Publications
Detecting small asteroids with the Space Surveillance Telescope
September 1, 2014
Journal Article
Published in:
Icarus, Vol. 239, 2014, pp. 253-9.
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The ability of the Space Surveillance Telescope (SST) to find small (2-15 m diameter) NEAs suitable for the NASA asteroid retrieval mission is investigated. Orbits from a simulated population of targetable small asteroids were propagated and observations with the SST were simulated. Different search patterns and telescope time allocation cases were considered, as well as losses due to FOV gaps and weather. It is concluded that a full-time, dedicated survey at the SST is likely necessary to find a useful population of these NEAs within the mission launch timeframe, especially if an object must be observed on >1 night at SST to qualify as a detection. The simulations were also performed for an identical telescope in the southern hemisphere, which is found to produce results very similar to the SST in New Mexico due to significant (~80%) overlap in the population of objects detected at each site. In addition to considering the SST's ability to detect small NEAs, a parallel study was performed focusing on >100 m diameter objects. This work shows that even with limited telescope time (3 nights per month) a substantial number of these larger objects would be detected.
Summary
The ability of the Space Surveillance Telescope (SST) to find small (2-15 m diameter) NEAs suitable for the NASA asteroid retrieval mission is investigated. Orbits from a simulated population of targetable small asteroids were propagated and observations with the SST were simulated. Different search patterns and telescope time allocation cases...
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Exploring the variable sky with LINEAR. III. classification of periodic light curves
October 1, 2013
Journal Article
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Astron. J., Vol. 146, No. 101, October 2013.
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We describe the construction of a highly reliable sample of ~7000 optically faint periodic variable stars with light curves obtained by the asteroid survey LINEAR across 10,000 deg^2 of the northern sky. The majority of these variables have not been cataloged yet. The sample flux limit is several magnitudes fainter than most other wide-angle surveys; the photometric errors range from ~0.03 mag at r = 15 to ~0.20 mag at r = 18. Light curves include on average 250 data points, collected over about 25 million objects, we selected ~200,000 most probable candidate variables with r
Summary
We describe the construction of a highly reliable sample of ~7000 optically faint periodic variable stars with light curves obtained by the asteroid survey LINEAR across 10,000 deg^2 of the northern sky. The majority of these variables have not been cataloged yet. The sample flux limit is several magnitudes fainter...
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Exploring the variable sky with LINEAR. II. Halo structure and substructure traces by RR Lyrae stars to 30 kpc
August 1, 2013
Journal Article
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Astron. J., Vol. 146, No. 21, August 2013.
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We present a sample of ~5000 RR Lyrae stars selected from the recalibrated LINEAR data set and detected at heliocentric distances between 5 kpc and 30 kpc over ~8000 deg^2 of sky. The coordinates and light curve properties, such as period and Oosterhoff type, are made publicly available. We analyze in detail the light curve properties and Galactic distribution of the subset of ~4000 type ab RR Lyrae (RRab) stars, including a search for new halo substructures and the number density distribution as a function of Oosterhoff type. We find evidence for the Oosterhoff dichotomy among field RR Lyrae stars, with the ratio of the type II and I subsamples of about 1:4, but with a weaker separation than for globular cluster stars. The wide sky coverage and depth of this sample allow unique constraints for the number density distribution of halo RRab stars as a function of galactocentric distance: it can be described as an oblate ellipsoid with an axis ratio q = 0.63 and with either a single or a double power law with a power-law index in the range -2 to -3. Consistent with previous studies, we find that the Oosterhoff type II subsample has a steeper number density profile than the Oosterhoff type I subsample. Using the group-finding algorithm EnLink, we detected seven candidate halo groups, only one of which is statistically spurious. Three of these groups are near globular clusters (M53/NGC 5053, M3, M13), and one is near a known halo substructure (Virgo Stellar Stream); the remaining three groups do not seem to be near any known halo substructures or globular clusters and seem to have a higher ratio of Oosterhoff type II to Oosterhoff type I RRab stars than what is found in the halo. The extended morphology and the position (outside the tidal radius) of some of the groups near globular clusters are suggestive of tidal streams possibly originating from globular clusters. Spectroscopic follow-up of detected halo groups is encouraged.
Summary
We present a sample of ~5000 RR Lyrae stars selected from the recalibrated LINEAR data set and detected at heliocentric distances between 5 kpc and 30 kpc over ~8000 deg^2 of sky. The coordinates and light curve properties, such as period and Oosterhoff type, are made publicly available. We analyze...
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Characterizing the optical variability of bright blazars: variability-based selection of fermi active galactic nuclei
November 20, 2012
Journal Article
Published in:
Astrophys. J., Vol. 760, No. 1, 20 November 2012, pp. 51-61.
