Venus: Galileo's first planetary flyby.

Venus: Galileo's first planetary flyby 

As the spacecraft Galileo swung past Venus last week, getting a gravitational boost in speed on its way to a 1995 rendezvous with Jupiter, NASA officials expected the craft's camera shutter to click 16 times. The number had reached 56 when data showed the shutter still clicking, and it had progressed to 468 by the time commands sent by ground controllers and by Galileo's own computer stopped it.

Engineers controlling the mission from NASA's Jet Propulsion Laboratory in Pasadena, Calif., were relieved to discover that the problem resulted from a correctable software mistake rather than a malfunction in the camera itself or an incoming solar proton disrupting Galileo's computer memory. About 65 more photos remained on the week's shooting schedule at Venus, since the shutter is designed to operate 150,000 times.

Most of Galileo's scientific data will not reach Earth until the craft's tape recorder can play them back in October (when the proper antenna onboard deploys). But some results have reached Earth already, including three of the photos and a batch of near-infrared mapping data.

Galileo's two ultraviolet spectrometers have shown, as expected, that hydrogen atoms at Earth's distance from the sun last about 1 million seconds (almost 12 days) before ionizing, says Charles W. Hord of the University of Colorado in Boulder, and that a cubic centimeter of space contains an average of about 1 interstellar hydrogen atom.

Because Galileo follows a flight path that will let its ultraviolet instruments observe from both inside and outside Earth's orbit, scientists will have a unique chance to investigate the controversial theory that numerous small comets are being vaporized by running into Earth's atmosphere. If the theory is correct, more hydrogen should appear in data from Galileo's present "inside" position -- looking through the presumed vaporized comets -- than will show up after the craft has crossed beyond Earth's orbit.

Galileo is already reporting on the ultraviolet spectra of stars (the one measured so far is Kappa Velorum, says Hord) and on impacts by space dust. The largest piece yet detected weighed about one hundred-millionth of a gram and measured about 26 microns across.