is wise the northern hemisphere more densely created than the southern hemisphere. the reason is because of the impact that excavated this material happened to occur deep in the southern hemisphere is helpful. so some of that material was thrown out with so much energy that it left the vicinity and went elsewhere including two here on earth. some of it just like if you had a big impact here on earth, some other stuff would the stuff would fly up and come down some place else. so it landed elsewhere in the southern hemisphere and it raised the crater that are ready form there, it resurfaced it. so you can think of it as the northern hemisphere recur records for billion years of stuff falling on it where is the southern hemisphere had its record wiped clean so it only requires 1,000,000,000 years.

does that make sense? >> yes. thank you. >> you welcome, i apologize for my wordiness. >> hello. i have a good intuition for chemical rocket engine, i watch a lot of videos and heard the audio, but i don't have a good intuition for what the ion engine is like. >> nobody does. >> is like darth vader. >> i'm wondering what it would sound like, what would happen if i put my hand a foot away from it, what if i stood tempe away from it. >> so, for all intensive purposes, the ion engine can only work in a backing change -- chamber. in space nobody can hear you. you cannot hear an ion engine and space. space. it does not make a sound. what would it feel like if you put your hand in front of a? there is a lot of energy in

these very high velocity the on ions. kilowatts. this is a highly efficient system. so, of course it is not perfectly efficient so not all of the electrical energy goes into that being, but that beam, but it is very high-energy. so if putting your hand in a vacuum of space with tolerable you would not appreciate the effect of the high energy xeon ions impinging on your skin. it would be very damaging. it is very energetic. and what if you stood tempe away from it? standing tempe away from it is no problem. we have a vacuum chamber's and as long as the vacuum chamber is less than 20 feet across it is easy to be within 10 feet of it. we are in the safety of the laboratory outside.

so it produces that the glow in that picture is the benefit of effective photography. with your ie you can see it but it is not exactly playing dean. it just is this gentle glow as these ions exit with a velocity that is so high you cannot detect their motion both because of their velocity and tiny size. >> thank thank you so much. >> you're welcome. >> hello. with the ion engine, how long did it take you to make it? i have a second question. >> how long did it take to fabricated, or how long did it take to manufacture it or how long did it take to make it from return destiny? >> fabricate. >> like most of the components on the spacecraft, these things take i don't know exactly how long but around one year or so,

most things take six months or couple of years to make. you have to produce these things to very high, very strict standards. you do not want them, you want them to operate correctly in space. they go through extensive testing. the total time from when he start with the raw material until you have the ion engine in the spacecraft ready to go is years because you test the individual engine and you put it on the spacecraft and tested on the spacecraft. then you test at the whole with the engine on it. so we started building the spacecraft in 2005 and do not launch it until 2007.

does that address your question. >> yes. also, why did you use it xeon as a propellant? >> there is a number of reasons for choosing xeon. i should say that really testify on engines did use other propellants, cesium and mercury were common ones. xenon has a number of advantages. it is one of the so-called noble gases or inert gases. it is not chemically reactive. that means that when technicians are loading it on the spacecraft there is no risk to their health. they do not have to wear special protection in case there is a leak. another benefit of it is that there is a mechanism that we understand well, by which a little xeon xeon leaks out of the engine at low velocity. so the spacecraft ends up almost

having this sort of a cloud of xeon around it. because it xeon is a nerd that does not present a risk to any of the other spacecraft systems, chemically reactive compound that could degrade optical surfaces like damage the camera or the solar rays, or interfere with the electronics, or even affect the temperature of services. everything on the spacecraft is designed with great care. xeon will not interfere with any of that. i can see that you're eager for something else but i've already proven i'm wordy so i need to tell you couple more things. xeon is very easy to store. we launched with 937 pounds contained in just 71 gallons. so a tank is about 1 yard across and about that tall. we need need to be able to store this very effectively because space is at a premium in the spacecraft. >> okay, thank you. >> you're welcome. >> hello.

