TEGA Status and More - by Deborah Bass
Thanks to all those who posted comments! I’m glad to see that there is
so much interest in the Phoenix mission! I wanted to address a few key
First off, some staff from the Mars Science Laboratory project
will be writing blog entries, so please hold your questions about that mission until
the end of summer, when those blogs begin!
Phoenix’s Thermal and Evolved-Gas Analyzer, or TEGA, has given the team some
head-scratchers, and those challenges continue. In my last blog I talked about lumpy
soil, sprinkling and delivery mechanisms. TEGA has been using a method to agitate its
cells to help move the soil down from the collection area into the oven as well. Well, it
turns out there was a short circuit in a TEGA cell number four when we used the agitator
on that cell in June. We used that same agitator for repeated periods of many minutes
each time while we were getting the first soil sample into TEGA. Project engineers
determined the likely cause was running the screen-agitator longer than we ever had done
in pre-launch testing. Running the circuit for such a long time caused some wire
insulation to get too warm, causing a short. That short in itself did not cause or
threaten any problem with operating TEGA, just on the “grounded” portion of the circuit.
It was on the return part of the circuit, between where the current does its work and the
ground connection. And then that short apparently healed itself when doors for cell
number zero were opened on July 19! However, the occurrence of any short raised concerns
that another short circuit might possibly occur, and if it did, it might be a more
harmful one. That concern still exists, and has prompted at least two precautions — a
decision to change sampling strategy to treat each TEGA sample as if it could be the
last, and an operational rule to avoid running a screen-agitation for more than three
minutes without a cool-down period before resuming.
Trying to get samples into the chemistry experiment was a big topic during development. When Peter Smith proposed to send Phoenix to the Martian arctic, the intention was to use as many already-developed pieces as possible. New methods of delivering material to the chemistry experiments on the lander deck had to be simple, because the original design was to use the scoop to dump soil. However, based on pre-launch testing, the original method of scraping/scooping the soil to generate a sample didn’t appear to work on ground that is frozen so hard that the ice and soil behaves like cement! The Phoenix team has been doing many tests to ensure that the alternative method, using a little Dremel-like tool called the rasp, works. These tests were done on analogs of extra-hard Martian soil, but there is still nothing like testing with the real stuff on Mars. The Phoenix team had established that the rasp will acquire enough icy soil to deliver a proper sample to TEGA. Those on-Mars tests have taken a long time, as expected. Mars continues to amaze the science and engineering team - the Martian soil is behaving unlike any sample the team used in practice back on Earth! The exciting news is that the team was able to get a sample with a bit of ice into TEGA after all!
Another item came up regarding better ways to clear off the ice table. I’ll tell you that the Phoenix development team wrestled with this topic for quite some time. Field studies show that a brush is the best way to remove loose soil from a region a geologist wishes to sample. The problem of course, is that then the brush gets dirty! The ability to clean the brush for further use becomes the problem. The soil on Mars is very, very sticky due to small particle sizes, salts and ice that appear to be acting as cements, and electrostatic properties that cause dust-sized particles to be charged and stick to each other that way too. The team could not come up with a reasonable, relatively inexpensive brush/cleaning mechanism in the short development cycle that the Phoenix mission undertook. (Remember that the mission was only approved in August 2003!) The notion of using the scraping blade on the robotic arm was deemed the most expedient, least costly way to clean surfaces.
Hope this answers some of your questions. Thanks for all of your interest!