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We investigate the use of optical photometric variability to select and identify blazars in large-scale time-domain surveys, in part to aid in the identification of blazar counterparts to the ~30% of y -ray sources in the Fermi 2FGL catalog still lacking reliable associations. Using data from the optical LINEAR asteroid survey, we characterize the optical variability of blazars by fitting a damped random walk model to individual light curves with two main model parameters, the characteristic timescales of variability T , and driving amplitudes on short timescales ^sigma. Imposing cuts on minimum T and ^sigma allows for blazar selection with high efficiency E and completeness C. To test the efficacy of this approach, we apply this method to optically variable LINEAR objects that fall within the several arcminute error ellipses of y -ray sources in the Fermi 2FGL catalog. Despite the extreme stellar contamination at the shallow depth of the LINEAR survey, we are able to recover previously associated optical counterparts to Fermi active galactic nuclei with E >/ 88% and C = 88% in Fermi 95% confidence error ellipses having semimajor axis r < 8'. We find that the suggested radio counterpart to Fermi source 2FGL J1649.6+5238 has optical variability consistent with other y -ray blazars and is likely to be the y -ray source. Our results suggest that the variability of the non-thermal jet emission in blazars is stochastic in nature, with unique variability properties due to the effects of relativistic beaming. After correcting for beaming, we estimate that the characteristic timescale of blazar variability is ~3 years in the rest frame of the jet, in contrast with the ~320 day disk flux timescale observed in quasars. The variability-based selection method presented will be useful for blazar identification in time-domain optical surveys and is also a probe of jet physics.
Summary
We investigate the use of optical photometric variability to select and identify blazars in large-scale time-domain surveys, in part to aid in the identification of blazar counterparts to the ~30% of y -ray sources in the Fermi 2FGL catalog still lacking reliable associations. Using data from the optical LINEAR asteroid...
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Exploring the variable sky with LINEAR : photometric recalibration with the Sloan Digital Sky Survey
December 1, 2011
Journal Article
Published in:
Astronomical J., Vol. 142, No. 6, December 2011.
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We describe photometric recalibration of data obtained by the asteroid survey LINEAR. Although LINEAR was designed for astrometric discovery of moving objects, the data set described here contains over 5 billion photometric measurements for about 25 million objects, mostly stars. We use Sloan Digital Sky Survey (SDSS) data from the overlapping ~10,000 deg2 of sky to recalibrate LINEAR photometry and achieve errors of 0.03 mag for sources not limited by photon statistics with errors of 0.2 mag at r ~ 18. With its 200 observations per object on average, LINEAR data provide time domain information for the brightest four magnitudes of the SDSS survey. At the same time, LINEAR extends the deepest similar wide-area variability survey, the Northern Sky Variability Survey, by 3 mag.We briefly discuss the properties of about 7000 visually confirmed periodic variables, dominated by roughly equal fractions of RR Lyrae stars and eclipsing binary stars, and analyze their distribution in optical and infrared color?color diagrams. The LINEAR data set is publicly available from the SkyDOT Web site.
Summary
We describe photometric recalibration of data obtained by the asteroid survey LINEAR. Although LINEAR was designed for astrometric discovery of moving objects, the data set described here contains over 5 billion photometric measurements for about 25 million objects, mostly stars. We use Sloan Digital Sky Survey (SDSS) data from the...
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Pre-discovery observations of disrupting asteroid P/2010 A2
July 1, 2011
Journal Article
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Astronom. J., Vol. 142, No. 29, July 2011.
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Solar system object P/2010 A2 is the first-noticed example of the aftermath of a recently disrupted asteroid, probably resulting from a collision. Nearly a year elapsed between its inferred initiation in early 2009 and its eventual detection in early 2010. Here, we use new observations to assess the factors underlying the visibility, especially to understand the delayed discovery. We present pre-discovery observations from the LINEAR telescope and set limits to the early-time brightness from SOHO and STEREO satellite coronagraphic images. Consideration of the circumstances of discovery of P/2010 A2 suggests that similar objects must be common, and that future all-sky surveys will reveal them in large numbers.
Summary
Solar system object P/2010 A2 is the first-noticed example of the aftermath of a recently disrupted asteroid, probably resulting from a collision. Nearly a year elapsed between its inferred initiation in early 2009 and its eventual detection in early 2010. Here, we use new observations to assess the factors underlying...
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Detecting asteroids with a multi-hypothesis velocity matched filter
July 17, 2008
Conference Paper
Published in:
ACM 2008, 10th Asteroids, Comets Meteors Mtg., 14-18 July 2008.
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We present a novel approach to image processing for optical detection of faint asteroids. Traditional methods of asteroid detection require observations in multiple frames taken over a period of time, but are limited by the signal-to-noise ratio in a single frame. Our approach is based on a velocity matched filter (VMF), which combines the signal from multiple frames in order to increase the aggregate SNR for dim objects. By generating a series of hypotheses about the apparent velocities of potential objects, we create a set of highly sensitive velocity-specific filters, the results of which are combined to achieve complete coverage of the search space. Each filter collapses a set of sidereal frames into a single frame through a shifted sum operation, thus aggregating the signal from the entire frameset and increasing SNR for objects matching the hypothesized velocity. We also present additional signal processing steps designed to filter out a variety of noise sources such as stars, spacecraft, and background gradients.