thank you for the talk. i was curious as the end of the mission approach, where there any particular last things that you wanted to see there? also, about the end of life plan , would you just float off into space? >> i'm not sure if i heard you, were there any particular things we wanted to see near the end of the mission as we were approaching. >> yes, did that he ball. >> so that is actually a fun question. but it turns out there weren't. the reason was that even before the end of the prime mission, i should take it back. don, like all mass emissions has a well-defined set set of objectives. you personally, you and i, and all other taxpayers have made an investment in what nasa does. so so we have to make sure that investment is done responsibly.

you don't just build the spacecraft, launch until it to do good things. we things. we agree that it is worth this investment to accomplish a certain set of objectives. so we had a well defined set of objectives. we surpass, not just met, but surpass those objectives i think by either february or march. i think it was february of this year. the end of the prime mission was not until june. so by the time the mission ended, we were already just a static with this rich trove of data we were returning. sure, the cosmos is endlessly fascinating and there will always be interesting things to look at. we were in the very fortunate position of not needing urgently to jesse one more thing. certainly things we would like to see in that is why it is so wonderful that nasa has chosen to extend the mission and continue its operation.

there was nothing that by the end of the prime mission that we felt we were sort of rushing to see. does that answer the question? >> yes. >> i think the second question you asked is what, what is going to happen to the spacecraft. it is is going to stay in orbit just as surely as the moon stays in orbit around the earth or the earth stays in orbit around the sun. it is the moon of series. we will continue operating it as long as two and porton criteria are met. as long as the spacecraft remains healthy and productive, that in itself is two criteria. and that there, as long as nasa continues to choose to invest its precious and limited funding into dawn. in any case, with a spacecraft can please it's operational

operational lifetime and i could explain that people are interested why that will occur, it will just become well i like to think it will become and a nerd celestial monument to human creativity and ingenuity and orbit around. it is not going to go anywhere else, it will remain in orbit. >> thank you. >> good evening. thank you very much. that you make our dreams about space true. >> if i could just interest interrupt you. it's not just your dreams, it's all of our dreams. it is so cool. >> yes, it is exciting. the question is, what was the biggest problem this spacecraft had happened and how did you solve this problem? >> did someone tell you test

that question? nobody did i know. >> the question is was the biggest problem that happened to the spacecraft are in the mission. the nice thing is if you ask anybody associated with the dawn mission, you'll get the same answer. we. we did have a big problem. i will jump to the end and tell you the mission was successful so it's okay. but the spacecraft have devices called reaction wheels. these are discs about this big that are electrically spun and they are light gyroscopes. there is a phenomenon that maybe you will remember from high school physics where you take a spinning bicycle wheel and hold it on a shaft like this, the wheel spins like this, and it looks like nobody went to the same high school i did. you you sit on a barstool and as you rotate the wheel you spit on the stool. it has to do with what experts call conservation of

angular momentum. the point is, with these discs, as we change the speed of which they spend we can turn, rotate the spacecraft. in a zero gravity frictionless space there is no other way to turn. if you have a wheel like this and you change it speed the spacecraft will turn around it. so that is how we orient the spacecraft. and other spacecraft as well around the world, not around the world, not all of them but many are oriented in the same way. so we need three of those because they are three dimensions, up, down, left, right, or pitch, low. we need three and for redundancy we have for because we do not want the mission to fail because of just some random failure. however, to have failed. we did not build the spacecraft to be able to tolerate two failures. failures like that could be catastrophic for the mission. so 11 failed in june 2010, 1 failed

in august 2012. it was when we were in the process of breaking out of orbit from vesta to begin the journey to siri. there is a really no especially good reason that the mission should have been successful following that second failure. one of the things again that is so cool about these missions is that we found a way. one of our mottos is, if it is not impossible it is not worth doing. [laughter] that is what makes nasa so neat. so we found ways to control the spacecraft to fly the spacecraft in ways that we had truly never thought of. never even considered. when the spacecraft was on earth or even in its vicinity. i will just give you one aspect of that. it is much longer story but i

