Traci Watson

Special for USA TODAY

The anxiety isn't over yet.

After a nerve-racking 7-hour descent, a miniature spacecraft drifted softly onto the surface of a comet more than 300 million miles away from Earth on Wednesday morning. It was the first craft in history to land on a comet, and the apparently successful feat provoked hugs, cheers and joyous laughter in the control room in Germany.

But the jubilation over the touchdown of the washing-machine-size ship, known as Philae, was quickly overshadowed. Within an hour after the landing was confirmed, engineers learned that contrary to plan, Philae had failed to fire its two harpoons into the comet's dusty soil. The harpoons were designed to anchor the spacecraft firmly to the comet's surface, partly due to fears that the craft could ricochet off the strange terrain and back into space.

Preliminary data suggest that the lander came down "abruptly" on one of its three legs and softly on the other two, then "bounced slightly off" the comet before settling onto the surface again, Finnish Meteorological Institute research manager Walter Schmidt told USA TODAY. Schmidt heads the team responsible for a Philae scientific instrument that has sensors in the spacecraft's feet.

"Maybe today we didn't just land once, we even landed twice," Philae lander manager Stephan Ulamec joked at a news conference Wednesday. There are some signs that the spacecraft started to rotate after lifting off again, but if so, it has stopped rotating, Ulamec said.

One option is try to fire the harpoons again. Even if the harpoons don't work, a good number of the 10 scientific instruments on Philae could still gather data, Schmidt said. But the situation is grimmer for Philae's drill and the probes on the harpoons unless the harpoons can be coaxed to function properly.

The experiments on Philae are designed to reveal the comet's composition, how it changes as it approaches the sun and more. Such clues could help scientists understand conditions in the solar system more than 4 billion years ago, when the comet formed.

The harpoon problems clouded a day of both anxiety and triumph for the flight control team. The little spaceship cast off at roughly 4 a.m. ET from the mother ship that had carried it close to the comet. Then, as mission personnel waited in anxious suspense, Philae descended at a leisurely pace toward the comet's rugged surface. There was no way to steer it on its way down, no way to change course and no going back.

A few minutes past 11 a.m. ET, engineers received word that the spaceship, which belongs to the European Space Agency, had alighted safely on the comet's crater-scarred and boulder-filled landscape.

"We can't be happier than we are now," Andrea Accomazzo, Rosetta flight director, said, before the harpoon problem became known.

The landing was "a difficult procedure. … There are so many uncertainties," said Gostar Klingelhoefer of Germany's Johannes Gutenberg University and the head of an experiment that will analyze the chemical composition of the comet's surface. The initial report of a successful landing "was a big relief, I have to say."

Even if the harpoons never fire, many consider the mission a victory. Google honored the lander's touchdown with a Google Doodle, and NASA'S director of planetary science, Jim Green, lauded the mission as "audacious."

"The number one thing: PHILAE IS A SUCCESS. Full stop," tweeted Emily Lakdawalla, a planetary scientist who blogs for the Planetary Society, an advocacy group that champions space exploration and space science.

Philae and its carrier spacecraft Rosetta spent a decade just getting to the comet, launching from Earth in 2004 and finally pulling up in August next to comet Churyumov-Gerasimenko, also known as comet 67P. The comet is tracing a 6 1/2-year oval around the solar system.

Rosetta's arrival at 67P gave scientists a thrill: The comet wasn't potato-shaped as had been expected but a complex rubber-duck shape. For mission engineers, the comet's profile was a nasty surprise rather than an intriguing revelation. That irregular shape made it trickier for Rosetta to orbit the comet and drop the lander in exactly the right place on the mountain-size comet.

The comet's terrain also posed challenges. It is pocked with deep depressions and dotted with building-size boulders, and it boasts slopes so steep that they make the toughest ski resort look tame. On its first landing, at least, Philae came down in the dead center of the designated landing zone, which has relatively forgiving terrain.

Scientists are now working against time. Philae's initial battery life is only 64 hours, and no one knows whether after the battery dies the craft's solar panels will work well enough to power all the experimental gear for an extended time.

Researchers hope to keep gathering information from Philae until March of next year, when the spacecraft is projected to become too hot to continue. Eventually managers may command Rosetta to make a soft landing on the comet's surface too, reuniting the two spacecraft once more.