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mantiss
2006-Feb-01, 06:15 PM
Asteroids Near Jupiter are Really Comets (http://www.space.com/scienceastronomy/060201_jupiter_comet.html)

Amusing find, opening new vistas to possibilities...


Using the Keck II Laser Telescope in Hawaii, astronomers found that the two objects, 617 Patroclus and its companion, Menoetius, had a density of only 0.8 grams per cubic centimeters—only a third that of rock.

Kullat Nunu
2006-Feb-01, 06:35 PM
This doesn't make them comets yet.

Very low density indicates they are loose rubble piles. This is not unheard of, because several main belt asteroids whose mass have been measured have also proved to have very small densities (not that low, though). On the other hand, all of them are binary asteroids, so we may have a strong observational bias here; a collision that creates the asteroid satellites also breaks up the asteroid itself. Mathilde is exception not having a satellite, but with a very long rotational period is probably not a typical asteroid.

mantiss
2006-Feb-01, 06:49 PM
Well I hope they have spectrographic data backing their claim, as you said, low density doesn't make them comets yet, they have to have at least the main constituents.

Hopefully the Nature article is more beefed up.

Doodler
2006-Feb-01, 08:01 PM
http://www.cnn.com/2006/TECH/space/02/01/jupiter.comets/index.html

If the Trojans really are kuiper style comets, then New Horizons might not even need an extended mission. These would be much easier to reach, I'd think.

Disinfo Agent
2006-Feb-01, 08:23 PM
The pair orbit around each other while floating 465 million miles (750 million kilometers) from Jupiter in one of the gas planet's two so-called Lagrange points. So far, so good...


At these points, the gravitational field of Jupiter and the sun are perfectly balanced, and objects can be captured and brought to relative rest. "Relative rest". Hmm... :think:


Jupiter has two Lagrange points, one in front and the other behind as the planet orbits the Sun.Bad! :wall:

ToSeek
2006-Feb-01, 09:13 PM
Threads merged.

galacsi
2006-Feb-01, 09:47 PM
Using the Keck II Laser Telescope in Hawaii, astronomers found that the two objects, 617 Patroclus and its companion, Menoetius, had a density of only 0.8 grams per cubic centimeters—only a third that of rock.


I am quite puzzled : How a "Laser telescope" can mesure the density of some asteroids ? Because this is a binary system and the Keck was able to determine their dimensions ? And from the rotation speed they deduced the mass ? then density ?

May be somebody can help with a little math ?

Cordialement.

Galacsi.

Kullat Nunu
2006-Feb-02, 12:39 AM
"Laser" means here adaptive optics guide laser. Mass can be deduced from the orbital speed of the components around common barycenter. If the asteroids' disks are resolved, their dimensions and volume can be measured, too. And mass/volume = density.

ToSeek
2006-Feb-02, 05:27 PM
Analysis from New Scientist:

Icy Trojan asteroids boost planet-forming theory (http://www.newscientistspace.com/article/dn8663)


A pair of asteroids that orbit in lockstep around Jupiter are more likely to be dirt-covered ice balls than rocky rubble piles, a new study suggests. The results back a new theory of how the giant planets formed.

mantiss
2006-Feb-02, 06:12 PM
So it would be interesting to get there and check it out, since they could harbor pristine formation-era material. Would be interesting to get an ice core and bring it back :think:

hhEb09'1
2006-Feb-02, 06:20 PM
Bad! :wall:I don't know what you're getting so worked up over :)

The article is saying "asteroids near Jupiter" but they're at the Lagrange points? The L5 and L4 points are 60 degrees away--in other words, the three: Jupiter, L5, and the Sun, form an equilateral triangle. The Sun is as "near" to Jupiter as the asteroids are.

Disinfo Agent
2006-Feb-02, 06:38 PM
There are five Lagrange points (http://en.wikipedia.org/wiki/Lagrange_points), not two. O.K., so only two of them are stable and relevant to this case, but it still seemed like an inaccuracy. I also thought the sentence could be a bit misleading:

Jupiter has two Lagrange points, one in front and the other behind as the planet orbits the Sun.I know what they mean, but 'one in front and the other behind' could be misinterpreted as 'one Lagrange point between Jupiter and the Sun, and the other with Jupiter between it and the Sun' (in other words, L1 and L2, instead of L4 and L5).