Summary
We present a novel approach to image processing for optical detection of faint asteroids. Traditional methods of asteroid detection require observations in multiple frames taken over a period of time, but are limited by the signal-to-noise ratio in a single frame. Our approach is based on a velocity matched filter...
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Bias-corrected population, size distribution, and impact hazard for the near-Earth objects
August 1, 2004
Journal Article
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Icarus, Vol. 170, No. 2, August 2004, pp. 295-311.
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Utilizing the largest available data sets for the observed taxonomic and albedo distributions of the near-Earth object population, we model the bias-corrected population. Diameter-limited fractional abundances of the taxonomic complexes are A-0.2%; C-10%, D-17%, O-0.5%, Q-14%, R-0.1%, S-22%, U-0.4%, V-1%, X-34%. In a diameter-limited sample, ~30% of the NEO population has jovian Tisserand parameter less than 3, where the D-types and X-types dominate. The large contribution from the X-types is surprising and highlights the need to better understand this group with more albedo measurements. Combining the C, D, and X complexes into a "dark" group and the others into a "bright" group yields a debiased darkto- bright ratio of ~1.6. Overall, the bias-corrected mean albedo for the NEO population is 0.14 +/-0.02, for which an H magnitude of 17.8 +/-0.1 translates to a diameter of 1 km, in close agreement with Morbidelli et al. Coupling this bias corrected taxonomic and albedo model with the H magnitude dependent size distribution of yields a diameter distribution with 1090 +/-180 NEOs with diameters larger than 1 km. As of 2004 June, the Spaceguard Survey has discovered 56% of the NEOs larger than 1 km. Using our size distribution model, and orbital distribution of we calculate the frequency of impacts into the Earth and the Moon. Globally destructive collisions (~10 ^21 J) of asteroids 1 km or larger strike the Earth once every 0.60 +/-0.1 Myr on average. Regionally destructive collisions with impact energy greater than 4 x 10 ^18 J (~200 m diameter) strike the Earth every 56,000 +/-6000 yr. Collisions in the range of the Tunguska event (4-8 x 10^16 J) occur every 2000-3000 yr. These values represent the average time between randomly spaced impacts; actual impacts could occur more or less closely spaced solely by chance. As a verification of these impact rates, the crater production function of Shoemaker et al. has been updated by combining this new population model with a crater formation model to find that the observed crater production function on both the Earth and Moon agrees with the rate of crater production expected from the current population of NEOs.
Summary
Utilizing the largest available data sets for the observed taxonomic and albedo distributions of the near-Earth object population, we model the bias-corrected population. Diameter-limited fractional abundances of the taxonomic complexes are A-0.2%; C-10%, D-17%, O-0.5%, Q-14%, R-0.1%, S-22%, U-0.4%, V-1%, X-34%. In a diameter-limited sample, ~30% of the NEO population...
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The Lincoln Near-Earth Asteroid Research (LINEAR) Program
January 1, 1998
Journal Article
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Lincoln Laboratory Journal, Vol. 11, No. 1, 1998, pp. 27-40.
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Lincoln Laboratory has been developing electro-optical space-surveillance technology to detect, characterize, and catalog satellites for more than forty years. Recent advances in highly sensitive, large-format charge-coupled devices (CCDs) allow this technology to be applied to detecting and cataloging asteroids, including near-Earth objects (NEOs). When equipped with a new Lincoln Laboratory focal-plane camera and signal processing technology, the 1-m U.S. Air Force ground-based electro-optical deep-space surveillance (GEODSS) telescopes can conduct sensitive large-coverage searches for Earth-crossing and main-belt asteroids. Field measurements indicate that these enhanced telescopes can achieve a limiting magnitude of 22 over a 2-deg2 field of view with less than 100 sec of integration. This sensitivity rivals that of much larger telescopes equipped with commercial cameras. Working two years under U.S. Air Force sponsorship, we have developed technology for asteroid search operations at the Lincoln Laboratory Experimental Test Site near Socorro, New Mexico. By using a new large-format 2560 X 1960-pixel frame-transfer CCD camera, we have discovered over 10,000 asteroids, including 53 NEOs and 4 comets as designated by the Minor Planet Center (MPC). In March 1998, the Lincoln Near-Earth Asteroid Research (LINEAR) program provided over 150,000 observations of asteroids--nearly 90% of the world's asteroid observations that month--to the MPC, which resulted in the discovery of 13 NEOs and 1 comet. The MPC indicates that the LINEAR program outperforms all asteroid search programs operated to date.
Summary
Lincoln Laboratory has been developing electro-optical space-surveillance technology to detect, characterize, and catalog satellites for more than forty years. Recent advances in highly sensitive, large-format charge-coupled devices (CCDs) allow this technology to be applied to detecting and cataloging asteroids, including near-Earth objects (NEOs). When equipped with a new Lincoln Laboratory...
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