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Three Million Miles Left
Brent Shockley Today we're about three days from landing on Mars, with three million miles left in our 422 million mile trip from Earth to Mars. I continue to be amazed by the fact the Phoenix will finally be arriving at Mars after so many years of work by teams spread across the world, and impressed by all the great response to the blog so far. The spacecraft is operating well for us as the teams continue to work on the final landing simulations and trajectory analyses. There have been a ton of great questions posted in the comments on the blog, so I'll take this opportunity to answer as many as I can.

One excellent question that came through in the comments was whether any of the orbiters already at Mars will try to take an image of Phoenix during EDL. I had heard rumors of this recently, and confirmed with our Project System Engineer that the decision was made last week that the Mars Reconnaissance Orbiter will attempt to take a picture of Phoenix using its HiRISE (High Resolution Imaging Experiment) camera during the parachute phase of EDL (when our parachute is deployed as we descend to the surface). However this is an extremely tricky task with a lot of variables, not the least of which are an uncertainty surrounding the pointing accuracy of camera itself, the uncertainty of knowingly exactly where Phoenix is in the sky, and the timing of taking the picture at just the right moment in order to capture Phoenix in the image frame. Hence estimates suggest that there may only be a 25% chance that we can capture an image of Phoenix during the parachute phase.

artist concept of Phoenix landing on Mars Artist's concept of Phoenix deploying its parachute. Bryce from Mr. Wright's 4th grade class in Wisconsin asked whether we are 100% sure that there is water in the northern polar region. Scientists and engineers alike are always hesitant to say anything is 100% certain, but the results of the Gamma Ray Spectrometer experiment on the 2001 Mars Odyssey spacecraft has scientists convinced enough that there is subsurface water ice in this region that Phoenix has been designed specifically to explore this possibility. So the real question scientists are asking is not whether there is water ice, but rather how deep it is beneath the surface. In order to answer this question, Phoenix has a robotic arm that is capable of digging below the surface and collecting samples for our other instruments designed to study soils and gases. The robot arm also carries what we refer to as an Icy Soil Acquisition Device that would be best described as a drill that can take samples of ice that may be rock-hard. Click here to learn more about the Phoenix instruments and what they can do.

This brings me to another interesting question from Kira in Poland who asked why we don't have a little rover that could use Phoenix as a garage/charging/computer station. The answer to this is that each mission is designed to address certain questions in the scientific community. The Mars Exploration Rovers, Spirit and Opportunity, address questions that require the ability to actually travel across the Martian plains and explore different areas. The Phoenix lander, on the other hand, is designed to answer questions that require us to dig down at a single location on Mars. The seven-foot robotic arm Phoenix uses to do this requires a more stable base than a rover can provide. Since we already have rovers on Mars, and our objectives don't require the ability to rove to new locations, Phoenix instead devotes its resources to a more stable platform and an advanced set of instruments geared towards understanding as much as possible about a specific location.

water map of Mars A map from Mars Odyssey showing water content in the very top of the Martian soil. Blue at the poles shows the soil is about 30 percent or more water.
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Another reader asked how Phoenix determines its orientation relative to the ground during the powered descent phase during which we use thrusters to bring Phoenix to gentle rest on the surface. Phoenix has a radar system that keeps track of our altitude and velocity during descent and feeds this information into our guidance, navigation and control system. This same system also includes what is called an inertial measurement unit that helps keep track of our attitude as well. These parts all work together to assure a safe landing.

A number of people have also asked about Phoenix's ability to survive the Martian winter. Being at the north pole, as winter arrives the sun barely rises on the horizon. Because Phoenix relies on solar power, we will no longer be able to operate our electronics and keep things warm. We expect that the electronics won't survive the harsh conditions once winter arrives, there is a contingency for the possibility that it might survive (however unlikely that may be). Should the electronics, solar arrays and batteries survive the winter, Phoenix has the capability to recharge the depleted batteries and "phone home." After a successful landing and surface mission, this would be frosting on the cake!

Jim from Michigan asked what timing mechanism is used for all the deployments that have to occur during EDL (parachute, heat shield, lander legs, etc). This is all done autonomously by the spacecraft using a sequence that was assembled and uploaded long before EDL. It takes into consideration the latest parameters regarding the atmosphere and the spacecraft trajectory, as well as a variety of inputs coming from our guidance, navigation and control system in order to make these events happen right when they need to.

Phoenix Extends its Legs An artist's concept showing Phoenix with its three legs extended.
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Another question asked why we have three legs instead of five. The answer to this is quite simply that three legs are the minimum number required in order to be stable. Thus three legs make the most efficient use of our mass and resources.

Some readers are also wondering what happens to the various parts that we jettison during EDL, such as the cruise stage, heat shield, parachute and backshell. Since we separate from the cruise stage just before entering the atmosphere, it will burn up on its own in the Martian atmosphere. The heat shield, backshell and parachute will simply fall to ground. If you've been following the Mars Exploration Rovers for a while, you may recall that the rovers had their own backshells and had an to opportunity (no pun intended) to see what effect EDL had on them.

One reader asked whether our thrusters would have any impact on the environment surrounding Phoenix as we land. To be sure, Phoenix will kick up quite a dust cloud as it settles onto the surface. In fact, we wait 15 minutes for the dust to settle before opening our solar arrays and making other critical deployments. Although the thrusters could melt subsurface ice if we ran them continuously in the landed state, they shut off almost immediately upon touchdown. This means that our thrusters should have little if any impact on the natural environment.

Finally, another reader inquired as to how we make our turn to entry after separating from our cruise stage. This is done using a combination of thrusters that put out one pound of force each, as well as a set of more powerful thrusters that exert five pounds of force each. Since this maneuver doesn't require much thrust, we could just use the smaller thrusters, but since it's such a critical maneuver, the bigger thrusters are kept at the ready, just in case.

Thanks again for the all the great questions and comments! We're all happy to see that people around the world are sharing in our excitement as landing day approaches.

Brent Shockley
Phoenix Configuration and Information Management Engineer

To learn more about the spacecraft and the mission, check out the following sites:

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