Probe travels to surface of Saturn's moon Titan Friday
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: January 12, 2005
KENNEDY SPACE CENTER, Fla. (CBS) - In one of the boldest space missions ever attempted, a small European-built probe will slam into the atmosphere of Saturn's mysterious moon Titan Friday for a two-and-a-half hour parachute descent to its smog-shrouded surface.
Researchers are hopeful Huygens will answer their most pressing questions: whether hydrocarbons fall like rain and form pools of liquid ethane and similar compounds on the moon's frigid surface; and what erosional or depositional processes are responsible for covering up impact craters and producing a relatively flat, mountain-free surface.
No matter what Huygens actually sees on the surface, scientists expect to gain insights into the workings of a thick, complex atmosphere that in some respects mirrors Earth's shortly after the planet's birth.
"We ought to be able to see a pretty good panorama of the area that the Huygens probe is going to land in," Jonathan Lunine, a planetary scientist at the University of Arizona, said in a recent interview. "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."
Huygens was released from Cassini on Christmas Eve, placed on a collision course with Titan that was set up to ensure the proper atmospheric entry angle. David Southwood, the European Space Agency's science director, thanked NASA for the lift, saying "now all our hopes and expectations are focused on getting the first in situ data from a new world we’ve been dreaming of exploring for decades."
During flybys of Titan in late October and again in December, Cassini's powerful cameras, an imaging radar system and other instruments mapped the surface in unprecedented detail, revealing a relatively flat terrain and unusual, sharply defined features that defied easy explanation.
Only a handful of large crater-like circular structures were apparent and researchers did not see the sort of specular reflections one might expect from sunlight glinting off a liquid surface.
Whether Huygens will detect standing lakes or pools is a major question mark going into Friday's descent. So far, said Torrence Johnson, a Cassini imaging team member at the Jet Propulsion Laboratory in Pasadena, Calif., "we don't have any evidence (for liquids)."
"Just like we don't have any clear evidence of something we know is a crater, there's nothing there that anybody's willing to hang their hat on yet that yes, we absolutely have a liquid surface," he said in a telephone interview Monday.
"What we saw (in the initial Cassini flybys), of course, was a surface that was much stranger than most of us thought we would see," he said. "The real story so far has been the things we didn't see."
Like large craters, hills or mountain ranges and obvious lakes or river-like structures. But that doesn't mean liquids aren't there. Just that Cassini didn't spot them in its initial looks at the moon.
"It's a distinct possibility that I could be the very first scientist to carry out oceanography on an outer planet of the solar system," said John Zarnecki, principal scientist for the Huygens Science Surface Package. "But equally the probe could land with a thud on hard ground or squelch into a morass of extraterrestrial slime - no one knows for sure.
"In any event, the instruments onboard have been designed to handle a range of possibilities," he said in a statement. "Let's just say that, after a seven-year voyage and twenty years of planning, design and build, I will be extremely pleased to land, whatever the surface."
"I think we're going to have to wait several flybys, maybe even several years, before we get a really good indication of what's going on," said Carolyn Porco, the Cassini imaging team leader. "What Huygens will do, of course, is give us a very exquisitely detailed view of one place. So their information will (provide) the 'ground truth' for helping us interpret what we see."
Spinning at 7 rpm for stability, Huygens will slam into the atmosphere Friday at an altitude of 789 miles, traveling at some 12,400 mph. A thick carbon composite heat shield will protect the craft from the fierce heat of atmospheric friction - nearly 3,500 degrees - which will quickly slow the probe to more benign speeds. Maximum deceleration is expected to be around 16 Gs.
When the velocity has dropped to about 870 mph, Huygens' aft cover will be pulled away by a pilot chute and the spacecraft's 27-foot-wide main parachute will deploy 2.5 seconds later.
The main chute will be jettisoned 15 minutes later and from that point on, Huygens will ride beneath a smaller 9.8-foot-wide parachute. Impact on the surface at some 11 mph is expected about two-and-a-half hours after entry begins. Regardless of how long Huygens might survive on the surface, Cassini will sink below Titan's horizon about 30 minutes after touchdown.
Assuming the 705-pound Huygens doesn't sink in a hydrocarbon lake, "we have good confidence the probe will survive landing," said European Space Agency mission manager Jean-Pierre Lebreton. "The landing speed is very low."
Here is a detailed timeline of major entry events on Jan. 14 (in EST; all times represent when an event occurs relative to signals received on Earth). Explanations for key events from the European Space Agency's Huygens website; Cassini timeline events provided by the Jet Propulsion Laboratory:
02:33 AM (-02h40m) - Cassini solid state recorders prepped for support
05:17 AM (+00h04m) - Pilot chute: 106-118 miles altitude; Mach 1.5; The parachute deploys when Huygens detects that it has slowed to 895 mph, at about 112 miles above Titan's surface. The pilot parachute is the probe's smallest, only 8.5 feet in diameter. Its sole purpose is to pull off the probe's rear cover, which protected Huygens from the frictional heat of entry. 2.5 seconds after the pilot parachute is deployed, the rear cover is released and the pilot parachute is pulled away. The main parachute, which is 27.2 feet in diameter, unfurls.
