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Planetary News: Phoenix (2007)Phoenix Prepares for August Launch to Mars' Northern PlainsBy Emily Stewart LakdawallaJuly 12, 2007
With the Dawn mission vacating the launch pad, the way has been cleared for Phoenix, the next Mars lander, to catch its departure window for Mars. Phoenix is the first in NASA's Scout program of low-cost Mars missions. Its suite of science instruments, designed to perform in-depth chemical, mineralogical, and morphologic analysis on samples of rock and ice dug from a trench, is mounted on NASA's equivalent of a "used car," a lander platform originally built for the canceled Mars Surveyor 2001 mission. Every 25 months, as Earth approaches slower Mars from behind, there is a few-week opportunity to send spacecraft outward bound to the Red Planet. Since the 1996 launch of Mars Global Surveyor, every biannual Earth-to-Mars launch opportunity (five of them to date) has seen at least one and as many as three launches. Phoenix will be the only Mars-bound spacecraft launching in the 2007 opportunity, with a launch period opening August 3 and closing August 24. Its cruise will take it on a long "Type II" interplanetary trajectory, traveling more than halfway round the solar system, to arrive at Mars 10 months later, on May 25, 2008 (if launch occurs on or before August 18) or June 5, 2008 (if launch occurs on or after August 19). NASA's Dawn mission recently had to accept a delay to a September launch in part because of foul weather prevailing at the Florida launch site during Dawn's afternoon launch windows. At a press briefing held on Monday at NASA Headquarters, Barry Goldstein, the mission's project manager, said that "on Phoenix we made a conscious decision to launch early in the morning to avoid the afternoon thunderstorms that are traditional in Florida that time of year. So on August 3rd, when our opportunity starts, we are going to be launching at 5:30 in the morning [EDT]. Our launch opportunity goes to the 24th of August. We roll back about 12 minutes a day if we should miss each of the first days." Phoenix will land on Mars using retrorockets and landing legs, and if it's successful, it will be the first soft lander to touch down on Mars in three decades; the last such landings were the two Vikings in 1976, and the only other recent soft landing attempt, Mars Polar Lander, ended in failure. At the press briefing, Mars Exploration Program director Doug McCuistion stated that the mission team has focused especially hard on testing Phoenix' entry, descent, and landing systems, but was cautious rather than confident about landing success: "Entry, descent, and landing is a challenge no matter what mission you're working on; it has a tendency to throw you curveballs. We used a lot of the Mars Exploration Rover information to ensure we understood the systems for Phoenix. But Mars is hard. The global success rate for landing on Mars is under 50%; the NASA success rate is over 50%. [The soft landing design] provides some unique advantages, but it also adds some risk. Hopefully Mars sends us a good day." That extensive testing of Phoenix' entry, descent, and landing systems has exposed a worrisome fault in a key data-handling card on the lander. Goldstein explained that the problem card interfaces with both the spacecraft's Descent Imager, MARDI, and its inertial measurement unit (or "gyro"), which is critical for maintaining the orientation of the lander during its final rocket-assisted descent. A statement in the mission's press kit reads that there is "a small possibility of triggering loss of some vital engineering data if [the card] receives imaging data during a critical phase of final descent." Goldstein explained further that "Propagating the attitude as we descend to Mars is quite important. Losing any of those measurements from the gyro would not be a good or happy thing for us to do. So we decided not to go down this path. The situation is actually temperature-dependent. The team at Lockheed was able to narrow it down…to a tight temperature band at which the problem was occurring. We feel pretty confident that we won't be in that temperature band, but what we can't predict is how things will change over time. So the path of least regret is to minimize the impact by eliminating the problem completely." Eliminating the problem completely means that only one image will be taken by MARDI during the descent, instead of 20 as originally planned. McCuistion played down the negative consequences of this reduction: "It is disappointing. But one important thing is that MARDI is not in the mission success criteria. We have other ways to come about this kind of data. Mars Reconnaissance Orbiter nearly has that resolution and it will be imaging Phoenix as it's on the surface and on the way down, hopefully, if we're in the right location. So there are ways we can get the context of the Phoenix environment through the Mars Reconnaissance Orbiter." Indeed, Goldstein reminded, MARDI was designed for a lander to be launched in 2001, when there were no orbital spacecraft that could capture photos with sufficient resolution to definitively identify Phoenix sitting on the patterned ground of Mars' northern plains. In 2008, however, Mars Reconaissance Orbiter's sharp-eyed HiRISE camera should be able to produce detailed images of Phoenix on Mars within days of its landing, so the MARDI images are not actually critical to locating the landing site as they were with the Mars Exploration Rovers. The single MARDI image is planned for a time close to the end of the descent, when it will be able to achieve better resolution of features on the surface than HiRISE can from orbit. But without earlier images, it may be quite difficult to place the MARDI image into its proper broader context. Still, the late discovery of the problem is a concern, the severe reduction of the MARDI data is frustrating, and there is another cost: the probable loss of what might have been the first sounds from