Hacker News Reader

Comment by Aqua_Geek | Hacker News Reader

> Estimates are that Schiaparelli dropped from a height of between 2 and 4 kilometres, therefore impacting at a considerable speed, greater than 300 km/h.

Ouch. It will be interesting to read the results of the investigation as to why the landing thrusters turned off prematurely.

View on HN

According to the landing sequence...

...the lander was supposed to detach from the aeroshell at about 1km up. So if it fell from 2 to 4km up, that implies that it detached too early.

This article:

https://spaceflightnow.com/2016/10/20/last-data-from-schiapa...

...says that the telemetry shows that the engines did fire, for at least a few seconds.

My uninformed hypothesis: the lander detached early for some reason, and started falling. As it approached the ground, the radar saw the ground coming, and it fired the engines for the final braking. But because it had been falling so far, it was now going too fast for the engines to stop it in time, and it hit the ground hard. (Mars doesn't really do terminal velocities.)

It says they have 600MB of telemetry. That's quite impressive.

> (Mars doesn't really do terminal velocities.)

It depends on many factors, but as an order of magnitude terminal velocity on Mars is about 1000 km/h (speed of sound on Earth at sea level).

https://spaceflightsystems.grc.nasa.gov/education/rocket/ter...

That's if you were to start from velocity zero (e.g. fall off a balloon). If you start from orbital velocity, you have to slow down to terminal velocity, which takes a very long time.

Yes, it seems they made quite a dent on Mars

I'm not so sure that this should be interpreted as "thrusters turning off prematurely" rather, it getting to the point of shut-off sooner (and faster) OR it stopped because they hit the ground

I read in some of the reports (can't remember if official ESA, news, or twitter) that they were turned on for just 3 seconds (instead of the inteded ~50), plus the parachute was cut off earlier too. So, as this report from ESA says, it fell from at least a 1000 times higher than intended (the cut off altitude was 2m or so).

a 1000 times... that sounds rather metric.

It's an order of magnitude. 1e3, not 1.000e3 which would indeed be suspicious.

That is when it switched from thrusters to lithobraking.

Lithobraking has actually been a feature in at least one Mars mission: the pair of impactor probes that piggybacked on NASA's unlucky Mars Polar Lander mission of 1999 [1]. The MPL itself, as is quite well known, ended up doing an unplanned lithobraking maneuver after its retrorockets shut down too soon (!) due to insufficient integration testing. The impactors were never heard of either; the exact failure mode remains unclear.

[1] https://en.wikipedia.org/wiki/Deep_Space_2

A proven method of matching planetary velocity.

Reply on news.ycombinator.com