Alan Stern

Pluto’s global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data

Abstract On July 14th 2015, NASA’s New Horizons mission gave us an unprecedented detailed view of the Pluto system. The complex compositional diversity of Pluto’s encounter hemisphere was revealed by the Ralph/LEISA infrared spectrometer on board of New Horizons. We present compositional maps of Pluto defining the spatial distribution of the abundance and textural properties of the volatiles methane and nitrogen ices and non-volatiles water ice and tholin. These results are obtained by applying a pixel-by-pixel Hapke radiative transfer model to the LEISA scans. Our analysis focuses mainly on the large scale latitudinal variations of methane and nitrogen ices and aims at setting observational constraints to volatile transport models. Specifically, we find three latitudinal bands: the first, enriched in methane, extends from the pole to 55°N, the second dominated by nitrogen, continues south to 35°N, and the third, composed again mainly of methane, reaches 20°N. We demonstrate that the distribution of volatiles across these surface units can be explained by differences in insolation over the past few decades. The latitudinal pattern is broken by Sputnik Planitia, a large reservoir of volatiles, with nitrogen playing the most important role. The physical properties of methane and nitrogen in this region are suggestive of the presence of a cold trap or possible volatile stratification. Furthermore our modeling results point to a possible sublimation transport of nitrogen from the northwest edge of Sputnik Planitia toward the south.

New Horizons: The First Reconnaissance Mission to Bodies in the Kuiper Belt

NASA has long been planning a mission of exploration to Pluto-Charon and the Kuiper Belt (e.g., Terrile et al., 1997). In 2001 NASA selected such a mission (NASA, 2001), called New Horizons, for design and development. New Horizons is now funded and planning a launch in January 2006. The mission plans to carry 8 scientific sensors and make flybys of Pluto-Charon and one or more KBOs. Statistical Monte Carlo simulations indicate that New Horizons has sufficient fuel to reach one or more KBOs with diameters exceeding 35 km. If launched as planned in 2006, the mission will use a Jovian gravity assist, arriving at Pluto-Charon in 2015 or 2016; if launched in its backup window in 2007, a Jovian gravity assist is not feasible and arrival will be later – 2019. Below we briefly summarize the New Horizons mission, concentrating on its role in Kuiper Belt exploration.

Optical Structure and Proper-Motion Age of the Oxygen-rich Supernova Remnant 1E 0102–7219 in the Small Magellanic Cloud*

We present new optical emission-line images of the young SNR 1E 0102-7219 in the SMC obtained with the ACS on HST. This object is a member of the oxygen-rich class of SNRs showing strong oxygen, neon, and other metal-line emissions in its optical and X-ray spectra, and an absence of hydrogen and helium. The progenitor of 1E 0102-7219 may have been a Wolf-Rayet star that underwent considerable mass loss prior to exploding as a Type Ib/c or IIL/b supernova. The ejecta in this SNR are generally fast-moving (V > 1000 km s-1) and emit as they are compressed and heated in the reverse shock. In 2003 we obtained optical [O III], Hα, and continuum images with the ACS Wide Field Camera. The [O III] image through the F475W filter captures the full velocity range of the ejecta and shows considerable high-velocity emission projected in the middle of the SNR that was Doppler-shifted out of the narrow F502N bandpass of a previous WFPC2 image from 1995. Using these two epochs separated by ~8.5 yr, we measure the transverse expansion of the ejecta around the outer rim in this SNR for the first time at visible wavelengths. From proper-motion measurements of 12 ejecta filaments, we estimate a mean expansion velocity for the bright ejecta of ~2000 km s-1 and an inferred kinematic age for the SNR of ~2050 ± 600 yr. The age we derive from HST data is about twice that inferred by Hughes et al. from X-ray data, although our 1 σ error bars overlap. Our proper-motion age is consistent with an independent optical kinematic age derived by Eriksen et al. in 2003 using spatially resolved [O III] radial-velocity data. We derive an expansion center that lies very close to conspicuous X-ray and radio hot spots, which could indicate the presence of a compact remnant (neutron star or black hole).

The Pluto reconnaissance flyby mission

Although Pluto was discovered in 1930, our modern view of this distant world only began taking shape in 1976. Prior to that time, the technology for studying this faint, 14th magnitude object was simply too immature. Virtually all that was known before 1976 was that Pluto circles the Sun in an unusually eccentric 248-year orbit (ranging from 29.5 to 49.4 AU) that is tilted far (17°) from the ecliptic—the plane in which the other planets orbit, that Plutos rotation period is 6.39 days, that its intrinsic surface color is reddish, and that its rotational lightcurve is the strangest and most variegated of the planets. Since 1976, however, the pace and diversity of discoveries have increased dramatically. First, methane ice was discovered on Plutos surface, which gave a clear indication that Pluto was formed in the outer solar system rather than simply ejected to it from another region. Next came the discovery of Plutos large satellite Charon (usually pronounced “Sharon”) in 1978. Lying just 19,400 km from Pluto (<1 arcsecond as seen from Earth), Charon orbits Pluto every 6.387 days—Plutos rotation period. Thus, unlike any other satellite in the solar system, Charon orbits Pluto at its synchronous orbit.

