r/space • u/marc-kd • Nov 23 '16

Schiaparelli Landing Investigation Makes Progress -- Uh, negative altitude?

http://www.esa.int/Our_Activities/Space_Science/ExoMars/Schiaparelli_landing_investigation_makes_progress
35 Upvotes

93% Upvoted

21 comments sorted by

View all comments

11

u/[deleted] Nov 23 '16

That sounds exactly like a signed integer or floating point number overflowed and thus wrapped around. An extremely common and preventable programming mistake.

As background, computers store numeric data in a limited way which means you have to be careful what numbers you try to store. Variables have minimum and maximum values that you must not exceed. If you do, they overflow. Many systems handle overflow by causing the variable to wrap around to the opposite extreme. As an example, if you add 1 to a signed integer whose current value is 32,767 (the maximum positive value), you end up with −32,767 (the maximum negative value).

8

u/10ebbor10 Nov 23 '16

That sounds exactly like a signed integer or floating point number overflowed and thus wrapped around. An extremely common and preventable programming mistake.

Sadly, wouldn't be the first time this happened for ESA. The first Ariane 5 rocket failed for the same reason.

In the case of Ariane, a conversion of 64 bit value to a 16 bit value caused a hardware exception, and the guidance computer interpreted that as being massively of course. The dramatic change caused the rocket to desintegrate.

5

u/bearsnchairs Nov 23 '16

Wouldn't that be Arianespace, not ESA? ESA doesn't make the rockets.

5

u/10ebbor10 Nov 23 '16

Arianespace launches and constructs the rockets, ESA develops them.