London (CNN) The comet probe Philae may have bounced when it landed, the European Space Agency said Wednesday.

It is the first time a soft landing has been achieved on a comet -- but there was a hitch.

Comet 67P has a very weak gravity, so anchoring harpoons were designed to shoot into the comet to fix the spacecraft to the surface. They failed to fire and Philae is not firmly secure, ESA says.

Philae lander manager Stephan Ulamec said the probe may have lifted off again and turned.

"So maybe we didn't land once -- we landed twice," he told a news conference.

"Did we land in a soft sandbox or is there something else happening? We don't fully understand what happened," he said.

Ulamec said scientists remain upbeat as they are still receiving data from the spacecraft and they hope to learn more Thursday.

Shortly after landing was confirmed, the probe tweeted: "Touchdown! My new address: 67P!" Later, it tweeted again: "I'm on the surface but my harpoons did not fire."

I'm on the surface but my harpoons did not fire. My team is hard at work now trying to determine why. #CometLanding — Philae Lander (@Philae2014) November 12, 2014

Led by ESA with a consortium of partners including NASA, scientists on the Rosetta comet-chasing mission hope to learn more about the composition of comets and how they interact with the solar wind -- high energy particles blasted into space by the sun.

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Speaking to CNN shortly after the landing, ESA lander system engineer Laurence O'Rourke said they were trying to check the orientation of the craft, to see "how we landed and where we landed."

"We are extremely happy," he said. "To get the signal it had touched the surface was a major achievement -- it was quite extraordinary. Philae is already taking measurements, sniffing the comet."

He also said the probe could start drilling into the surface and analyzing the material as soon as Thursday.

The comet is currently 500 million kilometers (310 million miles) from Earth and pictures from the Rosetta mission to track it on its orbit around the sun have amazed scientists.

NASA's chief scientist, Ellen Stofan, told CNN: "That comet is the most bizarre, wonderful thing I have ever seen. Those images have just blown me away. Philae was such a huge success before it even did anything, so kudos to ESA and the scientific community.

"No one has ever gotten data like Rosetta has gotten. No one has ever been able to land on a comet the way Philae just did."

