60 research outputs found
On LAM's and SAM's for Halley's rotation
Non principal axis rotation for comet Halley is inferred from dual periodicities evident in the observations. The modes where the spin axis precesses around the axis of minimum moment of inertia (long axis mode or LAM) and where it precesses around the axis of maximum moment of inertia (short axis mode or SAM) are described from an inertial point of view. The currently favored LAM model for Halley's rotation state satisfies observational and dynamical constraints that apparently no SAM can satisfy. But it cannot reproduce the observed post perihelion brightening through seasonal illumination of localized sources on the nucleus, whereas a SAM can easily produce post or pre perihelion brightening by this mechanism. However, the likelihood of a LAM rotation for elongated nuclei of periodic comets such as Halley together with Halley's extreme post perihelion behavior far from the Sun suggest that Halley's post perihelion brightening may be due to effects other than seasonal illumination of localized sources, and therefore such brightening may not constrain its rotation state
Scientific Report Number 2
I. INTRODUCTION
The earlier orbits and ephemerides for the Soviet satellites were not sufficiently
accurate to be very useful in making observations in Alaska. Extrapolations
from our own observations gave better predictions. This merely pointed out the fact
that rough observations of meridian transits at high latitudes will give better values
of the inclination of the orbit than precision observations at low latitudes. Hence,
it was decided to observe visually the meridian transits estimating the altitude by
noting the position with respect to the stars or using crude alidade measurements.
The times of the earlier observations were observed on a watch or clock and the clock
correction obtained from WWV. Later the times were determined with the aid of stop
watches, taking time intervals from WWV signals.
This rather meager program of optical observations of the Soviet satellites was
undertaken to give supplementary data for use of the radio observations, and particularly
to assist in the prediction of position of the satellite so that the 61-foot
radar of Stanford Research Institute could be set accurately enough to observe it
(the beam width at the half-power points is about 3°).
This report contains primarily the visual observations made at the Geophysical
Institute by various members of the staff, and a series of observations by Olaf
Halverson at Nome, Alaska. In addition there is a short discussion of the geometry
of the trajectory, the illumination of a circumpolar satellite, and a note on the
evaluation of Brouwer's moment factors.Air Force Contract No. AF 30(635)-2887
Project No. 5535 - Task 45774
Rome Air Development Center
Griffiss Air Force Base
Rome, New YorkI. Introduction -- II. Visual Observations -- III. Geometry of the Trajectory -- IV. Seasonal Illumination of a Circumpolar Earth Satellite at its
Extreme Latitude Orbit-Point. -- V. Tentative Evaluation of Brouwer's Moment Factors.Ye
Gemini and Lowell observations of 67P/Churyumov−Gerasimenko during the <i>Rosetta</i> mission
We present observations of comet 67P/Churyumov−Gerasimenko acquired in support of the Rosetta mission. We obtained usable data on 68 nights from 2014 September until 2016 May, with data acquired regularly whenever the comet was observable. We collected an extensive set of near-IR J, H and Ks data throughout the apparition plus visible-light images in g', r', i' and z' when the comet was fainter. We also obtained broad-band R and narrow-band CN filter observations when the comet was brightest using telescopes at Lowell Observatory. The
appearance was dominated by a central condensation and the tail until 2015 June. From 2015 August onwards, there were clear asymmetries in the coma, which enhancements revealed to be due to the presence of up to three features (i.e. jets). The features were similar in all broad-band filters; CN images did not show these features but were instead broadly enhanced in the southeastern hemisphere. Modelling using the parameters from Vincent et al. replicated the dust morphology reasonably well, indicating that the pole orientation and locations of active areas have been relatively unchanged over at least the last three apparitions. The dust production, as measured by A(0°)fρ peaked ∼30 d after perihelion and was consistent with
predictions from previous apparitions. A(0°)fρ as a function of heliocentric distance was well fitted by a power law with slope −4.2 from 35 to 120 d post-perihelion. We detected photometric evidence of apparent outbursts on 2015 August 22 and 2015 September 19, although neither was discernible morphologically in this data set
Align, Perturb and Decouple: Toward Better Leverage of Difference Information for RSI Change Detection
Change detection is a widely adopted technique in remote sense imagery (RSI)
analysis in the discovery of long-term geomorphic evolution. To highlight the
areas of semantic changes, previous effort mostly pays attention to learning
representative feature descriptors of a single image, while the difference
information is either modeled with simple difference operations or implicitly
embedded via feature interactions. Nevertheless, such difference modeling can
be noisy since it suffers from non-semantic changes and lacks explicit guidance
from image content or context. In this paper, we revisit the importance of
feature difference for change detection in RSI, and propose a series of
operations to fully exploit the difference information: Alignment, Perturbation
and Decoupling (APD). Firstly, alignment leverages contextual similarity to
compensate for the non-semantic difference in feature space. Next, a difference
module trained with semantic-wise perturbation is adopted to learn more
generalized change estimators, which reversely bootstraps feature extraction
and prediction. Finally, a decoupled dual-decoder structure is designed to
predict semantic changes in both content-aware and content-agnostic manners.
Extensive experiments are conducted on benchmarks of LEVIR-CD, WHU-CD and
DSIFN-CD, demonstrating our proposed operations bring significant improvement
and achieve competitive results under similar comparative conditions. Code is
available at https://github.com/wangsp1999/CD-Research/tree/main/openAPDComment: To appear in IJCAI 202
HCGMNET: A Hierarchical Change Guiding Map Network For Change Detection
Very-high-resolution (VHR) remote sensing (RS) image change detection (CD)
has been a challenging task for its very rich spatial information and sample
imbalance problem. In this paper, we have proposed a hierarchical change
guiding map network (HCGMNet) for change detection. The model uses hierarchical
convolution operations to extract multiscale features, continuously merges
multi-scale features layer by layer to improve the expression of global and
local information, and guides the model to gradually refine edge features and
comprehensive performance by a change guide module (CGM), which is a
self-attention with changing guide map. Extensive experiments on two CD
datasets show that the proposed HCGMNet architecture achieves better CD
performance than existing state-of-the-art (SOTA) CD methods
The Increasing Rotation Period of Comet 10P/Tempel 2
We imaged comet 10P/Tempel 2 on 32 nights from 1999 April through 2000 March.
R-band lightcurves were obtained on 11 of these nights from 1999 April through
1999 June, prior to both the onset of significant coma activity and perihelion.
Phasing of the data yields a double-peaked lightcurve and indicates a nucleus
rotational period of 8.941 +/- 0.002 hr with a peak-to-peak amplitude of ~0.75
mag. Our data are sufficient to rule out all other possible double-peaked
solutions as well as the single- and triple- peaked solutions. This rotation
period agrees with one of five possible solutions found in post-perihelion data
from 1994 by Mueller and Ferrin (1996, Icarus, 123, 463-477), and unambiguously
eliminates their remaining four solutions. We applied our same techniques to
published lightcurves from 1988 which were obtained at an equivalent orbital
position and viewing geometry as in 1999. We found a rotation period of 8.932
+/- 0.001 hr in 1988, consistent with the findings of previous authors and
incompatible with our 1999 solution. This reveals that Tempel 2 spun-down by
~32 s between 1988 and 1999 (two intervening perihelion passages). If the
spin-down is due to a systematic torque, then the rotation period prior to
perihelion during the 2010 apparition is expected to be an additional 32 s
longer than in 1999.Comment: Accepted by The Astronomical Journal; 22 pages of text, 3 tables, 6
figure
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