The end of Phoenix
Posted by Emily Lakdawalla
11-11-2008 16:46 CST
Topics:Phoenix is gone. We knew this would happen, and we even knew approximately when it would happen; the mathematics of solar input, solar panel output, and power consumption was relatively straightforward, so we knew from the moment that Phoenix survived its landing that it would most likely die some time in November.
But its sojourn on was so brief, barely five months, just 60 days past the end of its 90-day nominal mission. The American Mars missions that survive their launch and landing or orbit insertion have set a ridiculous standard for longevity. The Viking 2 orbiter lasted only two years, but the other orbiter lasted four, and the landers an amazing four and six years; Mars Global Surveyor lasted nearly ten; Pathfinder outdid its 30-sol nominal mission by a factor of nearly three; Odyssey is still going strong after seven years. The Mars Exploration Rovers are in a class all their own, having survived almost twenty times as long as their warranties stated. There's no reason to doubt that Mars Reconnaissance Orbiter will last for years to come (though, as Odyssey is aging and there's no new Mars telecom orbiter under development, the health of Mars Reconnaissance Orbiter is going to become increasingly important).
When most Mars missions last so long, can Phoenix have accomplished enough during its 151 sols on Mars to make the effort and expense worth it? It was seven years ago, project scientist Peter Smith said during yesterday's press conference, that he first came up with the idea of using a mothballed spacecraft from a cancelled mission to explore Mars' north pole. And the whole mission cost nearly half a billion dollars, or around three million dollars per day of operations on Mars.So, what has Phoenix accomplished? Probably the number one thing it did was to prove right the conclusions drawn from orbital evidence, that the flat, oddly patterned northern plains of Mars really are underlain by a vast deposit of water ice. Mars today is a very dry place, but orbiters detected a signal of large amounts of the element hydrogen just beneath the surface. Water seemed the likeliest suspect, but there was no way to prove it except by investigating directly. Phoenix detected it, and moreover, Phoenix dug into it, scraped it, and measured its effects on soil.
Phoenix did gather all the data that it was sent to Mars to retrieve, accomplishing everything on its list of mission success criteria in August. I had a blog entry prepared for that event but the mission didn't ballyhoo it so I never figured out when to post it -- I think now is finally the appropriate time. What does "mission success" actually mean? When I posted earlier about Phoenix' progress toward Mission Success I asked Mark Adler (formerly Mission Manager for Spirit, Lead Mission Engineer on Cassini-Huygens, and now Chief Mission Concept Architect for JPL) to refresh my memory on this one. He told me
"From the official documents, full and minimum mission success criteria are used mostly during development (i.e. before launch). If you find that you cannot meet your originally stated minimum mission success criteria, that may trigger a termination review. Full mission success criteria are used to drive the design capability during development. An oversimplified explanation would be that you design the system to absolutely assure that you can meet the minimum criteria, and you design it to not preclude meeting the full criteria if things go well and everything is working. For the latter, you make sure that you have the resources (consumables, energy, time) to allow for full mission success. In both cases, the criteria are considered against the duration of the primary mission." That is, even though it's reasonable to expect a healthy spacecraft to last beyond its primary mission, it is designed to be able to accomplish "full success" within the time constraint of its primary mission.
Mark went on: "Once you're in operations, you try to meet the minimum and full mission success criteria, but they are generally negotiable as the mission responds to real events. The criteria are not allowed to force you to do anything stupid, or something that would reduce the science return. E.g. if you have a driving distance mission success criterion on a rover, you wouldn't be forced to drive away from a really scientifically interesting spot just to meet the distance criterion before the primary mission is over." This was relevant for Phoenix, too; they hadn't quite gotten to the number of TEGA samples needed for mission success by the end of their first 90 sols, but they weren't forced to go grab another sample from a less-desirable location, using up one precious cell, just to squeeze that last sample in before the 90-sol mark.
