"After 10 years, five months and four days travelling towards our destination, looping around the sun five times and clocking up 6.4 billion kilometers, we are delighted to announce finally we are here," said Jean-Jacques Dordain, ESA's director General.

 The latest images of the comet taken by Rosetta are available at http://www.nasa.gov/rosetta

Comet 67P/Churyumov-Gerasimenko and Rosetta are 252 million miles (405 million kilometers) from Earth, about halfway between the orbits of Jupiter and Mars. The comet is in an elliptical, 6.5-year orbit that takes it from beyond Jupiter at its farthest point, to between the orbits of Mars and Earth at its closest to the sun. Rosetta will accompany the comet for over a year as it swings around the sun and back out towards Jupiter again.

Rosetta is 62 miles (100 kilometers) from the comet's surface. Over the next six weeks, it will fly two triangular-shaped trajectories in front of the comet, first at the 62-mile (100-kilometer) altitude and then down at 31 miles (50 kilometers). At the same time, the spacecraft's suite of instruments will provide a detailed scientific study of the comet, scanning the surface to identify a target site for its comet lander, Philae. Eventually, Rosetta will attempt a close, near-circular orbit at 19 miles (30 kilometers) and, depending on the activity of the comet, may come even closer.

"Over the next few months, in addition to characterizing the comet nucleus and setting the bar for the rest of the mission, we will begin final preparations for another space history first: landing on a comet," said Matt Taylor, Rosetta's project scientist from the European Space Agency's Science and Technology Centre in Noordwijk, The Netherlands.

As many as five possible landing sites will be identified by late August, before the primary site is identified in mid-September. The final timeline for the sequence of events for deploying Philae -- currently expected for Nov. 11 -- will be confirmed by the middle of October.

Close up detail focusing on a smooth region on the ‘base’ of the ‘body’ section of comet 67P/Churyumov-Gerasimenko. The image was taken by Rosetta’s Onboard Scientific Imaging System (OSIRIS) on August 6, 2014. Credits: ESA/Rosetta/MPS for OSIRIS Team

Comets are considered to be primitive building blocks of the solar system and may have helped to "seed" Earth with water, perhaps even the ingredients for life. But many fundamental questions about these enigmatic objects remain, and through a comprehensive, in situ study of the comet, Rosetta aims to unlock the secrets within.

The three U.S. instruments aboard the spacecraft are the Microwave Instrument for Rosetta Orbiter (MIRO), an ultraviolet spectrometer called Alice, and the Ion and Electron Sensor (IES). They are part of a suite of 11 science instruments aboard the Rosetta orbiter.

MIRO is designed to provide data on how gas and dust leave the surface of the nucleus to form the coma and tail that give comets their intrinsic beauty. Studying the surface temperature and evolution of the coma and tail provides information on how the comet evolves as it approaches and leaves the vicinity of the sun.

Alice will analyze gases in the comet's coma, which is the bright envelope of gas around the nucleus of the comet developed as it approaches the sun. Alice also will measure the rate at which the comet produces water, carbon monoxide and carbon dioxide. These measurements will provide valuable information about the surface composition of the nucleus.

The instrument also will measure the amount of argon present, an important clue about the temperature of the solar system at the time the comet's nucleus originally formed more than 4.6 billion years ago.

IES is part of a suite of five instruments to analyze the plasma environment of the comet, particularly the coma. The instrument will measure the charged particles in the sun's outer atmosphere, or solar wind, as they interact with the gas flowing out from the comet while Rosetta is drawing nearer to the comet's nucleus.

NASA also provided part of the electronics package for the Double Focusing Mass Spectrometer, which is part of the Swiss-built Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument. ROSINA will be the first instrument in space with sufficient resolution to be able to distinguish between molecular nitrogen and carbon monoxide, two molecules with approximately the same mass. Clear identification of nitrogen will help scientists understand conditions at the time the solar system was formed.

U.S. scientists are partnering on several non-U.S. instruments and are involved in seven of the mission's 21 instrument collaborations. NASA's Deep Space Network is supporting ESA's Ground Station Network for spacecraft tracking and navigation.

Launched in March 2004, Rosetta was reactivated in January 2014 after a record 957 days in hibernation. Composed of an orbiter and lander, Rosetta's objectives upon arrival at comet 67P/Churyumov-Gerasimenko in August are to study the celestial object up close in unprecedented detail, prepare for landing a probe on the comet's nucleus in November, and track its changes as it sweeps past the sun.