Photos of a comet racing toward an astonishingly close encounter with Mars are helping scientists to better estimate the comet's size, according to a NASA team that includes Penn State astronomers. The photos were made with a telescope on board NASA's Swift satellite, for which Penn State controls the science and flight operations from the Mission Operations Center at the University Park campus.

The photos captured by Swift's Ultraviolet/Optical Telescope are the first to reveal how much water the comet named Siding Spring is producing as it is beginning to get heated up by the sun during its first-known voyage through our inner solar system. The comet is named after the Siding Spring Observatory in Australia, which first detected it a year and a half ago.

"Comet observations are one of the biggest challenges that face our team of science planners for the Swift satellite," said Michael Siegel, the Penn State researcher who is the instrument leader for Swift's Ultraviolet/Optical Telescope (UVOT). "Observing comets requires a very detailed instrument setup, and Swift was designed to look at the most distant objects in the universe, not the closest ones like this new comet in our own solar system. In addition comets move, which requires us to carefully time the observations to point Swift at precise parts of the sky exactly at the time when we estimate the comet will move into these areas."

The leader for Swift operations, Deputy Mission Director Margaret Chester, is a Penn State scientist.

"Margaret usually takes the lead on making sure we get exactly the data needed for these observations," Siegel said. "Her hard work is paying off as we see the gorgeous images coming off the Swift spacecraft and the ground-breaking science coming out of its observations."

The comet makes its closest approach to Mars on Oct. 19, passing just 83,000 miles (132,000 km) from the Red Planet -- so close that gas and dust in the outermost reaches of the comet's atmosphere, or coma, will interact with the atmosphere of Mars. For comparison, the closest recorded approach to Earth by a comet was by the now-defunct comet Lexell, which on July 1, 1770, swept to within 1.4 million miles (2.3 million km) or about six times farther than the distance between Earth and the moon. During its Mars flyby, comet Siding Spring will pass more than 16 times closer than this distance.

New comets like Siding Spring, formally known as C/2013 A1, contain some of the most ancient material scientists can study. The solid nucleus of a comet is a clump of frozen gases mixed with dust often described as a "dirty snowball." Comets cast off gas and dust whenever they venture near enough to the sun for the frozen material to be transformed into gas by the sun's heat. Different gases stream from the nucleus as the comet approaches the sun, carrying with them large quantities of dust that brighten the comet by reflecting sunlight. By about two and a half times Earth's distance from the Sun, the comet has warmed enough that water becomes the primary gas emitted by the nucleus.

Between May 27 and 29, Swift's Ultraviolet/Optical Telescope (UVOT) captured a sequence of images as comet Siding Spring cruised through the constellation Eridanus at a distance of about 2.46 AU (229 million miles or 368 million km) from the sun. While this telescope cannot detect water molecules directly, it can detect the light emitted by fragments formed when ultraviolet sunlight breaks up water -- specifically, hydrogen atoms and hydroxyl (OH) molecules.

"Based on our observations, we calculate that at the time of the observations the comet was producing about 2 billion billion billion water molecules, equivalent to about 13 gallons or 49 liters, each second," said team member Tony Farnham, a senior research scientist at the University of Maryland College Park. At this rate, comet Siding Spring could fill an Olympic-size swimming pool in about 14 hours. These observations are part of a two-year-long Swift campaign to watch how the comet's activity develops during its travels. The lead researcher for this campaign is Dennis Bodewits, an astronomer at the University of Maryland College Park.

These Swift observations are part of a larger study to investigate the activity and evolution of new comets, which show distinct brightening characteristics as they approach the sun not seen in other comets. Near the time of the comet's close encounter with Mars, five spacecraft will be orbiting the Red Planet: Mars Reconnaissance Orbiter, Mars Express, Mars Odyssey, Mars Atmosphere and Volatile Evolution, and Mars Orbital Mission. This systematic study will help astronomers to better understand how the activity of these comets changes with repeated heating by the sun.

Scientists have established that the comet poses no danger to spacecraft now in orbit around Mars. These space missions are expected to serve as a provisional comet-observation fleet to take advantage of this unprecedented opportunity for space science.