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Monthly Archives: September 2014

Adapted from a European Space Agency press release, September 26, 2014:

ESA’s Rosetta mission will deploy its lander, Philae, to the surface of Comet 67P/Churyumov–Gerasimenko on November 12, 2014. Philae’s landing site, currently known as Site J, is located on the smaller of the comet’s two ‘lobes’, with a backup site on the larger lobe. The sites were selected just six weeks after Rosetta arrived at the comet on August 6, following its 10-year journey through the Solar System.

Philae’s primary landing site


The main focus to date has been to survey 67P/Churyumov–Gerasimenko in order to prepare for the first ever attempt to soft-land on a comet. Site J was chosen unanimously over four other candidate sites as the primary landing site because the majority of terrain within a square kilometre area has slopes of less than 30° relative to the local vertical and because there are relatively few large boulders. The area also receives sufficient daily illumination to recharge Philae and continue surface science operations beyond the initial 64-hour battery-powered phase.

Final confirmation of the primary landing site and its landing scenario will be made on October 14 after a formal review, which will include the results of additional high-resolution analysis of the landing site and its back-up conducted in the meantime. Should the backup site be chosen at this stage, the landing attempt can still take place on November 12.

Links: the ESA press release, including links to further resources on Rosetta and Philae and this short movie showing Philae’s planned descent.

Author Alex Filippenko’s recent talk “Discovering Our Celestial Connections: New data on Exploding Stars, Exoplanets, and Black Holes from UC’s Lick Observatory”, recorded at LinkedIn headquarters, is available to view below or here (approx. 1 hr 27 minutes).

Adapted from a CERN press release, September 18, 2014:

The Alpha Magnetic Spectrometer (AMS) collaboration has recently presented its latest results. These are based on the analysis of 41 billion particles detected with the space-based AMS detector aboard the International Space Station. The results provide new insights into the nature of the mysterious excess of positrons observed in the flux of cosmic rays, which, according to some models, might be evidence of dark matter (see Section 16.4, p. 428). The findings are published in the journal Physical Review Letters.

AMS aboard the International Space Station

Credit: NASA

Cosmic rays are particles commonly present in the Universe, consisting mainly of protons and electrons, but there are also many other kinds of particles, including positrons. Positrons are the antimatter counterparts of electrons, with the same mass but opposite charge. The presence of some positrons in space can be explained from the collisions of cosmic rays, but this phenomenon would only produce a tiny portion of antimatter in the overall cosmic ray spectrum. Since antimatter is extremely rare in the universe, any significant excess of antimatter particles recorded in the flux of energetic cosmic rays indicates the existence of a new source of positrons. Very dense stars, such as pulsars, are potential candidates.

The AMS experiment is able to map the flux of cosmic rays with unprecedented precision and in the results published last week, the collaboration presents new data at energies never before recorded. The AMS collaboration has analyzed 41 billion primary cosmic ray events among which 10 million have been identified as electrons and positrons. The distribution of these events in the energy range of 0.5 to 500 GeV shows a well-measured increase of positrons from 8 GeV with no preferred incoming direction in space.  The energy at which the positron fraction ceases to increase has been measured to be 275±32 GeV.

This rate of decrease after the “cut-off energy” is very important to physicists as it could be an indicator that the excess of positrons is the signature of dark matter particles annihilating into pairs of electrons and positrons. Although the current measurements could be explained by objects such as pulsars, they are also tantalizingly consistent with dark matter particles with mass of the order of 1 TeV. Different models on the nature of dark matter predict different behaviour of the positron excess above the positron fraction expected from ordinary cosmic ray collisions. Therefore, results at higher energies will be of crucial importance in the near future to evaluate if the signal is from dark matter or from a cosmic source.

Links: Full CERN press release.

In Physics Today, July 2014, Clive Speake and Terry Quinn describe how measurements of G, Newton’s constant of gravitation, have not improved in accuracy as much as might have been expected since Cavendish used the idea to ‘weigh the Earth’ over 200 years ago.

Link: the Physics Today article.

The International System of Units (SI, from the French Système International d’Unités) is about to be radically redefined.  The seven familiar base units will be replaced by constants of nature that are, on the whole less familiar to the general public. No more will mass be defined by a kilogram in a vault in Paris.


Credit: NIST

David Newell of the National Institute of Standards and Technology (NIST) explains the proposed changes in this article in Physics Today.


A new interactive posting from National Geographic tells the story of the twin Voyager spacecraft, sent to explore the outer planets and the edge of our Solar System.

Links: The Voyager feature on the National Geographic website; a short movie.