Joseph W. Gangestad

Charon's radius and atmospheric constraints from observations of a stellar occultation

The physical characteristics of Pluto and its moon, Charon, provide insight into the evolution of the outer Solar System. Although previous measurements have constrained the masses of these bodies, their radii and densities have remained uncertain. The observation of a stellar occultation by Charon in 1980 established a lower limit on its radius of 600 km (ref. 3) (later refined to 601.5 km; ref. 4) and suggested a possible atmosphere. Subsequent, mutual event modelling yielded a range of 600–650 km (ref. 5), corresponding to a density of 1.56 ± 0.22 g cm-3 (refs 2, 5). Here we report multiple-station observations of a stellar occultation by Charon. From these data, we find a mean radius of 606 ± 8 km, a bulk density of 1.72 ± 0.15 g cm-3, and rock-mass fraction 0.63 ± 0.05. We do not detect a significant atmosphere and place 3σ upper limits on atmospheric number densities for candidate gases. These results seem to be consistent with collisional formation for the Pluto–Charon system in which the precursor objects may have been differentiated, and they leave open the possibility of atmospheric retention by the largest objects in the outer Solar System.

POETS: Portable Occultation, Eclipse, and Transit System

Occultations of stars by small bodies in the outer solar system are opportunities to make high- resolution measurements of their geometries and orbital elements and to detect or probe their atmospheres. Such events are limited in space and time, so it is desirable to deploy highly capable camera systems on multiple fixed and/or portable telescopes anywhere in the world, potentially on short notice. Similar considerations apply to planetary transits and solar eclipses. We have designed a camera system called POETS (Portable Occultation, Eclipse, and Transit System), which is optimized for occultation and related observations, and have assembled five such systems. The core of this system is the Andor Technology DV-887 (now DU-897) frame-transfercamera, featuring a high frame rate, minimal dead time, high quantum efficiency, and low read noise. An electron- multiplying mode lowers effective read noise to below 1 e ! pixel ! 1 and is capable of photon counting. Each POETS includes a compact GPS timing system with microsecond accuracy, and a high-performance computer system capable of sustained fast frame rates. Each POETS is designed to be transportable as carry-on luggage and is adaptable to a wide variety of sites. POETS were deployed for the first time for the 2005 July 11 Charon occultation event, and they performed extremely well on telescopes with apertures from 0.6 to 6.5 m. Three POETS were subsequently deployed for the 2006 March 29 total solar eclipse, and five for the 2006 June 12 Pluto occultation.

Propellantless Stationkeeping at Enceladus via the Electromagnetic Lorentz Force

An introduction to the dynamics of an electrostatically charged spacecraft in two- and three-body regimes is presented, with particular attention to a promising application at Enceladus. Equilibrium solutions to the equations of motion are found, and the stability of the orbits assessed. The perturbative Lorentz force, produced by interaction of the spacecraft with a planetary magnetic field, creates trajectories that are not possible with Newtonian gravity alone. A heuristic, analytical control law with single-variable scalar feedback allows a charged spacecraft to remain near the collinear Lagrange points on either side of Enceladus. The mission feasibility depends on the charge requirement, which is primarily affected by the navigational accuracy. For example, an insertion error into the cis-Enceladus Lagrange point of 10 km in position and 1 m/s in velocity requires a specific charge of approximately 0.05 C/kg. Other significant gravitating bodies in the Saturnian system do not have a substantive effect on the charge requirements or on the stability. Given a sufficiently accurate insertion, a charged spacecraft could maintain station near Enceladus without propellant.

Inclination Change in Low-Earth Orbit via the Geomagnetic Lorentz Force

An electrostatically charged spacecraft is subject to Lorentz force perturbations as it orbits a central body with a magnetic field. On an artificially charged spacecraft, these perturbations may be harnessed for orbital maneuvering and are particularly potent for plane-change maneuvers. The Gaussian form of the variational equations is used to develop a control law for inclination change and to predict the capability of such maneuvers. An analytical expression provides the relative evolution of the semimajor axis and the inclination when the Lorentz force is used to change the inclination of a nearly circular orbit. Despite causing significant changes in the semimajor axis, the Lorentz force may enable substantial propellant savings for inclination change maneuvers. A constellation reconfiguration example is presented in which a specific charge magnitude of 0.01 C/kg leads to a 95 % savings over an impulsive maneuver for the redeployment of a low-Earth-orbit constellation from 34 to 55 deg inclination.

Analytical Expressions that Characterize Propellantless Capture with Electrostatically Charged Spacecraft

Spacecraft that intentionally maintain an electrostatic charge on their surface within a planetary magnetic field can manipulate the induced Lorentz force to perform propellantless maneuvers. Analytical expressions are developed that describe the process of capture with the Lorentz force and that demonstrate coupling among orbital elements (e.g., the eccentricity versus semilatus rectum) when the Lorentz force is the only perturbation on a Keplerian orbit. In equatorial orbits with a dipolar magnetic field, the relative evolution of the orbital elements notably depends on neither the charge nor on the magnetic field strength. The analytical solutions are applied to a capture at Jupiter, where, for example, a Galileo-like arrival requires ~0.2 C/kg of charge to effect capture. The analytical solutions agree with numerical propagations to within a fraction of a percent. A previous study, which numerically explored the parameter space of the Jupiter capture problem, identified trends and constraints on the motion that can now be explained by the analytical theory.