But what scientists didn't know as they watched Philae's excruciatingly slow, seven-hour descent was that this "ultimate anchor" was in fact just as non-functional as the thruster. When Philae hit, it ultimately stayed and was reported to be stable on Friday morning, even sending back the first picture ever taken on the surface of a comet. But its stability wasn't due to any anchor or thruster. A well-placed landing and luck, it seems, played a significant role. Even then, it's unclear what exactly happened on Thursday morning as the jubilation of the historic moment gave way to anxiety over Philae's future and mission. The first picture sent back of the Philae lander on the comet. Credit:ESA "The not so good news is that the anchoring harpoons did not fire," the Telegraph quoted Philae's landing manager, Stephan Ulamec, saying. "So the lander is not anchored to the surface. Did we just land in a soft-sand box and everything is fine? Or is there something else happening? We still do not fully understand what has happened." He then illustrated the full extent of the landing's opacity: "Some of the data indicated that the lander may have lifted off again. It touched down and was rebounding. So maybe today, we didn't just land once, we landed twice." In fact, Philae landed three times. According to the research team's magnetic field analysis, the probe hit like a bouncy ball. It first struck the comet's craggy surface at 15:33 UTC (Co-ordinated Universal Time, which is 11 hours behind AEDT) then bounced more than a kilometre back into space before floating back down to the surface two hours later at 17:26. It bounced again, then finally settled at 17:33. "Does this mean Philae is not stable on the surface?" one concerned person asked the team on Twitter. "It's stable now!" researchers responded. The initial confusion was because Philae lost contact with Earth post-impact, leaving scientists fumbling around looking for it. When they finally found it, uncertainty undercut the elation: The probe wasn't attached to the soft surface of a madly-spinning comet. According to the mission's landing blueprint, Philae's "pyro-driven harpoons" were supposed to have sunk nearly two and half metres into the surface. But instead, the probe was just sitting there.

The concern involves gravity — or the lack of it. "The comet's gravity is so weak that an attempt to take a brisk walk across its surface could launch a hypothetical pedestrian back into space," the Christian Science Monitor commented. The world watches: French leaders wear 3D glasses to view the landing. Credit:Reuters The entire business was disconcerting for some analysts. Landing manager Ulamec "confirms the harpoons did NOT fire," science writer Emily Lakdawalla wrote, adding the situation was "clearly worrisome." "There is much they currently do not know. … It is entirely possible that tomorrow will come and we will not hear from Philae." Without the harpoons, the washing-machine-size craft will have to rely nearly exclusively on the ice screws at the bottom of its landing legs to grip the comet. It's unclear whether those are strong enough to hold indefinitely. It "could mean that we are sitting in soft material and we are not anchored," Ulamec added. A picture of the surface of 67P/Churyumov-Gerasimenko, taken 40 metres from the comet during Philae's descent. Credit:AFP/ESA

The lander settled in a shadowy part of the comet near a cliff, where it is only getting one and a half hours of sunlight a day. At its planned landing site, Philae would have gotten six or seven hours of sunlight. The difference is crucial because Philae will need to rely on solar power after its batteries run out, and this will affect how much work the probe can do on the surface, mission officials said. Scientists also said Philae landed with two legs on the ground and one foot in the air during its final touchdown. Ulamec said the lander has the capability to make a little "hop" on the surface, which could help it get into a better position, but the manoeuvre would be risky and it is not likely ESA will try it. Officials are also wary of drilling, in case the force of the drills on such a low gravity body cause the lander to move again. On a mission: Rosetta's view of Philae, as the lander was released. Credit:AFP/ESA So now, scientists must confront yet another difficult choice: Should they let the craft alone, hope it sticks to the comet and try to collect as much data as they can? Or should they try to fire the harpoons again? The latter choice is fraught: the thruster, in addition to its responsibility of counteracting the recoil of impact, was also intended to fight the upward kick from firing the harpoons. While the craft may not be in a place where it can get as much sunlight as researchers had budgeted for, lead lander scientist Jean-Pierre Bibring says it's still "in a gorgeous place".

This five-image montage was used to try and identify where Philae finally landed. Credit:AP "It's amazing where we are," Bibring said. "We are at the limit of what humankind can do." Washington Post, LA Times, Reuters, Bloomberg