Here are the first images from the MESSENGER spacecraft’s Monday flyby of Mercury.

Image of craters on Mercury taken Oct. 6. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

MESSENGER will fly by the planet once more in September 2009. The spacecraft is scheduled to enter orbit around Mercury on March 18, 2011.

In the meantime, the Cassini spacecraft is getting ready to execute two more flybys of Saturn’s moon Enceladus (pronounced in-SELL-uh-dus) this month. Enceladus, you may recall, is the moon that is spewing cold geysers of water into space, which suggests to scientists there is liquid water (possibly even an ocean) under its surface.

The first flyby, set for Thursday October 9th, is arguably the more exciting of the two. Cassini will pass just 16 miles over the surface of the moon, directly through the geyser plume. The emphasis on this flyby will be to use the on-board science instruments to learn more about its composition. Data from previous flybys indicate that, in addition to water vapor, water ice, and dust, the plume also contains trace amounts of organic chemicals. The presence of organics has certainly perked up the antennae of the astrobiology community. In only a short period of time this little moon has shot to near the top of the list of promising places to look for extra-terrestrial microbial life.

The second Enceladus flyby of the month is set for Oct. 31. Cassini will fly 122 miles over the surface, and use on-board cameras to photograph surface fractures in the south polar region.

–Kate Tobin, Sr. Producer, CNN Science & Technology

NASA’s MESSENGER spacecraft will execute the second of three planned Mercury flybys on Monday, as it loops through the inner solar system on a trajectory that will take it into orbit around that planet in 2011.

MESSENGER image of Mercury. Source: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington

Mercury has been relatively scantily studied up until now because it is so close to the Sun and it is very difficult to get there. Only one spacecraft, Mariner 10, has flown by it before before — three passes back in 1974 and 1975.

What we know about the planet is that its surface is heavily cratered, with plains formed by volcanic eruptions. It has an extremely thin atmosphere. It is a place of temperature extremes: 840 degrees Fahrenheit during the day, -275 degrees at night. It is only about 43 million miles from the Sun — so from the surface of Mercury, the Sun would appear about two and a half times as large in the sky as it does on Earth. It revolves around the Sun once every 59 Earth days, and rotates on its axis very slowly…once every 176 Earth days. So on Mercury, a day is longer than 2 of Mercury’s years!

The first MESSENGER flyby occurred on January 14. The probe sent back pictures of approximately 20% of the surface that had never been photographed before. Monday’s flyby will have the spacecraft passing just 125 miles above the surface. The overarching goals of the mission are to photograph the planet in its entirety, and to learn more about its composition, structure, and magnetic field.

–Kate Tobin, Sr. Producer, CNN Science & Technology