The Microwave Instrument for the Rosetta Orbiter (MIRO) made the first observations of water vapor streaming from the comet on June 6 when the spacecraft was about 220,000 miles (350,000km) from the comet.
Since the initial detection, water vapor has been found every time MIRO has been pointed toward the comet.
“We always knew we would see water vapor outgassing from the comet, but we were surprised at how early we detected it,” said Sam Gulkis from NASA’s Jet Propulsion Laboratory in Pasadena, California.
“At this rate, the comet would fill an Olympic-sized swimming pool in about 100 days. But as it gets closer to the Sun, the gas production rate will increase significantly. With Rosetta, we have an amazing vantage point to observe these changes up close and learn more about exactly why they happen.”
Water is a major volatile component of comets, along with carbon monoxide, methanol, and ammonia. MIRO is designed to help determine the abundance of each of these ingredients in order to understand the nature of the comet’s nucleus, the process of outgassing, and where they originate on the surface.
These gases stream away from the nucleus carrying dust, forming the comet’s surrounding “coma.” As the comet moves closer to the Sun, its coma will expand, and eventually pressure from the solar wind will cause some of the material to stream out into a long tail.
Rosetta will be there to watch these developments up close. The comet and Rosetta will make its nearest approach to the Sun in August 2015 between the orbits of Earth and Mars.
Determining changes in the production rate of water vapor and other gases as the icy object moves around the Sun is important for comet science. But it is also vital for mission planning because once Rosetta is closer to the comet, the outflow of gases may alter the craft’s trajectory.
“Our comet is coming out of its deep-space slumber and beginning to put on a show for Rosetta’s science instruments,” said Matt Taylor from ESA. “Rosetta’s engineers will also be using MIRO’s observations to help them plan for future mission events when we are operating close to the comet’s nucleus.”
Today, the spacecraft is within 45,000 miles (72,000km) of its destination. Six out of a total of 10 rendezvous maneuvers still need to be carried out to ensure that Rosetta arrives at a distance of just 62 miles (100km) from the nucleus August 6.