Mars. Malin Space Science Systems, which built MARDI, incorporated into the instrument a tiny microphone that will be sensing the rush of air during Phoenix' descent. If microphone data are collected, The Planetary Society will be partnering with Malin Space Science Systems to work with the sounds, in an attempt to recover from the loss of the Society's Mars Microphone with the crash of Mars Polar Lander. McCuistion remained philosophical about the problem. "It's important to recognize that we bought a used car. This [hardware] has been around for a while. We know a lot about it, and these guys have tried to learn everything about it, but the bottom line is, you buy a used car and you might have some front-end alignment problems that you wouldn't know about until later when your tires are wearing. So there's a lot of things you find out late in the game, and this just happened to be an unfortunate one." Scratching the SurfaceOnce Phoenix does land, it will embark on a mission that is fundamentally different to those of Pathfinder and the Mars Exploration Rovers, said the mission's principal investigator, Peter Smith of the University of Arizona. From the outset, Phoenix's lifetime is sharply limited by the season, because it is a solar-powered spacecraft landing near the north pole of a planet whose axis is tilted slightly more than Earth's. Phoenix will enjoy continuous summer daylight for much of its nominal three-month mission, but as fall comes to Mars' northern hemisphere the Sun will sink lower and lower on the horizon, eventually starving the lander of power and causing its death. Phoenix is landing near the north pole not to survey the scenery but to see what's beneath the surface, Smith said. "If you look at the [landing site], you notice that it really doesn't matter much whether we land to the left or the right, it's very uniform. We want to spend the short amount of time that we have in the northern polar summer digging down under the surface rather than exploring horizontally. This is a vertical mission. There's a lot of excitement not just at the beginning, when we see the first pictures, but toward the end of the first three months as we begin to understand the mineralogy, chemistry, and habitability of the northern plains."
Based upon mineralogical information from the Mars Odyssey orbiter and high-resolution images from the Mars Reconnaissance Orbiter, Smith and his team are confident that the subsurface at their landing site will contain significant quantities of ice within reach of their robotic digging arm. But they still do not know how deep they will have to go to reach that ice, and that uncertainty means that they are not sure how many trenches they will get to dig. "We have a fixed amount of time. We are taking three months for the digging process and our science analysis. The arm is seven to eight feet [2.35 meters] long and we can reach between the two solar panels. So we have something like eight square meters of surface area that we can attack with our robotic arm, and the question is, how deep is the ice? We won't know that before we get there. We suspect the ice is only a few inches deep, and in that case we'll probably do a lot of horizontal digging and clear an area. If the ice turns out to be quite deep, we may spend a fair amount of time digging down two or three feet [up to a meter] below the surface. We can actually dig down to three feet [a meter] below the surface, but that takes a fair amount of time. So either one deep trench or a lot of clearing horizontally." Once it exposes rock, soil, and ice from beneath the surface, Phoenix will bring to bear a powerful suite of in-situ analysis tools. The instruments on the lander include:
One common question about the Phoenix mission is why they are not landing on Mars' ice cap, where there is certainly ice available for study at the surface. Smith answered, "We specifically did not want to land on the exposed ice on the north polar cap. It's a very difficult environment, and it's not the situation we're looking for where we can see the effect of water on soil, because it's the transition of soil from a volcanic mixture to clays and the effect of liquid water [that we are interested in]. You would not be able to tell that if you landed on solid ice. We also think [the ice-soil interface] is a more likely environment for microbes on Mars, more of a habitable zone." The mission team is closely focused on accomplishing as much as possible in their three-month nominal mission, because they do not know how long the lander will survive beyond that. The landing occurs in late May of 2008; digging will begin one week after landing. The summer solstice, and highest power levels, occur about a month after landing, on June 25, 2008, and the nominal mission ends at the end of September. It is likely the lander will survive longer than that -- into October, probably November, but possibly not December. Permanent polar darkness will come to Phoenix' landing site not long after the equinox on December 26, 2008. As the equinox arrives, so, too will seasonal cloud cover and even ice. Will Phoenix see its landing site ice up before it goes silent forever? "We wondered about this ourselves," Smith said. "We are dependent on solar power. As we get later into the season, the solar power gets less and less. It becomes a game as to how you can limp along with less and less energy, perhaps only operating every third day and allowing the batteries to charge. I'd be thrilled if we could last long enough to see the ice starting to form in winter. On Mars, when the Sun sets, it doesn't get dark immediately like on the Earth. Actually, light pipes around the limb of the planet and you can get a significant amount of light even after the Sun is low on the horizon. If we are lucky enough to be tilted toward the south, that would be a big help also. So we are hopeful that we can survive long enough to see ice [form], but that's probably unlikely." |
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