As New Horizons prepared for launch, NASA presented a webcast in which scientists answered questions from the public. In this edited transcript, mission systems engineer David Kusnierkiewicz talks about the technology that will take the spacecraft to Pluto and beyond.

Q: What factors influenced the design of the spacecraft? What materials is New Horizons made of?

David Kusnierkiewicz (DK): The materials it's made of are pretty ordinary materials to make a spacecraft out of. The structure is made out of aluminum. The large six-and-a-half foot dish antenna is made out of a carbon composite material. But the reason it looks the way it does is because of the number of different requirements and factors that have to be taken into account when designing a spacecraft.

First of all, we have the nuclear power source, which we'd like to maintain at some distance from the electronics because there is some radiation that comes off of it, and you want to minimize the exposure to the electronics and the instruments to that radiation environment.

Secondly, the large high-gain antenna has to be pointed at Earth when we're spin-stabilized, so the spacecraft needs to be balanced, and that drives where we put certain components on the spacecraft.

Probably the most important are the instruments, which have to be accommodated; they have to have the proper fields of view in order to see what they want to see, like Pluto and Charon as we fly by. And then we have to fit inside the launch vehicle. At the same time, we try to keep everything as simple as possible.

So we fit all that together, and the design you see is what we came up with, which incidentally bears a strong resemblance to the Ulysses spacecraft, which also was a nuclear power-source spacecraft.

Ulysses was a cooperative effort between NASA and the European Space Agency; its mission is to study the sun. It also has a large dish antenna like we do. They tried to solve the same sorts of problems we did, and came up with pretty much the same solution.

Q: The Voyager spacecraft had Intel 4004 processors onboard. What kind of CPUs are onboard New Horizons?

DK: The processors on New Horizons are specialized processors for space applications. They're made to withstand the radiation environment of space, they've been qualified for space flight, and they've all been flown before. They're not processors you've really ever heard of before. They're not as powerful as the processors you have in your computers today at home, but they're the right processors to get the job done.

Q: What type of communication systems does the spacecraft have to send data back to Earth from such a long distance?

DK: The communication system communicates what we refer to as "X-band" frequencies. That's about 8 gigahertz. Of course, we have that large dish antenna that you see in the pictures of the spacecraft; that's six-and-a-half feet across. The data rates we have vary, depending on our distance from Earth. They start out at about 104 kilobits per second, and at Pluto they'll be down to about one kilobit per second. That doesn't sound like much, but you have to realize this is a hard thing we'll be doing, and it's quite an accomplishment to achieve what we have.

Q: Could you tell me more about the guidance system on the spacecraft? Is there a big possibility that it could collide with a Kuiper Belt object? How are the scientists going to guide it through the Kuiper Belt?

DK: When we're at Pluto, it takes the radio signal four-and-a-half hours to get to the Earth. So if we send a command to the spacecraft from the Earth, it'll be nine hours before we get a response from the spacecraft knowing that the command was taken and has been executed. That's a long time to wait, but that's what you have to do when you're dealing with distances this extreme.

As far as guiding the spacecraft through the Kuiper Belt, it's true that there are hundreds of thousands of Kuiper Belt objects. We don't even know all of them yet. But space is so large that the chances of hitting an object as we go through there are really very small.

You have to remember that, before we get to the Kuiper Belt, we'll also be traveling through the asteroid belt located between Mars and Jupiter, which also has hundreds of thousands of objects in it. But again, because space is so large the chances of hitting something are very small. But you can be sure that if we realize ahead of time that a collision might occur, we're going to take action to prevent that.

Q: What is the speed of New Horizons before and after the Jupiter flyby?

DK: Before the Jupiter flyby, the spacecraft is moving at about 43,000 miles per hour, with respect to the sun. Jupiter increases our speed by about 20 percent, to 52,000 miles per hour.

Q: Without the slingshot boost from Jupiter, what would be the normal time for New Horizons to reach Pluto?

DK: Without the Jupiter gravity assist, our arrival time at Pluto would be in the year 2019 or 2020. So Jupiter enables us to arrive 4 to 5 years earlier than without it.

Q: If New Horizons is going to reach Pluto in 2015, children who are 15 to 16 years old today could be the ones who control the spacecraft. Is there a plan about how the mission will be carried out in the future, because many of the scientists who are now working on the mission may not be present at the time the spacecraft reaches Pluto?

DK: Let me assure you that every scientist that I know of working on the job today fully intends to be working on the job when we arrive at Pluto to get all that data back. But it is true that over the course of such a long mission, people will probably leave the job. We have many experienced people working right now operating the spacecraft and supporting it, and it's a conscious effort to capture all that knowledge and manage it over the life of the mission so we don't lose that knowledge.

This is part of an effort we refer to as longevity planning, and there are a number of aspects to it, including archiving all the documentation that exists today. We have training sessions to certify our operators of the spacecraft, and as new operators come onboard they will be trained and certified to continue operating the spacecraft.

A lot of those training sessions today are being recorded on DVD, and they'll be archived so that people can refer to those later on. We have a simulator on the ground which is used for training, and also to the help new operators when they come onboard. And then finally we do have a lot of rehearsals planned before the actual encounter with Pluto, which will serve not only to assess the status of the spacecraft, but to help train or retrain people to conduct the encounter.

Q: Is New Horizons spacecraft going to drop a lander on the surface of Pluto or Charon to explore them?

DK: No. A lander would mean extra mass in the spacecraft, and it takes a lot of energy to get a spacecraft to Pluto. Adding a lander or anything else on the spacecraft would make it that much harder to get to Pluto when we want to get there.

And, in fact, we can accomplish the science we need to accomplish without a lander, with just the instrument complement that we have onboard. Also, a lander adds an extra degree of complexity to the mission. We try to keep things as simple as possible in order to achieve our scientific goals.

Q: New Horizons is nuclear powered and so it will be capable of transmitting data after completing its original mission as the Voyager probes did. How many extra years could the probe function?

DK: We have enough power onboard the spacecraft to last us at least until the year 2026, which is 10 or 11 years past the Pluto encounter. So we'll continue to do as much interesting science as we can, as power onboard the spacecraft permits us, even after the Kuiper Belt encounters occur.

Q: Are there any plans for the spacecraft to make a flyby past Sedna, or will the spacecraft lose communication with Earth by that time because it is twice the distance between Pluto and Earth?

DK: In fact, Sedna is in the wrong direction from the direction we're going. So there's no possibility of us doing any kind of encounter with Sedna.

Q: Will the course of the spacecraft be changed in case a new planet is found during the period 2006 to 2015, and which can be approached by the spacecraft?

DK: Our first mission is to go to Pluto. You can be assured that if any new, really exciting discovery is made known on the way out there, we would do what we could in order to encounter that.

That's the approach we're taking with the Kuiper Belt objects � those specific targets haven't been selected yet. We're going to wait and see to select the most interesting target that we can get to with the fuel we have onboard the spacecraft.

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