Scientists elated with quality of data from Huygens
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: January 15, 2005
A missing computer command - apparently the result of human error - caused the loss of half the pictures taken by Europe's Huygens probe as it descended to the surface of Saturn's moon Titan. But project officials said today the 350 pictures that made it back, along with high-quality data from the spacecraft's other instruments and unexpected measurements by Earth-based radio telescopes, should fulfill all of the mission's primary objectives.
Researchers today unveiled a dramatic 360-degree panorama shot by Huygens descent imager showing the probe's path across the moon as it descended toward touchdown. The mosaic put in context three photos released Friday showing channels, an apparent shoreline, an unusual heart-shaped feature and what appeared to be ice blocks at the landing site itself.
You can see our montage of Huygens pictures showing the landing area here.
The mosaic shows the channels and shoreline behind Cassini with the landing site, near the heart-shaped feature, dead ahead. The channels in light-shaded, elevated terrain flow down toward what look like a shoreline along large expanses of dark, smooth-looking areas. It's not yet known whether ethane, methane or other hydrocarbons exist in liquid form in these areas, but additional data analysis may resolve the matter in the weeks and months ahead.
"It's almost impossible to resist the speculation that this flat, dark material is some kind of drainage channel, that we're seeing some kind of a shoreline," said University of Arizona researcher Martin Tomasko, principal investigator with the descent imager team. "We don't know if this still has liquid in it or whether the liquid has drained away or drained into the surface. You have the feeling maybe this was wet not so long ago."
Tomasko also unveiled a color version of a photo released Friday that showed what appeared to be ice boulders strewn across the surface near the Huygens landing site. The boulders, Tomasko said today, were actually closer in size to pebbles, based on additional data analysis. But he said depressions in the surface material around the small rock-like chunks of ice indicated possible erosion patterns due to the flow of some as-yet-unseen liquid.
Huygens entered Titan's thick nitrogen atmosphere around 5:13 a.m. Friday. John Zarnecki, principal investigator of the surface science package, said it took the spacecraft two hours 27 minutes and 50 seconds to complete its parachute descent to the surface. It hit that surface at a velocity of 10.1 mph and experienced a very brief impact deceleration of 15 Gs. The jolt knocked one sensor off line, but it came back to life on its own a few minutes later.
A "penetrometer" on the bottom of the probe extended six inches into the frigid soil. That data, coupled with the deceleration experienced by Huygens as it hit the ground, provided new insights into the nature of the surface material at the landing site.
"What we're seeing is, we think, a material which might have a thin crust followed by a region of relatively uniform consistency," said Zarnecki. "In terms of this (impact) force, the closest analog that I can give you - and remember, this is not suggesting these are the materials we hit, but that the mechanical consistency is similar - then I would say wet sand or clay are materials which give a similar sort of trace."
Huygens sampled the atmosphere as it floated toward the surface in winds measured at about 16 mph between six and 12 miles altitude. A microphone even recorded the sound of the wind rushing past. On-board instruments detected a thick methane haze, or cloud deck, 11 to 12 miles above the ground where atmospheric pressure measured about 7.3 pounds per square inch.
The outside temperature when the descent began was 70.5 Kelvin, or minus 332.8 degrees Fahrenheit, while the temperature on the surface was slightly warmer: 93.8 Kelvin, or minus 290.8 degrees Fahrenheit.
Huygens was programmed to transmit telemetry and scientific data to NASA's Cassini Saturn orbiter for relay to Earth using two redundant S-band radio systems. Channel A was the sole path for an experiment to measure wind speeds by studying tiny frequency changes caused by Huygens' motion. In one other deliberate departure from full redundancy, pictures from Tomasko's descent imager were split up, with each channel carrying 350 pictures.
As it turned out, Cassini never listened to channel A because of a software commanding error. The receiver on the orbiter was never commanded to turn on, according to officials with the European Space Agency.
"We should remember we're human and we should learn lessons, so I will institute an ESA inquiry on how the command came to be missing," David Southwood, director of science for the European Space Agency, told reporters today. "I'm not going to say any more about that, I'm not going to speculate (about blame)."
In an obvious reference to NASA and earlier news reports, he did say "there have been some erroneous messages implicating one of the other space agencies involved. No. It's an ESA responsibility."
According to published reports, an ESA official said earlier that the missing command was part of a software load developed by ESA for the Huygens mission and that it was executed by Cassini as delivered.
"There isn't any doubt that the command was missing," Southwood said today. "But I'm not going to say any more because the point of an inquiry is to find out. We will certainly have NASA representation on the inquiry, but I don't want to make a big thing about it."
Tomasko said that before the mission began, his team debated whether to send all pictures and spectral data in two independent sets using channels A and B to ensure full redundancy. In the end, they decided to send spectral data through both channels but to double their picture output by sending different photos through each radio. The loss of channel A means the team only gathered 350 pictures instead of the 700 planned.
"So we do have some holes in our panoramic mosaics, but we have a lot of overlap in our coverage and I think we can still do a fine job," he said. "I think the quality of the images will continue to get better ... as we assemble these mosaics in the days and weeks ahead."
Even the lost wind measurement data will be made up, thanks to a remarkable effort on the ground to monitor a faint carrier signal broadcast by Huygens - the equivalent of a cell phone call at a distance of 751 million miles - using a network of 18 radio telescopes around the world. That data, which not as precise as the Doppler information that was lost, should fill in the blanks.
Leonid Gurvits, a researcher with the Netherlands-based Joint Institute for Very Long Baseline Interferometry in Europe, said his team achieved wind speed accuracy levels of one meter per second, or 2.2 mph even though Huygens was three-quarters of a billion miles away.
"Just as there are malign gods, there are benign gods and with an extraordinary effort that i still frankly can't believe, the radio astronomers of the world gathered together to look at the little telephone-level signal coming from the other side of the solar system and we're expecting that will be able, with an enormous amount of work ... we will get back wind profiles as we need to get our full picture of the Titan atmosphere."