Early tomorrow morning, the European Space Agency's Rosetta spacecraft will deploy its comet lander, "Philae." A little over seven hours later (8 a.m. PST/11 a.m. EST), the experiment-laden, harpoon-firing Philae is scheduled to touch down on the surface of comet 67P/Churyumov-Gerasimenko.

It will be the first time in history that a spacecraft has attempted a soft landing on a comet. Rosetta is an international mission led by the European Space Agency (ESA), with instruments provided by its member states, and additional support and instruments provided by NASA.

"I know it sounds like something out of Moby Dick, but when you think about the gravity field of a comet, it makes a lot of sense to harpoon one," said Art Chmielewski, project manager for the U.S. participation in Rosetta, from NASA's Jet Propulsion Laboratory in Pasadena, California.

"Comet 67P has approximately 100,000 times less gravity than Earth does. So, if you don't want to float away, you have to go to extraordinary measures to attach yourself to its dusty surface. The Philae lander has two harpoons, shock-absorbing landing gear, and a drill located on each of the lander's three feet. It even has a small, upward-firing rocket engine. All this to help keep it on the surface."

The descent of Philae begins at 1:03 a.m. PST (4:03 a.m. EST) when Rosetta releases the 220-pound (100-kilogram) Philae from an altitude of about 14 miles (23 kilometers) from the center of the comet's nucleus.

As Philae descends, it will fall slowly without propulsion or guidance, gradually gathering speed in the comet's weak gravitational field. During the seven-hour descent, the lander will take images and conduct science experiments, sampling the environment close to the comet. It will take a "farewell" image of the Rosetta orbiter shortly after separation, along with a number of images as it approaches the comet surface.

The targeted landing site is called Agilkia after an island in the Nile River in southern of Egypt where ancient buildings from the Nile's flooded Philae island were relocated. Once the lander has touched down and safely anchored, it will begin a primary science mission, which extends to about two-and-a-half days.

Philae will take a panorama of its surroundings and perform on-the-spot analysis of the composition of the comet's surface. It can drill samples from a depth of nine inches (23 centimeters) and feed them to the onboard laboratory for analysis. The lander will also measure electrical and mechanical characteristics of the surface of the nucleus.

After the Philae landing is completed, Rosetta will begin the next major part of its mission, the escort phase. The orbiter will continue to maneuver around the comet at walking pace, collecting dust and gas samples and making remote sensing observations as the comet warms up and the nucleus and its environment evolve.

The comet will reach its closest point to the sun (perihelion) in August 2015. Rosetta will then track the waning of activity as the comet heads back toward the cold, far reaches of the outer solar system, through 2015.

Three NASA science instruments are aboard the Rosetta spacecraft; 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 on the 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. MIRO has the ability to study water, carbon monoxide, ammonia and methanol.

Alice will analyze gases in the comet's coma, which is the bright envelope of gas around the nucleus of the comet developed as a comet 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.

Alice 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 is 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 26 instrument collaborations. NASA's Deep Space Network is supporting ESA's Ground Station Network for spacecraft tracking and navigation.

"Landing a spacecraft on a comet is a remarkable challenge and we wish them well," said Claudia Alexander, project scientist for the U.S. Rosetta project from JPL.

"It is the overture, opening for what has already become an exciting mission of exploration. Whatever happens tomorrow, the main part of the Rosetta mission will continue with the Rosetta orbiter monitoring the comet as it sweeps through the inner-solar system."

Comets are time capsules containing primitive material left over from the epoch when the sun and its planets formed. Rosetta's lander will obtain the first images taken from a comet's surface and will provide comprehensive analysis of the comet's possible primordial composition by drilling into the surface.

Rosetta also will be the first spacecraft to witness at close proximity how a comet changes as it is subjected to the increasing intensity of the sun's radiation. Observations will help scientists learn more about the origin and evolution of our solar system and the role comets may have played in seeding Earth with water, and perhaps even life.