• How much will the impactor weigh?
• Why is the impactor made of copper?
• Do the spacecraft (flyby spacecraft and impactor) have names?
• How is the impactor propelled after separation?
• Whipple shields?!?
• What happens to the flyby spacecraft after it's finished collecting data?
• Why is everyone dressed as in a clean environment, down to the shoe covers and there are several members in street clothes/shoes with their faces revealed? Shouldn't all of them due to the risk of contamination be equally dressed?
• How were the design for the spacecraft and impactor developed?

Q: How much will the impactor weigh?

     The impactor has a mass of 370 kg, which corresponds to a weight of 816 pounds.

Q: Why is the impactor made of copper?

     Copper was chosen because it will cause the least interference with the measurements that will be made during the impact, will not leave a residue that would confuse potential future measurements, and can be made into a structurally strong impactor. In particular, all the inner shells of electrons for copper are completely filled. This means that it reacts very slowly with other elements, such as with the oxygen in cometary water, and it will end up producing relatively few bright emission lines in the spectrum of the vaporized materials. Other materials such as aluminum would produce far more and stronger emission lines (mostly due to aluminum oxides). There are only a few materials that satisfy this criterion and copper is the least expensive of them that is structurally sound. The material used to make the impactor is actually a copper alloy with about 3% beryllium to make the copper more stiff.

Q: Do the spacecraft (flyby spacecraft and impactor) have names?

     No, for the time being we refer to them as the flyby spacecraft and impactor and they make up the flight system. We may decide on names for them in the future. What do you think we should call them? Send your ideas to E. Warner.

Q: How is the impactor propelled after separation?

     The impactor, as the flyby spacecraft, is propelled by the expulsion of nitrogen, hydrogen and ammonia gases created during the decomposition of the liquid propellant hydrazine, N₂H₄:

2 N₂H₄ —> H₂ + N₂ + 2 NH₃

     Hydrazine is used because of its very fast reaction rate, allowing for the quick generation of gas.

Q: Whipple shields?!?

     No, these are not named after the gentleman who used to squeeze toilet paper in the television commercials. Instead, they are named after Dr. Fred Whipple, who originated the "dirty snowball" model of comets, and also proposed the idea of protecting spacecraft from the debris left behind by comets as they pass through space.

Q: What happens to the flyby spacecraft after it's finished collecting data?

     There are no plans to use the spacecraft for any more data collection after this mission is completed. The craft will enter a stable orbit around the sun, becoming another member of our solar system.

Q: Why is everyone dressed as in a clean environment, down to the shoe covers and there are several members in street clothes/shoes with their faces revealed? Shouldn't all of them due to the risk of contamination be equally dressed?

     Your question is a good one, and it is one that we always get asked whenever we release a picture from our cleanroom. The answer is: Deep Impact is being assembled and tested in a Class 100,000 cleanroom (meaning 100,000 particles per million liters of air). The clothing requirement for a Class 100,000 cleanroom is smocks only. But whenever an engineer or technician will be within 1 meter of flight hardware (like the Deep Impact spacecraft), the clothing requirement increases to hair bouffants, face masks, and gloves. And, if at any time the engineer or technician will be in an elevated position above the cleanroom floor (on a ladder or stool), the clothing requirement goes to full coveralls, boots, bouffant, face mask, and gloves.

     So, in the picture of the Deep Impact stacking operation, you see Joe and Alec in full coveralls, because they are the technicians that go up on the ladders to attach/detach the lifting crane from the Flyby spacecraft. You see Dave (holding the crane controls) and Lorna (standing next to Dave) in smocks, hair/face covers, and gloves, because they are working within 1 meter of the spacecraft. And in the picture background, do you see Arden standing with his arms crossed and his face mask down? Notice that he is behind the black barrier line, which is 3 meters from the spacecraft. The masks get steamy after a while, and Arden has moved to a safe area and has dropped his mask to give his face a break.

     The clothing requirements are in place to prevent contamination of the flight hardware from foreign object debris (FOD). So the closer you are going to get to the hardware, the more protection you must wear.

      -- Monte Henderson, Deputy Program Manager, Deep Impact

Q: How were the design for the spacecraft and impactor developed?

     Carl Buck, Flight Systems Engineer gives an answer.

     A spacecraft is the culmination of a few very general, and many very detailed requirements. These range from "it has to go to a comet" to "it has to take pictures of the comet nucleus at 3.5 meters resolution." A spacecraft designed to impact and image a comet has different requirements than one that will softly land a rover on the surface of Mars. For example, the requirements for a fast and agile sports car would be much different than those for an SUV for off-road use. These very general requirements outlining use of the vehicle would result in a lot of other requirements dictating everything from its size and style, to the stiffness of the suspension and type of tires to use.

     So, in the case of Deep Impact, we were required to design two spacecrafts with very different requirements into one vehicle. We had to design the Impactor to track the comet nucleus for the last twenty-four hours prior to impact. This resulted in the requirements for a spacecraft with a small camera, a propulsion system, a small battery to last one day, and a computer to control all of it. The Flyby spacecraft, on the other hand, has to operate for the six months it takes to get from the Earth to the comet, and then take pictures from far away of the Impactor hitting the comet. The long trip means the Flyby spacecraft has to have solar arrays to keep charging the battery, and it has to have a really big telescope to see the comet and crater from a distance. The Flyby also has to hold onto the Impactor while they are both launched on a single rocket, and then be able to let go of it when the time comes to let it hit the comet.

     So, we took what started with a few general requirements from the scientists about what they wanted to know about comets, and these turned into thousands of requirements for all the components that make up the two spacecrafts. When we were done designing these vehicles, assembling them, and testing them to prove we meet all these requirements, we had what you now see built today.