In a way, "mission success" is really just a bureaucratic detail -- it's sort of like a passing grade. A mission is in real trouble if it doesn't "pass," but most people on a mission would aspire to do more than just pass. And Phoenix did do that, making several surprising discoveries: the almost entirely unexpected observation of perchlorate salts in the soil; the soil's unexpectedly high pH and low salinity, and the detection of actual water-ice snow falling from the sky were all mentioned at yesterday's press briefing. Phoenix returned 25,000 images, and analyzed samples in five of the eight TEGA cells, about twice as many as mission success called for. MECA just met its mission success criteria for the wet chemistry lab, but analyzed nine different samples in its microscopy station, three times as many as mission success required.That's not to say there weren't disappointments. I'd say that the biggest disappointment, bar none, is that despite focusing on the effort continuously for more than 30 sols of the primary mission, they never managed to get that ice-rich soil sample into the TEGA instrument. Without a sample containing lots of ice, it's my understanding that they couldn't do an analysis of the ratio of hydrogen isotopes in the ice, which could tell them about whether there's a lot of exchange between this subsurface ice reservoir and the atmosphere, or if the subsurface ice reservoir is dynamically isolated from the atmosphere. I think it's disappointing both because the measurement would have been important and because they spent so very much of their precious -- and short -- time on Mars trying and failing to get that sample. If they'd known they wouldn't succeed, of course, they could have done a lot of other valuable things with that time, things that didn't get done by the end of Phoenix's 151 sols.
That's the problem, though; the best engineers and scientists couldn't predict just how hard it would be to handle these samples of soil and ice. And, in a way, Phoenix's struggle with, and failure to, handle those ice-rich soil samples is another accomplishment of the mission, if a backhanded one. Phoenix was the first in a new line of Mars missions in the "Scout" program. Scout missions are the least expensive of the line of Mars missions, somewhat like (but even cheaper than) the Discovery line of missions to places elsewhere in the solar system. If you want to try out a new idea, it makes the most sense to try it on a cheap mission. Pathfinder (a Discovery mission) was the first to successfully try a rover on Mars; Phoenix was the first to try to handle Martian ice, and the first to carry such a complicated suite of instruments. Phoenix's experience, both the accomplishments and the disappointments, will inform future missions. No doubt the designers of the Mars Science Laboratory sample handling hardware were paying close attention to Phoenix' trials and tribulations.
Also, we've barely scratched the surface of what Phoenix's scientific analyses can and will produce in the coming months and years. There hasn't been a single article, poster, or oral presentation on Phoenix science results to an open audience of scientists as yet. The first such presentations will happen in just over a month, at the American Geophysical Union meeting in San Francisco. That's when we'll begin to get an idea of the legacy that Phoenix has left us. Peter mentioned during yesterday's press briefing that the teams were working on papers for publication in Science -- hopefully the not-yet-published status of those papers won't affect their willingness to share their preliminary science results at the AGU meeting.Many of you have written to me asking whether Phoenix will be able to come back to life after the Martian winter is over. The good news is that if Phoenix suffers no major damage during the winter, it could not only regain contact, it could even regain the use of its robotic arm. The bad news is that the extremely cold temperatures that prevail at Phoenix' location during the winter make survival exceedingly unlikely. The materials used in Phoenix' electronics will go through what Barry Goldstein referred to as a "glass transition," where they lose their elasticity and become as brittle as glass, most likely cracking. Also, the lander will be encased in carbon dioxide ice; depending on how that ice forms and disappears, its weight could crack off Phoenix' solar panels, without which the spacecraft has no hope of returning to life. So, this is most likely the end, for good, of Phoenix. But it's not the last we've seen of Phoenix. Mars Reconnaissance Orbiter, which produced such an amazing image of Phoenix landing, has taken numerous photos of Phoenix sitting on the surface since then, and just because Phoenix is silent is no reason for the orbiter to stop now. It'll keep watching the landing site as autumn arrives, and it will start watching again in the spring, to see what the carbon dioxide ice cap has done to our little emissary.
Take a moment to pause in whatever you're doing and send your thoughts toward Mars, to think of a human-built artifact sitting on a desolate, frosty plain, with the Sun low in the sky to the south. Though it may not talk to us again, it'll sit there forever -- or at least until some future human arrives to restore it and place it in a museum.
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