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Comment by LordWinstanley | Hacker News Reader

>>>...Then thrusters, designed to decelerate the craft for 30 seconds until it was metres off the ground, engaged for only around 3 seconds before they were commanded to switch off, because the lander's computer thought it was on the ground...

>>...The lander even switched on its suite of instruments, ready to record Mars’s weather and electrical field...

It's a shame. But, reading that, I can't help imagining the lander in the role of the Blue Whale from Hitchhiker's Guide to the Galaxy:

"I wonder what this big flat thing rushing toward me is? I think I'll call it 'The Ground'"

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Given the history of ESA landers, the more appropriate quote might be "Oh no, not again!"

It really puts into perspective what good job NASA does at what many call a "boring" mission.

Considering NASA has a budget thats 3-4x that of ESA, I'd be a little less quick to judge if I were you.

> Considering NASA has a budget thats 3-4x that of ESA, I'd be a little less quick to judge if I were you.

I don't think GP was judging ESA. I think they were merely pointing out how impressive NASA's success rate has been lately.

Truth, its especially commendable that most of their projects keep soldiering on well past EOL.

Not to take away from that but I suspect that there are a lot of good reasons to underpromise and overdeliver. I also wonder to what degree budgeting has something to do with it. Budget for a shorter lifetime given that you can usually end up extending missions if the equipment is still operational.

Not to take away from the great work of NASA engineers but EOL is kind of a misleading term when it comes to space science missions. At lot of mission end times are based off of funding and the lowest level science goals of the mission. While those are being completed they gather a case for continuing the mission and usually get it. Of course the main cost is getting the stuff there in the first place so why not add a relatively small amount to make it last?

No, but given the short lifespan of these things due to radiation, literally other-wordly conditions, and the fact that you can never repair it, the fact that a rover on Mars can last a decade is a technical marvel.

You also have to pre-program the entire flight and find out half an hour later if your parachutes and retro-rockets deployed, or you made a relatively small crater.

Fair enough. How about the UK's Open University that was marginally more successful with Beagle 2 for £66 million.

It went riding on another orbiter, Mars Express, but at least reached the surface intact.

They're spending 1.4B on the two ExoMars missions (and that's not counting the Proton rockets, which Russia's contributing).

NASA's budget is larger, but they're doing more missions (and spending a lot on SLS and James Webb). I suspect blaming the ExoMars crash on budget is inaccurate.

It seems like ExoMars spent years shopping around between the US and Russia for launch, and major budget problems, which caused it to be redesigned more than once. I guess NASA doesn't usually have problems like that? https://en.wikipedia.org/wiki/ExoMars#History

NASA's Mars Polar Impacter, er, Lander, also shut off it's descent engine too early, believing it was on the ground, and slammed into Mars. This stuff is hard. The Wikipedia entry gives the details:

https://en.wikipedia.org/wiki/Mars_Polar_Lander#Landing_atte...

They don't have altimeter's/range finders?

I wonder what causes the confusion.

They do have radar altimeters, but radar altimeters face occasional issues. So they usually also bake in various failsafe measures. Sadly the failsafe measures tend to be the thing which dooms spacecraft, usually in odd ways unpredictable until an actual spacecraft is hurtling towards the ground.

That is too bad. I wonder if a laser in the front would be useless as the heat build up as it enters the atmosphere would make the "bounce-back" hard to receive.

I think a space program that has lost a Mars-bound spacecraft because different engineering teams could not decide on if they are to use metric or imperial units of measurement has nothing to gloat about.

It's not gloating, it's just pointing out that ESA has failed repeatedly where NASA has had repeated success. No need to take it personally.

It's very strange because, if would have been a glitch that would cause the program to halt unexpectadly, the engines would have remained running. But from the data, it actually entered into "on the ground" state initiating its exploratory procedure.

But what kind of "on the ground" test condition could have made it to do that ?

The fact that it was "designed to decelerate the craft for 30 seconds until it was metres off the ground" but instead it ran only 3, raises many questions. Not to say that there are zillions of ways to actually test that you are on the ground or not..

> if would have been a glitch that would cause the program to halt unexpectadly, the engines would have remained running

I don't know about that. I've worked on systems where there is some hardware with thruster duration that software is writing. So if software crashes, the thrusters stop. (But I have never worked on a lander). If software is running the control loop, things would get very bad if the engines kept running at their previous output levels for 27 seconds. Thrusters are never aligned perfectly, who knows what kind of rate the lander had, there may be wind pushing it, etc. If a thruster blindly kept running, the lander may have spun up, flipped over, etc.

Ok, good point.

But the point that I wanted to make is that a software crash wouldn't explain why the lander behaved as if it was on the ground. So it's more probable that it reached the "on the ground" condition.

If that condition was adequately programmed, the chances that some sensor glitch could have triggered it are quite low. So the probability is higher that it was either badly programmed or maybe someone flipped a bit somewhere ..

Yeah I agree, it seems like that between an accelerometer and gyro that it should be fairly clear when you reach the ground condition. Maybe a switches that engages when a lander legs are pressed or something similar as well.

The lander didn't have sensors on its feet, so my guess is there may have been two conditions to check for "on the ground": either a vertical speed of 0m/s (gyroscope), or an altitude of 0m (radar).

Given that the parachute was deployed at the correct time and its deployment was governed by air speed (to be deployed around 1mach), the gyroscopes were likely working correctly. The radar detection wasn't available until after deployment of the parachute (more specifically, after ejecting the heat shield, which occurred at the same time), so if the machine's decision was based on faulty input, the doppler radar is likely to blame.

