreasing the brightness leaves a few suspicious spots. I guess it will take some statistical work to determine the chances that those spots represent something real as opposed to random noise.

[AndyG]

The thing is, these snowballs have been described as "fluffy aggregates of very small particles of ice, like a dandelion puff" and up to basketball size - which must mean most are smaller than the wavelength of a typical radar signal.

I suspect Arecibo stood no chance spotting these.

Andy

[djellison]

Oh - I don't for one second think we're going to 'see' them in those visualizations of the radar return for the Nucleus. If there's anything present, it'll be in the signal in some other way.

[ynyralmaen]

Snowballs like this have been detected by radar before, e.g. Hyakutake, as reported in this paper in 1997. A trail of these objects were also observed from Earth, including by amateurs, as an anti-sunward "spike"; larger telescopes resolved mini-comae surrounding 1-10m-sized "snowballs", as reported here.

[cotopaxi]

So keeping the sample moderately cool (in terms of cometary volatiles; some would evaporate at 250 K), doesn't appear to be the big problem.

The required temperature depends on the objective, though. While only a small fraction of H2O will arrive on Earth in gaseous form at 200 or 250 K, the structure of the sample may have changed completely. This is because the equilibrium between sublimation and condensation is maintained, and at a temperature as low as 150 K most of the ice will have sublimated and recondensed at least once during a 5 year cruise. When studying a comet nucleus sample return for ESA's first cosmic vision call, we estimated that a temperature of 135 K would be needed to keep the sample unmodified (assuming that water ice and dust dominate the microscopic structure).
That does not mean to say that a sample return at higher temperatures would not be valuable, of course. And for the "snowflake collection" a higher temperature could be reasonable I guess because at the time of collection the sample will be relatively warm anyway.