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Milky Way is about half as massive as Andromeda galaxy, say astronomers

An image of the Andromeda galaxy, Messier 31.  Image credit: Adam Evans

An image of the Andromeda galaxy, Messier 31. Image credit: Adam Evans

Dark matter makes up 90% of the matter in both systems, the study finds.

Forest path

View larger. | Photo: T.  Richardsen

View larger. | Photo: T. Richardsen

A misty forest path … Where does it lead? Tommy Richardsen captured this image in Sørkjosen, Troms, Northern Norway.

Constellation Scutum named for a Polish king

After the waxing crescent moon sets at late evening on Wednesday, look for one of summer’s most beautiful celestial sights. Be sure you are looking in a dark country sky, far away from the the glare of city lights. You’ll find a hazy pathway stretched across the sky. This band is the starlit trail of our own Milky Way galaxy. We in the N. Hemisphere look southward in late in the evening – toward the richest part of the Milky Way – to find a very small constellation called Scutum the Shield.

101 geysers on Enceladus, ice moon of Saturn

This view looks across the geyser basin of Saturn's moon Enceladus, along fractures spewing water vapor and ice particles into space. Cassini scientists have pinpointed the source locations of about 100 geysers and gained new insights into what powers them. Image credit: NASA/JPL-Caltech/SSI

This view looks across the geyser basin of Saturn’s moon Enceladus. What you see here are fractures spewing water vapor and ice particles into space. Cassini scientists have now pinpointed some 101 geysers on Enceladus. They believe a large underground ocean is the geysers’ dominant source. Image via NASA/JPL-Caltech/SSI

Using data from NASA’s Cassini spacecraft – which has been orbiting Saturn, weaving among its moons, since 2004 – scientists have now mapped 101 distinct geysers erupting from the surface of Enceladus, an ice world and arguably Saturn’s most fascinating moon. They’ve concluded that the only plausible source for the geysers is an underground sea now known to exist beneath Enceladus’ icy surface.

We’re in the early days of Earth’s sixth great extinction, say biologists

Photo credit: John Spooner/Flickr

Photo credit: John Spooner/Flickr

An international team of scientists cautions that the loss and decline of animals is contributing to what appears to be the early days of the planet’s sixth mass biological extinction event.

Since 1500, more than 320 terrestrial vertebrates have become extinct. Populations of the remaining species show a 25 percent average decline in abundance. The situation is similarly dire for invertebrate animal life.

Mars rover set an off-world driving distance record this week

Image credit:

Image credit:

The chart above provides a comparison of the distances driven by various wheeled vehicles on the surface of Mars and Earth’s moon. On July 27, the Opportunity rover passed 25 miles driven – an off-world driving distance record. Opportunity has gone farther than any other earthly wheeled vehicle on another world. It’s especially remarkable, NASA says, because Opportunity was intended to drive about one kilometer (.6 mile) and was never designed for distance.

Star Skat is near radiant for Delta Aquarid meteors

When you stand on a railroad track, you can see the illusion of tracks converging in the distance.  Likewise, the paths of meteors in a single meteor shower appear to converge at a point on the sky's dome.  This is the shower's radiant point.

When you stand on a railroad track, you can see the illusion of tracks converging in the distance. Likewise, the paths of meteors in a single meteor shower appear to converge at a point on the sky’s dome. This is the shower’s radiant point.

How to spot it, plus an explanation of why meteors in annual showers have radiant points.

Super-close views of Mercury

Image via NASA / JHU / APL MESSENGER spacecraft

Close image of Mercury acquired by the MESSENGER spacecraft on June 11, 2014. 69.84 degrees North & 48.47 degrees East. Resolution 2.9 meters. Image via NASA / JHU / APL MESSENGER spacecraft

The image above is one of the highest resolution images ever returned of Mercury. MESSENGER acquired it on June 11, 2014. Last Friday – July 25, 2014 – MESSENGER’s periherm (closest point to Mercury) dropped below 100 kilometers / 62 miles. MESSSENGER is the first manmade object ever to come this close to Mercury.

Young moon and star Regulus low in west after sunset July 29

2014-july-29-regulus-moon-night-sky-chart

As day ebbs into night on July 29, will you spot the young lunar crescent and the star Regulus as they make their fleeting appearance over the western horizon at dusk? From mid-northern North American latitudes, you might catch the elusive couple about 45 to 60 minutes after sunset – if you’re blessed with an unobstructed western horizon and crystal-clear skies. Binoculars may be helpful.

As Comet Siding Spring approaches Mars, NASA gets ready

An illustration of the inner part of the orbit of comet C/2013 A1 Siding Spring.  On October 19, 2014, the comet will have a close pass of the planet Mars.  Its nucleus will miss Mars by about 82,000 miles (132,000 kilometers). The comet's trail of dust particles shed by the nucleus might be wide enough to reach Mars or might also miss it.   Image via NASA/JPL.

An illustration of the inner part of the orbit of comet C/2013 A1 Siding Spring. Image via NASA/JPL.

One of the most anticipated astronomical events of 2014 is the close passage of Comet C/2013 A1 Siding Spring to the planet Mars on October 19, 2014. The comet’s tiny nucleus, or core, will miss Mars by about 82,000 miles (132,000 kilometers). As comets travel through space, though, they leave behind a trail of dust particles, and this trail of debris might be wide enough to reach Mars and encounter its thin atmosphere … or might miss it, too. Follow the links inside to learn more.