[X] My Assistant

[Seryddwr]

On a light note, has anyone made any plans for 5 August? It is, after all, less than one hundred days away now, and I'm sure Mars is getting very big in the window for ol' Curiosity.

I hope they'll get a webcast going. I'll be frantically clicking on 'refresh' on here come what may!...

[MOD NOTE: Topic moved to Chit Chat for obvious reasons.]

[brellis]

I remain - uh, Curious - about the factor of atmosphere in calculating the trajectory of the landing of MSL. When was the likely atmospheric effect established, and from what data? Because, parachute? On Mars?? Just ask weathermen on earth, that atmosphere can change a lot!

[Drkskywxlt]

I remain - uh, Curious - about the factor of atmosphere in calculating the trajectory of the landing of MSL. When was the likely atmospheric effect established, and from what data? Because, parachute? On Mars?? Just ask weathermen on earth, that atmosphere can change a lot!

MSL will actively control its descent, watch the EDL video on Youtube to see that. That's what allows it to have such a small landing ellipse relative to past missions. But, there still is atmospheric data that is incorporated into the plan. That data comes primarily from general circulation modeling, augmented with mesoscale modeling. These models have been "tuned" based on available atmospheric data (e.g., Viking landers, TES, MCS, etc...). The models provide a range of expected density, pressure and wind profiles during descent and that constrains the engineering problem. The atmospheric density on Mars is far more variable than on Earth, but it is of course much much less in absolute terms.

[MahFL]

I was under the impression the only actively guided part was the initial ballistic entry using the Nitrogen jets on the back shell. After that it's not guided, as one of the team said later they hope to add more active guidance and keep shrinking the landing circle to 7 km or so on future missions. The initial guidance though of course helps make the landing circle a bit smaller this time.

[djellison]

I was under the impression the only actively guided part was the initial ballistic entry using the Nitrogen jets on the back shell.

Here's the best single document describing EDL
http://trs-new.jpl.nasa.gov/dspace/handle/2014/41629

Best section is this

The MSL entry guidance algorithm is divided into four phases. Entry interface marks the start of guided entry:
guidance is initialized in the pre-bank phase and the controller commands bank attitude hold until the sensed acceleration exceeds 0.5 Earth g’s.
Once the sensed acceleration exceeds the specified trigger limit, the range control phase begins. During the range control phase, the bank angle is commanded to minimize predicted downrange error at parachute deployment.
Throughout this phase, cross-range error is maintained with a manageable deadband limit by executing bank reversals as necessary.
Peak heating and peak deceleration occur during this guidance phase.
Once the navigated relative velocity drops below about 900 m/s, guidance transitions to a heading alignment phase to minimize residual cross-range error before parachute deployment.
Just prior to parachute deployment, the vehicle angle of attack is adjusted to 0° by ejecting balance masses while the azimuth is aligned for better radar performance later during parachute descent. Parachute deployment is triggered at a navigated velocity of over 450 m/s.

A small point - it's not nitrogen - it's hydrazine thrusters. Infact the 8 thrusters on the back of the backshell to control attitude / roll during guided entry are the same model of thruster used in clusters for Phoenix terminal descent ( Aerojet MR-107U )

This document is more specifically about the guided entry phase - including some nice charts of sim-runs
http://www.planetaryprobe.eu/IPPW7/proceed...ssion5/p453.pdf