Cassini and Huygens craft are well equipped for science
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: June 12, 2004
Remote Sensing Instruments
Fields and Particles
It all makes for an enormously complicated ballet.
"On Galileo, you pointed the antenna to Earth and you just left it there," said program manager Bob Mitchell. "You had a scan platform, you had a spinning section for the fields and particles instruments, the radio science guys were always happy because the antenna was pointed to Earth and you just left it in that configuration and you let it fly and you collected data all the time.
"For us, you've got four optical remote sensing instruments that want to point, if you're lucky, in the same direction. You've got six fields and particles instruments that generally want to point someplace different. ... You've got radar that wants to point off perpendicular to where the ORS (remote sensing) guys want to look. You've got an aeronomy instrument that wants to point someplace different yet and so you've got this constant tension among the different investigations about where they're going to point this thing."
"They'll do 15 minutes of imaging of this and 15 minutes of that and maybe two hours for (a movie of Saturn's atmosphere)," Webster said. "Then, we'll go off in different places and maybe they'll do a little mosaic. Of course, the interesting ones where everybody wants to be in the act are right around the icy satellite flybys and the Titans, and they've done a lot of horse trading over the years.
"So they kind of work out times so that radar can take a little bit and maybe radio science, which has to point back to Earth can take a little bit and the ORS instruments can turn and take their pictures. It's a highly choreographed scenario that they work out on a daily basis. George Balanchine doesn't have anything on us!"
Engineering and science data will be stored on two 2.2-gigabyte digital recorders that also hold backup copies of flight software for use as needed. After completing the day's science observations, Cassini will re-orient itself, aim its high-gain antenna toward Earth and spend eight hours or more transmitting stored data to NASA's Deep Space Network antennas in Australia, Spain and California. When the largest dishes are used, data rates of up to 165,900 bits per second are possible, allowing scientists to receive up to four gigabytes of data per day.
"I've been with this thing since it was a hunk of aluminum," Webster said in an interview. "Sometimes I take out pictures of the cabling and just marvel at the way this thing was cabled and put together. It's been a joy to fly and it was a joy to build."
The remote sensing instruments will provide the spectacular pictures that will most appeal to the public. The cameras and spectrometers "will be addressing questions like what does the surface of Titan look like underneath its veil of haze? Are there craters there? Are there lakes?" project scientist Dennis Matson said before launch. "When we come close to the rings of Saturn and the surfaces of icy satellites and look at them at high resolution, what are they really going to look like? What are they composed of? What is the composition of Saturn's atmosphere? How does that composition change from one place to another across Saturn? How does it change with time?"
The magnetosphere of the planet "is a gigantic magnetic bubble that surrounds Saturn," he said. "It's very complicated, it has structure to it, there are many different neighborhoods in the magnetosphere where special processes happen. We will be visiting those places, measuring what's going on there, and for some of them we'll actually be able to take pictures of the processes as they occur.
"Saturn also has a big magnetic puzzle. It's magnetic field is almost exactly aligned with its rotation axis. Our theory of magnetic dynamos says this is something that can't occur. So in the course of Cassini's measurement of the magnetic field around Saturn, we'll be addressing some fundamental questions about the nature of its magnetic field and how it arises and we'll learn some lessons. I think those lessons will turn out to be useful in terms of understanding the magnetic field here at the Earth, a place where we still do not understand why the magnetic field flips from time to time."
Cassini's camera systems, of course, will provide the most spectacular results for the lay person. Several hundred thousand images are expected to be beamed back to Earth with a maximum resolution of two kilometers per pixel when looking at Saturn's rings, twice as good as images from the Voyager spacecraft.
"Dynamically speaking, the ring system of Saturn shares a lot of common traits with systems as large as the spiral galaxies, which are trillions of times bigger," Porco said at a pre-launch news conference. "So in addressing questions about ring systems we are actually asking questions that are truly universal in nature."
"Some of those features we think we do understand. They seem to be dynamically related to the satellites of Saturn, both satellites embedded within the rings and those external to the rings. And if our theories are correct, the dynamical interactions between the satellites and the rings should lead to discernible changes in the orbits of both ring particles and the satellites.
"So a primary objective of the imaging system is going to be to refine the orbits of those satellites and actually search for the changes that have occurred between the Voyager epoch and Cassini's arrival there. And with that, we should have a direct measurement of the rate at which the rings are evolving and extrapolating from that, a much better estimate for how long the rings have been around and even, perhaps, a prediction for how long they will stick around in the future."
