When it came out of hibernation after seven months of silence back in mid-June, scientists at the European Space Agency were jubilant about what Philae, the first probe ever landed successfully on a comet, could tell them.

But now Philae has gone silent again for nearly the past two weeks, and scientists at ESA are unsure when they'll be able to re-establish contact, or even if they'll be able to.

The agency has two possible explanations for the sudden silence:

Comet 67P/C-G, photographed on July 7, 2015.

(ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)

"The position of Philae may have shifted slightly, perhaps by changing its orientation with respect to the surface in its current location," the scientists say in a .

"The lander is likely situated on uneven terrain, and even a slight change in its position – perhaps triggered by gas emission from the comet – could mean that its antenna position has also now changed with respect to its surroundings."

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Alternatively, one of the lander's two transmission units may not be working -- and they already know one of those units is damaged.

The lander made history last November when, after a 10-year-long mission, the ESA successfully landed it on , now hurtling around our solar system and set to make its closest approach to the Sun in mid-August.

Philae could only send data back to Earth for a few days, however. When it landed, the probe bounced and ended up in the shadow of a cliff on the comet, depriving its solar panels of enough sunlight to recharge its batteries. After about 60 hours, Philae was forced to shut down its systems and go silent.

In June, t and was able to send data for about 85 seconds that was relayed from Rosetta, the mother ship for Philae that is orbiting the comet and transmits the information collected from the surface.

The probe appears to have been awake for several days before it communicated with the scientists at ESA back in June, perhaps as a result of its closer proximity to the Sun, allowing it to use its solar panels again to recharge its batteries.

"Philae is obviously still functional, because it sends us data, even if it does so at ," said Stephan Ulamec, a scientist with the German Aerospace Agency and project manager for the Philae lander.

"Several times we were afraid that the lander would remain off – but it has repeatedly taught us otherwise," he added.

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