Being part of a natural thermoregulation process for all mammals, perspiration is the skin’s response to increasing outside temperatures, with humans releasing as much as four liters of water per hour in the form of sweat, often as a result of direct exposure to the Sun. Despite not being alive, inanimate celestial objects like comets can “sweat” too, outgassing huge quantities of water ice, dust, and other volatile elements while nearing the perihelion point in their orbits around the Sun, creating their characteristic atmosphere, or coma, in the process. The European Space Agency’s Rosetta spacecraft, which is en route to comet 67P/Churyumov–Gerasimenko, has found that the icy celestial wanderer is already releasing huge amounts of water vapor into space, even at its current distance of 3.7 Astronomical Units from the Sun.
One of the most fascinating planetary science missions of the decade, ESA’s Rosetta spacecraft is currently chasing 67P/Churyumov–Gerasimenko through the inner Solar System for an orbital rendezvous later this summer, which will hopefully make it the first ever robotic spacecraft to orbit and land on a comet. Having launched in March 2004, Rosetta spent most of its time quietly conducting a series of gravity assists of Earth and Mars, while gaining the necessary speed in order to catch up with the 6.5-year-long orbit of 67P/Churyumov–Gerasimenko around the Sun. Following a 31-month deep-space slumber, Rosetta was successfully brought out of hibernation on Jan. 20, signaling the start of its final approach toward the two-mile-wide (4.5-km) comet, which is currently midway between the orbits of Mars and Jupiter, accelerating toward the Sun prior to its perihelion in August 2015.
With the spacecraft successfully re-awakened, the mission’s science team initiated a commissioning phase of Rosetta’s payload of 11 science instruments, three of which were provided by NASA, as well as that of the onboard Philae lander which is scheduled to soft-land on the comet’s surface in late 2014. This two-month commissioning phase allowed scientists to confirm that all of the spacecraft systems were operating nominally, while giving the green light for the start of routine science operations in mid-May. Rosetta has been observing the comet with its onboard instruments ever since, including the Optical, Spectroscopic, and Infrared Remote Imaging System, or OSIRIS, which has been taking a series of images of the comet’s nucleus, allowing the spacecraft to fine-tune its trajectory as it approaches ever closer to its intended target. Most notably, these observations showed that the activity on 67P/Churyumov–Gerasimenko has winded down significantly following the formation of the comet’s tail in April, when the comet was brighter. “After its onset of activity at the end April, our images are currently showing a comet back at rest,” says Dr. Holger Sierks, a planetary scientist at the Max Planck Institute for Solar System Research in Göttingen, Germany, and Principal Investigator for the OSIRIS instrument. “The variable activity of the comet shows it definitely has personality, which makes us all the more eager to get there to learn just how it ticks,” adds Matt Taylor, Rosetta’s Project Scientist.
The comet’s volatile personality became more evident during a series of observations in June, when NASA’s Microwave Instrument for Rosetta Orbiter, or MIRO, detected a resurgence of activity, in the form of streams of water vapor outgassing from the nucleus. Such outgassing activity is a normal process for a comet when it is closer to the Sun on its highly elliptical orbit, resulting from the sublimation of all the volatile materials from the comet’s nucleus like water vapor, carbon monoxide, and ammonia, due to the heat from the nearby Sun. But in the current point in its orbit, approximately 362 million miles (583 million km) from the Sun, midway between Mars and Jupiter, 67P/Churyumov–Gerasimenko was expected to be less active. Yet Rosetta’s MIRO instrument has been constantly detecting excessive amounts of water vapor streaming from the comet ever since it first detected the water signatures in early June, from a distance of 217,480 miles (350,000 km) away. Scientists have calculated that the amount of the observed water outgassing is the equivalent of two small glasses of water spewed out into space every second. “We always knew we would see water vapor outgassing from the comet, but we were surprised at how early we detected it,” says Sam Gulkis, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and Principal Investigator for the MIRO instrument. “At this rate, the comet would fill an Olympic-size 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.”
The study of 67P/Churyumov–Gerasimenko’s activities in real-time and its nucleus environment and overall properties will be Rosetta’s main mission, when the spacecraft finally enters orbit around the comet on Aug. 4 and releases its onboard Philae lander in November, for a daring touchdown on the comet’s surface. Before this happens, however, the spacecraft must first safely reach 67P/Churyumov–Gerasimenko. To that end, mission designers have planned for a total of 10 thruster burns, officially known as orbit correction manoeuvres, or OCMs, that will allow Rosetta to change its orbital trajectory in order to match that of 67P/Churyumov–Gerasimenko and decelerate its speed enough so as to achieve orbit. “By doing the manoeuvres, we continuously change Rosetta’s speed so that on 6 August, they have the same speed and position,” says Rosetta Flight Director Andrea Accomazzo. The first four, which have been dubbed “the big burns” because of their significance for reducing the spacecraft’s speed, have been already completed successfully. Another 6 OCMs remain, with the last one on Aug. 6 being the final orbital insertion maneuver.
Meanwhile, mission planners will increase the rate of images of 67P/Churyumov–Gerasimenko with Rosetta’s onboard OSIRIS instrument to one per week, starting the first week of July until the orbit insertion day. With the distance between the spacecraft and the comet ever decreasing, scientists will have the opportunity to get more detailed views of the comet’s nucleus, which will provide some important science, before the start of orbital observations later this year. “Our comet is coming out of its deep-space slumber and beginning to put on a show for Rosetta’s science instruments,” says Taylor. “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.”
With Rosetta fast approaching its final destination, the following weeks promise to be truly exciting for planetary science, as we gear up for the first orbiting and, eventually, landing on a comet which, if successful, will be one of the greatest technical achievements in the history of space exploration.
Video Credit: ESA