Setting the scene: what did we know before Rosetta? Meech, K. J.
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
07/2017, Letnik:
375, Številka:
2097
Journal Article
Recenzirano
Odprti dostop
This paper provides an overview of our state of knowledge about comets prior to the Rosetta mission encounter. Starting with the historical perspective, this paper discusses the development of comet ...science up to the modern era of space exploration. The extent to which comets are tracers of solar system formation processes or preserve pristine interstellar material has been investigated for over four decades. There is increasing evidence that in contrast with the distinct dynamical comet reservoirs we see today, comet formation regions strongly overlapped in the protoplanetary disc and there was significant migration of material in the disc during the epoch of comet formation. Comet nuclei are now known to be very low-density highly porous bodies, with very low thermal inertia, and have a range of sizes which exhibit a deficiency of very small bodies. The low thermal inertia suggests that comets may preserve pristine materials close to the surface, and that this might be accessible to sample return missions.
This article is part of the themed issue ‘Cometary science after Rosetta’.
Deep Impact: Excavating Comet Tempel 1 A'Hearn, M. F; Belton, M. J. S; Delamere, W. A ...
Science (American Association for the Advancement of Science),
10/2005, Letnik:
310, Številka:
5746
Journal Article
Recenzirano
Deep Impact collided with comet Tempel 1, excavating a crater controlled by gravity. The comet's outer layer is composed of 1- to 100-micrometer fine particles with negligible strength (<65 pascals). ...Local gravitational field and average nucleus density (600 kilograms per cubic meter) are estimated from ejecta fallback. Initial ejecta were hot (>1000 kelvins). A large increase in organic material occurred during and after the event, with smaller changes in carbon dioxide relative to water. On approach, the spacecraft observed frequent natural outbursts, a mean radius of 3.0 ± 0.1 kilometers, smooth and rough terrain, scarps, and impact craters. A thermal map indicates a surface in equilibrium with sunlight.
Spitzer Space Telescope imaging spectrometer observations of comet 9P/Tempel 1 during the Deep Impact encounter returned detailed, highly structured, 5- to 35-micrometer spectra of the ejecta. ...Emission signatures due to amorphous and crystalline silicates, amorphous carbon, carbonates, phyllosilicates, polycyclic aromatic hydrocarbons, water gas and ice, and sulfides were found. Good agreement is seen between the ejecta spectra and the material emitted from comet C/1995 O1 (Hale-Bopp) and the circumstellar material around the young stellar object HD100546. The atomic abundance of the observed material is consistent with solar and C1 chondritic abundances, and the dust-to-gas ratio was determined to be greater than or equal to 1.3. The presence of the observed mix of materials requires efficient methods of annealing amorphous silicates and mixing of high- and low-temperature phases over large distances in the early protosolar nebula.
•We show results from a Spitzer mid-IR survey of Jupiter-family (JF) comets.•We present 89 new radii and 57 new beaming parameters for the nuclei.•Mean beaming parameter is 1.03±0.11, so ensemble ...thermal inertia is low.•Our independent cumulative size distribution is similar to earlier work.•There are likely low-perihelion, large JF nuclei still undiscovered.
We present results from SEPPCoN, an on-going Survey of the Ensemble Physical Properties of Cometary Nuclei. In this report we discuss mid-infrared measurements of the thermal emission from 89 nuclei of Jupiter-family comets (JFCs). All data were obtained in 2006 and 2007 using imaging capabilities of the Spitzer Space Telescope. The comets were typically 4–5AU from the Sun when observed and most showed only a point-source with little or no extended emission from dust. For those comets showing dust, we used image processing to photometrically extract the nuclei. For all 89 comets, we present new effective radii, and for 57 comets we present beaming parameters. Thus our survey provides the largest compilation of radiometrically-derived physical properties of nuclei to date. We have six main conclusions: (a) The average beaming parameter of the JFC population is 1.03±0.11, consistent with unity; coupled with the large distance of the nuclei from the Sun, this indicates that most nuclei have Tempel 1-like thermal inertia. Only two of the 57 nuclei had outlying values (in a statistical sense) of infrared beaming. (b) The known JFC population is not complete even at 3km radius, and even for comets that approach to ∼2AU from the Sun and so ought to be more discoverable. Several recently-discovered comets in our survey have small perihelia and large (above ∼2km) radii. (c) With our radii, we derive an independent estimate of the JFC nuclear cumulative size distribution (CSD), and we find that it has a power-law slope of around −1.9, with the exact value depending on the bounds in radius. (d) This power-law is close to that derived by others from visible-wavelength observations that assume a fixed geometric albedo, suggesting that there is no strong dependence of geometric albedo with radius. (e) The observed CSD shows a hint of structure with an excess of comets with radii 3–6km. (f) Our CSD is consistent with the idea that the intrinsic size distribution of the JFC population is not a simple power-law and lacks many sub-kilometer objects.
