W. M. Owen

Imaging of Titan from the Cassini spacecraft

Titan, the largest moon of Saturn, is the only satellite in the Solar System with a substantial atmosphere. The atmosphere is poorly understood and obscures the surface, leading to intense speculation about Titans nature. Here we present observations of Titan from the imaging science experiment onboard the Cassini spacecraft that address some of these issues. The images reveal intricate surface albedo features that suggest aeolian, tectonic and fluvial processes; they also show a few circular features that could be impact structures. These observations imply that substantial surface modification has occurred over Titans history. We have not directly detected liquids on the surface to date. Convective clouds are found to be common near the south pole, and the motion of mid-latitude clouds consistently indicates eastward winds, from which we infer that the troposphere is rotating faster than the surface. A detached haze at an altitude of 500 km is 150–200 km higher than that observed by Voyager, and more tenuous haze layers are also resolved.

THE GRAVITY FIELD OF THE SATURNIAN SYSTEM FROM SATELLITE OBSERVATIONS AND SPACECRAFT TRACKING DATA

We present values for the masses of Saturn and its major satellites, the zonal harmonics in the spherical harmonic expansion of Saturns gravitational potential, and the orientation of the pole of Saturn. We determined these values using an extensive data set: satellite astrometry from Earth-based observatories and the Hubble Space Telescope; Earth-based, Voyager 1, and Voyager 2 ring occultation measurements; Doppler tracking data from Pioneer 11; and Doppler tracking, radiometric range, and imaging data from Voyager 1, Voyager 2, and Cassini.

The landing of the NEAR-Shoemaker spacecraft on asteroid 433 Eros.

The NEAR-Shoemaker spacecraft was designed to provide a comprehensive characterization of the S-type asteroid 433 Eros (refs 1,2,3), an irregularly shaped body with approximate dimensions of 34 × 13 × 13 km. Following the completion of its year-long investigation, the mission was terminated with a controlled descent to its surface, in order to provide extremely high resolution images. Here we report the results of the descent on 12 February 2001, during which 70 images were obtained. The landing area is marked by a paucity of small craters and an abundance of ‘ejecta blocks’. The properties and distribution of ejecta blocks are discussed in a companion paper. The last sequence of images reveals a transition from the blocky surface to a smooth area, which we interpret as a ‘pond’. Properties of the ‘ponds’ are discussed in a second companion paper. The closest image, from an altitude of 129 m, shows the interior of a 100-m-diameter crater at 1-cm resolution.

THE ORBITS OF SATURN'S SMALL SATELLITES DERIVED FROM COMBINED HISTORIC AND CASSINI IMAGING OBSERVATIONS

We report on the orbits of the small, inner Saturnian satellites, either recovered or newly discovered in recent Cassini imaging observations. The orbits presented here reflect improvements over our previously published values in that the time base of Cassini observations has been extended, and numerical orbital integrations have been performed in those cases in which simple precessing elliptical, inclined orbit solutions were found to be inadequate. Using combined Cassini and Voyager observations, we obtain an eccentricity for Pan 7 times smaller than previously reported because of the predominance of higher quality Cassini data in the fit. The orbit of the small satellite (S/2005 S1 [Daphnis]) discovered by Cassini in the Keeler gap in the outer A ring appears to be circular and coplanar; no external perturbations are apparent. Refined orbits of Atlas, Prometheus, Pandora, Janus, and Epimetheus are based on Cassini , Voyager, Hubble Space Telescope, and Earth-based data and a numerical integration perturbed by all the massive satellites and each other. Atlas is significantly perturbed by Prometheus, and to a lesser extent by Pandora, through high-wavenumber mean-motion resonances. Orbital integrations involving Atlas yield a mass of GMAtlas = (0.44 ± 0.04) × 10-3 km3 s -2, 3 times larger than reported previously (GM is the product of the Newtonian constant of gravitation G and the satellite mass M). Orbital integrations show that Methone is perturbed by Mimas, Pallene is perturbed by Enceladus, and Polydeuces librates around Diones L5 point with a period of about 791 days. We report on the nature and orbits of bodies sighted in the F ring, two of which may have persisted for a year or more.

