ISS Transit of Venus

May 31, 2012: In 1768, when James Cook sailed out of Plymouth harbor to observe the Transit of Venus in Tahiti, the trip was tantamount to a voyage through space. The remote island had just been "discovered" a year earlier, and by all accounts it was as strange and alien to Europeans as the stars themselves. Cook's pinpoint navigation to Tahiti and his subsequent observations of Venus crossing the South Pacific sun in 1769 have inspired explorers for centuries.

One of those explorers is about to beat Cook at his own game.

High above Earth, astronaut Don Pettit is preparing to photograph the June 5th Transit of Venus from space itself.

"I've been planning this for a while," says Pettit, who serves as Flight Engineer onboard the International Space Station. "I knew the Transit of Venus would occur during my rotation, so I brought a solar filter with me when my expedition left for the ISS in December 2011."

Because transits of Venus come in pairs that occur once every 100 years or so, humans have rarely had the chance to photograph the apparition from Earth, much less from Earth orbit.

"The Expedition 31 crew will be the first people in history to see a Venus transit from space, and Pettit will be the first to photograph one," says Mario Runco, Jr. of the Johnson Space Center (JSC). Runco, an astronaut himself who flew aboard three shuttle missions, is an expert in the optics of spacecraft windows. Along with his wife Susan Runco, who is the coordinator for astronaut photography at JSC, Mario is helping Pettit gather the best possible images of the transit.

Pettit will be pointing his camera through the side windows of the space station's cupola, an ESA-built observatory module that provides a wide-angle view of Earth and the cosmos. Its seven windows are used by the crew to operate the station's robotic arm, coordinate space dockings, and take science-grade photos of the Earth and sky. It's also a favorite "hangout" for off-duty astronauts who find the view exhilarating.

"For this transit, Don will be removing the non-optical quality, internal protective window panes known as 'scratch panes,' which really make crisp, sharp, and clear images impossible," says Runco. "Removing those panes is a huge plus when it comes to details that will be seen in the imagery of the sun."

Pettit describes the camera system: "I'll be using a high-end Nikon D2Xs camera and an 800mm lens with a full-aperture white light solar filter."

"Even with this great camera system, the images would be quite soft if the scratch panes were not removed," notes Runco. "This is only the third time that we'll be [shooting through] the Cupola's optical quality windows. I'm hoping this becomes routine in the future."

This month's transit is the bookend of a 2004-2012 pair. Astronauts were onboard the ISS in 2004, but they did not see the transit, mainly because they had no solar filters onboard. Tiny Venus covers a small fraction of the solar disk, so the sun is still painfully bright to the human eye even at mid-transit. Pettit's foresight to bring a solar filter with him makes all the difference.

How would Cook feel about all this?

"I don't think James Cook should be too envious," says Runco. "After all, he did get an all-expense paid trip to Tahiti out of the deal."

Don's photos will be rapidly posted to the web during the transit. The historic webcast begins on June 5th at approximately 3 pm PDT.


Author:Dr. Tony Phillips| Production editor: Dr. Tony Phillips | Credit: Science@NASA