HP image story for Tuesday 5/6 When Mercury transits across the face of Sun early Wednesday, May 7, most people will miss the rare event because they lack the time or inclination to create a safe viewing setup. If this sounds like you, you're in luck: The SOHO spacecraft will be watching for you, and the images will be available on the Web, almost in real time.
Observers in Asia, Africa and Europe will have the best natural, outdoor view. The transit begins in the wee hours of the morning for people in the United States.
The transit occurs because Mercury is closer to the Sun than Earth. Now and then -- only about 13 times a century -- the inner planet crosses the visible disk of the Sun, as viewed from Earth.
A transit is observationally challenging. Mercury's apparent size is only about 1/160 of the Sun's diameter, for one thing. And looking directly at the Sun -- either with your eyes or through a telescope -- is extremely dangerous and can cause permanent eye damage.
Avid observers will set up telescopes to project the image on a wall or other flat surface. For most people, however, SOHO (Solar and Heliospheric Observatory) can serve as a nifty set of virtual eyes.
Paal Brekke, SOHO Deputy Project Scientist for the European Space Agency (ESA), said the spacecraft's MDI (Michelson Doppler Imager) camera will record an image a minute, and the images should be posted to the Web every 5-10 minutes, after processing.
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Even someone like Brekke, who works out of NASA's Goddard Space Flight Center and has an obvious interest in events like this, the Internet is the way to go.
"I do not plan to go out since the Sun is very low in the sky here on the East Coast and it is very hard to find an elevated viewing area to set up a telescope," Brekke said. "So I will rely on the SOHO images from my home PC."
SOHO is a joint project between NASA and ESA. The spacecraft orbits a special point in space a million miles (1.6 million kilometers) from Earth, in line with the Sun. Scientists use its observations to predict space weather that can affect Earth. SOHO has also serendipitously discovered more than 500 comets.
Transits of Venus, the other inner planet, are even rarer.
"The last Venus transit was in 1882, so no one alive has seen one," said Fred Espenak, a noted eclipse observer and NASA astronomer. "Happily, there will be a Venus transit June 8, 2004, so this year's Mercury transit can be taken as an appetizer for the main course."
SOHO's observations will be posted at http://soho.nascom.nasa.gov/hotshots/2003_05_07/.
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