Up in the Air: Mercury, Venus, Saturn shine in May sky

On April 30, NASA’s Messenger space probe bit the dust on the planet Mercury. After a decade-long mission of studying the closest planet to the sun, the spacecraft ran out of options for adjusting its pointing and spiraled into the surface.

Messenger had been only the second spacecraft – after a couple of fly-bys by NASA’s Mariner 10 in the 1970s – to get close-up views and data. It yielded many unexpected discoveries, as these missions tend to do, including evidence of volcanic activity and the signature of ice in some craters at Mercury’s north pole that never see direct sunlight. This, on top of making the best images and maps of the planet.

You can see Mercury yourself right now: It is close to its highest position for the year in the western dusk sky. Look for it low in the west-northwest, about a fist’s-width at arm’s length above the horizon, as soon after sunset as the darkening sky allows. Higher in the sky, in the same direction, you will see the much-brighter planet Venus. Mercury is a dark planet, reflecting about the same percent of sunlight as the moon – around 12 percent – while Venus is covered in bright clouds that reflect three quarters of the sunlight.

Higher yet in the sky you will find Jupiter and, later in the evening, Saturn rises in the southwestern sky. Only Mars is unavailable, hidden in the glare of the sun. We could throw in Uranus and Neptune in the dawn sky, but you can’t see them without a telescope.

NASA has explored all of these planets and even has the New Horizons space probe headed to a summertime encounter with the celestial body formerly known as the planet Pluto. We can expect the unexpected again.

Why explore the planets? Besides satisfying our species’ natural curiosity, we need to understand the other planets to perhaps get a glimpse of possible fates for Earth. Undeniable evidence now exists that Mars once had a lot of water, enough to flow over its surface and to produce sediments. How did Mars lose its water? Those clouds on Venus are sulfuric acid (think of that stuff that corrodes your car battery terminals) and are in a mostly carbon dioxide atmosphere (think fossil fuel combustion product). Venus is trapped in a runaway greenhouse effect that keeps it extremely hot. Could these scenarios be part of our future?

We have a lot to learn from studying the other planets. After all, it is hard to generalize your knowledge of planets based on a sample of one: Earth. And as important, they may teach us a lesson on why we need to take care of ours.

This story was originally published May 10, 2015 at 12:44 AM with the headline "Up in the Air: Mercury, Venus, Saturn shine in May sky."