It is also possible that the sensors were working correctly, but the inputs were processed incorrectly. I'd expect that an issue like that could have been caught while testing the lander on earth...

edit: it's also unlikely the engines would have kept firing after computer malfunction: there were multiple (9 iirc) thrusters all around the craft and they were fired independently using pulse-width modulation. This approach was intended to both slow down the descent and to stabilize its course/orientation.

Don't most landers have a mechanical probe extending down from the landing gear to indicate contact with ground? I wonder if this lander was so equipped?

Unless they used a ground proximity radar which sent back false pings due to overheating caused by the too early discarded heat shield?

I'll wait for the final crash analysis and report, but I trust that the lessons learned here will significantly improve the chances of future missions, if at least by adding some more redundancies into the mix.

See my post above about a similar NASA incident. The ground contact in that case was indicated by sensors on the landing gear. They appear to have recorded a false positive when the gear were deployed. Different spacecraft use different methods, I don't know what technology Schiaparelli used.

At least it wasn't a unit-conversion problem. It would be really shameful for an _European_ lander to mix up an altitude reading between feet and meters.

But it's not the case, isn't it?

It may very well be.

Maybe they used an US-developed microcontroller library and screwed up the unit setting.

How realistic is a dry run via an earth simulation from high aptitude to make sure units are sane?

Given that Mar's atmosphere masses about 25 terratonnes compared to Earth's 5148 terratonnes, and Mar's atmospheric pressure is about 1% that of Earth's sea level, you'd need to do high altitude testing at about 30 miles up, which is about 5 Mount Everests.

Much simpler to test fire the engines in a lab somewhere to make sure they run, and test the software in a simulation of the landing. It'll probably come out in the next year or so that there was either just a flat error in the code, or that the simulation was incorrect in some way.

Surely in 2016 no one is using imperial units as a default for anything even remotely related to aerospace programming.

I work at NASA and sadly, we do still use Imperial units for many things. It's a travesty.

In military aviation mission planning, it's even worse. We have to support any damn-fool unit some random major might want to see on the printed reports. A length might need to display in feet, yards, meters, kilometers, statute miles, nautical miles, US flight level (hundreds of feet by barometer reading), AGL vs. MSL, etc. Sometimes, people mix customary, nautical, and SI units, such as by showing distances in km, altitudes in feet, and speed in knots (NM/h).

If I were ever to somehow become absolute tyrant over the entire Earth, my first edict would make open and notorious use of non-SI units a capital crime, then I'd go live in my imperial hermit shack on a remote mountainside (paradoxically, one with a >1 Gb/s network connection) until my minions finished mopping up the last of the blood.

I enjoy the literary irony here of you imposing a ban on Imperial units and yet you'd go live in an Imperial shack.

Does the software store unit information and calculates with units (like say 1 m / 1 s = 1 m/s) or does it directly operate on the numerical value (so just 1/1 = 1)?

I think the first way should be fast enough with today's processors (but IIRC the processors on the Rovers are slower).

.. I don't understand how you guys can get something up in space. That's amazing. It's either a remarkable feat of engineering or an gigantic 42 gallons barrel of liquid luck.

That seems exceedingly unlikely. No-one in Europe would even consider using anything other than metric units even for buying their groceries, let alone a critical engineering task.

That said, _if_ there were any US companies involved then perhaps there might have been complications. Many years ago I worked on guidance software for tunnelling machines and we had a US client who insisted that we reprogrammed the entire system for them to work in "centifeet", i.e. 100th of a foot or approx 3mm.

I well remember the chaos and arguments caused by adopting such a ridiculous chimera of a unit, and was struck by how resistant some people are to adopting something as universal and transferrable as SI units.

10ths (and hundredths of an inch) are completely standard in US surveying. For example: https://www.amazon.com/Lufkin-FE200D-Engineers-Fiberglass-Me...

Interesting to know - 100th of an inch is extremely small - certainly not visible with my eyesight! Seems odd to need that for surveying.

There are actually a fair few problems with SI units. The candela is fairly ridiculous, the kilogram needs to not depend on a physical object (it's also a little silly to have a prefixed base unit). The ampere is not practical to measure with its current definition, and apparently with the current definitions of Hz and radians, 1 Hz = 1/s = 1 radians/second. Other people take issue with the mole, as well.

SI is an improvement but it is not quite perfect. As long as we have a choice of imperfect systems, we have to pick which is most convenient.

> it's also a little silly to have a prefixed base unit

I somewhat agree with that - I had the exact same conversation with my kids very recently.

Re. physical object: well, whatever physical reference artefact is used, to all intents and purposes the physical stuff is water, with the basically sensible equivalence 1L = 1kg = 1000cc. As you point out, it's not perfect because of the somewhat arbitrary scales (kg, cc), and it gets worse when you bring temperature into it (I believe it's supposed to be measured at 4 degrees C, but I'm not totally sure of that).

You're right in that it is not perfect, but at least it is a fair attempt at coherence.

I believe it's supposed to be measured at 4 degrees C

Correct; it's because at that temperature water is at its densest: https://en.wikipedia.org/wiki/Water#States

re: the kilogram issue:

Redefining the Kilogram with the DIY Watt Balance: https://www.youtube.com/watch?v=ewQkE8t0xgQ

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