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Re: Magnetized Meteorites?
>I have seen Canyon Diablos that will attract small
>bits of metal. Connection? Pretty nifty!
Thanks, that should be easy enough to check out.
>Thanks for the interesting posts. Could
>you post something about the Ida/Dactyl
>orbit connection? Thanks,
The text appended below is from the Galileo home page I've mentioned before:
http://www.jpl.nasa.gov/galileo/mess35/DACTYL.html
I'll let you go look at the diagram and photos off the home page.
Ron Baalke
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Solving for Dactyl's Orbit and Ida's Density
Galileo's recent discovery that Ida has a satellite (now known as Dactyl)
suggests that satellites orbiting asteroids may be a commonplace occurrence.
In the Solid-State Imaging (SSI) camera data returned from the Ida
encounter, Dactyl and Ida appear in 47 images. Their locations in these
images were used to estimate Dactyl's orbit and Ida's bulk density, which is
of great interest because it may indicate whether Ida is composed of rocks
that have been thermally processed deep within a collisionally destroyed
planetesimal. Density calculations were based on an Ida volume of 16,100
cubic kilometers (112 percent), which was determined from an accurate model
of the shape of Ida. While Dactyl's orbit is of interest, its greatest
significance is in providing the first accurate estimate of an S-type
asteroid's density--another achievement by Galileo!
Initial attempts to apply classical astronomical orbit-fitting methods to
estimate Dactyl's orbit, assuming a reasonable value for Ida's density,
suffered from numerical problems caused by the Galileo-to-Ida line of sight
being nearly in the plane of Dactyl's orbit for most of the images. Mike
Belton, SSI Team leader, then asked the Navigation Team to apply their
orbit-determination methods to the problem, which led to this challenging
and enjoyable search.
Since our objective was to determine preliminary estimates for Dactyl's
orbit and Ida's density, the analysis was simplified by assuming that
Dactyl's orbit was affected only by Ida's gravity acting as a point mass.
The problem was to find an orbit for Dactyl that was consistent with the
locations of Ida and Dactyl in the SSI images. Much of the analysis involved
reducing the raw data associated with the images (exposure time,
camera-pointing direction, positions of Ida, etc.) to a form usable by a new
computer program that could estimate Dactyl's orbit. Another large part of
the task involved actually writing and debugging this new program, which is
constructed of QUICK commands. (QUICK is an easy-to-use, versatile processor
from JPL's Multimission Analysis Software Library.)
It became clear almost immediately that the mass/density of Ida could not be
solved at the same time as Dactyl's orbit. Instead, a series of Dactyl
orbits were generated for a range of Ida mass/density values--from 1.5 to
4.0 grams per cubic centimeter. For each density value, there is a unique
orbit; over this range of densities, these orbits differ greatly. For Ida
densities less than about 2.1 grams per cubic centimeter, the orbits are
just barely hyperbolic. For higher Ida densities, the orbits are elliptical
with a large apoapsis (farthest point from Ida), a periapsis (nearest point
to Ida) of around 80-85 km, and periods that range from just over a day to
many tens of days. At a density of about 2.8 grams per cubic centimeter,,
the orbit is nearly circular (about 82 by 98 km) with a period of about 27
hours. For even higher densities, the elliptical orbits have apoapses of
about 95-100 km, with periapses that decrease with increasing density. For
an Ida density greater than about 2.9 grams per cubic centimeter, the
periapsis is less than about 75 km and the period is less than 24 hours. The
geometry for a range of orbit solutions is shown in the accompanying figure.
Since this view is from the spin pole of Ida, the motion of Dactyl and Ida's
rotation are both counterclockwise.
The figure shows that when points recorded at the same time on each Dactyl
orbit are connected, they are parallel to the center line through Ida that
points to the spacecraft. All of the images but the very last were taken
when Galileo was thousands--or even hundreds of thousands--of kilometers
from Ida and nearly in its equatorial plane, so that the spacecraft was
viewing the Ida-Dactyl system from the lower right part of the figure. For
scale, the long axis of Ida is 58 km, and Ida is shown as it would be
oriented at the time of Galileo's closest approach. Thus, the figure only
covers a few hundred kilometers around Ida.
The last image mosaic was taken when Galileo was almost at its closest
approach to Ida and included parts of both Ida and Dactyl in separate
images. At that point, Galileo was essentially looking down on Dactyl's
orbit plane (essentially the plane of the figure), and Dactyl was at the
point where all possible orbits cross. The lowest parallel line connects the
points on each orbit at 5 hours prior to closest approach, or about the time
of the earliest image. Since Dactyl was viewed for only a fraction of its
orbit and from a nearly edge-on vantage point, all of the orbits shown fit
the observations equally well. If one imagines being on the Galileo
spacecraft looking at Ida and Dactyl, then all of the orbit solutions would
have appeared the same during the 5-hour approach, since the differences
between them are all along the line of sight (the parallel lines in the
figure).
Thus, for a given mass/density of Ida, a unique and well-determined two-body
conic orbit can be found. However, this alone does not help us find the
unknown density of Ida. Only by applying the dynamics of motion about a
non-point-mass Ida and using our knowledge of the general distribution of
asteroidal material in the entire asteroid belt can the range of possible
mass/density values for Ida be reduced.
Dynamical studies show that orbits with periapses less than about 75 km from
Ida are unstable and either collide with, or escape from, Ida--thus, orbit
solutions are not physically possible that correspond to an Ida density of
about 2.9 grams per cubic centimeter or greater. At the other extreme,
hyperbolic and even highly elliptical orbits around Ida are very unlikely.
The observed speed of Dactyl around Ida for any of the orbit solutions is no
more than about 10 m/s, about the speed of a fast run or a slowly thrown
baseball. Calculations indicate that the chance of a random piece of
asteroidal material the size of Dactyl passing by Ida at that speed, just
when Galileo was observing it, are about 1 in 1019. In addition, if Dactyl
were in a hyperbolic or highly elliptical orbit, it should have been seen by
the Hubble Space Telescope (HST) when it observed the region around Ida over
an 8-hour period on April 26, 1994 . HST would have easily seen Dactyl had
it been more than about 700 km from Ida. Combining these two restrictions
gives a preliminary estimate for Ida's density of 2.1 to 2.9 grams per cubic
centimeter. Allowing for a 12-percent uncertainty in the modeled volume of
Ida increases the range to 1.9 to 3.2 grams per cubic centimeter.
This density range is surprisingly well constrained and suggests that Ida is
fairly porous and/or made of fairly light rocks. This result already
excludes several classes of dense igneous rocks that had previously been
suggested as the primary components of Ida's composition.
Further work on the long-term stability of orbits that fit these
observations, as well as a more precise analysis of the SSI images
themselves, may lead to a better determination of both the density of Ida
and the orbit of Dactyl. These, combined with other ongoing work involving
the color, spectral properties, and geology of Ida's surface are expected to
lead to major advances in our knowledge of the nature of asteroids and what
they can tell us about the birth of the planets.
Dennis V. Byrnes
Louis A. D'Amario
Galileo Navigation Team
Diagram of Possible Dactyl Orbits
Dactyl by Idashine photo
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