[X] My Assistant

[PFK]

Folks
Forgive my inorance, or inattention if its already been pointed out, but is it possible to hazard a guess as to the scale involved in the nearby features above ie what is the approx width of the closest of the polygons?

[remcook]

"Smith said such pithy things as: "I know it looks a little like a parking lot, but that's a safe place to land. That makes it exactly where we want to be. Underneath this surface, I guarantee there's ice. You can see lots of pebbles, and soil, and all these troughs you see in between the polygons. These are probably about 15 feet [5 meters] across."

"This is probably the cutest polygon that I have ever seen...." "

(from Emily's blog)

So, when's the next batch coming?

[PFK]

Excellent, thank you. That makes the front square just about the same size as the patch of lawn I see right now if I look out of my window - really does help to (quite literally!) bring home the magnitude of what has been acheived.

[MichaelT]

Folks
Forgive my inorance, or inattention if its already been pointed out, but is it possible to hazard a guess as to the scale involved in the nearby features above ie what is the approx width of the closest of the polygons?

If you go to this page http://fawkes3.lpl.arizona.edu/images.php?gID=440&cID=8 and mouse over an image, the pointing elevation of the camera is shown. From that and the fact that the cameras are situated about 2 m above the surface, plus the knowledge of the field of view of the camera (14°, see here: http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2156.pdf), you can calculate the distances and the scales of the objects (assuming that the lander is level and the ground, too, which it pretty much is).

Distance of the center of the image from the camera:
d_c = h / sin(a)

Top
d_t = h / sin(-b/2 - a)

Bottom
d_b = h / sin(b/2 - a)

Where a is the pointing angle (eg. a = -27.1185° for the lowest part of the mosaic), b the field of fiew (b = 14°) and h is the camera height (h = 2 m).

So the center of the lowest frame (see below) has a distance from the camera of about 4 m.

The width of the frame at center is:

w_c = 2 d tan(b/2) = 2 h tan(b/2) / sin(a)

So the width of the field is about 1 m and so is the polygon in the lower part of the mosaic. The larger pebbles to the left (image below) are about 5-8 cm in diameter.

I hope I got the maths approximately right. It seems the polygon is a little smaller than said by Smith?

Michael

[Ulysses]

Great landing, awesome pics, congrats to all involved. Still a bit of a shame to be at the pole without skis!

[rlorenz]

For my first post I have a question: does anyone know what was the event that triggered the parachute deploy? I'm wondering if it was a specific altitude, speed, density, or something else. If it's something like that, then the Martian atmosphere models will have to be recalculated. The 7 seconds delay of the parachute deploy at that speed caused a significant deviation from the center of the landing ellipse.

For lots of background you can read Ball et al, 'Planetary Landers and Entry Probes'

Usually the specification on the parachute is for deployment in an allowed dynamic pressure range
(i.e. not too high density*speed^2 or you shred the thing when it deploys) and a Mach number
range for probe/parachute stability (typically Mach 1.4 - as you get to the transonic regime, the
sphere-cone entry configuration can start to tumble, so you deploy before you slow down to this
point)

But, you cant measure Mach directly. So for a range of plausible models you calculate the entry
deceleration history, and it turns out that there is typically a function of the form

Right_Time_to_deploy = time_g_falls_below_X + the_time_between_g_rising_above_y_and_falling_below_X
(with maybe an extra offset term or something)

This can be implemented with simple logic and g-switches (literally microswitches with weights on springs)
or more typically an accelerometer (both Huygens and Galileo used this approach - for Huygens the accels
were prime, with g-switches as a backup). I think in Phoenix's case it was probably a full-up IMU (i.e.
accels integrated to derive speed)

Interestingly (see Harland and Lorenz 'Space System Failures') on Galileo the two switches were
miswired, and the parachute was deployed late (meaning the descent data only began from
lower down than desired).

I think for Phoenix the hardware and software is rather more sophisticated, so a simple miswiring like
this is unlikely to explain the late chute for Phoenix - it may be as you say an atmospheric profile issue, but
it's too early to know.

[vikingmars]

Recreating a synthetic green + filter corrections, here is my interpretation of the Mars REAL colors at the Phoenix landing site as they should be seen...
Congrats to trhe whole Phoenix team + especially to Mark Lemmon the SSI designer !
Enjoy !

Attached thumbnail(s)

 

[pechisbeque]

For lots of background you can read Ball et al, 'Planetary Landers and Entry Probes'
[...]
I think for Phoenix the hardware and software is rather more sophisticated, so a simple miswiring like
this is unlikely to explain the late chute for Phoenix - it may be as you say an atmospheric profile issue, but
it's too early to know.

Thanks for the clarification, very informative. I will try to get my hands on those two books.

[algorimancer]

...to the scale involved in the nearby features...

MichaelT's response will have to do for now.

I've been fixing some last minute bugs in my rangefinder application as updated for Phoenix. Just at the moment I only see only one stereo image pair (of the landing pad). I've got two camera models for Phoenix, one (official) which is giving me some clearly incorrect values, and another (less official) which seems more consistent but seems to show the pad is rather closer than I would have expected. Does anyone know the actual diameter of those landing pads? Does something near 30 centimeters sound correct? Sounds big to me.

[Steve G]

A pity Madame Curie couldn't have hitched a ride. Perfect terrain for her!

[Guest_Zvezdichko_*]

Here's a simple mosaic of 3 images, created by me. It's not very good, I know.

Attached thumbnail(s)

 

[nprev]

I think for Phoenix the hardware and software is rather more sophisticated, so a simple miswiring like
this is unlikely to explain the late chute for Phoenix - it may be as you say an atmospheric profile issue, but
it's too early to know.

VERY interesting, Ralph; I need to get these books, too.

Question: On Phoenix, was the descent radar turned on prior to parachute deployment? Everything happened so rapidly that I lost track. Reason I ask is that I can see a combination of IMU deceleration data & ground proximity to cue chute deployment. The only other thing I can think of if the radar was NOT on is that they were relying on IMU data alone, which would include a Martian "geoid" model for surface reference (assuming that ambient atmospheric pressure was not used as a reference, and can't see why it would be based on the highly variable density of the upper atmosphere). Or did they do it all based just on deceleration data, which seems quite risky?

[ugordan]

Radar was enabled after parachute deployment and heatshield jettison. I believe the parachute timing was made using IMU delta-V integration.

BTW, it was great to wake up in the morning and already see a 3 frame color mosaic. Looking at that color release, even though it's false color (in the sense that natural color was inferred from it), it's the probably the best first color image from a lander on Mars I've seen. It actually looks natural color straight-off, no funky blue skies or accidental IR instead of red filters

The whole thing up until now seems to be so perfect in fact that it's got me worried. It's almost too good to be true!

[Mogster]

A pity Madame Curie couldn't have hitched a ride. Perfect terrain for her!

Looks like perfect terrain for a static lander to me, the same flat tundra stretching for miles. I don't see the advantage of a small rover.

looking at that first pic Phoenix seems ideal. Mobility would always be better but on this terrain for the extra cost how much would be gained?

[imipak]

is it just me or has Phoenix broken thro' a surface crust? Is that a bit of the ice crust poking up?

That analglyph view is a really nice composition, with the leg pulling the eye into the surface. I'm probably seeing things, but it almost looks to me as if there's a dish-like depression around the pad, and if it didn't so much touchdown as "squishdown". It's almost like a crater rim...

Is the ice crust poking up that you refer to the bit just to the right of the triangular hole on the shadow?