This is a photo composite of the encounter of Comet Siding Spring with Mars on October 19, 2014. Separate Hubble Space Telescope images of Mars and the comet have been combined together into a single picture. This is a composite image because a single exposure of the stellar background, Comet Siding Spring, and Mars would be problematic because the objects are all moving with respect to each other and the background stars. Hubble can only track one planetary target at a time. Also, Mars is actually 10,000 times brighter than the comet, and the exposure here has been adjusted so that details on the Red Planet can be seen.
Solar physics: Solar atmosphere is a hotbed of activity
Nature 514, 7523 (2014). doi:10.1038/514406a
Explosions of plasma in the Sun's atmosphere can reach temperatures of nearly 100,000 °C, much hotter than scientists had expected.The finding is one of several about the region between the solar surface and the uppermost edge of the Sun's atmosphere, or corona, revealed by
Astronomy: Hurling comets around a planetary nursery
Nature 514, 7523 (2014). doi:10.1038/514440a
Authors: Aki Roberge
An analysis of hundreds of star-grazing comets in a young planetary system shows that they form two families: a group of old, dried-out comets and a younger group probably related to the break-up of a larger planetary body. See Letter p.462
Two families of exocomets in the β Pictoris system
Nature 514, 7523 (2014). doi:10.1038/nature13849
Authors: F. Kiefer, A. Lecavelier des Etangs, J. Boissier, A. Vidal-Madjar, H. Beust, A.-M. Lagrange, G. Hébrard & R. Ferlet
The young planetary system surrounding the star β Pictoris harbours active minor bodies. These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as happened early in the history of our Solar System. Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies, that is, exocomets. Here we report an analysis of more than 1,000 archival spectra gathered between 2003 and 2011, which provides a sample of about 6,000 variable absorption signatures arising from exocomets transiting the disk of the parent star. Statistical analysis of the observed properties of these exocomets allows us to identify two populations with different physical properties. One family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted (that is, strongly depleted in volatiles) comets trapped in a mean motion resonance with a massive planet. Another family consists of exocomets producing deep absorption lines, which may be related to the recent fragmentation of one or a few parent bodies. Our results show that the evaporating bodies observed for decades in the β Pictoris system are analogous to the comets in our own Solar System.
Characterizing and predicting the magnetic environment leading to solar eruptions
Nature 514, 7523 (2014). doi:10.1038/nature13815
Authors: Tahar Amari, Aurélien Canou & Jean-Jacques Aly
The physical mechanism responsible for coronal mass ejections has been uncertain for many years, in large part because of the difficulty of knowing the three-dimensional magnetic field in the low corona. Two possible models have emerged. In the first, a twisted flux rope moves out of equilibrium or becomes unstable, and the subsequent reconnection then powers the ejection. In the second, a new flux rope forms as a result of the reconnection of the magnetic lines of an arcade (a group of arches of field lines) during the eruption itself. Observational support for both mechanisms has been claimed. Here we report modelling which demonstrates that twisted flux ropes lead to the ejection, in support of the first model. After seeing a coronal mass ejection, we use the observed photospheric magnetic field in that region from four days earlier as a boundary condition to determine the magnetic field configuration. The field evolves slowly before the eruption, such that it can be treated effectively as a static solution. We find that on the fourth day a flux rope forms and grows (increasing its free energy). This solution then becomes the initial condition as we let the model evolve dynamically under conditions driven by photospheric changes (such as flux cancellation). When the magnetic energy stored in the configuration is too high, no equilibrium is possible and the flux rope is ‘squeezed’ upwards. The subsequent reconnection drives a mass ejection.
Sun’s stroke keeps Kepler online
Nature 514, 7523 (2014). http://www.nature.com/doifinder/10.1038/514414a
Author: Mark Zastrow
Space telescope beats mechanical failures to begin a second mission that will trace new celestial targets.