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.

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 !

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

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.

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

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

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!

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?

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?

I agree with this. The first picture has done a lot to assuage my fears we would be frustrated by tantalizing targets just out of reach of the lander.

there may be some interesting targets in the area that is not yet seen

Very true... who knows what interesting things are waiting to be seen to Phoenix's sides, or behind it? I wonder if we'll be treated to any new images at the media briefing later today? (7pm British Summer Time)

Fantastic night last night, thanks to everyone who made it so special. Special "Thanks, and well done!"s to Doug and Emily for all their hard work. Very proud of you both. And a huge night for UMSF too. Welcome to all our new members!

When the SSI mast is/was expected to be extended?

Yes, that's what they said last night. IMU = Inertial Measurement Unit.

It was either Barry Goldstein or Ed Sedivy who remarked that it seemed that the radar came on late but when he checked back with the guys at LockMart, they said it was within the normal range -- it was just time stretching out in their excitement

Thanks all for your kind words and for sticking with me through the various attacks on the website and ustream channel!

--Emily

Just before the opening of RA's Biobarrier

Whendo we expect to see the LIDAR in action?

I'd have to echo Emily's words from her blog: please consider joining the Planetary Society!

Just wondering about something, thoughts appreciated...

In her excellent report on last night, Emily advises that this HiRISE image is, apparrently, dead centre on the landing zone...

I've just been looking at it with IAS Viewer and found this at 1:1 scale...

Click to view attachment

... which looks like this if you zoom in on it x2...

Click to view attachment

Could that be a jagged rock we're mistaking for the backshell..?

Hi, all. First post here, though I've been following the site for a few months now.

Thank you to everyone who made last night such a great time - all good fun switching between Mars Live, Nasa TV, and Emily's TV.

I do have one question about the image showing the horizon: given that Phoenix is tilted at a quarter degree, doesn't that mean it's sitting on a hill? The horizon looks to be tilted by several degrees there.

David

full inline quote removed

I take it thats pre-landing HiRISE imaging Stu?

Just want to comment here that it was slinted who identified that image. (Thanks, slinted, for all the helpful links in the chat channel.)

--Emily

Yep, no post-landing HiRISE images have been released yet, as far as I know...

Gordan, thanks for the illumination, and Emily, thanks for the confirmation.

Hate to critique something that worked...great!!!...but given the extreme variability of the density of the upper atmosphere of Mars it seems that this technique could be refined a bit better. One approach that might be cost-effective and not incur a mass penalty or add additional physical complexity would be to add a reference Martian geoid (basically a mathematical model of Mars' 'sea level' surface if it were spherical/slightly oblate and featureless) to the navigation system as a model for estimated absolute altitude based on IMU data, which can also provide distance & vector relative to such a geoid traveled since entry. An "and-gate" approach to parachute cueing might provide better targeting accuracy, provided that the inputs have the required redundancy.

I pulled out a pre-launch test mosaic that Mark put onto the pre-landing blog and the SSI webpage - and then put our pieces onto it from last night - gives it all a bit of context.

Not "several degrees" but 1.5º if I did my math correctly. In any case, it's not a quarter of a degree so I have the same question.

Were those really attacks and not just the result of heavy traffic? I can't understand attacks. It does not compute.

Thanks Doug -- that one went straight into the blog

And yes, there were attacks. Our site handles high traffic just fine; it's managed by a company whose clients are mostly major motion picture studios whose websites have to stream movie trailers, so bandwidth is the least of our problems (though not always the least of our expenses ) Fortunately the team was on duty last night and was really able to recover from the attacks very quickly -- it just seemed to take forever from my perspective, as it happened, of course, right during the landing!

We seem to be all better this morning though.

--Emily

Emily are the press conferences still going to be coming from JPL today? Or has it all moved out to Arizona now that science has taken over?

Attacks during the landing...incredible. I don't think a swear jar large enough to absorb the financial penalties incurred by me describing the fully accurate obscene attributes and activities of such alleged people could ever be built...

Thanks for persevering, Emily; you were beyond great!

I am certain that at least Monday and Tuesday there will be briefings at JPL. I'm a bit confused as to the rest of the week -- one pre-landing schedule I saw had them returning to UA on Wednesday, but JPL had press briefings all week on their calendar. I'll get clarification when I go there today. I believe Peter Smith has already returned to Tucson.

--Emily

30cm is a bit big. A caption on Phoenix image gallery says -
"Each footpad is about the size of a large dinner plate, measuring 11.5 inches from rim to rim. The base of the footpad is shaped like the bottom of a shallow bowl to provide stability."

My idle, sleep-deprived speculation continues...

Still wondering about this "rock"... feel free to jump in, anyone... I've even found a way to line up the lander's POV with the "sharp rock" and two vague mounds/humps on the horizon and features on the HiRISE image.

Click to view attachment

Probably nothing, but I'd appreciate input from more informed members.

Nick, it's not the altitude that triggers the parachute (and really it shouldn't even be, as Ralph pointed out), it's parachute dynamic pressure margin (don't go too fast!) on one side and not too slow a speed on the other side. Both of these can be perfectly well accomodated by a good IMU that integrates from a known initial velocity. All you need to do is make sure there's plenty of altitude left for the parachute to do its job and Phoenix did this. IIRC, the chute was opened at around 12 km and the majority of remaining velocity was lost soon thereafter. It was a more gentle descent after that.

To cope with varying atmospheric density, an active entry of some kind might be envisioned that adjusts lift/drag effects on the heatshield to compensate for modeled/measured deceleration profile. That's just arm-waving on my side, though.

Paging Phil Stooke... Mr. Stooke please pick up the white courtesy phone.

