Scientists await descent
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: January 13, 2005
Editor's Note... Saturn currently is 751 million miles from Earth. One-way light time, i.e., the time needed for a radio signal to travel from the Cassini probe to Earth, is roughly 67 minutes. To avoid confusion, all times in the story below refer to "Earth received time," or ERT. That is when an event, in theory, can be confirmed, not the time the event actually occurred.
The European Space Agency's Huygens probe, closing in on Saturn's mysterious moon Titan, is racing toward a dramatic, high-speed plunge into the moon's thick nitrogen atmosphere early Friday for a long-awaited parachute descent to an enigmatic surface.
Over the next three to four minutes, the 705-pound probe is expected to endure peak heating of nearly 3,500 degrees and a braking force of some 16 Gs, slowing the craft to a more sedate 895 mph. At that point - 5:17 a.m. - the probe should release a small 8.5-foot pilot chute that will pull away a protective rear cover.
Just 2.5 seconds later, the craft's 27.2-foot-wide main parachute will unfurl. For the first time since pre-release tests, an on-board radio will begin transmitting data to Cassini around 5:18 a.m. as the NASA spacecraft streaks past Titan some 37,000 miles away.
Huygens is expected to reach the surface of Titan around 7:31 a.m. But with its main antenna pointed toward the landing zone, Cassini will be out of contact with Earth, leaving anxious engineers and scientists in the dark until the NASA craft turns back toward Earth and begins transmitting recorded data around 10:17 a.m. That's nearly three hours after Huygens touches down.
"The probe is going to transmit data in realtime, Cassini will listen to them, but because Cassini is pointing its high gain antenna towards Titan, we would not be able to receive any data," said Claudio Sollazzo, ESA's Huygens operations manager. "So we will be here waiting for this data, but the mission will be over even before Cassini can turn back toward the Earth."
But if they're lucky, scientists may get a near-realtime confirmation the probe survived the initial phases of its descent from large radio telescopes that will be monitoring the descent from Earth.
No data will be received, but a carrier signal may be barely detectable. If so, project engineers and scientists will at least know Huygens survived entry.
"This will indicate the probe is active, the probe is alive, the probe has entered the atmosphere of Titan, the main parachute has been deployed, the back cover has been removed and the computer has been activated," Jean-Pierre Lebreton, ESA's Huygens project scientist, told reporters Thursday. "It does not mean more than this (but) it tells us the probe is alive and descending under parachute."
Whether a carrier signal is detected or not, it will be a moment of high drama for the hundreds of scientists and engineers from Europe and the United States who have spent two decades planning Huygens' Titan descent.
"I've been involved in this mission almost 21 years," said Lebreton. "I'm very excited, but I must say I'm also quite confident we're going to see something very special."
Shaun Standley, an ESA systems engineer stationed at the Jet Propulsion Laboratory in Pasadena, Calif., echoed Lebreton's confidence, saying on the eve of entry, "at this stage, it's very, very, very exciting for all of us."
"The reason it's exciting and not stressful is we know this is the absolute best that can be done," he said in a telephone interview. "This represents a pinnacle achievement. It's the best we can do and I'm looking forward to everybody sharing in that when these images and the rest of the science comes in."
Titan was discovered by Dutch astronomer Christiaan Huygens in 1655. Larger than both Pluto and Mercury, Titan has a dense nitrogen atmosphere and chemical processes that scientists believe mirror conditions on Earth shortly after the planet's formation. Given the chemical constituents, the low temperatures and high pressures at the surface, theorist predict hydrocarbon rain and, possibly, lakes or streams of ethane and related compounds on the surface. Or, perhaps, some sort of hydrocarbon sludge.
Cassini's instruments failed to detect any obvious signs of liquids during two flybys of Titan in October and December. But Cassini was unable to resolve features smaller than about six tenths of a mile across and small pools or streams remain a possibility.
Swinging beneath its main parachute in the hazy sky 90 to 100 miles above Titan, the half-dozen instruments aboard Huygens will begin sampling the atmosphere within moments of main chute deploy, probing its chemistry, looking for lightning and taking panoramic photographs. Nineteen minutes after entry, the main parachute will be released and a 9.8-foot drogue parachute will deploy to make sure the craft reaches the surface at the right time.
Built at the University of Arizona, the probe's innovative camera is known as the Descent Imager/Spectral Radiometer, or DISR. At an altitude of 90 miles above the surface, DISR will be able to detect surface features 500 feet across. But as it gets closer and closer to the surface, resolution will improve dramatically.
"From 50 or 70 kilometers altitude (31 to 44 miles), we expect to break underneath the haze layer that's made it so difficult to see the surface of Titan from the orbiter," said Martin Tomasko, DISR principal investigator at the University of Arizona's Lunar and Planetary Laboratory. "And from that point on down, we'll have better than 20 times the best resolution that's been obtained so far from the orbiter.
"The views we've seen from the orbiter have been really tantalizing, they've given us the feeling that we're looking at Titan through this veil of haze and it's been extremely difficult to interpret these images to understand what the physical processes are that formed the surface, what's really going on. It's our hope that with the DISR cameras, basically the eyes in Huygens ... for every kilometer that we descend, we'll have that much less haze between us and the ground and we hope to have progressively clearer and clearer views and progressively higher and higher resolution."
Just before Huygens hits the surface, DISR should be able to resolve features smaller than a yard across. And it will continue transmitting from whatever surface it lands on.
"If we land in a liquid, we may see us floating around in the liquid, we may see some features on the horizon," Tomasko said. "If we land on a solid surface we'll only see something from our aspect on the surface. In any case, the main mission will be from 150 kilometers (93 miles) to the ground. That's where we expect to take our basic data and those data should provide a spectacular new view of Titan and hopefully a much clearer understanding of this new, mysterious world."
Here is a list of instruments aboard Huygens from the European Space Agency's mission website:
If Huygens lands in a liquid, ultra-low temperatures will sap the craft's batteries faster than if it lands on a solid surface. On a solid surface, the batteries could, in theory, last two hours beyond the planned landing time, allowing the craft to beam data back to Cassini until the mothership disappears from view over Titan's horizon.
"We expect to get a total of about 750 images that have a lot of overlap in them and we expect to be able to put the closest images into context, in the view, with the images from farther away, from higher altitudes," Tomasko said. "So I think we'll be able to paint a pretty complete picture of the surface that is visible to the DISR cameras.
As for DISR's ability to distinguish between dark liquids and solid surface features, Tomasko said "we carry our own lamp and in the last couple of hundred meters, we will measure the specular reflectivity of the material we're looking at as a function of wavelength. And we have already in our library the reflective spectra expected from liquid methane, liquid ethane, various mixtures of the two."
Also among the expected post-landing data are sounds from a microphone that might capture the rustling of frigid nitrogen winds or lapping waves.
Sollazzo said Cassini will record four redundant sets of probe data that will be transmitted to Earth four times "just to make sure we do not lose any bit of information."
"So the first complete set will be here in the control center at 1900 local time (7 p.m. Central Europe Time, or 1 p.m. EST)," he said. "Within 40 minutes or even less, the control team will be able to deliver the science data from Huygens to our PIs (principal investigators). The team has already set up all their equipment and all their instrumentation to analyze the data realtime."
Here is a detailed timeline of major entry events (in EST/Earth received time). 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