The Deep Ecliptic Survey (DES)-a search optimized for the discovery of Kuiper belt objects (KBOs) with the Blanco and Mayall 4 m telescopes at the Cerro Tololo Inter-American Observatory and Kitt ...Peak National Observatory-has covered 550 deg2 from its inception in 1998 through the end of 2003. This survey has a mean 50% sensitivity at VR magnitude 22.5. We report here the discoveries of 320 designated KBOs and Centaurs for the period 2000 March through 2003 December and describe improvements to our discovery and recovery procedures. Our data and the data products needed to reproduce our analyses in this paper are available through the NOAO survey database. Here we present a dynamical classification scheme, based on the behavior of orbital integrations over 10 Myr. The dynamical classes, in order of testing, are 'Resonant,' 'Centaur,' 'Scattered-Near,' 'Scattered-Extended,' and 'Classical.' (These terms are capitalized when referring to our rigorous definitions.) Of the 382 total designated KBOs discovered by the DES, a subset of 196 objects have sufficiently accurate orbits for dynamical classification. Summary information is given for an additional 240 undesignated objects also discovered by the DES from its inception through the end of 2003. The number of classified DES objects (uncorrected for observational bias) are Classical, 96; Resonant, 54; Scattered-Near, 24; Scattered-Extended, 9; and Centaur, 13. We use subsets of the DES objects (which can have observational biases removed) and larger samples to perform dynamical analyses on the Kuiper belt. The first of these is a determination of the Kuiper belt plane (KBP), for which the Classical objects with inclinations less than 5° from the mean orbit pole yield a pole at R.A. = 27392 ± 062 and decl. = 6670 ± 020 (J2000), consistent with the invariable plane of the solar system. A general method for removing observational biases from the DES data set is presented and used to find a provisional magnitude distribution and the distribution of orbital inclinations relative to the KBP. A power-law model fit to the cumulative magnitude distribution of all KBOs discovered by the DES in the VR filter yields an index of 0.86 ± 0.10 (with the efficiency parameters for the DES fitted simultaneously with the population power law). With the DES sensitivity parameters fixed, we derive power-law indices of 0.74 ± 0.0,0.52 ± 0.08, and 0.74 ± 0.15, respectively, for the Classical, Resonant, and Scattered classes. Plans for calibration of the DES detection efficiency function and DES magnitudes are discussed. The inclination distribution confirms the presence of 'hot' and 'cold' populations; when the geometric sin i factor is removed from the inclination distribution function, the cold population shows a concentrated 'core' with a full width at half-maximum of approximately 46, while the hot population appears as a 'halo,' extending beyond 30°. The inclination distribution is used to infer the KBO distribution in the sky, as a function of latitude relative to the KBP. This inferred latitude distribution is reasonably consistent with the latitude distribution derived from direct observation, but the agreement is not perfect. We find no clear boundary between the Classical and Scattered classes either in their orbital inclinations with respect to the KBP or in their power-law indices in their respective magnitude distributions. This leaves open the possibility that common processes have shaped the distribution of orbital parameters for the two classes.
This open-label phase III study assessed the addition of Toll-like receptor 9-activating oligodeoxynucleotide PF-3512676 to gemcitabine/cisplatin chemotherapy in patients with non-small-cell lung ...cancer (NSCLC).
Chemotherapy-naive patients with stage IIIB or IV NSCLC were randomized (1:1) to receive six or fewer 3-week cycles of i.v. gemcitabine (1250 mg/m2 on days 1 and 8) and cisplatin alone (75 mg/m2 on day 1, control arm) or combined with s.c. PF-3512676 0.2 mg/kg on days 8 and 15 of each chemotherapy cycle and weekly thereafter until progression or unacceptable toxicity (experimental arm). No crossover was planned. The primary end point was overall survival (OS).
A total of 839 patients were randomized. Baseline demographics were well balanced. Median OS (11.0 versus 10.7 months; P = 0.98) and median progression-free survival (PFS) (both 5.1 months) were similar between groups. Grade ≥3 hematologic adverse events (AEs), injection-site reactions, and influenza-like symptoms were more frequently reported among patients receiving PF-3512676. At the first-interim analysis, the Data Safety Monitoring Committee recommended study discontinuation. Administration of PF-3512676 was halted based on efficacy futility and increased grade ≥3 AEs (experimental arm).
Addition of PF-3512676 to gemcitabine/cisplatin chemotherapy did not improve OS or PFS but did increase toxicity.