I am still stunned by the correspondence between the planned mission objectives and the first views of the landing site -- looks (to my fevered imagination!) exactly like the type of heaving terrain beneath which one would expect to find ice!

Infestations of trolls, etc, in the chat channel were unfortunate but not too surprising, alas. But an attack against TPS really boggles me, both as a TPS member and professionally speaking, as someone who works in security. It wasn't a normal "DoS" attack where a site's swamped with spurious, but otherwise normal, requests, because Emily referred to rebooting the server to bring the site back. I don't know of any public "crash your server" exploits in the wild that work against Apache 2.0.59 as used on planetary.org . There's no money to be made from such an attack, which suggest sociopathic types ("script kiddies") with too much time on their hands... Although come to think of it, there's a thriving industry in DoS for blackmail purposes, especially around one-off events with high traffic peaks (hitting betting sites on the day of a huge sporting event was a favourite tactic a year or two back.)

Well, anyway, last night saw some of the absolute best the Internet can offer, as well as a couple of glimpses of the worst. (The Internet, plus dedicated, unbelievably knowledgeable people like Emily and Doug of course!) It bears repetition: enormous kudos to Emily, Doug and the Selsey crew, and all the UMSFers out there, as well as of course to the project team. So many memorable moments... and what a fabulous atmosphere, from Sir Patrick Moore's shirt and monocle, to the banter in the chat rooms (especially as the beers went down!) I'd love to see some traffic stats from umsf as well - did you pick up many new user registrations?

PS Next press briefing is 7pm BST / 1800 UTC on NASA TV I think?

PPS 11.5 inches == 29.21 cm

I noticed that as well - I also get 1.5 degrees. Could it be the SSI mast is tilted a degree or so from vertical?

Gotcha; thanks, man! I'm overthinking this; too used to INS as applied to military airdrop systems, I guess.

That was quite a night. I actually decided to sleep through the event as I had to hack some application into submission at work this morning so I couldn't take the day off and needed to be focussed. Somehow my internal timer managed to boot my brain exactly 10 minutes before touchdown ERT. After taking a look at the clock it dawned on me what time it was. I quickly rolled out of bed, put on some clothes, ran to the living room and turned on the TV. Just in time to hear that atmospheric entry was confirmed. Back in bed 5 minutes after touch down. Deep sleep failed but mission accomplished

I wasn't that surprised by the trolls and the attacks, as the internet is no different to real life: there are always going to be pathetic little people with pathetic little lives who are drawn to the brighter, richer lives of people better than themselves, just as moths are drawn to a flame. They can't stand to see people enjoying themselves, so they bang on community hall windows when people are having meetings, uproot flower displays, type abuse and worse in chat rooms, etc. Last night some low life somewhere saw that a group of people had come together to experience something fantastic, something unique, something they had no chance of grasping the significance of, so they decided to spoil it, just because they could. It's sad but it's not rocket science.

But compare that with the goodwill found in the chat rooms, the sheer delight on Emily's face when she was in her element at JPL, and the joy on the Phoenix team's faces during the press briefing and, well, we know who won, don't we?

- I agree that's what I see in that anaglyth



I was thinking more the big bit at 10:00ish, but that bit to the right of the shadow looks good as well.

PPPS 0.79 cm is a bit!!

It was a terrific afternoon/evening when it's all said and done; best time I've had ever during an EDL!

Would add one note: Some clown asked Emily what I, at least, felt was an intrusive and highly personal question when she was beginning her first 'cast, and I called him on it. He wasn't ejected, and behaved himself from then on. Since Emily has intimated that she'd like to do more live 'casts, I think it is incumbent upon us all to ensure that she is allowed to do so without having to put up with that sort of nonsense, and of course this should be the rule for all people who are kind enough to provide their time and expertise for our collective enlightenment.

Stu... right on!!! I had the same dream and it was a late night for me ..... HAD to stay up for those pics.

Yes, can hardly wait for the full panorama.....

And I would also like to comment on what a fantastic community we have here with UMSF.... what a joy it is to know that
out there are all you wonderful folks who love space exploration as deeply as I do. What an added joy it gave last night as the Phoneix team made history.

Doug and Emily continue to do an awesome job.... as do so many of you poets and image mages!!!!

Now a mission op questions.... since we really do not KNOW how deep the ice is, are they going to dig a rehersal trench just to give them that benchmark and then dig an adjacent sampling trench? They only have so many chambers to use for soil/ice analysis. Any one know the answer... or is this a stupid question? Still a bit whumped from last night's late night excitement.

Craig

I'm surprised by the amount of material inside the landing pad. A lot more than the Vikings and with the ground "frozen solid" as we expected tells me the top layer is quite loose and dry. I could be dead wrong, of course, but that's my impression. As for my earlier comment about wanting to land close to the parachute, it would be interesting to see a human made object interacting theith the martian environment over the lifetime of the lander. Will it ever flutter in the breeze, moved to a different direction? It would be a usefull passive experiment. I was dissapointed when Oppy didn't travers over to its chute which wasn't that far away.

I think I remember someone asking about that shortly after Oppy landed.... and I think that they were concerned about the rover becoming tangled in the tethers attaching the parachute to the backshell should it move in the wind for instance.

I was amzed at Emily's knowledge. She always had an immediate answer to every question!

The images were taken to the NNW. Landing position shows Heimdall to NE. The sol 2 part of the initial low-resolution site pan could show something on the horizon.

Mark

I think you'll be very sorry if you miss the briefing. Should be a fantastic new goody.

Interesting thought, Deimos.

Does anyone know why they wanted to land with the solar arrays aligned E-W? If they are horizontal, their azimuth shouldn't make any difference to power levels. Perhaps they were thinking about shadowing of the masts onto the arrays?