PARIS — The European Space Agency on Sept. 15 announced the selection of a target site for its 100-kilogram Philae comet lander and began the difficult task of evoking the mission’s historic importance and at the same time dialing down expectations for its success.

In a briefing at the 20-member agency’s headquarters here, managers of the Rosetta comet-chaser mission, whose main spacecraft is now circling Comet 67P at a distance of just 30 kilometers, said the selected site is the best of a difficult set of options given the nature and shape of the comet.

The selected target zone, labeled “J” as ESA was narrowing the landing choices, is on the “head” of the comet, whose odd shape has been compared to a rubber duck.

Mission managers recalled that when Rosetta was designed 15 years ago or more, no one had any idea of what kind of comet the probe would encounter. Its size, gravity and surface properties all were unknown.

“This is the best landing place on this comet,” Rosetta Flight Director Andrea Accomazzo said. “This is what we have to live with.”

The Rosetta mothership is expected to close to within 20 kilometers of Comet 67P, and then to 10 kilometers, in the coming weeks before final decisions are made on whether the “J” site is in fact the best of the lot. The Nov. 11 landing date should be confirmed Sept. 26. ESA’s Rosetta Lander Consortium will make final decisions Oct. 12 and Oct. 14 in concert with ESA managers.

Once released from Rosetta at a distance of 20 kilometers from the comet, the Philae lander will take about seven hours to drop onto its surface under a force of gravity so slight that ESA officials said it is equivalent to dropping a 1-gram object on Earth. The speed at touchdown is expected to be 1 meter per second.

If the “J” site is ultimately rejected, a backup site will be used and the landing could be delayed by as many as 28 days.

Rosetta and Philae each carry about half the mission’s total of 20 experiments. Whatever happens with the landing, Rosetta has already delivered a rich set of data that comet scientists will be evaluating for years.

In addition, Rosetta will continue to follow Comet 67P for more than a year as it makes its way toward the sun and then past it.

Rosetta Mission Manager Fred Jansen said ESA had made a detailed risk assessment of the Philae mission when its target was presumed to be a smooth, round comet that presented no unforeseen landing challenges. In that scenario, he said, the chances of success — that is, the likelihood that Philae would be able to attach itself stably to the comet and conduct its experiments — was judged to be 70-75 percent.

“The double structure of [Comet 67P] complicates the mission,” Jansen said, adding that “none of the sites meet all the criteria” that had been set for landing.

Jansen said the detailed knowledge of Comet 67P has come so recently that there has been no time to perform a full risk analysis. He declined to give a probability of success but the clear implication was that it is far less than 70 percent.

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Peter B. de Selding was the Paris bureau chief for SpaceNews.