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Spacecraft Status and Questions Answered
Brent Shockley It’s now Wednesday, and Phoenix is fast approaching Mars. In fact, if you were hitching a ride on Phoenix, right now Mars would look about a third the size of the full moon viewed from Earth. Thanks to everyone for the all the great responses to the blog so far. It’s great to see that people across the world are as excited as we are about exploring Mars. Thanks especially to Mr. Wright’s 4th grade class in Kaukauna, Wisconsin for all their comments and wishes. I came across lots of great questions in the comments, which I’ll try to answer now.

One reader asked why our Project is called Phoenix. The Phoenix lander was initially built for the 2001 Mars Surveyor Program. Following the cancellation of that program, however, the partially built lander was put in storage in a Class 100,000 clean room. Instead of a lander-orbiter pair that year, JPL launched the 2001 Mars Odyssey orbiter which is still going strong and will actually relay data to Earth for Phoenix during EDL. In 2003, a proposal revived the 2001 lander, complete with a new suite of instruments – some from the lost Mars Polar Lander and Mars 2001, plus some new. The name Phoenix, in reference to the mythological bird rising from the ashes, therefore seemed fitting.

It's probably also worth mentioning at this point, in case there's any confusion out there, that Phoenix is a robotic spacecraft. Although Phoenix will send data back to Earth from Mars, there aren't any humans on board the spacecraft.

Another question many people are asking is why we’ve chosen powered descent (thrusters) instead of airbags like those used on Pathfinder in 1997 and the Mars Exploration Rovers, Spirit and Opportunity, in 2004. There are a number of trade-offs that have to be made when selecting a touchdown system. These include the tolerance of the payload to the force of the impact, the complexity of the deceleration system, the ability of the landing gear to absorb shock, and efficiency of the delivered mass. A powered descent system, though more complex, allows the spacecraft to reach the surface with a final velocity of only five miles per hour. An airbag system, on the other hand, would result in an impact velocity of 30 miles per hour, which the Phoenix scientific instruments couldn’t survive. Mass is another consideration. Whereas the landing system accounted for 58% of the total landed mass for the Mars Exploration Rovers, the Phoenix landing system (legs and propulsion system) accounts for only 16% of the total landed mass. This allows us to devote more mass to our scientific payload.

comparison of sizes for Mars rovers and landers This shows relative sizes of four different landers. Mars Science Lab is scheduled to launch in 2009. As the landers get bigger, engineers want to use different landing systems.
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Another popular question is why Phoenix is only expected to operate for three months on the surface of Mars. This is due to the location of our landing site. As luck would have it, the most interesting place for the Phoenix to explore -- the polar region due to the discovery of subsurface water ice by the Mars Odyssey orbiter -- is also a location where it would be very difficult for Phoenix to survive the Martian winter. Phoenix uses solar arrays to charge its batteries and provide power to our heaters and instruments. Much like how the days grow short during the winter in Alaska, the days will also get shorter when the Martian winter arrives in November later this year. As the sun gets low on the Martian horizon, Phoenix receives less sunlight on its solar arrays and therefore generates less power with which to heat and operate our electronics. Furthermore, as it continues to get colder, the carbon dioxide will eventually condense out of the atmosphere to create a polar ice cap that is expected to encase the spacecraft from February to November of 2009. The cold temperatures and loss of power means that Phoenix probably won’t survive the Martian winter.

map showing landing site for Phoenix and other rovers and landers This is a map showing the landing locations on Mars of several robotic missions, including Phoenix.
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There was also a question in regard to where visitors can view the landings. As you can imagine, JPL is going to be a flurry of activity on landing day. As much as we love sharing the excitement and anticipation of landing on Mars, the Laboratory won’t be open to the general public. However, there are a number of museums and science centers around the country that are sponsoring live events that people can attend. Check out the list here. There will also be live coverage on landing day on NASA TV (click here for NASA TV on the web) and various outside news outlets, and of course on my blog (and several other blogs as well).

As Phoenix continues on the final leg of its voyage to Mars, engineers at JPL continue to monitor not only the spacecraft but also weather conditions at the landing site. As I mentioned in my previous entry, operations teams successfully executed a trajectory correction maneuver last Saturday that puts Phoenix on a course for our desired landing location. Between now and Sunday, teams will continue to monitor and evaluate the trajectory of the spacecraft to determine whether another maneuver is required. In addition to this, however, teams are also keeping a watchful eye on the weather at our landing site using data provided by the Odyssey and Mars Reconnaissance Orbiter spacecraft already at Mars. Much like conditions on Earth, the weather on Mars fluctuates and has an impact on the parameters we provide to the spacecraft during EDL. Therefore engineers at JPL monitor the weather at the landing site up until landing and feed those conditions into the simulations run on the super-computers here at JPL in order to see how it might affect our landing and how we need to adjust the parameters we send to the spacecraft.

Phoenix is getting closer, but there is obviously still plenty of work to be done, so stay tuned!

Brent Shockley
Phoenix Configuration and Information Management Engineer