ESA Science & Technology - Publication Archive
Published online 11 February 2016
Context. Dust jets (i.e., fuzzy collimated streams of cometary material arising from the nucleus) have been observed in situ on all comets since the Giotto mission flew by comet 1P/Halley in 1986, and yet their formation mechanism remains unknown. Several solutions have been proposed involving either specific properties of the active areas or the local topography to create and focus the gas and dust flows. While the nucleus morphology seems to be responsible for the larger features, high resolution imagery has shown that broad streams are composed of many smaller jets (a few meters wide) that connect directly to the nucleus surface.
Aims. We monitored these jets at high resolution and over several months to understand what the physical processes are that drive their formation and how this affects the surface.
Methods. Using many images of the same areas with different viewing angles, we performed a 3-dimensional reconstruction of collimated jets and linked them precisely to their sources on the nucleus.
Results. We show here observational evidence that the northern hemisphere jets of comet 67P/Churyumov-Gerasimenko arise from areas with sharp topographic changes and describe the physical processes involved. We propose a model in which active cliffs are the main source of jet-like features and therefore of the regions eroding the fastest on comets. We suggest that this is a common mechanism taking place on all comets.
Aims. We investigate whether quasars can be used to independently verify the parallax zero-point in early data reductions. This is not trivially possible as the observation interval is too short to disentangle parallax and proper motion for the quasar subset.
Methods. We repeat TGAS simulations but additionally include simulated Gaia observations of quasars from ground-based surveys. All observations are simulated with basic angle variations. To obtain a full astrometric solution for the quasars in TGAS we explore the use of prior information for their proper motions.
Results. It is possible to determine the parallax zero-point for the quasars with a few μas uncertainty, and it agrees to a similar precision with the zero-point for the Tycho-2 stars. The proposed strategy is robust even for quasars exhibiting significant spurious proper motion due to a variable source structure, or when the quasar subset is contaminated with stars misidentified as quasars.
Conclusions. Using prior information about quasar proper motions we could provide an independent verification of the parallax zero-point in early solutions based on less than one year of Gaia data.
This document presents the proposed activities to be initiated in 2016 in the Exploration Technology Programme (ETP, funded by MREP-2). It also summaries the current state of MREP-2 Programme's technology programme as a whole, providing an overview of all running and proposed Technology Development Activities supporting the implementation of the Programme.
This document is provided for information only and is subject to future updates.
Version issued December 2015: Download document (pdf, 2.9 MB)
This media kit contains background information of use to journalists and reporters covering the LISA Pathfinder mission.
Why LISA Pathfinder?
Mission at a glance
A challenging build
What LISA Pathfinder is doing and how
Paving the way for gravitational-wave observatories in space
100 years of general relativity
LISA Pathfinder in the context of great physics experiments
Updated in June 2016 to reflect the detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and to account for operational milestones of LISA Pathfinder.
Aims. We derive for the first time the size-frequency distribution of boulders on a comet, 67P/Churyumov-Gerasimenko (67P), computed from the images taken by the Rosetta/OSIRIS imaging system. We highlight the possible physical processes that lead to these boulder size distributions.
Methods. We used images acquired by the OSIRIS Narrow Angle Camera, NAC, on 5 and 6 August 2014. The scale of these images (2.44−2.03 m/px) is such that boulders ≥7 m can be identified and manually extracted from the datasets with the software ArcGIS. We derived both global and localized size-frequency distributions. The three-pixel sampling detection, coupled with the favorable shadowing of the surface (observation phase angle ranging from 48° to 53°), enables unequivocally detecting boulders scattered all over the illuminated side of 67P.
Results. We identify 3546 boulders larger than 7 m on the imaged surface (36.4 km²), with a global number density of nearly 100/km² and a cumulative size-frequency distribution represented by a power-law with index of -3.6 +0.2/-0.3. The two lobes of 67P appear to have slightly different distributions, with an index of -3.5 +0.2/-0.3 for the main lobe (body) and -4.0 +0.3/-0.2 for the small lobe (head). The steeper distribution of the small lobe might be due to a more pervasive fracturing. The difference of the distribution for the connecting region (neck) is much more significant, with an index value of -2.2 +0.2/-0.2. We propose that the boulder field located in the neck area is the result of blocks falling from the contiguous Hathor cliff.
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Context. Here we describe a new model of the dust streams of comet 67P/Churyumov-Gerasimenko that has been developed using the Interplanetary Meteoroid Environment for Exploration (IMEX). This is a new universal model for recently created cometary meteoroid streams in the inner solar system.
Aims. The model can be used to investigate characteristics of cometary trails: here we describe the model and apply it to the trail of comet 67P/Churyumov-Gerasimenko to develop our understanding of the trail and assess the reliability of the model.
Methods. Our IMEX model provides trajectories for a large number of dust particles released from ~400 short-period comets. We use this to generate optical depth profiles of the dust trail of comet 67P/Churyumov-Gerasimenko and compare these to Spitzer observations of the trail of this comet from 2004 and 2006.
Results. We find that our model can match the observed trails if we use very low ejection velocities, a differential size distribution index of α ≈ -3.7, and a dust production rate of 300–500 kg s-1 at perihelion. The trail is dominated by mm-sized particles and can contain a large proportion of dust produced before the most recent apparition. We demonstrate the strength of IMEX in providing time-resolved histories of meteoroid streams. We find that the passage of Mars through the stream in 2062 creates visible gaps. This indicates the utility of this model in providing insight into the dynamical evolution of streams and trails, as well as impact hazard assessment for spacecraft on interplanetary missions.