After a 10-year journey that included 31 tense months of hibernation, the European Space Agency's Rosetta spacecraft began traveling in tandem with comet 67P/Churyumov–Gerasimenko Wednesday, heralding the start of the most detailed space-based observations of a comet ever made.
The announcement came at 5:29 a.m. Eastern Daylight Time when Rosetta's operations engineer Silvian Lodiot shouted, "We're at the comet!" as he monitored data coming in from the craft some 342 million miles away.
Comet 67P/Churyumov–Gerasimenko is a relic of the early solar system whose orbit around the sun sends it as far out as 5.7 astronomical units, or 5.7 times the distance between Earth and the sun, and brings it as close as 1.2 AU from the sun. The Rosetta mission has been timed to observe the comet's behavior before, during, and after its closest approach, which will occur in August 2015.
Initially, Rosetta will trace two triangular paths in front of the oddly shaped comet at successively closer distances. The goal is to gather images that will be used to select a landing site for Rosetta's lander, Philae. The craft also will measure the comet's mass, which until now has been estimated through modeling.
Armed with the comet's mass, the mission team will be able to calculate the comet's gravitational field, crucial for finally placing Rosetta into an orbit around the comet and for landing Philae on the nucleus's surface.
Mission planners hope to place Rosetta into a nearly circular orbit about 18 to 19 miles above the nucleus. If all goes well, this will be the first spacecraft to orbit and release a lander onto a comet nucleus.
Comets, as well as asteroids, are thought to represent the extra parts left over after planets formed some 4.6 billion years ago. The opportunity to study the structure and behavior of a comet nucleus is expected to yield insights into how the planets formed, and at least in Earth's case, how some of the planet's water might have been delivered.
Mission controllers awakened Rosetta from its Rip Van Winkle-like slumber Jan. 20, and the closer the craft has gotten to 67P/Churyumov–Gerasimenko, the more bizarre the nucleus has appeared.
Based on Hubble Space Telescope images, it was thought to be a block of rubble and ice 2.5 miles long and 2.2 miles wide. As Rosetta has gotten closer, however, images have grown sharper, yielding a shape more akin to a potato, and finally to a child's rubber ducky.
How that shape came to be is one of the mysteries Rosetta will help solve.
"We don't know if it's a contact binary or if it's carved like this" by eruptions of gas from the interior as the comet heats during its closest approaches to the sun, said Holger Sierks, the lead investigator for Rosetta's remote imaging system, known as OSIRIS, during a pre-arrival program at ESA's European Space Operations Center in Darmstadt, Germany.
For asteroids and comets, a contact binary forms through the gentle merger of two separate objects.
An image of the comet taken Aug. 3 shows the nucleus's two lobes, a deep bight in the nucleus about 3,300 feet deep, cliffs some 500 feet high, and house-sized boulders, as well as smooth areas.
The image of the nucleus is a tantalizing foretaste of what Rosetta will offer during the mission. It represents the most detailed picture yet taken from space of a comet's core, said Dr. Sierks, a researcher with the Max Planck Institute for Solar System Research in Göttingen, Germany.
In addition, mission scientists have found that the maximum temperature on the surface of the nucleus is higher than expected – about 64 degrees below zero Fahrenheit.
"We were surprised to see this large temperature on the surface," said Fabrizio Capaccioni, a researcher with Italy's Institute for Space Astrophysics and Planetology in Rome. Dr. Capaccioni is the lead scientist for VIRTIS, Rosetta's Visible and Infrared Thermal Imaging Spectrometer.
The temperature reading suggests that for the first inch or two, the nucleus's surface "is porous, dusty, and with no ice," he said.
In addition, researchers have been using Rosetta's small microwave radio telescope, dubbed MIRO, to analyze the gas the comet is shedding, including water vapor. For now, the emissions amount to about two small glassfuls of water per second.
Sam Gulkis, a researcher at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the lead scientist for MIRO, noted that the instrument has shown that very little gas erupting from the dark face of the comet as the nucleus rotates once every 12.7 hours.
Overall, Rosetta is carrying 11 instruments to make a variety of measurements, while Philae carries another 10 instruments. Among Philae's tasks: send radar signals through the nucleus to Rosetta to open a window on its internal structure.
Overall, "Rosetta is the sexiest science mission, the sexiest space mission that's ever been," enthused Matt Taylor, the mission's project scientist.
Rosetta was launched in March 2004 – a delay of just over a year, which resulted in a change of targets. The craft was initially slated for launch in January 2003 and a rendezvous with comet 46P/Wirtanen in 2011. But the type of rocket that was to loft it, an Ariane V, was destroyed three minutes into an attempt to loft a communications satellite in December 2002. This forced Rosetta's postponement.
Rosetta used the gravitational kicks from two Earth flybys and one Mars flyby to give it the speed it needed to reach its rendezvous point with the comet in time to observe the comet during its closest approach to the sun.
Along the way, the probe performed close flybys of two asteroids, 2867 Steins and 21 Lutetia, to the delight of researchers studying these objects.
Before the craft could rendezvous with 67P/Churyumov–Gerasimenko, however, it had to travel beyond Jupiter – too far for its solar panels to provide power to all of the craft's systems. So controllers put the craft to sleep in June 2011 – providing power only to the solar-panel controls, the craft's main computers, and enough heaters to keep the electronics from freezing.
In many ways, the runup to Rosetta's Great Awakening was far more tense than Wednesday's rendezvous, mission officials said. Thirty-one months is a long time to be out of contact with a spacecraft.
If all goes well, mission planners anticipate selecting five potential landing sites for Philae by the end of August, with the finalist selected in September. Plans call for Rosetta to release the lander Nov. 11 for its touchdown on 67P/Churyumov–Gerasimenko.
Once Rosetta begins to orbit the comet, it is expected to continue its observations until December 2015.