Bakersfield Night Sky – August 16, 2014
By Nick Strobel
The Rosetta mission arrived successfully at Comet 67P/Churyumov-Gerasinmenko on August 6th. It is now executing a series of triangular-shaped orbits to close in on the comet from its current 100-kilometer distance to just 10 kilometers. It will map the comet ice-dust nucleus in detail and analyze its composition with a whole suite of instruments including a mass spectrometer and microscopic imager of the dust particles in the comet's atmosphere (called a "coma"). The detailed mapping is needed to find a location for the Philae Lander to land on the comet nucleus in November.
The comet nucleus looks like two round pieces stuck together with a narrow bridge of ice-rock material, so it is technically known as a "contact binary". The smaller round piece has a large crater on its end that makes it look like a Klingon bird of prey spacecraft without the wings, according to Mark McCaughrean, the ESA Senior Science Advisor and Star Trek fan. The nucleus is about 2.2 by 2.5 miles in dimension and is darker than charcoal, so the images you see on the web have been light stretched in order to see the details. There is a mix of craters and smooth areas on the surface like that found on Comet Tempel 1. The smooth areas are dusty---probably the result of material deposited from outgassing and several of the craters, at least, are vents from which the ice-dust material comes. Other craters are the result of impacts. The Rosetta mission website is at www.esa.int/Our_Activities/Space_Science/Rosetta and mission updates are first posted on the Rosetta blog at http://blogs.esa.int/rosetta/ .
In other comet news, NASA and ESA have completed maneuvering the spacecraft orbiting Mars into positions that will keep them safe from the very close flyby of Comet Siding Spring (C/2013 A1) by Mars on October 19th. The "2013 A1" means the comet was discovered in early January 2013. The comet wasn't found until so recently because it came from underneath the plane of the solar system. At the time of discovery the comet was still farther from the Sun than Jupiter. The comet nucleus is about 1 to 2 kilometers in size. It will pass within 82,000 miles of Mars, so there's no chance of it colliding with the Mars or the spacecraft but the Mars orbiter spacecraft teams are more worried about the dust particles in the comet's coma moving at over 35 miles/second hitting part of the spacecraft. All it would take is one small grain travelling at 35 mps (125,000 mph) hitting a critical hardware component to end the orbiter mission. Mars' atmosphere is thick enough to effectively shield the rovers on the ground. The orbiters and the Curiosity rover at least will be observing the comet from their protective positions.
Another orbiter will join the current team when MAVEN arrives at Mars on September 21st. MAVEN stands for "Mars Atmosphere and Volatile EvolutioN." It does not have instruments such as a camera to observe Comet Siding Spring. MAVEN will be focussed on Mars' upper atmosphere, ionosphere, and interactions with the Sun and solar wind. MAVEN will have a very elliptical orbit that allows it to swoop down close enough to directly sample the upper atmosphere. The scientists will use the data from all of the instruments to figure out how Mars' atmosphere slowly escapes to space now and they also hope that the data will enable them to figure out how Mars' atmosphere became so thin in the past. Features such as dry river beds and eroded crater walls tell us that Mars' atmosphere was once warm enough and thick enough for liquid water to flow across the surface.
One of the ways that a planet atmosphere can be lost is through impacts blasting part of the atmosphere away. While Comet Siding Spring (C/2013 A1) won't hit Mars, the dust particles in the coma will heat up the atmosphere some; probably enough for MAVEN to measure. MAVEN was already set to study the steadier, longer lasting processes of thermal escape and solar wind blasting of Mars' atmosphere, so the arrival of Siding Spring so close to Mars should be an unexpected bonus (if it doesn't damage the spacecraft). Also arriving in September will be India's Mars Orbiter Mission (MOM). It will study Mars' surface and atmosphere. To find out more about Comet Siding Spring's close flyby of Mars and what we're doing to protect our spacecraft at Mars, go to http://mars.jpl.nasa.gov/comets/sidingspring/ . More information about MAVEN is available at http://lasp.colorado.edu/home/maven/ .
Two planets in more immediate news are the two blazing bright ones in our pre-dawn sky: Venus and Jupiter. Over the past week Venus has been plunging down closer to the Sun and Jupiter has been inching upward away from the Sun as seen from Earth. They are now close enough together on our sky to easily fit within the field of view of a pair of binoculars. On the pre-dawn morning of August 18th, they will be just 0.3 degrees apart from each other (about a pinky width at arm's length). Look down low in the east at about 45 minutes before sunrise (see the first star chart below). With binoculars you may also be able to see the Beehive Cluster just to the left of the two planets depending on sky conditions (see inset A of the first star chart below).
Jupiter and Venus will fit within the field of view of your binoculars until the morning of August 22nd. On the morning of the 23rd a very thin Waning Crescent Moon will form a compact triangle with Jupiter and Venus (see inset B of the first star chart below). Venus will be the bright point at lower left and Jupiter will be the top point of the triangle. At the end of the month a Waxing Crescent Moon will form a beautiful triangle with the evening sky planets, Mars and Saturn. Before then you will see Mars pass under Saturn from August 23rd to 26th. They will both fit comfortably within the same field of your binoculars during that time and be about the same brightness, so you'll need to use their color to figure out which is which (or remember that Saturn is above Mars). See the second chart below for the evening sky positions of Mars and Saturn in the southwest among the stars of Libra.
Two last things to quickly mention: the free public star party at Panorama Park hosted by the Kern Astronomical Society on September 6th and the "Stars of the Four Seasons" classoffered through the Levan Institute for Lifelong Learning. The stars class will highlight the major constellations of the four seasons and throw in a little astrophysics of how we determine the properties of the stars.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com