3,985 research outputs found
Keck Pencil-Beam Survey for Faint Kuiper Belt Objects
We present the results of a pencil-beam survey of the Kuiper Belt using the
Keck 10-m telescope. A single 0.01 square degree field is imaged 29 times for a
total integration time of 4.8 hr. Combining exposures in software allows the
detection of Kuiper Belt Objects (KBOs) having visual magnitude V < 27.9. Two
new KBOs are discovered. One object having V = 25.5 lies at a probable
heliocentric distance d = 33 AU. The second object at V = 27.2 is located at d
= 44 AU. Both KBOs have diameters of about 50 km, assuming comet-like albedos
of 4%.
Data from all surveys are pooled to construct the luminosity function from
red magnitude R = 20 to 27. The cumulative number of objects per square degree,
N (< R), is fitted to a power law of the form log_(10) N = 0.52 (R - 23.5).
Differences between power laws reported in the literature are due mainly to
which survey data are incorporated, and not to the method of fitting. The
luminosity function is consistent with a power-law size distribution for
objects having diameters s = 50 to 500 km; dn ~ s^(-q) ds, where the
differential size index q = 3.6 +/- 0.1. The distribution is such that the
smallest objects possess most of the surface area, but the largest bodies
contain the bulk of the mass. Though our inferred size index nearly matches
that derived by Dohnanyi (1969), it is unknown whether catastrophic collisions
are responsible for shaping the size distribution. Implications of the absence
of detections of classical KBOs beyond 50 AU are discussed.Comment: Accepted to AJ. Final proof-edited version: references added,
discussion of G98 revised in sections 4.3 and 5.
Rotation and Color Properties of the Nucleus of Comet 2P/Encke
We present results from CCD observations of comet 2P/Encke acquired at
Steward Observatory's 2.3m Bok Telescope on Kitt Peak obtained in Oct. 2002,
when the comet was near aphelion. Rotational lightcurves in B, V and R-filters
were acquired over two nights of observations, and analysed to study the
physical and color properties of the nucleus. The average apparent R-filter
magnitude across both nights corresponds to a mean effective radius of 3.95 +/-
0.06 km. The rotational lightcurve results in a nucleus axial ratio a/b >= 1.44
+/- 0.06 and semi-axes lengths of [3.60 +/- 0.09] x [5.20 +/- 0.13] km. Our
data includes the first detailed time series multi-color measurements of a
cometary nucleus, and significant color variations were seen. The average color
indices across both nights are: (V-R) = 0.39 +/- 0.06 and (B-V) = 0.73 +/- 0.06
(R_mean = 19.76 +/- 0.03). We linked our data with the September 2002 data from
Fernandez et al. (2005) - taken just 2-3 weeks before the current data set -
and we show that a rotation period of 11.083 +/- 0.003hours works extrememly
well for the combined data set.Comment: Accepted for publication in Icarus (Dec 2006). 27 page
A Titan exploration study: Science, technology, and mission planning options, volume 2
For abstract, see Vol.
Cloning of terminal transferase cDNA by antibody screening
A cDNA library was prepared from a terminal deoxynucleotidyltransferase-containing thymoma in the phage vector λgt11. By screening plaques with anti-terminal transferase antibody, positive clones were identified of which some had β-galactosidase-cDNA fusion proteins identifiable after electrophoretic fractionation by immunoblotting with anti-terminal transferase antibody. The predominant class of cross-hybridizing clones was determined to represent cDNA for terminal transferase by showing that one representative clone hybridized to a 2200-nucleotide mRNA in close-matched enzyme-positive but not to enzyme-negative cells and that the cDNA selected a mRNA that translated to give a protein of the size and antigenic characteristics of terminal transferase. Only a small amount of genomic DNA hybridized to the longest available clone, indicating that the sequence is virtually unique in the mouse genome
A Titan exploration study: Science, technology and mission planning options, volume 1
Mission concepts and technology advancements that can be used in the exploration of the outer planet satellites were examined. Titan, the seventh satellite of Saturn was selected as the target of interest. Science objectives for Titan exploration were identified, and recommended science payloads for four basic mission modes were developed (orbiter, atmospheric probe, surface penetrator and lander). Trial spacecraft and mission designs were produced for the various mission modes. Using these trial designs as a base, technology excursions were then made to find solutions to the problems resulting from these conventional approaches and to uncover new science, technology and mission planning options. Several mission modes were developed that take advantage of the unique conditions expected at Titan. They include a combined orbiter, atmosphere probe and lander vehicle, a combined probe and surface penetrator configuration and concepts for advanced remote sensing orbiters
Triton's surface age and impactor population revisited in light of Kuiper Belt fluxes: Evidence for small Kuiper Belt objects and recent geological activity
Neptune's largest satellite, Triton, is one of the most fascinating and
enigmatic bodies in the solar system. Among its numerous interesting traits,
Triton appears to have far fewer craters than would be expected if its surface
was primordial. Here we combine the best available crater count data for Triton
with improved estimates of impact rates by including the Kuiper Belt as a
source of impactors. We find that the population of impactors creating the
smallest observed craters on Triton must be sub-km in scale, and that this
