On March 18, a NASA spacecraft called Messenger will, fingers crossed, go into orbit around the planet Mercury. Let’s take a look at the planet and the spacecraft.

Mercury is the planet closest to the Sun, being some 36 million miles away (Earth is 93 million miles away). It is also the smallest planet with a diameter of just over 3,000 miles, not much larger than our Moon. In fact, if you think of our Moon as made of an iron core that is covered by a 400-mile-thick layer of rock, you have Mercury. Being so close to the Sun, Mercury whizzes around it very quickly: a Mercury year lasts just 88 Earth days. However, it has a very long day: one Mercury day equals 59 Earth days. Finally, its orbit is more elliptical than Earth’s. Put these dry facts together and you get some interesting effects. Let’s put on our spacesuits and go to Mercury.

We are standing in Caloris Basin, which has a diameter of over 900 miles. This is one of the largest impact craters in the solar system. It is just before dawn and it is very cold, the temperature being minus 300 degrees Fahrenheit. When the Sun appears, looming larger than it does from Earth, it is close enough that you can see tendrils of gas writhing on its surface. As the Sun very slowly rises, it gets larger in the sky, appearing twice as large as it does from Earth, and the temperature increases to 800 degrees Fahrenheit by noon. That is hot enough to melt lead. Because Mercury’s orbit isn’t quite circular and its day is so long, an interesting thing happens – the Sun stops moving across the sky, reverses its course, stops, reverses course again, then continues on its way. If this happens at sunset, you will see the Sun set, pop back up, then set again.

Back on Earth, if you look at our western horizon just after sunset on March 18, you will see a bright object: that will be Jupiter. But just above it and a little to the right will be a pinprick of light: that is Mercury. Messenger should go into orbit at 4:48 p.m. Unfortunately, at that time of the day, Mercury will be hidden in the glare of
the Sun.

Now let’s take a look at Messenger. This isn’t the first spacecraft to visit Mercury – that was done by Mariner 10, which made two flybys in 1974 and 1975. Messenger, however, will be the first to go into orbit around the planet. Spacecraft heading toward the Sun pick up speed. To lose some of that speed, Messenger had to take a rather convoluted path that took over six years to complete. Messenger’s yearlong mission is to examine the chemical composition of Mercury’s surface, its geological history, the nature of its magnetic field, and see what chemicals are hiding in the eternal cold of the deep craters at its poles.

Keith Turner will return for special editions of “Starry Skies” whenever an astronomical event worth reporting is on the horizon. A retired physicist, Keith is a longtime Marina resident and Marina Times contributor.