After two decades of preparation, $1.2 billion in taxpayer money spent and an astounding 4 billion miles traveled, a potentially disastrous problem emerged right before the Rosetta spacecraft deposited its landing probe onto a twirling comet hurtling through space 40 times faster than a bullet. It was the thruster on the top of probe. Something was wrong with it. A mechanism intended to counteract the recoil of impact, the thruster was meant to insure that the probe, Philae, didn’t hit the surface and bounce back into space.

Now it wasn’t firing.

Despite those concerns, the European Space Agency decided to proceed. They had come so far. They were so close. And besides, Philae still had its anchoring harpoons, which would shoot into the surface of the comet at a speed of 160 miles per hour, latching the probe onto the comet like a tick. Given the delicate feat before the scientists, such a device was vital.

“Delivering Philae to the comet’s surface requires all the finesse of depositing a pro-football linebacker on a trampoline without a bounce,” the Christian Science Monitor reported. “The harpoon represents the craft’s ultimate anchor.”

But what scientists didn’t know on Wednesday 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 the Philae hit, it ultimately stayed, and Thursday morning it was reported to be stable, but it 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 Wednesday as the jubilation of the historic moment gave way to anxiety over Philae’s future and mission.

“The not so good news is that the anchoring harpoons did not fire,” the Telegraph quoted Philae’s landing manager, Stephan Ulamec, as 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 (Coordinated Universal Time, which is five hours ahead of Eastern time), then bounced high 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. “Is 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 eight feet 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 entire business was disconcerting for some analysts. Landing manager Ulamec “confirms the harpoons did NOT fire,” science writer Emily Lakdawalla wrote, adding that 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.

And now, scientists must confront yet another 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.

The team is expected to release more information on that decision and on the status of the mission today. “Our big concern at the moment is whether we are standing stably” on the comet, a spokesman told the Telegraph. “We are considering if we need to retry shooting the anchors.”