The last time this century to observe Venus appearing to cross the face of the sun occurs on Tuesday and the National Aeronautics and Space Administration is using the occasion to promote public interest in science.

The “Venus transit” event will not occur again until 2117. Scientists are studying Venus, which has a surface temperature hot enough to melt lead, in hopes that the cloud-shrouded planet could provide clues to Earth’s future, including climate change.

The transit begins at about 6 p.m. on Tuesday and should last about seven hours, into early Wednesday. Would-be viewers are warned to use solar viewing glasses and not to stare directly at the sun, to avoid possible eye damage.

Venus transits have historically occurred in pairs, eight years apart, with more than a century elapsing before the next pairing. Prominent astronomers have been watching the events for centuries. North America’s last transit viewing was in June 2004.

One of science’s biggest questions about Venus is how it got so hot, with a surface temperature of around 800 degrees Fahrenheit, unlike the similarly sized Earth.

Venus orbits closer to the sun than Earth, and its constant cloud cover traps heat near the Venusian surface. Scientists are examining its atmosphere for more information.

“How long has Venus been radically different from Earth? Was it born that way, or did it develop into that?” said Steven Williams, an official with the Smithsonian’s National Air and Space Museum assigned to NASA to help publicize the Venus transit event. “Anytime you have two systems that are basically the same, and yet the outcome is really, really different, that’s a powerful situation to learn from.”

The two planets have similar mass and are made of similar elements.

More than 20 spacecraft have visited Venus, starting with NASA’s Mariner 2 in 1962. Some have used radar to map its topography, mostly rolling plains with a few active volcanoes. Any spacecraft that has landed on the surface was destroyed within a few hours by the harsh environment.

NASA’s Glenn Research Center, near Cleveland, gave reporters a tour Wednesday of a chamber that the agency uses to simulate the Venusian environment.

NASA is planning on sending additional spacecraft to Venus, on dates to be determined. Its scientists are partnering with European and Japanese counterparts.

Among those watching the transit Tuesday will be NASA’s Hubble Space Telescope and the European Space Agency’s Venus Express spacecraft that is orbiting Venus. Scientists expect to learn more about its atmosphere by analyzing sunlight that filters through it.

Contact this reporter at (937) 225-2242 or jnolan@DaytonDailyNews.com.