* Astronomy

Title: Small near-Earth asteroids in the Palomar Transient Factory survey: A real-time streak-detection system
Author: Adam Waszczak, Thomas A. Prince, Russ Laher, Frank Masci, Brian Bue, Umaa Rebbapragada, Tom Barlow, Jason Surace, George Helou, Shrinivas Kulkarni

Near-Earth asteroids (NEAs) in the 1-100 meter size range are estimated to be ~1,000 times more numerous than the ~15,000 currently-catalogued NEAs, most of which are in the 0.5-10 kilometer size range. Impacts from 10-100 meter size NEAs are not statistically life-threatening but may cause significant regional damage, while 1-10 meter size NEAs with low velocities relative to Earth are compelling targets for space missions. We describe the implementation and initial results of a real-time NEA-discovery system specialised for the detection of small, high angular rate (visually-streaked) NEAs in Palomar Transient Factory (PTF) images. PTF is a 1.2-m aperture, 7.3-deg² field-of-view optical survey designed primarily for the discovery of extragalactic transients (e.g., supernovae) in 60-second exposures reaching ~20.5 visual magnitude. Our real-time NEA discovery pipeline uses a machine-learned classifier to filter a large number of false-positive streak detections, permitting a human scanner to efficiently and remotely identify real asteroid streaks during the night. Upon recognition of a streaked NEA detection (typically within an hour of the discovery exposure), the scanner triggers follow-up with the same telescope and posts the observations to the Minor Planet Center for worldwide confirmation. We describe our ten initial confirmed discoveries, all small NEAs that passed 0.3-15 lunar distances from Earth. Lastly, we derive useful scaling laws for comparing streaked-NEA-detection capabilities of different surveys as a function of their hardware and survey-pattern characteristics. This work most directly informs estimates of the streak-detection capabilities of the Zwicky Transient Facility (ZTF, planned to succeed PTF in 2017), which will apply PTF's current resolution and sensitivity over a 47-deg² field-of-view.

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Title: The Need for Speed in Near-Earth Asteroid Characterisation
Author: J. L. Galache, C. L. Beeson, K. K. McLeod, M. Elvis

We have used Minor Planet Centre data and tools to explore the discovery circumstances and properties of the currently known population of over 10,000 NEAs, and to quantify the challenges for follow-up from ground-based telescopes. The increasing rate of discovery has grown to ~1,000/year as surveys have become more sensitive, by 1mag every ~7.5 years. However, discoveries of large (H =< 22) NEAs have remained stable at ~365/year over the past decade, at which rate the 2005 Congressional mandate to find 90% of 140m NEAs will not be met before 2030. Meanwhile, characterisation is falling farther behind: Fewer than 10% of NEAs are well characterised in terms of size, rotation periods, and spectra, and at current rates of follow-up it will take about a century to determine them even for the known population. Over 60% of NEAs have an orbital uncertainty parameter, U >= 4, making reacquisition more than a year following discovery difficult; for H > 22 this fraction is over 90%. We argue that rapid follow-up will be essential to characterise newly-discovered NEAs. Most new NEAs are found within 0.5mag of peak brightness and fade quickly, typically by 0.5/3.5/5mag after 1/4/6 weeks. About 80% have synodic periods of <3 years that bring them close to Earth several times a decade. However, follow-up observations on subsequent apparitions will be near impossible for the bulk of new discoveries, as these will be H > 22 NEAs that tend to return 100 times fainter. We show that for characterisation to keep pace with discovery would require: Visible spectroscopy within days with a dedicated >2m telescope; long-arc astrometry, used also for phase curves, with a >4m telescope; and fast-cadence (<min) lightcurves obtained within days with a >= 4m telescope. For the already-known large (H =< 22) NEAs, subsequent-apparition spectroscopy, astrometry, and photometry could be done with 1-2m telescopes.

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Title: Assessing the physical nature of near-Earth asteroids through their dynamical histories
Author: Julio A. Fernández, Andrea Sosa, Tabaré Gallardo, Jorge N. Gutiérrez

We analyse a sample of 139 near-Earth asteroids (NEAs), defined as those that reach perihelion distances q<1.3 au, and that also fulfill the conditions of approaching or crossing Jupiter's orbit (aphelion distances Q>4.8 au), having Tisserand parameters 2<T<3 and orbital periods P<20 yr. In order to compare the dynamics, we also analyse a sample of 42 Jupiter family comets (JFCs) in near-Earth orbits, i.e. with q<1.3 au. We integrated the orbits of these two samples for 10^4 yr in the past and in the future. We find that the great majority of the NEAs move on stable orbits during the considered period, and that a large proportion of them are in one of the main mean motion resonances with Jupiter, in particular the 2:1. We find a strong coupling between the perihelion distance and the inclination in the motion of most NEAs, due to Kozai mechanism, that generates many sungrazers. On the other hand, most JFCs are found to move on very unstable orbits, showing large variations in their perihelion distances in the last few 10^2 - 10^3 yr, which suggests a rather recent capture in their current near-Earth orbits. Even though most NEAs of our sample move in typical 'asteroidal' orbits, we detect a small group of NEAs whose orbits are highly unstable, resembling those of the JFCs. These are: 1997 SE5, 2000 DN1, 2001 XQ, 2002 GJ8, 2002 RN38, 2003 CC11, 2003 WY25, 2009 CR2, and 2011 OL51. These objects might be inactive comets, and indeed 2003 WY25 has been associated with comet Blanpain, and it is now designed as comet 289P/Blanpain. Under the assumption that these objects are inactive comets, we can set an upper limit of ~0.17 to the fraction of NEAs with Q>4.8 au of cometary origin, but it could be even lower if the NEAs in unstable orbits listed before turn out to be bona fide asteroids from the main belt.

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Title: Precovery of near-Earth asteroids by a citizen-science project of the Spanish Virtual Observatory
Authors: E. Solano, C. Rodrigo, R. Pulido, B. Carry

This article describes a citizen-science project conducted by the Spanish Virtual Observatory (SVO) to improve the orbits of near-Earth asteroids (NEAs) using data from astronomical archives. The list of NEAs maintained at the Minor Planet Centre (MPC) is checked daily to identify new objects or changes in the orbital parameters of already catalogued objects. Using NEODyS we compute the position and magnitude of these objects at the observing epochs of the 938 046 images comprising the Eighth Data Release of the Sloan Digitised Sky Survey (SDSS). If the object lies within the image boundaries and the magnitude is brighter than the limiting magnitude, then the associated image is visually inspected by the project's collaborators (the citizens) to confirm or discard the presence of the NEA. If confirmed, accurate coordinates and, sometimes, magnitudes are submitted to the MPC. Using this methodology, 3,226 registered users have made during the first fifteen months of the project more than 167,000 measurements which have improved the orbital elements of 551 NEAs (6% of the total number of this type of asteroids). Even more remarkable is the fact that these results have been obtained at zero cost to telescope time as NEAs were serendipitously observed while the survey was being carried out. This demonstrates the enormous scientific potential hidden in astronomical archives.
The great reception of the project as well as the results obtained makes it a valuable and reliable tool for improving the orbital parameters of near-Earth asteroids.

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