NASA plans Pluto-Kuiper Belt Mission

The trans-Neptunian region, which contains the binary planet Pluto-Charon and the myriad planetary embryos of the Kuiper Belt, is a scientific and intellectual frontier. In recent years, the Pluto-Charon system has become recognized as a key element for understanding the origin of the outer solar system. So, too, it has become apparent that Pluto-Charon is a scientific wonderland offering insights into exotic dynamics, the nature of primitive organic material, complex volatile transport processes, and hydrodynamic atmospheric escape, as well as rich surface and atmospheric chemistry. Plutos size, density albedo, surface composition, and atmosphere also make it a key comparator to Triton, Neptunes large and complex icy satellite.

Finding KBO Flyby Targets for New Horizons

Development of the New Horizons mission to Pluto and the Kuiper Belt is now fully funded by NASA (Stern and Spencer, this volume). If all goes well, New Horizons will be launched in January 2006, followed by a Jupiter gravity assist in 2007, with Pluto arrival expected in either 2015 or 2016, depending on the launch vehicle chosen. A backup launch date of early 2007, without a Jupiter flyby, would give a Pluto arrival in 2019 or 2020. In either case, a flyby of at least one Kuiper Belt object (KBO) is planned following the Pluto encounter, sometime before the spacecraft reaches a heliocentric distance of 50 AU, in 2021 or 2023 for the 2006 launch, and 2027 or 2029 for the 2007 launch. However, none of the almost 1000 currently-known KBOs will pass close enough to the spacecraft trajectory to be targeted by New Horizons, so the KBO flyby depends on finding a suitable target among the estimated 500,000 KBOs larger than 40 km in diameter. This paper discusses the issues involved in finding one or more KBO targets for New Horizons. The New Horizons team plans its own searches for mission KBOs but will welcome other U.S, or international team who wish to become involved in exchange for mission participation at the KBO. 1. The Number of Accessible KBOs We first determine how many KBOs of a given size or magnitude are likely to be accessible to the New Horizons spacecraft, given the amount of fuel available for targeting (measured in v, the velocity change that the fuel can provide). We assume the KBO sky density vs. brightness relation from Gladman et al. (2001) N = 100.69(M−23.5), where M is R magnitude and N is the KBOs per square degree brighter than that magnitude. Luu and Jewitt (2002) propose an only slightly different power law (N = 100.64(M−23.23)) which results in a very similar sky density of magnitude 26–27 objects. Neither set of authors sees strong evidence for a break of slope at small sizes to a shallower power law (as might be expected from a transition to a collisional size distribution), which would reduce the number of faint objects, at R magnitudes brighter than 26. Earth, Moon and Planets 92: 483–491, 2003.

Low-cost airborne astronomy imager to begin research phase

For decades, airborne astronomy and geophysical observations have proven useful adjuncts to ground-based and space-based instrumentation, particularly for optical and infrared studies [e.g., Larson, 1995].Compared to ground-based instruments, airborne research platforms offer superior atmospheric transmission, the ability to reach remote and often otherwise inaccessible locations over the Earth, and virtually guaranteed good weather for observing the sky.Airborne platforms also offer substantial cost advantages over space-based instruments.With funding from Southwest Research Institute (SwRI) and NASA, we have developed the hardware and tech-niques to routinely conduct valuable astronomical and aeronomical observations from high-performance, two-seater military-type aire raft. These platforms cost far less than larger, more conventional airborne platforms, offering savings that are often of an order of 10:1 per flight hour. Smaller platforms based throughout the world eliminate the need for expensive, campaignstyle movement of specialized large aircraft and logistics support teams, and can react faster to transient events. The Southwest Ultraviolet Imaging System-Airborne (SWUIS-A) imager has been flight tested in 14 airborne missions since 1997. With initial systems development and operational trials completed, a vigorous operational research phase is beginning.

Ralph: a visible/infrared imager for the New Horizons Pluto/Kuiper Belt mission

The instrument named Ralph is a visible/NIR imager and IR hyperspectral imager that would fly as one of the core instruments on New Horizons, NASAs mission to the Pluto/Charon system and the Kuiper Belt. It is a compact, power efficient, and robust instrument with excellent imaging characteristics and sensitivity, and is well suited to this longduration flyby reconnaissance mission.

Autonomy enables new science missions

The challenge of space flight in NASA’s future is to enable smaller, more frequent and intensive space exploration at much lower total cost without substantially decreasing mission reliability, capability, or the scientific return on investment. The most effective way to achieve this goal is to build intelligent capabilities into the spacecraft themselves. Our technological vision for meeting the challenge of returning quality science through limited communication bandwidth will actually put scientists in a more direct link with the spacecraft than they have enjoyed to date. Technologies such as pattern recognition and machine learning can place a part of the scientist’s awareness onboard the spacecraft to prioritize downlink or to autonomously trigger time-critical follow-up observations—particularly important in flyby missions—without ground interaction. Onboard knowledge discovery methods can be used to include candidate discoveries in each downlink for scientists’ scrutiny. Such capabilities will allo...

Viewing NASA's Mars Budget with Resignation

I would like to clarify several points in the News of the Week story (26 September, p. [1754][1]) by A. Lawler, “Rising costs could delay NASAs next mission to Mars and future launches.” When the National Research Councils Planetary Science Decadal Survey recommended the Mars Science Laboratory (MSL) mission for priority funding, it assigned a cost level of