[jmmcd]

> The Athena spacecraft was not exactly flying blind as it approached the lunar surface one week ago. The software on board did a credible job of recognizing nearby craters, even with elongated shadows over the terrain. However, the lander's altimeter had failed.

> So while Athena knew where it was relative to the surface of the Moon, the lander did not know how far it was above the surface.

This is really crappy writing. That second paragraph sounds like a self-contradiction. Unless "the lander" is a separate entity from "Athena"? Some publications refuse to use the same term twice, even if it introduces ambiguity as here.

[kryogen1c]

> That second paragraph sounds like a self-contradiction

Aside from the point you made, it actually IS a contradiction. Paraphrased:

>> Athena knew where it was relative to the surface of the Moon, but Athena did not know how far it was above the surface of the moon.

Relative position includes height/altitude? One understands from context, but this sentence does not carry meaning itself.

[HenryBemis]

I read this and I thought it was a bad writing, and I assumed (right or wrong) that (metaphor ensues):

The airplane's pilot could see where the ground was and he estimated it was 1000m-1200m below. But the instrument of the airplane were malfunctioning and reported that the ground was at 500m (or 5000m). This caused the plane to override/resist the pilot's manual landing efforts causing the plane to ultimately crash but not allowing the wheels to come down and the flaps to do turn.

So two different mechanisms, providing contradicting evidence, and the lack of a 'higher' deciding authority to use 'judgment' and/or other inputs to 'decide' which reported measurements are more accurate, and thus make the viable decision/take the viable action.

I get that for such devices every-gram-counts, and perhaps from now on some devices/mechanisms that are truly single-points-of-failure, will get 2x or 3x for redundancy/contingencies. If your ABC-camera fails, it's 'ok' you can still get images from your DEF and GHI will give you some data, reducing to 80% the output of the mission, but still not making it 5%. Your 'landing equipment' fails, and you get out only 5% of the value (flying/trajectory/telemetry of flight/perhaps some eclipse photos).

[EGG_CREAM]

In their defense, using Athena again here would have definitely sounded strange and not natural. But, they could have used the word “it” without introducing ambiguity, which they do 2 times already.

[randallsquared]

It knew where it was selenographically, which is what the author means by "relative to the surface of the moon", I think.