Does the Great Valley Group Contain Jurassic Strata? Reevaluation of the Age and Early Evolution of a Classic Forearc Basin

The presence of Cretaceous detrital zircon in Upper Jurassic strata of the Great Valley Group may require revision of the lower Great Valley Group chronostratigraphy, with significant implications for the Late Jurassic–Cretaceous evolution of the continental margin. Samples (n = 7) collected from 100 km along strike in the purported Tithonian strata of the Great Valley Group contain 20 Cretaceous detrital zircon grains, based on sensitive high-resolution ion microprobe age determinations. These results suggest that Great Valley Group deposition was largely Cretaceous, creating a discrepancy between biostratigraphy based on Buchia zones and chronostratigraphy based on radiometric age dates. These results extend the duration of the Great Valley Group basal unconformity, providing temporal separation between Great Valley forearc deposition and creation of the Coast Range Ophiolite. If Great Valley forearc deposition began in Cretaceous time, then sediment bypassed the developing forearc in the Late Jurassic, or the Franciscan subduction system did not fully develop until Cretaceous time. In addition to these constraints on the timing of deposition, pre-Mesozoic detrital zircon age signatures indicate that the Great Valley Group was linked to North America from its inception

Constraints on Gamma-ray Emission from the Galactic Plane at 300 TeV

We describe a new search for diffuse ultrahigh energy gamma-ray emission associated with molecular clouds in the galactic disk. The Chicago Air Shower Array (CASA), operating in coincidence with the Michigan muon array (MIA), has recorded over 2.2 x 10^{9} air showers from April 4, 1990 to October 7, 1995. We search for gamma rays based upon the muon content of air showers arriving from the direction of the galactic plane. We find no significant evidence for diffuse gamma-ray emission, and we set an upper limit on the ratio of gamma rays to normal hadronic cosmic rays at less than 2.4 x 10^{-5} at 310 TeV (90% confidence limit) from the galactic plane region: (50 degrees < l < 200 degrees); -5 degrees < b < 5 degrees). This limit places a strong constraint on models for emission from molecular clouds in the galaxy. We rule out significant spectral hardening in the outer galaxy, and conclude that emission from the plane at these energies is likely to be dominated by the decay of neutral pions resulting from cosmic rays interactions with passive target gas molecules.Comment: Astrophysical Journal, submitted, 11 pages, AASTeX Latex, 3 Postscript figure

Very high-energy observations of the two high-frequency peaked BL Lac objects 1ES 1218+304 and H 1426+428

We present results of very-high-energy gamma-ray observations (E > 160 GeV) of two high-frequency-peaked BL Lac (HBL) objects, 1ES 1218+304 and H 1426+428, with the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE). Both sources are very-high-energy gamma-ray emitters above 100 GeV, detected using ground-based Cherenkov telescopes. STACEE observations of 1ES 1218+304 and H 1426+428 did not produce detections; we present 99% CL flux upper limits for both sources, assuming spectral indices measured mostly at higher energies

OSETI with STACEE: A Search for Nanosecond Optical Transients from Nearby Stars

We have used the STACEE high-energy gamma-ray detector to look for fast blue-green laser pulses from the vicinity of 187 stars. The STACEE detector offers unprecedented light-collecting capability for the detection of nanosecond pulses from such lasers. We estimate STACEE's sensitivity to be approximately 10 photons per square meter at a wavelength of 420 nm. The stars have been chosen because their characteristics are such that they may harbor habitable planets and they are relatively close to Earth. Each star was observed for 10 minutes and we found no evidence for laser pulses in any of the data sets.Comment: 38 pages, 12 figures. Accepted for publication in Astrobiolog

A Comparison of Astronaut Near-Earth Object Missions

NASA intends to send astronauts to a near Earth object (NEO) in or around 2025. This is expected to involve a six month mission with a few weeks stay-time at the NEO. Problems with this concept include lack of abort modes, vulnerability to solar flares, and lack of resupply opportunities. Studies by the authors (the Asteroid Mining Group) and a recent workshop at JPL organized by the Keck Institute opens the door to an alternative that addresses these problems and creates additional opportunities. Both groups investigated the feasibility of bringing one of more small NEOs into Earth or Lunar orbit. Particularly for High Earth Orbits (HEO) or High Lunar Orbits (HLO), this appears feasible with near-term technology, especially high-propellant-velocity, low-thrust solar electric propulsion (SEP) inspace vehicles. This paper compares the currently planned mission with an alternative: bringing one or more NEOs into HEO or HLO using SEP and lunar gravity assist. An astronaut mission to the NEO is then similar to a mission to the Moon without a landing. Trip times are measured in days, the NEO can be used for solar flare protection for most of the mission, and resupply within a few days is practical. Furthermore, materials derived from the NEO, e.g., propellant, water, radiation shielding, metals, silicon, and others, are available for projects in cis-lunar space, including satellite refueling, habitats, and space solar power. The alternative mission also develops much of the technology, experience, and infrastructure needed to protect Earth from potentially hazardous NEOs. As an outcome of these studies we are proposing a process whereby early missions can lead to large-scale industrialization of cis-lunar space based on solar energy and asteroidal resources

Observations of the BL Lac Object 3C 66A with STACEE

We present the analysis and results of recent high-energy gamma-ray observations of the BL Lac object 3C 66A conducted with the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE). During the 2003-2004 observing season, STACEE extensively observed 3C 66A as part of a multiwavelength campaign on the source. A total of 33.7 hours of data was taken on the source, plus an equivalent-duration background observation. After cleaning the data set a total of 16.3 hours of live time remained, and a net on-source excess of 1134 events was seen against a background of 231742 events. At a significance of 2.2 standard deviations this excess is insufficient to claim a detection of 3C 66A, but is used to establish flux upper limits for the source.Comment: Accepted for publication in the Astrophysical Journa