Cometary Science Newsletter

Issue
17
Month
August 2016
Editor
Michael S. P. Kelley (msk@astro.umd.edu)

Conference Announcements

Announcements for cometary conferences or workshops. Limited to 2000 characters.

COMETS 2016 - New abstract deadline 22 July

Comets: A new vision after Rosetta/Philae and Rosetta SWT 2016 November 14 to 18, 2016 Toulouse, France Important notice: Abstract deadline is postponed to: July 22, 2016 Early registration deadline: October 1, 2016 For more information about the conference, visit http://comets2016toulouse.com If you have questions or need assistance during the registration process, contact Insight Outside at contact@comets2016toulouse.com with reference "Comets 2016" in the subject.

Comet Wirtanen - Observers' Workshop at DPS

The Comet Group at the University of Maryland is organizing a campaign to coordinate observations of Comet 46P/Wirtanen during its 2018 apparition. To kick off this campaign, we are holding a short workshop at the DPS meeting in Pasadena to raise awareness of this opportunity, and to assemble participants who may be interested in collaborations and coordinated studies, as well as public outreach and educational opportunities. The meeting will be held during the lunch break on Monday, October 17, from 12:30-2:00 in Room C102 in the Pasadena Convention Center.

Comet Wirtanen is a small, hyperactive comet, which in many respects is a near-twin of comet 103P/Hartley 2. It will pass within 0.077 AU of the Earth on December 16, 2018, just four days after reaching perihelion. This close approach is the highlight of an already excellent apparition, which offers prime conditions for high-resolution imaging, favorable circumstances for spectroscopic observations, and visibility allowing coverage for many hours per night for over a year around perihelion.

As the original target of the Rosetta mission and the target of the Comet Hopper Discovery Phase A mission proposal, Wirtanen is clearly a potential future spacecraft target. The upcoming apparition presents us with the opportunity to study this comet in a long-duration, systematic manner encompassing a wide range of techniques and wavelength regimes, to further characterize its physical and chemical properties, to investigate its long-term evolution through comparison to past data sets, and to perform comparative studies with Hartley 2 and other comets. This bright target also represents an opportunity to mobilize the amateur observing community to execute otherwise difficult-to-achieve science programs.

For more information, contact Tony Farnham (farnham@astro.umd.edu).

Refereed Articles

Abstracts of articles in press or recently published. Limited to 3000 characters.

Hubble Space Telescope Observations of Active Asteroid 324P/La Sagra

  • Jewitt, D. 1,2
  • Agarwal, J. 3
  • Weaver, H. 4
  • Mutchler, M. 5
  • Li, J. 1
  • Larson, S. 6
  1. Department of Earth, Planetary and Space Sciences, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095-1567
  2. Department of Physics and Astronomy, UCLA, 430 Portal Plaza, Los Angeles, CA90095-1547
  3. Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 G\"ottingen, Germany
  4. The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723
  5. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
  6. Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd. Tucson AZ 85721-0092

Hubble Space Telescope observations of active asteroid 324P/La Sagra near perihelion show continued mass loss consistent with the sublimation of near-surface ice. Isophotes of the coma measured from a vantage point below the orbital plane are best matched by steady emission of particles having a nominal size a ~100 μm. The inferred rate of mass loss, dMd/dt ~0.2 kg s-1, can be supplied by sublimation of water ice in thermal equilibrium with sunlight from an area as small as 930 m2, corresponding to about 0.2% of the nucleus surface. Observations taken from a vantage point only 0.6° from the orbital plane of 324P set a limit to the velocity of ejection of dust in the direction perpendicular to the plane, V⟂ < 1 m s-1. Short-term photometric variations of the near-nucleus region, if related to rotation of the underlying nucleus, rule out periods ≤ 3.8 hr and suggest that rotation probably does not play a central role in driving the observed mass loss. We estimate that, in the previous orbit, 324P lost about 4×107 kg in dust particles, corresponding to 6×10-5 of the mass of a 550 m spherical nucleus of assumed density ρ = 1000 kg m-3. If continued, mass loss at this rate would limit the lifetime of 324P to ~1.6×104 orbits (about 105 yr). To survive for the 100 Myr to 400 Myr timescales corresponding, respectively, to dynamical and collisional stability requires a duty cycle 2×10-4 ≤ fd ≤ 8×10-4. Unless its time in orbit is over-estimated by many orders of magnitude, 324P is revealed as a briefly-active member of a vast population of otherwise dormant ice-containing asteroids.