small-impactor population can be best fit by a differential power-law size
index near -3. Such results provide interesting, indirect probes of the unseen
small body population of the Kuiper Belt. Based on the modern, Kuiper Belt and
Oort Cloud impactor flux estimates, we also recalculate estimated ages for
several regions of Triton's surface imaged by Voyager 2, and find that Triton
was probably active on a time scale no greater than 0.1-0.3 Gyr ago (indicating
Triton was still active after some 90% to 98% of the age of the solar system),
and perhaps even more recently. The time-averaged volumetric resurfacing rate
on Triton implied by these results, 0.01 km yr or more, is likely
second only to Io and Europa in the outer solar system, and is within an order
of magnitude of estimates for Venus and for the Earth's intraplate zones. This
finding indicates that Triton likely remains a highly geologically active world
at present, some 4.5 Gyr after its formation. We briefly speculate on how such
a situation might obtain.Comment: 14 pages (TeX), plus 2 postscript figures Stern & McKinnon, 2000, AJ,
in pres
An interstellar precursor mission
A mission out of the planetary system, with launch about the year 2000, could provide valuable scientific data as well as test some of the technology for a later mission to another star. Primary scientific objectives for the precursor mission concern characteristics of the heliopause, the interstellar medium, stellar distances (by parallax measurements), low energy cosmic rays, interplanetary gas distribution, and mass of the solar system. Secondary objectives include investigation of Pluto. Candidate science instruments are suggested. Individual spacecraft systems for the mission were considered, technology requirements and problem areas noted, and a number of recommendations made for technology study and advanced development. The most critical technology needs include attainment of 50-yr spacecraft lifetime and development of a long-life NEP system
Recommended from our members
Production of liquid fuels and chemicals by microalgae. Final subcontract report
An overall objective of the project was to conceptually determine if simple open pond systems have application for the production of fuels from microalgae. To demonstrate the overall objective, work concentrated on showing the potential microalgal yields that are possible from an open pond system on a sustained basis. Furthermore, problems associated with this experimental system were documented and reported so that future endeavors shall benefit. Finally, operational costs were documented to permit preliminary economic analysis of the system. The major conclusions of this project can be summarized as follows: (1) Using two wildtype species in northern California a yearly average productivity of 15 gm/m/sup 2//day, or 24 tons/acre/yr can be obtained in water with TDS = 4 to 8 ppt. (2) This can probably be increased to 20 to 25 gm/m/sup 2//day or 32 to 40 tons/acre/y in southern California. (3) Productivity can probably be further increased by using competitive strains screened for low respiration rates, tolerances to high levels of dissolved oxygen, broad temperature optima, and resistance to photoinhibition. (4) In systems with randomized, turbulent mixing, productivity is independent of channel velocity at least for productivities up to 25 to 30 gm/m/sup 2//day and velocities from 1 to 30 cm/sec. (5) Storage product induction requires one to three days of growth in batch mode under n-depleted conditions. (6) Critical cost centers include CO/sub 2/ input, harvesting and system capital cost. (7) Media recycling, necessary for water conservation, has no adverse effects, at least in the short term for strains which do not excrete organics, and when the harvesting method is at least moderately effective for all algal forms which may be present. 8 refs., 28 figs., 56 tabs
Ergodicity, Decisions, and Partial Information
In the simplest sequential decision problem for an ergodic stochastic process
X, at each time n a decision u_n is made as a function of past observations
X_0,...,X_{n-1}, and a loss l(u_n,X_n) is incurred. In this setting, it is
known that one may choose (under a mild integrability assumption) a decision
strategy whose pathwise time-average loss is asymptotically smaller than that
of any other strategy. The corresponding problem in the case of partial
information proves to be much more delicate, however: if the process X is not
observable, but decisions must be based on the observation of a different
process Y, the existence of pathwise optimal strategies is not guaranteed.
The aim of this paper is to exhibit connections between pathwise optimal
strategies and notions from ergodic theory. The sequential decision problem is
developed in the general setting of an ergodic dynamical system (\Omega,B,P,T)
with partial information Y\subseteq B. The existence of pathwise optimal
strategies grounded in two basic properties: the conditional ergodic theory of
the dynamical system, and the complexity of the loss function. When the loss
function is not too complex, a general sufficient condition for the existence
of pathwise optimal strategies is that the dynamical system is a conditional
K-automorphism relative to the past observations \bigvee_n T^n Y. If the
conditional ergodicity assumption is strengthened, the complexity assumption
can be weakened. Several examples demonstrate the interplay between complexity
and ergodicity, which does not arise in the case of full information. Our
results also yield a decision-theoretic characterization of weak mixing in
ergodic theory, and establish pathwise optimality of ergodic nonlinear filters.Comment: 45 page
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