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Night Sky

April Night Sky

April 2011 Night Sky

—Charlie Christmann

The Messenger Arrived

On March 17, 2011, NASA’s Messenger spacecraft arrived at Mercury for the third time. And this time, it plans to stay awhile. After its launch from Earth early in August 2004, this spacecraft flew past Earth on August 2, 2005, Venus in October 2006 and June 2007, and Mercury in January 2008, October 2008, and September 2009 before finally settling into orbit around Mercury on March 17 this year.

Its long journey was necessary to save fuel. Flying by Earth and Venus used gravity to place Messenger on a trajectory to intersect Mercury. But the spacecraft was moving much too fast, 15,877 miles per hour relative to Mercury, to be nudged into orbit with the fuel available onboard. So, three passes of Mercury were used to slow down Messenger, so it could use its thruster to enter orbit. The final approach to Mercury was made at 8,640 miles per hour and required a 15-minute burn to slow down by 1,900 miles per hour, enough to be captured in orbit.

Mercury is the least studied planet in our solar system (remember, Pluto is no longer a planet). Unfortunately, Mercury is too close to the Sun to be seen by the Hubble Telescope. The only other spacecraft to visit was Mariner 10, which also flew by the planet three times in 1974 and 1975. Mariner was only able to photograph 45 percent of the surface.

Being the smallest planet in our solar system, Earth is more than five times larger and 18 times heavier than Mercury. It is just slightly smaller than the moons Ganymede and Titan. It is so close to the Sun that Mercury orbits in only 88 days, versus the Earth’s 365 days. The temperature ranges from -280º to +800º Fahrenheit.

What have we learned so far about the Sun’s closest planet? Well, first, Mercury does have a tenuous atmosphere of hydrogen, helium, oxygen, sodium, calcium, and potassium around it. They don’t form a stable atmosphere. Instead, there’s a constant flow of these atoms into orbit around Mercury, and then the Sun’s solar wind blasts it away into space. And Mercury has a small magnetic field. This magnetic field is similar to the one we have on the Earth, which protects our planet from the Sun’s solar wind. But Mercury cooled down a long time ago and doesn’t have the internal movement of magma like inside the Earth. So, where does the magnetic field come from? That is one of the big questions that NASA’s Messenger spacecraft will try to answer.

And, most interesting of all, considering its location so close to the blistering Sun, Mercury may have ice in deep impact craters near its polls, just like our Moon.

If you look at a photograph of Mercury without knowing what the picture was, you would be hard pressed not to think it was the Moon. The planet has extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. Craters on Mercury range in diameter from small bowl-shaped cavities to multiringed impact basins hundreds of miles across. Mercurian craters differ from lunar craters in that the area blanketed by their ejecta is much smaller. This is because of Mercury’s stronger surface gravity.

One interesting observation a visitor to Mercury would see from the surface would be sunrise and sunset. This planet rotates on its axis three times for every two orbits. At some longitudes, the observer would see the Sun rise and then gradually increase in apparent size as it slowly moves toward the zenith. At that point, the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while, the stars would be moving three times faster across the sky. Observers at other points on Mercury’s surface would see different but equally bizarre motions.

With Messenger now in orbit, many new and unexpected discoveries are sure to be found. Stay tuned as observations come in, and the data is analyzed.

Good Night, Sweet Shuttle

After almost 27 years in service, it is time to say good-bye to Space Shuttle Discovery. After 148 million miles, 39 missions, 5,830 orbits, and 365 days in orbit, it will be retired to the Smithsonian Institution’s National Air and Space Museum, replacing the Enterprise now on display there.

Shuttles Atlantis and Endeavour are still operational.

The Planets, Moon, and Other Events

  • Catch Mercury the last half of April before sunrise in the west, rising a full hour before the Sun by the 30th.
  • Venus will shine brightly in the east before sunrise. A Moon-Venus conjunction occurs on the 30th. Look for Venus six degrees below the waning crescent Moon 20 minutes before sunrise.
  • Look for Mars low in the east before sunrise, getting higher each morning throughout the month.
  • Jupiter, too, will be in the east before sunset, starting around the second week of April.
  • Saturn can be found high in the eastern sky after sunset this month. On the 3rd, Saturn reaches opposition, a point opposite in the sky from the Sun as seen from Earth. Saturn has a conjunction with the Moon on the 16th. Look two hours after sunset—Saturn will be eight degrees to the upper left of the waxing gibbous Moon.
  • On the 2nd, the Moon reaches apogee, its farthest point in orbit around Earth, and is 31117.03 miles further away than on March 19th. On the 16th, find Spica 11 degrees to the lower left of the Moon, and on the 17th, look for a Moon-Spica conjunction, with Spica six degrees above the Moon. The new Moon occurs at 8:32 a.m. on the 3rd. The first full Moon of spring occurs at 8:44 p.m. on the 17th. This full Moon is almost as good as the March 19th one. The Moon is just past perigee and will appear seven percent larger than average.
  • The Lyrid meteor shower peaks on Good Friday, the 22nd. Comet Thatcher is responsible for this shower. Look for less than 20 meteors per hour.




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