|MESSENGER Webcast: Mission Overview|
Question and Answer with James Leary|
Tiffany Nail: We're fortunate to have James Leary with us live in our NASA Direct studio. James, it's great to have you with us.
James Leary: Thank you, Tiffany
Nail: James, you have quite an impressive resume. You've worked on a number of Navy and Defense Department projects and you are a staff member with the Applied Physics Lab at Johns Hopkins University. And it's worth noting that you have your masters degree in mechanical engineering and you are currently working on your doctorate.
You are here with us today because you are leading the technical development of the MESSENGER spacecraft. Are you excited about MESSENGER, and can you share with us some of the complexities of MESSENGER?
Leary: I'm very excited, and one the major complexity that I deal with every day is working with a large number of amazing people and it's wonderful.
Nail: I bet this has been a labor of love for you. Could you tell us a little something about how the MESSENGER mission has gotten to this point and how long you've been involved with it?
Leary: We've been working on the MESSENGER spacecraft for about four years, and I've been there for about the past three and a half.
Nail: As you may know, the NASA Direct! question board has received dozens of questions from people around the planet. I'm sure our viewers are eager to get to those questions and hear your answers, so let's get started.
Our first question comes from Juan from Spain. MESSENGER uses a shield to protect itself against very high temperatures; are low temperatures dangerous for the space probe? What will you do to protect the probe against very low temperatures during some part of the orbit around Mercury, which is in the shadow?
Leary: Well, Juan, that's a very good question. MESSENGER has been designed to protect it against the very high temperatures of Mercury and the Sun, but it also does get very cold at times. When we fly by Venus one of our times, we're in shadow for about an hour, and the extremities of the spacecraft can get down to -240 degrees Fahrenheit, which is very cold. There are heaters throughout the spacecraft, which put hundreds of watts of heat through the spacecraft to keep it warm when the Sun's not there.
Nail: Rich from Elk Grove: How long will it take MESSENGER to arrive at Mercury? What path will the spacecraft take to get there?
Leary: Rich, it takes us about six and a half years to get to Mercury. We have to do a very complex tour of the inner Solar System where the year after launch, we go by Earth, and then Venus twice and have to fly Mercury three times. All of those are gravity assists that help to slow the spacecraft down enough that we have enough propellant in order to do a compulsive burn and get into orbit at Mercury in 2011.
Nail: Dimitri from Ormond Beach: Why does it take so long (seven years) for the spacecraft to reach Mercury?
Leary: It takes us the six and a half years because we have to do the large number of fly bys in order to get the propellant mass down to a level to where we can get into orbit.
Nail: Thomas from Holland: Why wasn't Mercury investigated earlier? It is, after all, an interesting planet to study.
Leary: The reason it's taken almost 30 years, there's two key challenges. Obviously, the thermal challenge is the one we worked. Mass is the other one that I just mentioned. The spacecraft had to be very lightweight, because over half of our mass is propellant, so we do that burn in order to get into Mercury orbit after we do all those fly bys of the other planets.
Nail: Junichi from Niihama-city:How difficult will it be to get MESSENGER into Mercury's orbit?
Leary: Well, after we do our six-and-a-half-year tour of going around all these planets, the burn is about 15 minutes that we have to do to get into Mercury orbit, which is short compared to all the other insertion burns.
Nail: Art from Los Altos: What are the planned orbital parameters once in orbit around Mercury (in other words the closest approach, farthest point, period, etc.), and how long is the primary mission planned for?
Leary: Once we go into Mercury orbit, we plan to take science data for a year. Our orbit's a 12-hour orbit, so we go around the planet twice in one Earth day. At our closest approach, we get down to about 120 miles from the surface, and it's very elliptical. At the farthest point out, we're about 9,000 miles. This elliptical orbit allows us to heat up when we get near the planet, and then have a long period of time of about 1,100 hours to cool down after we take our science mission.
Nail: Vance from Chardon:Would it be possible to do with MESSENGER at the end of its mission that was done with NEAR-Shoemaker on Eros by getting some very close up images of the surface before it crashes?
Leary: Yes. One of our possibilities for an extended mission, if the probe is successful over its year of getting all its science data, is to alter our orbit a little bit and get in a little closer to the planet to get better science measurements.
Nail: Bret from Dalton:What is the life of the spacecraft and will radiation have anything to do with how long it will last?
Leary: The spacecraft was designed for an eight-year lifetime. It can last longer than that. The radiation that MESSENGER sees going out to Mercury and near the Sun is nearly equivalent to what you would see in an Earth orbiter. The Earth traps radiation belt, so an Earth orbiter sees a lot of radiation, and MESSENGER actually has about the same level over its six-and-a-half-year journey.
Nail: George from Dallas:Why does NASA always use such slow computer processors in its probes? 25Mhz seems pretty slow, even his PDA is 400Mhz.
Leary: Well, one of the key reasons we use slower processors is radiation. The older processors use technologies that are more tolerant to radiation upsets.
Nail: Mark from Danville:Will you be using the Deep Space Network to communicate with the spacecraft? Will the spacecraft use the gravity assist from other planets?
Leary: Yes, we'll be using the Deep Space Network around the planet to communicate with the spacecraft over x-band.
Nail: Our last question comes from Kim from Winnemucca. Instead of a high gain antenna MESSENGER uses phased array antennas. How do they work and are they as effective as a high gain antenna?
Leary:The MESSENGER phased-array antennas allowed us to have a more reliable spacecraft. The phased-array antennas actually do not move and they're electronically steered, as opposed to other deep-space missions, which have a large gimbaled antenna. They don't have as high a data throughput as a high-gain gimbaled antenna, but they work well enough for our mission.
Nail: We appreciate you being here with us today, James, and look forward to a successful launch.
Leary: Thank you, Tiffany.