Photos: Rosetta: The comet chaser The Rosetta probe sent an unexpected final image back to Earth shortly before it made a controlled impact onto the surface of Comet 67P last September. Hide Caption 1 of 40 Photos: Rosetta: The comet chaser Philae has been found! – The Rosetta spacecraft's high-resolution camera took this image of the Philae lander on September 2, 2016. The lander is wedged into a dark crack on a comet, named 67P/Churyumov--Gerasimenko, hurtling through space. The discovery comes less than a month before the Rosetta mission's end. Hide Caption 2 of 40 Photos: Rosetta: The comet chaser Philae is wedged in a dark corner of Rosetta – "We are so happy to have finally imaged Philae, and to see it in such amazing detail," says Cecilia Tubiana of the OSIRIS camera team. She was the first person to see the images when they were downlinked from the Rosetta probe, according to the European Space Agency. Hide Caption 3 of 40 Photos: Rosetta: The comet chaser Philae found using images from Rosetta – The image is detailed enough that viewers can pick out features of Philae's 3-foot-wide (1 meter) body. Even two of its three legs can be seen. Hide Caption 4 of 40 Photos: Rosetta: The comet chaser One of the primary objectives of the Rosetta mission was to drop the Philae lander onto the comet. The probe was successfully deployed in November 2014, becoming the first probe to land on a comet. But Philae failed to grab onto the comet and bounced around. It fell silent a few days later. Then on June 13, 2015, Philae came out of hibernation and "spoke" to mission managers at the European Space Agency for 85 seconds. This photo above was taken by the lander's mothership, the Rosetta orbiter, after the lander started its descent to the comet. Hide Caption 5 of 40 Photos: Rosetta: The comet chaser The Rosetta spacecraft captured this image of a jet of white debris spraying from Comet 67P/Churyumov--Gerasimenko on July 29, 2015. Mission scientists said this was the brightest jet seen to date in the mission. The debris is mostly of ice coated with dark organic material. Hide Caption 6 of 40 Photos: Rosetta: The comet chaser This image of Comet 67P/Churyumov-Gerasimenko was taken by Rosetta on July 8, 2015 as the spacecraft and comet headed toward their closest approach to the sun. Rosetta was about 125 miles (201 kilometers) from the comet when it took this image. Hide Caption 7 of 40 Photos: Rosetta: The comet chaser Philae wakes up! Mission managers posted this cartoon of the lander yawning after it came out of hibernation on June 13, 2015. They also sent a series of tweets between the lander and its mothership, Rosetta. Hide Caption 8 of 40 Photos: Rosetta: The comet chaser Comet 67P/Churyumov-Gerasimenko is a Jupiter-family comet. Its 6.5 year journey around the Sun takes it from just beyond the orbit of Jupiter at its most distant, to between the orbits of Earth and Mars at its closest. The comet hails from the Kuiper Belt, but gravitational perturbations knocked it towards the Sun where interactions with Jupiter's gravity set it on its present-day orbit. Hide Caption 9 of 40 Photos: Rosetta: The comet chaser This image of Comet 67P/Churyumov-Gerasimenko was taken by Rosetta on June 5, 2015, while the spacecraft was about 129 miles (208 kilometers) from the comet's center. Hide Caption 10 of 40 Photos: Rosetta: The comet chaser Rosetta's navigation camera took this image of the comet on June 1, 2015. Hide Caption 11 of 40 Photos: Rosetta: The comet chaser The Rosetta Mission is tracking Comet 67P/Churyumov-Gerasimenko on its orbit around the sun. This image was taken on May 3, 2015 at a distance of about 84 miles (135 km) from the comet's center. Hide Caption 12 of 40 Photos: Rosetta: The comet chaser This image of Comet 67P/Churyumov-Gerasimenko was taken on April 15, 2015. Hide Caption 13 of 40 Photos: Rosetta: The comet chaser Rosetta snapped this wide-angle view of Comet 67P/Churyumov-Gerasimenko in September 2014. Rosetta was about 107 million miles (172 million kilometers) from Earth and about 92 million miles (148 million kilometers) from the sun when the photo was released. Hide Caption 14 of 40 Photos: Rosetta: The comet chaser A camera on Rosetta took this picture of Comet 67P/Churyumov-Gerasimenko on November 22, 2014, from a distance of about 19 miles (31 kilometers). The nucleus is deliberately overexposed to reveal jets of material spewing from the comet. The 2.5-mile-wide (4-kilometer) comet has shown a big increase in the amount of water its releasing, according to NASA. The space agency says about 40 ounces (1.2 liters) of water was being sprayed into space every second at the end of August 2014. Hide Caption 15 of 40 Photos: Rosetta: The comet chaser Rosetta took this picture of a section of the comet's two lobes from a distance of about 5 miles (8 kilometers) on October 14, 2014. Hide Caption 16 of 40 Photos: Rosetta: The comet chaser The Rosetta spacecraft's Philae lander is shown sitting on Comet 67P/Churyumov-Gerasimenko after becoming the first space probe to land on a comet on November 12, 2014. The probe's harpoons failed to fire, and Philae bounced a few times. The lander was able to send back images and data for 57 hours before losing power. Hide Caption 17 of 40 Photos: Rosetta: The comet chaser Rosetta's lander, Philae, wasn't able to get a good grip on the comet after it touched down. This mosaic shows Philae's movements as it bounced across the comet. Hide Caption 18 of 40 Photos: Rosetta: The comet chaser Philae snapped these images after landing, and mission scientists used them to create a panoramic view of the landing site. A graphic shows where the probe would be sitting in the photograph. Hide Caption 19 of 40 Photos: Rosetta: The comet chaser The image of Comet 67P/Churyumov-Gerasimenko was taken by a camera on the Philae lander during its descent to the comet on November 12, 2014. The lander was about 1.9 miles (3 kilometers) from the surface at the time. Philae touched down on the comet about seven hours later. Hide Caption 20 of 40 Photos: Rosetta: The comet chaser Rosetta's OSIRIS camera captured this parting shot of the Philae lander after separation. Hide Caption 21 of 40 Photos: Rosetta: The comet chaser Rosetta's lander Philae took this parting shot of its mother ship shortly after separation on November 12, 2014, as Philae headed for a landing on Comet 67P. While Philae is the first probe to land on a comet , Rosetta is the first to rendezvous with a comet and follow it around the sun. Hide Caption 22 of 40 Photos: Rosetta: The comet chaser This mosaic is made of four individual images taken about 20 miles (31.8 kilometers ) from the center of the comet on November 4, 2014. Hide Caption 23 of 40 Photos: Rosetta: The comet chaser Rosetta took this image of comet 67P/Churyumov-Gerasimenko on September 15, 2014. The box on the right shows where the lander was expected to touch down. Hide Caption 24 of 40 Photos: Rosetta: The comet chaser The spacecraft sent this image as it approached the comet on August 6, 2014. From a distance of nearly 81 miles (130 kilometers), it reveals detail of the smooth region on the comet's "body" section. Hide Caption 25 of 40 Photos: Rosetta: The comet chaser This image, captured August 7, 2014, shows the diversity of surface structures on the comet's nucleus. Hide Caption 26 of 40 Photos: Rosetta: The comet chaser The comet's "head" can be seen in the left of the frame as it casts a shadow over the "body" in this image released August 6, 2014. Hide Caption 27 of 40 Photos: Rosetta: The comet chaser This image of the comet was taken on August 1, 2014, as Rosetta closed in its target. Hide Caption 28 of 40 Photos: Rosetta: The comet chaser Rosetta's mission started on March 2, 2004, when it was launched on a European Ariane 5 rocket from Kourou, French Guiana. Hide Caption 29 of 40 Photos: Rosetta: The comet chaser Rosetta is named after the Rosetta Stone, the black basalt that provided the key to deciphering Egyptian hieroglyphs. Scientists think the mission will give them new clues about the origins of the solar system and life on Earth. The mission is spearheaded by the European Space Agency with key support from NASA. Hide Caption 30 of 40 Photos: Rosetta: The comet chaser This photo shows Rosetta being tested before it was wrapped in insulating blankets and loaded on a rocket for launch. Hide Caption 31 of 40 Photos: Rosetta: The comet chaser Rosetta has massive solar wings to power the spacecraft. They were unfurled and checked out at the European Space Agency's test facilities before being packed up for liftoff. Hide Caption 32 of 40 Photos: Rosetta: The comet chaser After its closest approach to Earth in November 2007, Rosetta captured this image of the planet. Hide Caption 33 of 40 Photos: Rosetta: The comet chaser Rosetta snapped this image of Earth in November 2009. The spacecraft was 393,328 miles from Earth. Hide Caption 34 of 40 Photos: Rosetta: The comet chaser Rosetta passed asteroid Steins in September 2008, giving scientists amazing close-ups of the asteroid's huge crater. The asteroid is about 3 miles in diameter. Hide Caption 35 of 40 Photos: Rosetta: The comet chaser Rosetta took this image of Mars as it looped through the solar system. Hide Caption 36 of 40 Photos: Rosetta: The comet chaser This image was taken by an instrument on Rosetta's Philae lander just minutes before the spacecraft made its closest approach to Mars. Part of Rosetta and its solar arrays are visible. Hide Caption 37 of 40 Photos: Rosetta: The comet chaser On July 10, 2010, Rosetta flew about 1,864 miles from asteroid Lutetia, which is 10 times larger than asteroid Steins. Hide Caption 38 of 40 Photos: Rosetta: The comet chaser Look closely at the top of this picture. See that dot? That's Saturn. Rosetta snapped the picture of asteroid Lutetia and captured Saturn in the background. Hide Caption 39 of 40 Photos: Rosetta: The comet chaser After taking pictures of Earth, Mars and asteroids, Rosetta was put into hibernation in May 2011 after it reached the outer part of the solar system. Mission managers woke it January 20, 2014. Hide Caption 40 of 40

