Phoenix Final Descent Trajectory Options

[vikingmars]

With a little image processing, one can easily see the final descent trajectory of the Phoenix Lander with its plumes disturbing the top layer of the Martian surface. After the release of the lander from the backshell, the effect is barely seen first, but is becoming more obvious, until the final strong pulses before landing. Any better estimates ? Enjoy !

[bgarlick]

Is it possible that what you are seeing is wind dispersal of kicked up dust? The parachute landed to the south of the backshell so presumably the wind was blowing from the north at the time of landing.

[fredk]

I'm not convinced - there are comparable dark areas elsewhere in your middle plot. Also, we've heard that the final horizontal velocity was apparently very small. Anyone know its direction? Since the parachute drifted south, perhaps the lander was also moving south before it zeroed the horizontal velocity? In that case any direct effect of exhaust on the ground should trail off to the north.

[bcory]

Also one must consider at what swing angle the decending package was at when the heat shield was jettisoned.

[kenny]

Phoenix was of course programmed to thrust away from the parachute after being released, to avoid it being blanketed by the chute after it landed. So it may well have been travelling in a direction unrelated to the wind which governed the chute's descent.

[MahFL]

Yes Phoenix was programmed to fly away. Mind you I am still surprised how close all the hardware landed together.

[vikingmars]

OK, good points. Maybe I'm wrong then : let's wait for the final EDL trajectory release by the engineers...

[ugordan]

Indeed. I was also surprised how close to the heatshield Phoenix landed. Keep in mind that the heatshield was jettisoned some 12-ish km and the velocity was supposed to be about 45 degrees to vertical/horizontal. Several minutes later the lander lands less than 200 meters of it.

Vikingmars, your plot would make sense if Phoenix descended slowly and had a large horizontal sliding velocity. What we know of is it basically came straight down.

[MahFL]

Then again the atmosphere is very very thin and all the objects would not be blown much by any wind.

[kwan3217]

Actually this is about what I expect. The powered descent is not random. The lander specifically looked at its radar to see which way it was travelling across the surface. From how the parachute is blown, I would say that it is basically from due north. Having measured that, after it separated, it purposely flew upwind (to the north) a ways as parachute avoidance, then tipped back over the other way to slow down its horizontal velocity and land. It flew straight upwind because it was supposed to.

There is a PDF slide presentation posted here which shows the details of this "backshell avoidance maneuver".

I guess what I'm saying is that I think your interpretation is plausible.

[dmuller]

There is a PDF slide presentation posted here which shows the details of this "backshell avoidance maneuver".

As a non-engineer, I just love the engineering English [engenglish???] (from above document):

... there is an increased probability the backshell/parachute will recontact the lander ...

Mmm yeah, recontact

I was actually hoping to include more of these events (BAM. gravity turn, alignment etc) into my real-time simulation but could not get the necessary information. Maybe next time.

[AndyG]

engenglish???

...Eng2lish?

Andy

[mhall]

Engl-ish ?

[Greg Hullender]

As a non-engineer, I just love the engineering English [engenglish???]

As an engineer and a linguist, I'd call that jargon, and what I find cool is that it follows its own domain-dependent rules. Consider how NASA uses the word "nominal." I like this one because I'm still puzzling over how they got "nominal" to mean "good," when it usually means "in name only." So if I said, "the orbiter is nominally functioning," that means that despite appearances to the contrary, I don't think it's working correctly, BUT, if I say "the orbiter is functioning nominally," that means there are no known problems, and I think it's working fine. Only the second is the "correct" way to use "nominal" when talking about space probes, and if you misuse it, you identify yourself as an outsider.

As for "recontact," I'd guess that arises from the fact that you can't say "contact," since these things are in contact to begin with. It crisply captures the idea of "to collide with, at any speed, but after separation." Standard English lacks such a term, since the circumstance is uncommon in daily life. (We'll ignore the fact that lots of people still think it's a solecism to use "contact" as a verb.) :-)

--Greg

[Adonis]

Indeed. I was also surprised how close to the heatshield Phoenix landed. Keep in mind that the heatshield was jettisoned some 12-ish km and the velocity was supposed to be about 45 degrees to vertical/horizontal. Several minutes later the lander lands less than 200 meters of it.

I got also surprised. I remember the same happened with both MERs: their heatshields got very close to the landers. There must be some reason for these so many cases. MER's heatshield and parachute design closely matches that of Phoenix. Perhaps the dynamic flight behavior of these heatshield when they're free from their landers are close to that of the parachute when hanging the lander.