will mention one of them because it comes back to one of the earlier questions. that is we have a very small supply of conventional rocket propellant called hydrazine. we have about 12 gallons of that on for. it was not not meant for the purpose for which we're using it. but we have these thrusters on the spacecraft, little thruster here and there. if you squirt squirt some out of this thruster that makes the space craft turn like this, if you squirt some out of this thruster it makes it turn like that. we had not intended to fly the spacecraft that way, but that is one of the ways that we are doing it. we did not have enough of this hydrazine, this chemical to fly the mission this way. so we undertook a very, very, very ambitious campaign. this was a huge amount of work by very dedicated the, creative, and capable team of men and women here at jpl and with our partners that orbital a tk, and we came up with ways to use this

hydrazine much more efficiently than we anticipated. so one year ago it would not really occur to us that we'll be in a position right now i been able to undertake an extensive mission because the hydrazine was so tight that we did not think it would last until now. >> it sounds like this problem. [inaudible] it is very, for spacecraft, it happens in an international space station. and a lot of times, why, what is the root cause for this problem? >> so the question is, it sounds like these devices like it gyroscopes have failures of them are common. >> yes. i mean it's a very common problem. in why doesn't happen every

time? >> that's a good question but i think it is somewhat of a misperception. there are myriad satellites and spacecraft with these devices that operate flawlessly, essentially endlessly. for notches years, but decades. i told you that was going to happen. so i will invite the audiovisual people to go to something else, or, thank you. truly a most spacecraft and satellites they operate flawlessly for years, and years. truly, many billions of revolutions. the ones that do not operate are the ones that make the news that you have heard about. really, they generally work very well. there happens to be a number of

satellites and spacecraft like dawn that used one particular design which we all found out too late, it was not reliable for endless years of operation after the rigors of a launch, the temperature changes, the radiation, the for bidding environments of space. so, those have not worked successfully on a number of spacecraft. spacecraft. but on most satellites they do. so it is ben this is batch of one. i want to point out here you have a device that is spinning, it's not it's not like an electrical circuit where there is no mechanical movement. this constant motion, for years, and years without stopping, that is a pretty challenging problem for people to make them so they

work reliably. so the truly overwhelming majority do work beautifully, unfortunately some don't. >> thank you very much. >> you're welcome. >> i would like to make a comment about, before i ask my question. >> okay i will say on behalf of the organizers here -- >> the venus spacecraft and it worked for 40 years successfully. the space station use control. >> which are different from reactors. >> but the same momentum. and now my question is about the electric propulsion. how much did the work on the

development of the ion propulsion been that important as we see it? and how much industry, particularly the aircraft that developed the ion propulsion. in the mid- 1970s for young engineers came to gpl to work on -- at jpl to work on propulsion and then everybody went to look for something else. >> is the question what is the history of the development of ion propulsion? and this electric motion that he is using is one of the other terms for. it it goes well back before that. the first recorded thoughts about ion propulsion was by robert goddard in 1906. the father of american rocketry.

koski, known as the father of cosmonaut x he published in paper on it. and in the 1950s they do very important work on ion propulsion. you are making the entirely accurate point that engineers at hughes aircraft and elsewhere did, it was worked on by people in nassau and private industry from the 50s up until the present. so there is a rich history there of many very creative and talented people contributing to the development of it. if i suggested that jpl was the organization that developed it or thought of it, or was the only one to work on it, i did not mean to suggest that. when i set i raise the question of how could we travel around the solar system more easily and less expensively, it was that we could turn to the ion propulsion

that brilliant people have been working on for nearly one century. >> it is very important for this travel, why did jpl not follow it exactly and it had such importance. >> i'd be happy to talk to about this more, but you may have some perhaps incomplete perceptions of jpl's role in the development. jpl is not solely responsible for the development of ion propulsion. this was a joint nasa industry development. we have taken advantage of the brilliant work that was done by many people to have the success. i think rather would get into the details of the technical history i think there are broader questions that would be of interest. come up

afterwards and we will talk. thank you. >> also propulsion is used for keeping between -- >> most people don't know about station keeping, but you are right. there many spacecrafts and satellites that use ion propulsion. dawn is not the only one. it's not the person or planetary -- i'm glad you're so interested and i'd be glad to talk to about it afterwards. you have have good knowledge of the history. >> jupiter in one of the first photos that you showed of the orbit around the planet the sun, jupiter showed that it goes through these pockets of the asteroid and i wonder why it did not clear that material? >> so are you referring to the slide that showed all the asteroid belt and both leading jupiter and following it?