Scientists believe the rings formed several hundred million years ago when one or more small moons broke apart in the grip of Saturn's gravity. Another camp believes a stray Kuiper Belt Object wandered too close and was ripped to shreds. Mathematical analyses show the debris from such a wreck would quickly spread out in a vast, thin disk. Subsequent collisions and impacts ground the fragments into smaller and smaller pieces, giving birth to the rings first glimpsed by Galileo in the early 1600s.
The Cassini spacecraft was named after the French astronomer Jean-Dominque Cassini, who discovered several of Saturn's moons and the broad gap in its rings that is a famous target for amateur astronomers. The European Titan probe was named after Christiaan Huygens, a Dutch scientist who discovered the cloud-shrouded moon in 1655 and who developed the first accurate theory explaining the structure of Saturn's rings.
"What makes it most interesting is the presence of methane, which makes up a few percent of the atmospheric composition," said Jean-Pierre Lebreton, European Space Agency project scientist. "Sun rays, cosmic rays and certain energetic particles break the methane and the nitrogen, which leads to a complex photochemistry which produces complex organic molecules. When going to Titan, we are looking for answers to many questions. One is what complex organic molecules nature makes from these two simple gases, nitrogen and methane? And how complex are molecules today on Titan."
The Huygens/Titan probe is equipped with six science instruments:
Assuming the 705-pound Huygens doesn't splash down in a hydrocarbon lake, "we have good confidence the probe will survive landing," said Lebreton. "The landing speed is very low and there is a very good probability the probe will survive landing and we have capability to do measurements for half an hour on the surface. During the three-hour measurement phase, the probe will transmit its data to the overflying orbiter."
During a post-launch test, engineers discovered the radio receiver could not cope with the Doppler shift in the frequency of the signal coming from Huygens due to Cassini's high relative velocity. Much like the pitch of a siren changes as a police car races past a stationary observer, the frequency of radio waves can shift a significant amount if relative velocities are high enough.
"Originally, the closing speed of Cassini coming up on Huygens, which is for all practical purposes sitting still once it's in the atmosphere, the closing speed was about 5.8 kilometers per second (13,000 mph)," Mitchell said in a recent interview. "And because we were coming in almost dead overhead and going off to the right at about 1,200 kilometers (746 miles) altitude."
The solution was to minimize the Doppler shift by reducing the relative velocities of the two spacecraft. That was accomplished by changing Cassini's trajectory slightly and delaying Huygens' release to Christmas Eve. During the Jan. 14 descent, Cassini now will be 37,300 miles from Titan and the difference in velocity between the two spacecraft will never be more than 8,500 mph.
"We have pretty solid evidence that's going to work," Mitchell said. "We did some tests where we used the Deep Space Network stations transmitting an S-band signal with telemetry modulated onto the carrier so that from the receiver's point of view on the Cassini spacecraft, it should have simulated the probe quite accurately. We adjusted the frequency, taking into account the motion of everything, so that the frequency of the received signal at the receiver should very closely if not exactly match the frequency that the receiver will see coming from Huygens."
Lunine can hardly wait.
"We ought to be able to see a pretty good panorama of the area that the Huygens probe is going to land in," he said in an interview, describing the descent. "Those pictures will continue all the way down to the surface, they'll be interrupted right at the end when the camera switches over to take what are called spectra, which will tell us about the composition of the surface. So we ought to be able to get a pretty good panorama to start with.
"We ought to be able to see whether the probe came down in an area that's mostly craters or other kinds of land forms. We ought to be able to get a hint of whether there might be pools or lakes of liquid in that area. It won't be immediately apparent whether dark places are liquid or solid, but depending on where the probe lands, we might get some direct information on that. And we might see clouds in the sky toward the horizon.
"There may be some detection of lightning," he said, "although there probably isn't a lot of lightning in Titan's atmosphere. And then after impact, or touchdown, if the antennas aren't pointed in a strange direction, we should be able to get some information about the surface. If we're lucky enough to land in liquid, then the probe should be bobbing up and down and there's a tilt meter that will tell us that. And we might be able to get samples of surface material because the probe will still be warm and anything like these liquid hydrocarbons will vaporize and go up into the sample inlets."