An Interview with Sami Asmar
Meet Dawn Team Member, Sami AsmarThe following interview took place in Pasadena, California on March 26, 2010, between Sami Asmar, member of the Dawn team at JPL, and Education and Public Outreach team members Whitney Cobb (Mid-continent Research for Education and Learning—McREL) and Education and Public Outreach Manger, Joe Wise.
WC: How does the period of time Dawn will be at Vesta, six to twelve months, factor into your work? I would think that you're spending 6 to 9 months orbiting it helps hugely.
SA: In fact, the quality of our gravity data depends on—in addition to the instrument precision that I talked about—two geometrical aspects: how close we are to the body, because gravity's sensitive as 1/r2; and how much time we spend orbiting. The longer we are there, the more global the coverage. We fill in the gaps and we remove errors and systematic effects—things of that nature. So, the gravity science team would like to be as close to and spend as much time as possible at Vesta.
WC: Tell us about different altitudes from which you will access the data.
SA: We'll be taking it at all altitudes, but the mission has two primary orbits: 1) HAMO (High Altitude Mapping Orbit) and 2) LAMO (Low Altitude Mapping Orbit). Our team benefits more at the low orbit, but we always acquire data whenever possible. You never know when something may go wrong and this might be the last bit of data that you'll have. So you can't take it for granted, saying, "I'll wait until we get closer."
So basically—and a lot of missions do this—they start with a higher orbit configuration, mostly in terms of general observations and the exploratory phase. A lot of instruments get most of what they need that way. In fact, some imagers and spectrometers might be more optimized for a certain distance. Then, they try to circle closer to the asteroid, lowering the altitude to increase the sensitivity for some investigations such as gravity. When you're closer you can also image specific surface features at the high resolution.
WC: Has the Gravity Science method been proven or tested?
SA: Oh, has it ever, especially in the beginning! At the beginning, we had to convince people that this is a legitimate method.
WC: Because you're measuring the differences in speed or position of spacecraft in millimeters?
SA: Less than millimeters—sometimes a few micrometers per second in terms of velocity, and this is over distances spanning the solar system. It is amazing.
WC: At a distance of …?
SA: This is a great point. We did this as far as Neptune, with the Voyager spacecraft, and it will be done eventually when New Horizons arrives at Pluto. At Neptune distances—we're talking tens of astronomical units—you can measure a change in the velocity of a spacecraft of a few micrometers per second.
WC: And extract data from it?
SA: And do proper interpretation of the gravity. It's incredible. We have this international infrastructure, which no individual team should replicate because we're using very precise atomic clocks to accomplish our objectives, and the clocks, called hydrogen masers, have a stability of, say, one part ten to the minus fifteen over a few minutes. You can't reinvent this for every mission.
I really should give credit to the largest infrastructure that's key to this, the Deep Space Network (DSN). The DSN is the set of these large antennas that receive the radio signals from space. They're as large as a football field, and they still track and move, an incredible engineering feat! Their sensitivity is amazing. If it wasn't for that, we would not be measuring these very small numbers. In fact, these numbers are so small we're running into relativistic effects, and that opened a window to a whole new science. So, now we're verifying Einstein's theory of general relativity by the same technique that was really built to do something else, because we have the precision to do that. I can go on and on about that—don't get me started!
GRAVITY SCIENCE BEYOND DAWN
What are your hopes for future studies and advancements in asteroid research?
SA: My interest extends beyond asteroids to using the gravity techniques applied specifically to the Moon to determine its interior structure. I'm also working on a project called GRAIL, Gravity Recovery and Interior Laboratory. The idea is to not only study the gravity of our moon to unprecedented accuracy, but to try to determine if it has a liquid core, because to this day we actually haven't answered that question conclusively. This additional data could help determine which of the competing theories of the Moon formation should really rise to the top. The core detection turns out to be very challenging and achieving that by the end of the mission—and it is a short one—would be a very significant accomplishment for our team.
WC: You've written a book about gravity science?
SA: That's right—it's a book about radio science. The biggest chapter in the book is about gravity science, but it also covers other aspects of radio science which we haven't really talked about. One example, as I mentioned earlier, is the testing of aspects of theory of general relativity. Other topics covered are atmospheric studies using radio science, radio propagation techniques, and studying the sun's solar corona. The book includes all these because they all have the same fundamental physics common to them.