Phoenix Final Descent Trajectory Options

[Juramike]

And altitude?

-- Pertinax

I'm pretty sure it's touching the surface.

[jmknapp]

I'm pretty sure it's touching the surface.

Although the present "altitude" reported by the PDS NAIF spice kernel (not sure how accurate) is -2.4 km! That may be correct, insofar as the zero-altitude reference is an idealized tri-axial ellipsoid, which no planet actually fits. So that would place the Phoenix landing site elevation somewhat below mean sea level, so to speak.

If I recall correctly, in one of the early press conferences Peter Smith mentioned that liquid water is actually stable in the landing area, due to the low elevation.

The lat and lon reported by the spice kernel is 66.217N 243.692E, but how accurate that is I don't know (predicted, actual, tentative?).

[akuo]

The position was already reported in an earlier post on this forum

68.218830N 234.250778E, IAU 2000 Areocentric

[edstrick]

At approximately 0 km elevation, the average pressure corresponds to the triple point of water (approximately, and not including seasonal atmosphere pressure variations). So at -100 meters (if this were exact and my arm waving numbers were exact), water 1/2 degree above freezing would not boil. It would evaporate damn fast, though, under most plausible conditions. At lower elevations, water some 5 to 10 deg F <half that for C> above freezing would not boil.

Regardless, it's REAL hard for plausible normal <not impact, for example> processes to supply enough energy to ice at the martian surface to melt it any faster than the water would likely evaporate.

[jmknapp]

The position was already reported in an earlier post on this forum

Thanks--is that link correct? The original reference doesn't seem to be there.

[akuo]

Works for me. I guess this is another way to put it:
http://www.unmannedspaceflight.com/index.p...st&p=115817

[jmknapp]

Hmmm... still don't see it. Anyway, the location you quoted is very plausible based on NAIF spice kernels having MRO pointing squarely at that point at 26MAY2008 22:04:30. The published photo that MRO took of the lander on the ground is stated to have been taken 22 hours after landing (25MAY2008 23:46), so that fits.

Here's a simulation of the view from MRO at that instant, with the HIRISE look direction marked with crosshairs (MGS 200m mosaic, equalized):



HIRISE had moved pretty far off-nadir to take that shot.

I still wonder if part of that mosaic was taken when there were significant clouds over the Phoenix site, and another part when it was clear.

[Pertinax]

I'm pretty sure it's touching the surface.

Thank you Juramike!

I think I might have misunderstood the initial question, thinking that is was asking if the Lat/Lon (and altitude) of Phoenix in flight at the instant MRO captured it was pin-pointed yet.

-- Pertinax

[kwan3217]

I just did my hourly check of the spice kernels at NAIF and found that the EDL reconstruction is posted!

http://naif.jpl.nasa.gov/naif/data_mars.html, phoenix/ck/phx_edl_rec_att.bc, phoenix/spk/phx_edl_rec_traj.bsp

Now all we need is an exact time for the entry parachute photograph and we can tell everything about it.

[dmuller]

Now all we need is an exact time for the entry parachute photograph and we can tell everything about it.

I havent had a look at it nor do I know what these files show (too busy at the moment), but parachute deploy should show a big spike in deceleration

[jmknapp]

parachute deploy should show a big spike in deceleration

Good thought. Here's a graph of the speed and acceleration:



But where along there did MRO take the picture? Anybody know the lat/lon of Heimdal Crater?

[gallen_53]

I just did my hourly check of the spice kernels at NAIF and found that the EDL reconstruction is posted!

http://naif.jpl.nasa.gov/naif/data_mars.html, phoenix/ck/phx_edl_rec_att.bc, phoenix/spk/phx_edl_rec_traj.bsp

Now all we need is an exact time for the entry parachute photograph and we can tell everything about it.

I also downloaded the files "phx_edl_rec_att.bc" and "phx_edl_rec_traj.bsp". They're binary files. What code did you use to get them into a readable form?

[gallen_53]

Hold on, I just found the NAIF website at http://naif.jpl.nasa.gov/naif/toolkit_C_PC..._GCC_32bit.html . I'm downloading the SPICE toolkit tarball while writing this post.

I just got the SPICE tool kit package up and running but it's not friendly. I want the tool kit to translate the Phoenix binary data files into text files describing the vehicle's trajectory in the inertial frame. How do I do that? Does the SPICE database only model the trajectory as a 3-DoF point mass or does it include 6-DoF information from the vehicle's IMU? Thanks!

[kwan3217]

I use the NAIF spice toolkit and IDL, but the one for C works fine too. the .bsp file includes the position and speed (and Spice will convert it to any coordinate system you feel like) while the .bc file contains orientation. Together it is 6DoF.

Look up examples on the NAIF site for spkezr_c for the position stuff.

[gallen_53]

Thank you. I'll try it in the morning. Do you know where to find the Phoenix mass model, i.e. moments and products of inertia, center of gravity, etc.? Is there an aerodynamic model specific to the Phoenix 70 deg. aeroshell including Cm, Cmq, etc. accessible on the web? This information would be in the Lockheed-Martin databook for Phoenix but they're probably keeping it close to their vest. Thanks again.