>> yes. >> that's a good question. so it doesn't go through those because those are going around the sun just as jupiter's going around the sun. but just so everybody else's following, remember that picture of the asteroid belt. he was observed that to see that there's two groups of asteroids ahead of and behind jupiter. those are called trojan asteroids, bodies that, think that orbit the sun or doesn't have to be orbiting the sun, but when one bodies orbiting another they turned out to be places in its orbit that a relatively stable for other bodies to orbit. so both ahead of them be behind jupiter to such places. asteroids that happen to have wandered through this place kind of get trapped there.

for experts there's a little bit of simplification. so it is not that jupiter's plowing through them, but, but they are orbiting the sun with jupiter and they are relatively stable. there are places that are like that for earthen orbit around the sun and even the moon's orbit orbit around the earth. >> and another thing, ecliptic oh -- >> they are close. >> thank you. >> we were going to answer questions that came from the left but there is another one first. >> is the ever going to be a spacecraft that travels into space that can show dawn to refill? >> so that there there ever gonna be another spacecraft to

refuel it. that's a good question. i guess i could answer that question by asking you a question. when you grow up would you be willing to make a spacecraft that will travel into space to refuel dawn? [applause]. and i hope the answer is yes. so nasa does not plan to do that, but we are waiting for smart, creative, energetic, enthusiastic people like you to come up with the great missions like that. but right now dawn, as i said is well over 1,000,000 times well over 1 million times further away than the space station. that's a pretty long way even for robotic that is a spacecraft without astronauts on board to go refuel another one, rather we are going to send a satellite,

spacecraft out there instead of carrying fuel it could carry advanced instruments and sensors in morse sophisticated cameras and things like that that we have not even thought of. nevertheless, it's a great idea. if you do it and i am old and retired, semi-whatever the future communication that there will be and tell me how dawn is doing. does that answer question? thank you. [applause]. so we have a couple of questions from people who were watching right now and the live stream and thank you for that. so i'm not very good at pronouncing some of these things so i will spell it. the ky a and it asks was there any other plan for them to visit a third body such as palace. palace pal las is another body

comparable to vesta and the answer to that is no. there was never such a plan but i presume the background for the question is for those's here that read about these things on the internet, i have been have been reading for years that dawn has actually has done studies of going to this other body. i don't know the origin of that rumor is but i can tell you that we never looked at it. i have read many places, we never looked at it. art targets were vesta and siri from the time we can see them. we never had any intent to go anywhere else. however as you may know from reading news recently, just in the last few months we gave nasa

the option of sending the spacecraft to another body for an extended mission possibility. so the extended mission after the completion of the primary mission. so we told nasa that if they were consider extending the mission we could continue to stay in orbit around siri or we could go to a different body. nasa considered the scientific merits of remaining in orbit, lingering, lingering and continuing to make the detailed observation or flying to a different body. they considered scientific merit and concluded that the best use of this precious resource that you anti- s taxpayers have funded is to continue at siri to make more measurements of, the only door planet in the solar system. i hope that answers your question. the other question i will tell you this is going to be the last one. it is from our -- it asked are we able to determine the mass or other properties of the jacked

which collided with vesta. i apologize for stumbling over that. remember that big crater, the 300-mile crater at the são paulo vesta which is the source of the media right, crashed into it about 1,000,000,000 years ago. it is estimated it is estimated that that body that crashed into it is around 30 or so miles in diameter. that's pretty big. if you imagine that question at your backyard, that probably would not be appreciated by you or your neighbors. that's a big thing. it's large compared to the object which crashed into earth and is primarily responsible for the extinction of the dinosaurs. another species 66 million years ago. that was a big job object and a big impact. that's the estimated

size of it. >> so once again, thank you thank you very much for coming to jpl. after hearing about the mission. [applause]. >> the c-span radio act makes it easy to follow the 2016 election election wherever you are. free to download from the apple i app store get up-to-the-minute schedule information for c-span radio and television. plus podcast times for public affairs, books and history programs. stay up-to-date on election coverage. c-span's radio that means you have c-span on the go.