“We are all holding our breath to see what discoveries await us in the observations near closest approach,” Michael A’Hearn, a University of Maryland astronomer and the mission’s principal investigator, said in a statement.

The Deep Impact spacecraft officially completed its mission in 2005 after it visited the comet Tempel 1, letting loose an 800-pound piece of equipment that crashed into that comet as it passed by. The collision allowed scientists to identify some of the minerals beneath the surface of Tempel 1.

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But the spacecraft still had plenty of maneuvering fuel left, so NASA approved a second mission, called Epoxi, that included a visit to a second comet. Three years ago, the mission had to shift course when the comet it was aiming for, Comet Boethin, could not be found. The backup target was Hartley 2.

The odd shape of Hartley 2 could mean that the comet, three-quarters of a mile wide, was originally two objects that somehow spiraled into each other and stuck.

Comets are believed to contain primordial materials that date back to the beginning of the solar system, some 4.5 billion years ago. “If we understand the comets really well, it will tell us how the planets got made,” Dr. A’Hearn said. “That’s why we choose comets to study.”

Epoxi is a combination of two acronyms: Epoch, or Extrasolar Planet Observation and Characterization, which has been using one of the spacecraft’s cameras to look at stars known to have planets, and Dixi, or Deep Impact Extended Investigation, for the second comet fly-by.

Back on Earth, NASA had less luck with the space shuttle Discovery, which was originally scheduled to launch on Monday but has been delayed by various setbacks. Most recently, bad weather at the Kennedy Space Center in Florida postponed the launching to Friday afternoon at the earliest.

The 11-day mission, which will take supplies to the International Space Station, is to be the 39th and last flight of the Discovery as the space shuttle program winds down.