Astronomer Robert Massey also called the landing a tremendous achievement.

"A comet is a bit like a time capsule, which is why it's special," he told CNN.

"It's an insight into the composition of the early solar system -- the ratio of chemicals.

"Even without Philae, we still have Rosetta in orbit around a comet, giving the ability to watch it at close quarters. There will be a lot of science from this," he said.

Weighing in at 220 pounds, it might be the size of a domestic washing machine but Philae is considerably smarter.

It is equipped with an array of experiments to photograph and test the surface of Comet 67P as well as finding out what happens when the roasting effect of the sun drives off gas and dust.

Built by a European consortium, led by the German Aerospace Research Institute (DLR), the landing probe has nine experiments.

According to details on ESA's Rosetta website, sensors on the lander will measure the density and thermal properties of the surface, gas analyzers will help to detect and identify any complex organic chemicals that might be present, while other tests will measure the magnetic field and interaction between the comet and solar wind.

Philae also carries a drill that can drive 20 centimeters (8 inches) into the comet and deliver material to its on-board ovens for testing.

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Scientists are already pleased with progress of the mission.

ESA project scientist Matt Taylor said: "The orbiter will remain alongside the comet for over a year, watching it grow in activity as it approaches the sun, getting to within 180 million kilometers (112 million miles) in summer next year, when the comet will be expelling hundreds of kilograms of material every second.

"It's got an awesome profile -- the adventure of the decade-long journey necessary to capture its prey, flying past the Earth, Mars and two asteroids on the way," he said.

Daniel Brown, an astronomy expert at Nottingham Trent University in the UK, said: "Although we have landed on planets, moons and asteroids, it has never been attempted for a nucleus of a comet -- and with good reason. These objects have a very low gravity, are loosely composed of ice, dust and rocks, and are very irregular in shape. They are temperamental in their behavior and notoriously difficult to predict.

"Comets such as 67P have already been exposed to the intense heat of the sun in their past orbits, resulting in their surface being altered, but going beneath the surface will give us an insight into unchanged material, allowing us to peak into the chemical composition of our early solar system.

"Apart from the amazing scientific results, the sheer challenge and ambition of such a mission is outstanding and illustrates how our space exploration of the solar system has become more advanced and successful. It gives us much to hope for in future missions."

And science fiction writer Alastair Reynolds said: "This is science fiction made real in terms of the achievement of the mission itself, but Rosetta is also taking us a step closer to answering science fiction's grandest question of all: Are we alone?"