Recovering the rotational light curve of Phoebe

We present rotational light-curve data for Saturns satellite Phoebe taken over the observing period prior to the Cassini missions encounter with that moon. We find a rotation period of 9.2735 ± 0.0006 hr, a factor of 25 improvement in the rotation periods uncertainty over previously published data. This improved rotation period measurement allow s us to correlate previously observed spectral features and colors with albedo features observed by Voyager and to predict which side of Phoebe will be observed by Cassini during its 2004 June 11 encounter. The light curve, sampled at subobserver latitudes farther south than achieved by Voyager, shows evidence of surface features that cannot be explained by previously published shape models or albedo maps and that may be located in the regions in Phoebes southern hemisphere that were unobserved by Voyager.

Recovery of near's mission to eros

Abstract On December 20, 1998, the main bipropellant thruster on the Near Earth Asteroid Rendezvous (NEAR) spacecraft was commanded to start a 15-minute burn. It was the first and largest of four planned maneuvers that would cancel NEARs velocity relative to (433) Eros to allow capture into orbit about the asteroid in January 1999. But the burn aborted and the spacecraft tumbled, causing loss of communication. The spacecraft corrected the problem, but used 29 kg of fuel in the process. A day later, NEAR was reacquired with just enough time to upload a sequence of commands to image Eros extensively as the spacecraft hurtled past the asteroid on December 23. The NEAR team designed a large bipropellant maneuver that successfully cancelled most of NEARs velocity relative to Eros on January 3, 1999. But then NEAR was almost 1 million kilometers from Eros, so a long “U-turn” journey was begun to return to the asteroid on February 14, 2000. The spacecraft remains healthy and enough fuel remains to enter orbit about Eros to complete all of the mission goals. This will be the first time that an interplanetary spacecraft has failed an orbital capture burn and returned to accomplish its objectives. This is possible because NEAR had a generous fuel supply and a robust contingency plan.

THE GM VALUES OF MIMAS AND TETHYS AND THE LIBRATION OF METHONE

We have determined the GM of the Saturnian satellite Mimas from an analysis of its resonance with Tethys and with the newly discovered satellite Methone (GM is the product of the Newtonian constant of gravitation G and the satellites mass M). Observations of the latter permit a factor of 5 improvement in our knowledge of Mimass GM; its value is 2.504 ± 0.002 km3 s-2. Tethyss GM was originally found from the Mimas-Tethys resonance. We, however, estimate it using observations of its two Lagrangian satellites, Calypso and Telesto; its value is 41.200 ± 0.007 km3 s-2.

High precision comet trajectory estimates: The Mars flyby of C/2013 A1 (Siding Spring)

Abstract The Mars flyby of C/2013 A1 (Siding Spring) represented a unique opportunity for imaging a long-period comet and resolving its nucleus and rotation state. Because of the small encounter distance and the high relative velocity, the goal of successfully observing C/2013 A1 from the Mars orbiting spacecraft posed strict accuracy requirements on the comet’s ephemeris. These requirements were hard to meet, as comets are known for being highly unpredictable: astrometric observations can be significantly biased and nongravitational perturbations affect comet trajectories. Therefore, even prior to the encounter, we remeasured a couple of hundred astrometric images obtained with ground-based and Earth-orbiting telescopes. We also observed the comet with the Mars Reconnaissance Orbiter’s High Resolution Imaging Science Experiment (HiRISE) camera on 2014 October 7. In particular, these HiRISE observations were decisive in securing the trajectory and revealed that out-of-plane nongravitational perturbations were larger than previously assumed. Though the resulting ephemeris predictions for the Mars encounter allowed observations of the comet from the Mars orbiting spacecraft, post-encounter observations show a discrepancy with the pre-encounter trajectory. We reconcile this discrepancy by employing the Rotating Jet Model, which is a higher fidelity model for cometary nongravitational perturbations and provides an estimate of C/2013 A1’s spin pole ( RA , DEC ) = ( 63 ° , 14 ° ) .