05:18 AM (+00h05m) - At about 99 miles above the surface, the front shield is released. Forty-two seconds after the pilot parachute is deployed, inlet ports are opened up for the Gas Chromatograph Mass Spectrometer and Aerosol Collector Pyrolyser instruments, and booms are extended to expose the Huygens Atmospheric Structure Instruments. The Descent Imager/Spectral Radiometer will capture its first panorama, and it will continue capturing images and spectral data throughout the descent. The Surface Science Package will also be switched on, measuring atmospheric properties.
05:32 AM (+00h19m) - Main parachute separates and drogue parachute deploys: The drogue parachute is 9.8 feet in diameter. At this level in the atmosphere, about 78 miles in altitude, the large main parachute would slow Huygens down so much that the batteries would not last for the entire descent to the surface. The drogue parachute will allow it to descend at the right pace to gather the maximum amount of data.
05:49 AM (+00h36m) - Surface proximity sensor activated: Until this point, all of Huygens's actions have been based on clock timers. At a height of 37 miles, it will be able to detect its own altitude using a pair of radar altimeters, which will be able to measure the exact distance to the surface. The probe will constantly monitor its spin rate and altitude and feed this information to the science instruments. All times after this are approximate.
05:56 AM (+00h43m) - Possible icing effects to Probe (31 miles)
06:57 AM (+01h41m) - Gas Chromatograph Mass Spectrometer begins sampling atmosphere: This is the last of Huygens's instruments to be activated fully. The descent is expected to take 137 minutes in total, plus or minus 15 minutes. Throughout its descent, the spacecraft will continue to spin at a rate of between 1 and 20 rotations per minute, allowing the camera and other instruments to see the entire panorama around the descending spacecraft.
07:19 AM (+02h06m) - Cassini closest approach: 37,282 miles flyby at 12,080 mph, 93 deg phase
07:30 AM (+02h17m) - Descent Imager/Spectral Radiometer lamp turned on: Close to the surface, Huygens's camera instrument will turn on a light. The light is particularly important for the 'Spectral Radiometer' part of the instrument to determine the composition of Titan's surface accurately.
07:34 AM (+02h21m) - Surface touchdown: This time may vary by plus or minus 15 minutes depending on how Titan's atmosphere and winds affect Huygens's parachuting descent. Huygens will hit the surface at a speed of 11.2-13.4 mph. Huygens could land on a hard surface of rock or ice or possibly land on an ethane sea. In either case, Huygens's Surface Science Package is designed to capture every piece of information about the surface that can be determined in the three remaining minutes that Huygens is designed to survive after landing.
09:44 AM (+04h31m) - Cassini stops collecting data; Huygens's landing site drops below Titan's horizon as seen by Cassini and the orbiter stops collecting data. Cassini will listen for Huygens's signal as long as there is the slightest possibility that it can be detected. Once Huygens's landing site disappears below the horizon, there's no more chance of signal, and Huygens's work is finished.
09:46 AM (+04h33m) - Cassini probe data partitions write protected
10:14 AM (+05h01m) - First data sent to Earth: Getting data from Cassini to Earth is now routine, but for the Huygens mission, additional safeguards are put in place to make sure that none of Huygens's data are lost. Giant radio antennas around the world will listen for Cassini as the orbiter relays repeated copies of Huygens data.
10:17 AM (+05h04m) - Probe data replay begins (Canberra: 66,360 bps)
07:07 AM (+01d02h) - Power on of orbiter instruments
Cassini braked into orbit around Saturn on July 1 after a seven-year voyage from Earth. The original flight plan called for Huygens to enter Titan's atmosphere in late November as Cassini streaked overhead at an altitude of just 746 miles. But engineers were forced to delay Huygens' arrival to this month because of an issue with the radio aboard the Cassini mothership that will be used to relay data from Huygens to Earth.
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)," Cassini program manager Bob 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."
The tests were successful and a potentially crippling design flaw was resolved with no significant loss of science. And so, the stage is set for a dramatic voyage of discovery.
"Whatever is there is going to look pretty good, I think," Johnson said. "The probe is spinning as it comes down, sort of a spin-scan imager looking out and down at an angle. Of course, the haze will get less as you go down.
"We had hoped that once it got down to within a hundred kilometers of the surface or something like that we'd start seeing things that looked like pictures out of an airplane window. Based on our data, I think that maybe they will still see a very hazy surface even at longer wavelengths at that type of altitude. Because one of the things that we found is that some of the scattering that's producing this fuzzy appearance on the surface is happening down under 10 kilometers. ... But at some point, we ought to start seeing the surface more clearly. It may be in the last 10 kilometers of descent."
Johnson said he was especially looking forward to finding out "what sort of topography there is. Is it all flat down there? Or are there hints of underlying topography?"
"I would hope we would be able to tell the difference between mantle material that's been covered up by soft aerosols and areas where there might be really flat places with lakes, all of which could be hidden in the data we're seeing now at the resolutions we have."
Whatever Huygens sees, "it could be pretty damn spectacular," Johnson said.
Huygens, of course, will send back much more than pictures. Here's a summary from NASA's Cassini press kit:
"Throughout the descent, Huygens' atmospheric structure instrument will measure the physical properties of the atmosphere," according to NASA's Cassini press kit. "The gas chromatograph and mass spectrometer will determine the chemical composition of the atmosphere as a function of altitude. The aerosol collector and pyrolyzer will capture aerosol particles - fine liquid or solid particles suspended in the atmosphere - heat them and send the resulting vapor to the chromatograph/spectrometer for analysis.