Does everyone remember the Huygens space probe which successfully landed on Saturn's moon Titan on January 14, 2005?(If not, read this: http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=530)
The Huygens probe successfully returned multiple images of the surface of this mysterious methane-covered moon which has been compared to a primordial earth.What is not commonly known is how the Huygens probe came dangerously close to being a complete scientific failure!
Two critical mission errors related to Huygens almost prevented any data being recovered from the landing mission. It was only due to some engineering ingenuity, perseverance, and some luck that we have any data at all from the atmosphere and surface of Titan!1. The first problem involved the radio communications link between the Huygens spaceprobe and the Cassini orbiter.
When the orbiter-lander mission was launched in October, 1997, extensive electrical and radio tests had been performed to ensure communication integrity between the spacecraft, both connected and separated.
However, this was a very basic test. A more complete full-scale communications test that simulated the actual flight conditions and telemetric data to be handled by the spacecraft had NOT been performed. This more elaborate kind of test was rejected for budgetary reasons, since it would have required disassembly and recertification of the components for spaceflight.
The lack of this comprehensive test bothered one of ESA's ground operations managers to the point that he asked one of ESA's global ground antenna engineers to send a special signal from earth to Cassini, one that simulating the type of signal Huygens would send if it were landing on Titan. What they found was astounding: The data was corrupted depending on the amount of Doppler shift (an effect where the frequency of a wave is changed due to the relative velocities of the wave's source to its recipient)
The effect of Doppler shift was critical: the planned flight plan meant that the Cassini orbiter would be traveling at 5.5 km/s relative to the probe at the time when data would be transmitted. The radio hardware used by the spacecraft had been designed to compensate for the Doppler effect, BUT ONLY for the carrier signal and not for the digital to analog encoding scheme used for the data carried on this signal.
Thanks to this finding BEFORE landing on Titan, the flight plan was changed so that the Doppler effect would be minimized. This changed the Titan landing time frame from November, 2004 to January, 2005.
Read a more comprehensive article related to this discovery that appeared in IEEE Spectrum: http://www.spectrum.ieee.org/WEBONLY/publicfeature/oct04/1004titan.html
See the NASA/ESA press release after discovering the problem in 2001: http://www.jpl.nasa.gov/releases/2001/cassini_010629.html
On to the second major mission error...2. Half of the data sent from Huygens to Cassini during the landing never made it.
Cassini has two data channels meant to receive data from Huygens, but before the critical moment of the landing transmission one of them was simply not turned on, the command not being sent from earth!
Fortunately, several of the largest radio telescopes on earth were pointed at Huygens at the moment of the transmission as part of a wind speed experiment. Cassini, combined with these earth-bound radio telescopes, was meant to measure different wind velocities of Titan's atmosphere by measuring the Doppler shift of the carrier signal (Cassini of course would have had to account also for its own high velocity)
After discovering that half of the Cassini data was missing, engineers decided it might be possible to reconstruct at least part of the missing Cassini data using the radio signals gathered directly from earth. The radio telescopes' data was not only able to provide the needed information about Titan's atmospheric wind velocity and direction, but also gave the first indication of the health of the Huygens spacecraft during landing. In addition, scientists were able to gather information as to the precise location of the lander on Titan using the earth data.http://www.nrao.edu/pr/2005/titanwinds/
So what do we learn from these flaws in the Cassini-Huygens mission?
That to err is human, but with some ingenuity we can persevere.
... (read rest of article).