What Would It Be Like Standing On A Comet?

[jaxtraw]

Hi all, haven't posted in ages, couldn't get my login to work when it moved from the "old place" but now it seems to be working again.

I write/draw a sci-fi online comic for a living, it's "adult" so I won't link or hint, heh. I'm thinking of having my characters visit a comet (it's in an alien star system but presume comets are much the same the universe over!). What I'm wondering is what the experience would actually be like in a real scientific scenario rather than Hollywood, to be on a comet which is busy creating a tail. Would there be great boiling jets of gas everywhere, or would there just be more of a gentle "haze" of gently sublimating ice? From what I've seen of missions photographing comets, there are jets, but if you were standing on the surface would they be spurting from tiny holes and cracks, or big great holes in the ice surface? That is, are they "jetty" at ground level or is the jet we see from a distance an agglomeration of, er "jetlets"?

Also, what's the surface like? Ice covered in darker dusty material? Bright ice? Mostly rock with some ice? Is there likely to be a regolith?

I'm thinking this comet is some distance from the star, just developing its tail, but I wonder generally how lively the surface gets as the comet approaches perihelion. Come to that, do we call it perihelion for stars other than the Sun?

Any help will be most helpful and appreciated

 

[MeteorWayne]

It's periastron for other stars

Each of the 5 comet nuclei we've seen is different.
The surface can be icy, dusty, or organic goo. There can be a few weak or strong jets here or there, or like the surface of Hartley 2 (See the last few pages of the Deep Impact/EPOXI thread in Missions and Launches for the latest) the sunfacing end seems to have dozens in the space of half a kilometer.

The jets can range from a few meters a second to hundreds, from almost invisible to like a geyser.

One overriding aspect would be the almost non existant gravity. A slightly too springy step could permanently remove you from the surface into your own orbit around the star. As could a well placed strong jet from below you.

Here's some nucleus images:

1P/Halley (16x8x8 km): http://neo.jpl.nasa.gov/images/halley.html
19P/Borelly (8x4 km): http://nmp.jpl.nasa.gov/ds1/images.html
9P/Tempel 1 (7.6x4.9 km) (See beginning of Deep Impact thread) and http://www.nasa.gov/mission_pages/deepimpact/main/
81P/Wild 2 (5.5x4.0x3.3 km): http://stardust.jpl.nasa.gov/photo/wild2.html

 

[centsworth_II]

During the press briefing after the Hartley 2 flyby, the panel was asked what it would be like to stand by, or in, one of the observed jets. One scientist said it probably wouldn't be forceful enough to eject you from the surface. Another interjected that it might. The first said yeah, it might, but that of course Hartley 2 is so small that it wouldn't take much force to do that.

 

[jaxtraw]

Thanks guys!

Is there any idea of what these jets would look like at groud level? I mean, would there be like a small hole or crack in the "crust" the size of your thumb, or the size of a football field? Are we talking a jet at the base of a few centimeters, or meters?

I'm just wondering why there are jets at all. It must indicate a non-icy surface with ice below sublimating through holes of some kind, right?

 

[MeteorWayne]

Yes, a non icy surface with cracks or holes as the ice below the surface sublimates and the pressure forces it's way out. As far as the size, the ones on Halley seemed to be pretty big; ~ 10 sq km. Those on Hartley 2 will require some more analysis, which will take months or years. Obviously much smaller since the whole asteroid is only 2.0 x 0.5 km in size! or about 3 sq km total surface area.

Halley, seems to lose much of it's mass as boulders of meter sized or more (though millions of smaller particles are ejected). A boulder hitting you in the tail could be an unpleasant experience. The ejection velocity for 1mm sized grains is estimated at 50 m/s, a 2 mm sized grain about 35 m/s. Water molecules are ejected at about 2.5 km/s! .

Just some numbers to work with....

 

[neilsox]

I'll guess it is sort of like standing in Yellowstone National Park, without the the trees, vegetation and atmosphere. Some of the jets in the park are dangerous and some are intermittent, but the danger is easily avoided using common sense. The jets = geysers range from centimeters to many meters. If it erupts as a super volcano, many kilometers. The comet may disassemble, if it passes though a strong gravity field = Roach limit. Neil

 

[MeteorWayne]

Comets can also be disrupted from too much ice sublimating at once increasing the internal pressure, a big impact, or too high a rotation rate.