(ProQuest: Formulae and/or non-USASCII text omitted)We report production rates for H sub(2) O and eight trace molecules (CO, C sub(2) H sub(6), CH sub(4), CH sub(3) OH, NH sub(3), H sub(2) CO, HCN, C ...sub(2) H sub(2)) in the dynamically new, Sun-grazing Comet C/2012 S1 (ISON), using high-resolution spectroscopy at Keck II and the NASA IRTF on 10 pre-perihelion dates encompassing heliocentric distances R sub(h)= 1.21-0.34 AU. Measured water production rates spanned two orders of magnitude, consistent with a long-term heliocentric power law Q... .Abundance ratios for CO, C sub(2) H sub(6), and CH sub(4) with respect to H sub(2) O remained constant with R sub(h) and below their corresponding mean values measured among a dominant sample of Oort Cloud comets. CH sub(3) OH was also depleted for R sub(h)> 0.5 AU, but was closer to its mean value for R sub(h)< or = 0.5 AU. The remaining four molecules exhibited higher abundance ratios within 0.5 AU: for R sub(h)> 0.8 AU, NH sub(3) and C sub(2) H sub(2) were consistent with their mean values while H sub(2) CO and HCN were depleted. For R sub(h)< 0.5 AU, all four were enriched, with NH sub(3), H sub(2) CO, and HCN increasing most. Spatial profiles of gas emission in ISON consistently peaked sunward of the dust continuum, which was asymmetric antisunward and remained singly peaked for all observations. NH sub(3) within 0.5 AU showed a broad spatial distribution, possibly indicating its release in the coma provided that optical depth effects were unimportant. The column abundance ratio NH sub(2)/H sub(2) O at 0.83 AU was close to the "typical" NH/OH from optical wavelengths, but was higher within 0.5 AU. Establishing its production rate and testing its parentage (e.g., NH sub(3)) require modeling of coma outflow.
We report the direct detection of solid water ice deposits exposed on the surface of comet 9P/Tempel 1, as observed by the Deep Impact mission. Three anomalously colored areas are shown to include ...water ice on the basis of their near-infrared spectra, which include diagnostic water ice absorptions at wavelengths of 1.5 and 2.0 micrometers. These absorptions are well modeled as a mixture of nearby non-ice regions and 3 to 6% water ice particles 10 to 50 micrometers in diameter. These particle sizes are larger than those ejected during the impact experiment, which suggests that the surface deposits are loose aggregates. The total area of exposed water ice is substantially less than that required to support the observed ambient outgassing from the comet, which likely has additional source regions below the surface.
We present radio, optical/NIR, and X-ray observations of the afterglow of the short-duration Swift and Konus-Wind GRB 130603B, and uncover a break in the radio and optical bands at approximately 0.5 ...day after the burst, best explained as a jet break with an inferred jet opening angle of approximately 4degrees-8degrees. GRB 130603B is only the third short GRB with a radio afterglow detection to date, and represents the first time that a jet break has been evident in the radio band. We model the temporal evolution of the spectral energy distribution to determine the burst explosion properties and find an isotropic-equivalent kinetic energy of approximately (0.6-1.7)x 10 super(51) erg and a circumburst density of approximately 5 x 10 super(-3)-30cm super(-3). From the inferred opening angle of GRB 130603B, we calculate beaming-corrected energies of E sub( gamma ) approximately (0.5-2) x 10 super(49) erg and E sub(K) approximately (0.1-1.6) x 10 super(49) erg. Along with previous measurements and lower limits we find a median opening angle of approximately 10degrees. Using the all-sky observed rate of 10 Gpc super(-3) yr super(-1), this implies a true short GRB rate of approximately 20 yr super(-1) within 200 Mpc, the Advanced LIGO/VIRGO sensitivity range for neutron star binary mergers. Finally, we uncover evidence for significant excess emission in the X-ray afterglow of GRB 130603B at > ~ 1 day and conclude that the additional energy component could be due to fail-back accretion or spin-down energy from a magnetar formed following the merger.
•Slope and albedo distributions of Themis, Beagle and Veritas families were compared.•Space weathering increases slopes and decreases albedos of C-complex asteroids.•Size related spectral and albedo ...trends appear to result from regolith variations.•The Beagle parent body may have been a mixture of ice and aqueously altered minerals.
We present visible spectroscopic and albedo data of the 2.3Gyr old Themis family and the <10Myr old Beagle sub-family. The slope and albedo variations between these two families indicate C-complex asteroids become redder and darker in response to space weathering. Our observations of Themis family members confirm previously observed trends where phyllosilicate absorption features are less common among small diameter objects. Similar trends in the albedos of large (>15km) and small (⩽15km) Themis members suggest these phyllosilicate feature and albedo trends result from regolith variations as a function of diameter. Observations of the Beagle asteroids show a small, but notable fraction of members with phyllosilicate features. The presence of phyllosilicates and the dynamical association of the main-belt comet 133P/Elst-Pizarro with the Beagle family imply the Beagle parent body was a heterogenous mixture of ice and aqueously altered minerals.
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