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Final Trajectory, Risk, Nerves, and Excitement
Brent Shockley As I write this, Phoenix is 27 hours from entry, descent, and landing on Mars. The spacecraft is in good health, and today teams are focusing on whether we need a final trajectory correction maneuver tonight. The navigation team has determined that the actual center of our landing ellipse has drifted some 10 kilometers from the desired target. Although this is still within an acceptable range, the team will continue to monitor our projected trajectory throughout the day and a final decision will be made this afternoon on whether a trajectory correction maneuver will be executed at 8 p.m. (Pacific time) tonight. If so, the spacecraft will rotate, do a short burn using its trajectory correction thrusters, and return to its original attitude. The entire maneuver takes about 11 minutes for the spacecraft to execute, and the thruster bursts are comparable to the spray from a can of spray paint.

For those of you who didn't have a chance to view the press conference from JPL this afternoon, here are some interesting analogies that were offered by our Mission Manager, Joe Guinn. One graphic showed what our landing ellipse would look like if it were on Earth. On Earth our landing ellipse would cover a distance from San Francisco to San Jose. For those of you on the East Coast, this would be just smaller than the size of Long Island. At a time when gas prices are soaring, Phoenix is getting good fuel economy at about 2 million miles per gallon.

This brings us back to a point on the distance traveled by Phoenix, which seems to raise some confusion. From launch until landing, Phoenix will have traveled 422 million miles with respect to the sun. At the time that it lands, however, Phoenix (and Mars) will only be about 170 million miles from Earth. Remember from my previous blog that everything in space is moving: Earth, Mars, Phoenix, and even our solar system and galaxy. As a result, Phoenix slowly spirals out to the orbit Mars occupies and eventually catches up with Mars. Since Earth has also moved with respect to the sun since we launched Phoenix, Earth is only 170 million miles from Mars at the time of landing even though Phoenix has traveled a total of 422 million miles. So no, we didn't head to Mars via the Kuiper belt.

One reader has also asked whether Phoenix has the ability to remove dust from its solar arrays. Although dust buildup is inevitable in the Martian environment, the onset of the Martian winter will limit our power supply and eventually bring our mission to its expected close long before dust buildup on the solar arrays becomes a significant problem. Hence we haven't included anything such as "windshield wipers" to clear dust.

Some readers are interested in more details regarding how we communicate with the spacecraft. Up until cruise stage separation just before EDL, we communicate with the spacecraft using an X-band radio located on the cruise stage itself. After cruise stage separation, Phoenix transmits data via UHF to the Mars Odyssey and Mars Reconnaissance Orbiter for relay back to Earth. A station at Green Bank, West Virginia will also listen for a carrier signal from Phoenix during EDL. By the time we reach Mars, it takes a signal from Earth about 15 minutes to travel 170 million miles at the speed of light to reach Phoenix, and 15 minutes for any signal from Phoenix to reach Earth. But it only takes Phoenix seven minutes once it enters the Martian atmosphere to land. This means that by the time we even begin to watch the data coming into Mission Control, EDL is already done! This is why Phoenix is designed to land completely autonomously.

Others reading the blog have rightly noted that our landing is not without risk. Indeed one boulder in the wrong place could result in a bad day on Mars for us. However, the Phoenix design team has gone to great lengths to manage these risks. High-resolution images from the Mars Reconnaissance Orbiter have allowed us to literally count all the large boulders in our landing area and determine that there aren't more than a few boulders per football field-sized area in our landing zone.

Some readers also ask whether it would have been less risky to use an airbag landing approach for Phoenix instead of powered descent. In truth, every landing system comes with its own strengths and weaknesses. The strength of Phoenix is that we can land a larger payload with a much more gentle landing than airbags provide. Indeed, this approach was successfully used for each of the two Viking landers. See my blog entry from May 21, 2008 to read more about the advantages and disadvantages of a powered descent versus airbags.

As our Project Manager Barry Goldstein conveyed in today's press conference, "test, test, test" has been the mantra on the Phoenix project in order to drive down risk. The Phoenix team has tirelessly prepared for landing through tests, simulations, and constant vigilance. Nonetheless, as you can imagine, everyone is on edge as we go into the final day. I for one can't wait to see what happens tomorrow. Although there are no guarantees in this business, Phoenix has behind it a great team that has done everything in its power to enable a safe landing.

Tomorrow I'll be filling you in on the action from JPL throughout the day and providing minute-by-minute updates from Mission Control as we anxiously await Phoenix's arrival at Mars. You can also watch the action live on NASA TV either on your TV, if you receive that channel, or on the NASA website if you don't. A variety of events will also be taking place around the country.Check out the list here.

Thanks again for the all wishes to everyone who's read and left a comment! We're all in for an exciting day tomorrow.

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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