Phobos: Doomed Moon of Mars
Astronomy Picture of the Day
Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1977 Viking orbiter image, the largest moon, Phobos, is seen to be a heavily cratered asteroid-like object. It is about 17 miles across and zips through the Martian sky completing an orbit in less than 8 hours. Phobos is doomed. It orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it could crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.
Two Views, Two Crescents
Astronomy Picture of the Day
Venus rose in a glowing dawn sky on November 5th, just before the Sun. For early morning risers, its brilliant crescent phase was best appreciated with binoculars or a small telescope. On that day the crescent Venus also appeared in close conjunction with another lovely crescent that hugs the eastern horizon in planet Earth's morning skies, the waning crescent Moon. The celestial photo-op is captured here from two locations. Left, separated by less than a degree, the two crescents hover above a sea of clouds. The picture was recorded from an Alpine mountain pass not far from Turin, Italy. On the right is a sharp telephoto view taken before an earlier sunrise, farther east in the Alborz Mountains of Iran. In steady skies the slender Moon is still sliding toward Venus, the bright planet's compact crescent just clearing the mountainous horizon. For now, the crescent phase of Venus remains easy to enjoy with binoculars in November's dawn skies. The first observations of the phases of Venus, made by Galileo with his telescope in 1610, agreed with the predictions of the heliocentric Copernican model of the Solar System.
By Stefano De Rosa
Tornadoes on the Sun
Astronomy Picture of the Day
Giant spinning clouds of gas, similar to Earth's tornadoes, have been found on the Sun. Solar tornadoes, however, can be larger than the entire Earth, and sustain wind gusts over 1000 times stronger than their Earth counterparts. The SOHO spacecraft has found that solar tornadoes start low in the Sun's atmosphere and spiral outwards, gathering speed as they enter the Solar System. Earthlings have more to fear from Earth's own weather phenomena, though, because the high speed particles that result from solar tornadoes are easily stopped by the Earth's thick atmosphere. Earthlings may have much to learn from solar tornadoes, including details of how the solar wind and corona are powered, and how to better predict future solar particle storms that could damage sensitive satellites.
Black Holes Signature From Advective Disks Research Credit:
Astronomy Picture of the Day
What does a black hole look like? If alone, a black hole would indeed appear quite black, but many black hole candidates are part of binary star systems. So how does a black hole binary system look different from a neutron star binary system? The above drawings indicate it is difficult to tell! Recent theoretical work, however, has provided a new way to tell them apart: advective accretion flows (ADAFs). A black hole system so equipped would appear much darker than a similar neutron star system. The difference is caused by the hot gas from the ADAF disk falling through the event horizon of the black hole and disappearing - gas that would have emitted much light were the central object only a neutron star. Recent observations of the soft X-ray transient V404 Cyg has yielded a spectrum much like an ADAF onto a black hole - and perhaps brighter than allowable from an ADAF onto a neutron star.
Light from the First Stars
Astronomy Picture of the Day
What were the first stars like? No one is yet sure. Our Sun is not a first-generation star. It is not even second generation. The first stars to appear in the universe likely came and went about 13 billion years ago. However, deep observations by the Spitzer Space Telescope in infrared light have detected a diffuse glow, possibly from first generation stars hundreds of times more massive than our Sun. The above image shows infrared background light with bright patches that might have originated from clusters of these first objects. Gray areas depict places where nearby foreground stars from our Milky Way Galaxy were digitally removed.
Phobos: Doomed Moon of Mars
Astronomy Picture of the Day
Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1977 Viking orbiter image, the largest moon, Phobos, is seen to be a heavily cratered asteroid-like object. It is about 17 miles across and zips through the Martian sky completing an orbit in less than 8 hours. Phobos is doomed. It orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it could crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.
Two Views, Two Crescents
Astronomy Picture of the Day
Venus rose in a glowing dawn sky on November 5th, just before the Sun. For early morning risers, its brilliant crescent phase was best appreciated with binoculars or a small telescope. On that day the crescent Venus also appeared in close conjunction with another lovely crescent that hugs the eastern horizon in planet Earth's morning skies, the waning crescent Moon. The celestial photo-op is captured here from two locations. Left, separated by less than a degree, the two crescents hover above a sea of clouds. The picture was recorded from an Alpine mountain pass not far from Turin, Italy. On the right is a sharp telephoto view taken before an earlier sunrise, farther east in the Alborz Mountains of Iran. In steady skies the slender Moon is still sliding toward Venus, the bright planet's compact crescent just clearing the mountainous horizon. For now, the crescent phase of Venus remains easy to enjoy with binoculars in November's dawn skies. The first observations of the phases of Venus, made by Galileo with his telescope in 1610, agreed with the predictions of the heliocentric Copernican model of the Solar System.