17 research outputs found
An N-body Integrator for Gravitating Planetary Rings, and the Outer Edge of Saturn's B Ring
A new symplectic N-body integrator is introduced, one designed to calculate
the global 360 degree evolution of a self-gravitating planetary ring that is in
orbit about an oblate planet. This freely-available code is called epi_int, and
it is distinct from other such codes in its use of streamlines to calculate the
effects of ring self-gravity. The great advantage of this approach is that the
perturbing forces arise from smooth wires of ring matter rather than discreet
particles, so there is very little gravitational scattering and so only a
modest number of particles are needed to simulate, say, the scalloped edge of a
resonantly confined ring or the propagation of spiral density waves.
The code is applied to the outer edge of Saturn's B ring, and a comparison of
Cassini measurements of the ring's forced response to simulations of Mimas'
resonant perturbations reveals that the B ring's surface density at its outer
edge is 195+-60 gm/cm^2 which, if the same everywhere across the ring would
mean that the B ring's mass is about 90% of Mimas' mass.
Cassini observations show that the B ring-edge has several free normal modes,
which are long-lived disturbances of the ring-edge that are not driven by any
known satellite resonances. Although the mechanism that excites or sustains
these normal modes is unknown, we can plant such a disturbance at a simulated
ring's edge, and find that these modes persist without any damping for more
than ~10^5 orbits or ~100 yrs despite the simulated ring's viscosity of 100
cm^2/sec. These simulations also indicate that impulsive disturbances at a ring
can excite long-lived normal modes, which suggests that an impact in the recent
past by perhaps a cloud of cometary debris might have excited these
disturbances which are quite common to many of Saturn's sharp-edged rings.Comment: 55 pages, 13 figures, accepted for publication in the Astrophysical
Journa
Dynamics of the Sharp Edges of Broad Planetary Rings
(Abridged) The following describes a model of a broad planetary ring whose
sharp edge is confined by a satellite's m^th Lindblad resonance (LR). This
model uses a streamline formalism to calculate the ring's internal forces,
namely, ring gravity, pressure, viscosity, as well as a hypothetical drag
force. The model calculates the streamlines' forced orbit elements and surface
density throughout the perturbed ring. The model is then applied to the outer
edge of Saturn's B ring, which is maintained by an m=2 inner LR with the
satellite Mimas. Ring models are used to illustrate how a ring's perturbed
state depends on the ring's physical properties: surface density, viscosity,
dispersion velocity, and the hypothetical drag force. A comparison of models to
the observed outer B ring suggests that the ring's surface density there is
between 10 and 280 gm/cm^2. The ring's edge also indicates where the viscous
torque counterbalances the perturbing satellite's gravitational torque on the
ring. But an examination of seemingly conventional viscous B ring models shows
that they all fail to balance these torques at the ring's edge. This is due
ring self-gravity and the fact that a viscous ring tends to be nearly
peri-aligned with the satellite, which reduces the satellite's torque on the
ring and makes the ring's edge more difficult to maintain. Nonetheless, the
following shows that a torque balance can still be achieved in a viscous B
ring, but only in an extreme case where the ratio of the ring's bulk/shear
viscosities satisfy ~10^4. However, if the dissipation of the ring's forced
motions is instead dominated by a weak drag force, then the satellite can exert
a much stronger torque that can counterbalance the ring's viscous torque.Comment: Accepted for publication in the Astrophysical Journal on April 3,
200
Physical characteristics and non-keplerian orbital motion of "propeller" moons embedded in Saturn's rings
We report the discovery of several large "propeller" moons in the outer part
of Saturn's A ring, objects large enough to be followed over the 5-year
duration of the Cassini mission. These are the first objects ever discovered
that can be tracked as individual moons, but do not orbit in empty space. We
infer sizes up to 1--2 km for the unseen moonlets at the center of the
propeller-shaped structures, though many structural and photometric properties
of propeller structures remain unclear. Finally, we demonstrate that some
propellers undergo sustained non-keplerian orbit motion. (Note: This arXiv
version of the paper contains supplementary tables that were left out of the
ApJL version due to lack of space).Comment: 9 pages, 4 figures; Published in ApJ
The instant sequencing task: Toward constraint-checking a complex spacecraft command sequence interactively
Robotic spacecraft are controlled by sets of commands called 'sequences.' These sequences must be checked against mission constraints. Making our existing constraint checking program faster would enable new capabilities in our uplink process. Therefore, we are rewriting this program to run on a parallel computer. To do so, we had to determine how to run constraint-checking algorithms in parallel and create a new method of specifying spacecraft models and constraints. This new specification gives us a means of representing flight systems and their predicted response to commands which could be used in a variety of applications throughout the command process, particularly during anomaly or high-activity operations. This commonality could reduce operations cost and risk for future complex missions. Lessons learned in applying some parts of this system to the TOPEX/Poseidon mission will be described
High-throughput RNA structure probing reveals critical folding events during early 60S ribosome assembly in yeast
While the protein composition of various yeast 60S ribosomal subunit assembly intermediates has been studied in detail, little is known about ribosomal RNA (rRNA) structural rearrangements that take place during early 60S assembly steps. Using a high-throughput RNA structure probing method, we provide nucleotide resolution insights into rRNA structural rearrangements during nucleolar 60S assembly. Our results suggest that many rRNA-folding steps, such as folding of 5.8S rRNA, occur at a very specific stage of assembly, and propose that downstream nuclear assembly events can only continue once 5.8S folding has been completed. Our maps of nucleotide flexibility enable making predictions about the establishment of protein-rRNA interactions, providing intriguing insights into the temporal order of protein-rRNA as well as long-range inter-domain rRNA interactions. These data argue that many distant domains in the rRNA can assemble simultaneously during early 60S assembly and underscore the enormous complexity of 60S synthesis.Ribosome biogenesis is a dynamic process that involves the ordered assembly of ribosomal proteins and numerous RNA structural rearrangements. Here the authors apply ChemModSeq, a high-throughput RNA structure probing method, to quantitatively measure changes in RNA flexibility during the nucleolar stages of 60S assembly in yeast
Submitted for publication in the Astrophysical Journal
The following describes a model of a broad planetary ring whose sharp edge is confined by a satelliteās m th Lindblad resonance (LR). This model uses the streamline formalism of Borderies et al. (1982, 1985) to calculate the ringās internal forces, namely, ring gravity, pressure, and viscosity. The model also allows for the possibility of a drag force that can affect small ring particles directly, and large ring particles indirectly via collisions with the small. The model calculates the streamlines ā forced eccentricities e, their longitudes of peripase ĖĻ, and the surface density Ļ throughout the perturbed ring. This model is then applied to the outer edge of Saturnās B ring, which is maintained by an m = 2 inner LR with the satellite Mimas. A suite of ring models are used to illustrate how a ringās perturbed state depends on the ringās physical properties: its surface density, its viscosity, the ring particles ā dispersion velocity, and the strength of the hypothetical drag force. A comparison of model results to the outer B ringās observed properties suggests that the ringās surface density there is 10 ļæ½ Ļ ļæ½ 28