The Astronomical Journal (In press)

NASA ADS: 2016arXiv160608522J

The Nature and Frequency of the Gas Outbursts in Comet 67P/Churyumov-Gerasimenko observed by the Alice Far-ultraviolet Spectrograph on Rosetta

  • Paul D. Feldman 1
  • Michael F. A'Hearn 2
  • Lori M. Feaga 2
  • Jean-Loup Bertaux 3
  • John Noonan 4
  • Joel Wm. Parker 4
  • Eric Schindhelm 4
  • Andrew J. Steffl 4
  • S. Alan Stern 4
  • Harold A. Weaver 5
  1. Johns Hopkins University, USA
  2. University of Maryland, USA
  3. LATMOS, France
  4. Southwest Research Institute, USA
  5. Johns Hopkins Applied Physics Laboratory, USA

Alice is a far-ultraviolet imaging spectrograph onboard Rosetta that, amongst multiple objectives, is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/Churyumov-Gerasimenko. The initial observations, made following orbit insertion in August 2014, showed emissions of atomic hydrogen and oxygen spatially localized close to the nucleus and attributed to photoelectron impact dissociation of H2O vapor. Weaker emissions from atomic carbon were subsequently detected and also attributed to electron impact dissociation, of CO2, the relative H I and C I line intensities reflecting the variation of CO2 to H2O column abundance along the line-of-sight through the coma. Beginning in mid-April 2015, Alice sporadically observed a number of outbursts above the sunward limb characterized by sudden increases in the atomic emissions, particularly the semi-forbidden O I 1356 multiplet, over a period of 10-30 minutes, without a corresponding enhancement in long wavelength solar reflected light characteristic of dust production. A large increase in the brightness ratio O I 1356/O I 1304 suggests O2 as the principal source of the additional gas. These outbursts do not correlate with any of the visible images of outbursts taken with either OSIRIS or the navigation camera. Beginning in June 2015 the nature of the Alice spectrum changed considerably with CO Fourth Positive band emission observed continuously, varying with pointing but otherwise fairly constant in time. However, CO does not appear to be a major driver of any of the observed outbursts.

The Astrophysical Journal Letters (Published)

DOI: 10.3847/2041-8205/825/1/L8 NASA ADS: 2016ApJ...825L...8F arXiv: 1606.05249

The end states of long-period comets and the origin of Halley-type comets

  • Julio A. Fernández 1
  • Tabaré Gallardo 1
  • Juan D. Young 1
  1. Departamento de Astronomía, Facultad de Ciencias, Universidad de la República, Iguá 4225, 11400 Montevideo, Uruguay

We analyze a sample of 73 old long-period comets (LPCs) (orbital periods 200 < P < 1000 yr) with perihelion distances q < 2.5 au, discovered in the period 1850-2014. We cloned the observed comets and also added fictitious LPCs with perihelia in the Jupiter's zone. We consider both a purely dynamical evolution and a physico-dynamical one with different physical lifetimes. We can fit the computed energy distribution of comets with q < 1.3 au to the observed one only within the energy range 0.01 < x < 0.04 au-1 (or periods 125 < P < 1000 yr), where the "energy" is taken as the inverse of the semimajor axis a, namely x ≡ 1/a. The best results are obtained for physical lifetimes of about 200-300 revolutions (for a comet with a standard q = 1 au). We find that neither a purely dynamical evolution, nor a physico-dynamical one can reproduce the long tail of larger binding energies (x >~ 0.04 au-1) that correspond to most Halley-type comets (HTCs) and Jupiter-family comets. We conclude that most HTCs are not the end states of the evolution of LPCs, but come from a different source, a flattened one that we identify with the Centaurs that are scattered to the inner planetary region from the trans-Neptunian belt. These results also show that the boundary between LPCs and HTCs should be located at an energy x ~ 0.04 au-1 (P ~ 125 yr), rather than the conventional classical boundary at P = 200 yr.

Monthly Notices of the Royal Astronomical Society (Published)

DOI: 10.1093/mnras/stw1532 arXiv: 1606.05603