Some Plotted Asteroid Data

[higginsdj]

Hi all,

I'm trying to extend myself beyond 'just' observing and have been plotting some asteroid data, specifically about asteroids and stumbled upon a trend (that is well known and documented) but I thought was interesting.
http://www.david-higgins.com/P1vD_2-D_1.GIF
The trend is that the higher the value of D_2/D_1 (the ratio of the diameter of the Moon to it's parent) the slower the spin rotation of the parent. At D_2/D_1 >= 0.8, the systems are completely synchronised, that is P1 = P2 = P_orb. I also plotted the size of the parents denoted by the size of the circles and it surprised me to note how many large asteroids there were with fast spin rates.

The 'mass' of smaller asteroids are those specifically covered by BINAST (the survey I belong to) whilst the really small ones represent the NEO's observed just prior to, and actually triggered the formation of, BINAST. Nothing ground breaking here, just thought it interesting to see it in black and white so to speak.

Cheers

[CraigS]

Hi David;

Interesting. You've got me thinking.
Do the binaries rotate solely due to gravitational attraction or has the motion come about in different ways ? I think everyone suspects the latter (?) but is it really so ?.

For example, I wonder how closely the moons' rotational velocities track that predicted by Keplarian motion ?

It might be possible to model the system the way they model stars rotating about a galaxy core. If they obey Kepler's third law, (ie the moon velocity should be inversely proportional to the square root of the radius), then at further out distances, the velocity should fall off with the sqrt of the radius.

I might be 'off beam' here, but it would be interesting to see the variations from the theory predictions.

Its just a thought … (I guess someone's already looked into all this).

Cheers & Rgds.

[higginsdj]

The binaries couldn't exist other than by gravitational attraction! Of course some come undone in the process and then eject the moon to become asteroid pairs (at least thats the theory). If you meant the formation process then thats still up in the air. It is believed that the primary evolutionary process for asteroids is collisional. Is that still the case today? Cratering and surface compositions of a handful of asteroids by direct observation can be explained by density and low velocity collisional processes (anything other than low velocity and asteroids do not possess the gravitation to either hold onto/capture a moon or even hold it's own ejecta in the case of collision). It is also beleived that ultimately, the collisional process will result in nothing but rubble pile asteroids. Are we close to that point now (astronomical timescale speaking of course)? Other than direct observation, is there a way to observe/prove it one way or another?

I was reading a paper last night that stated that for rubble pile (gravitational aggregates), collisional process caused craters through indentation/compression rather than ejecta. Of course they only had the 1 rubble pile asteroid (Mathilde) to work from...

As for Keplers third law, we assume so. Few binary systems have been imaged directly (optical, radar or spacecraft) so we can't say for sure. We use Newtons form of Keplers third law to model the system based on observed rotations (P1 and P_orb) and D_2/D_1. If we have Observed H and G values (Absolute Magnitude and Slope parameter) then we can reasonably accurately generate sizes but density has to be assumed. (For those we don't have an accurate H or G value we just use the MPC published values which leaves a large margin for error ~ 30% I think). We can derive a reasonable estimate of density from the discovery of the spin barrier for rubble piles. (around 2 gm/cm^2).

[CraigS]

I've been perusing through some information about Saturns' co-orbital moons, Epimetheus and Janus, recently. Fascinating stuff there .. and they've learned so much more from Cassini data, like about the 'propeller' features in the ring material caused by shepherd moon resonance and the impact effects on the surface features, and the clouds of ejecta. Different influences and complexities, (I guess) when compared with Asteroids, but also similar in other ways.

They also seem to trying to work out the internal density distribution from spin/eccentricities.

Oh well .. its all very interesting.

Cheers
PS: Epimetheus' mean density is only 0.69 ± 0.11 g/cm³, Janus' is 0.64 ± 0.06 g/cm³ (note the small error terms !), which I think, together with surface Albedo measurements result in the conclusion that they are very porous ! …. cheers

[Outbackmanyep]

Originally Posted by CraigS

PS: Epimetheus' mean density is only 0.69 ± 0.11 g/cm³, Janus' is 0.64 ± 0.06 g/cm³ (note the small error terms !), which I think, together with surface Albedo measurements result in the conclusion that they are very porous ! …. cheers

A nice striking view of a "porous" object i think would be Hyperion
http://wanderingspace.net/wp-content...1/hyperion.jpg

Its an amazing image!

[renormalised (Carl)]

Originally Posted by CraigS

Epimetheus' mean density is only 0.69 ± 0.11 g/cm³, Janus' is 0.64 ± 0.06 g/cm³ (note the small error terms !), which I think, together with surface Albedo measurements result in the conclusion that they are very porous ! …. cheers

Or made out of styrofoam!!!!

Very porous and made out of mostly ices.

[CraigS]

Hyperion:
Mean density: 0.5667±0.1025 g/cm3
From Wiki:

Hyperion's low density indicates that it is composed largely of water ice with only a small amount of rock. It is thought that Hyperion may be similar to a loosely accreted pile of rubble in its physical composition. However, unlike most of Saturn's moons, Hyperion has a low albedo (0.2–0.3), indicating that it is covered by at least a thin layer of dark material. This may be material from Phoebe (which is much darker) that got past Iapetus. Hyperion is redder than Phoebe and closely matches the color of the dark material on Iapetus.
Hyperion has a porosity of about 0.46.

Its a wobbler, too:

The 3:4 orbital resonance between Titan and Hyperion may also make a chaotic rotation more likely. The odd rotation probably accounts for the relative uniformity of Hyperion's surface, in contrast to many of Saturn's other moons which have contrasting trailing and leading hemispheres.

Very interesting ..

Cheers

[higginsdj]

Makes you wonder if it wasn't a captured comet nucleus. The craters being the result of sublimation on it's previous passes by the sun!

Cheers

[CraigS]

Could be a comet .. there are so many things happening out there. Was watching a doco last night on Saturn (for the umpteenth time) and I'm reminded of how much H2O is being poured into the outer regions (& rings) by Enceladus' cryovolcanism. There's heaps of ice out there, probably coming from Enceladus !!

Would be great to find out someday .. not in our lifetimes, I suspect.

Cheers

[renormalised (Carl)]

Could be an old Centaur that got captured by Saturn at some time in its history. Most likely very early on....it's stopped outgassing.

[Outbackmanyep]

The pits look like impact craters, but on a low density object the penetration would be quite deep. If you notice on that image there is a much larger "crater" which has been peppered by smaller impacts, and it seems the lower portion has broken off somehow, could have been disrupted from the impact or tidal stresses later on?
I say large crater because there seems to be a central uplift, although a lot more rounded than a lunar crater.

I doubt it's a comet nucleus but it seems in this universe anything is possible......

[higginsdj]

A 'recent' (~2002) paper on the topic indicated that impact craters on low density/porous objects more likely result in indentations rather than 'normal' cratering events ( ie via ejecta), the object being hit absorbing the impact. The large 'frontal crater' may not actually be a crater.

I'm reading a draft paper on a bifurcated asteroid which indicates that YORP can spin an elongated asteroid up and down, throwing a moon by rotational fission on spin up BUT that it is also possible that the system can be unstable and combined with YORP the spun off moon can reconnect to the parent. YORP can spin it up throw it off again. Note that each of these events can take several My, but given the age of the solar system, these events can occur a number of times. The bifurcated target we are looking at comprises a spherical component on the end of a prolate spheroid component with some 'overlap' at the transition. If the 2 components were simply gravitationally bound then the new fission event would leave some of the overlapping material attached to both components, giving the impression of a large crater with a very large central dome. In this case the dome was the original surface of the spherical component!

Cheers