453 lines
24 KiB
Markdown
453 lines
24 KiB
Markdown
---
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created_at: '2014-03-26T10:18:02.000Z'
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title: How a Swedish engineer saved a once-in-a-lifetime mission to Titan (2004)
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url: http://spectrum.ieee.org/aerospace/space-flight/titan-calling
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author: ablutop
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points: 165
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story_text: ''
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comment_text:
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num_comments: 51
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story_id:
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story_title:
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story_url:
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parent_id:
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created_at_i: 1395829082
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_tags:
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- story
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- author_ablutop
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- story_7472495
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objectID: '7472495'
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year: 2004
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---
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![1004titanf1](/img/10Cassinif1-1395852608172.jpg) Photo: Bert
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Bostelmann **Unsung Hero:** With the help of the engineering model of
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Huygens \[background\], Boris Smeds discovered a crippling
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communications problem.
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**Last June, scientists were thrilled** when NASA’s Cassini probe
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successfully began orbiting Saturn after a 3.5-billion-kilometer,
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seven-year journey across the solar system. The 6-ton spacecraft
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immediately started returning spectacular pictures of the planet, its
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rings, and its 30-plus moons. It was just the beginning of Cassini’s
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four-year tour of Saturn’s neighborhood, and while scientists expect
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amazing discoveries in the years to come, the most dramatic chapter in
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the mission’s history will happen this January, when scientists attempt
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to peek beneath the atmospheric veil that surrounds Saturn’s largest
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moon, Titan—a chapter that might have ended in disaster, save for one
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persistent engineer.
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In a collaboration with the European Space Agency, Cassini, in addition
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to its own suite of scientific instruments designed to scan Saturn and
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its moons, carries a hitchhiker—a lander probe called Huygens. A stubby
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cone 3 meters across, Huygens was built for a single purpose: to pierce
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the cloaking methane atmosphere of Titan and report its findings back to
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Cassini for relay to Earth.
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So it was quite a shock when Boris Smeds, a graying, Swedish, 26-year
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ESA veteran, who normally specializes in solving problems related to the
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agency’s network of ground stations, discovered in early 2000 that
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Cassini’s receiver was in danger of scrambling Huygens’s data beyond
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recognition.
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Making that discovery would lead Smeds from his desk in Darmstadt,
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Germany, to an antenna farm deep in California’s Mojave Desert, after he
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and his allies battled bureaucracy and disbelief to push through a test
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program tough enough to reveal the existence of Cassini-Huygens’s
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communications problem. In doing so, Smeds continued a glorious
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engineering tradition of rescuing deep-space missions from doom with
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sheer persistence, insight, and lots of improvisation.
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![1004titanf2](/img/1004titanf2-1395849582443.jpg) Photo: NASA-HQ-GRIN
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**Smoggy Sphere:** This image, taken by Cassini, of Saturn’s largest
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moon, Titan, shows the dense atmospherichaze of hydrocarbons that hides
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the surface.
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**Larger than the planet Mercury,** Titan appeared to the Voyager probes
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in the 1980s as a mysterious yellow-orange globe, its surface hidden by
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its soupy methane atmosphere. Cassini is equipped to peer through those
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clouds with special camera filters and radar, but really getting up
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close and personal with this enigmatic world is the job of ESA’s
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Huygens.
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Launched from Cassini, Huygens will soon slam into Titan’s atmosphere at
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21 000 kilometers per hour and begin a one-way, two-and-a-half hour
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descent to the surface, slowed by parachutes. The lander is fitted with
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cameras pointing down and sideways, instruments designed to unlock the
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atmosphere’s chemical secrets, and a microphone to pick up wind sounds.
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Investigators have speculated there might be seas of liquid methane and
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ethane on Titan, so Huygens has been designed to float. Although its
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batteries will be nearly exhausted by the time it finally reaches the
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surface, researchers hope it will be able to make a few measurements of
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the physical composition of the landing site \[see illustration,
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below\].
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[![1004titanf5](/img/1004titanf5-1395850143021.jpg)](/img/1004titanf5-1395850143021.jpg)
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Illustration: John MacNeill Huygens will slam into Titan’s thick
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atmosphere at 21 000 kilometers per hour. After drag has slowed the
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probe down to about 1440 km/h at a 180-km altitude, a pilot parachute
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will be deployed to pull out the 8.3-meter-diameter main parachute. The
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front shield will then eject, and the probe will begin to take readings.
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So that the descent doesn’t last more than two and a half hours and
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exhaust the probe’s batteries before it reaches the surface, the main
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parachute will be released after 15 minutes and a 3-meter parachute will
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be deployed. Huygens will hit the surface at around 20 km/h.Nestled
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behind its 3-meter-diameter front shield that protects the
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battery-powered probe from heat during the initial entry phase, Huygens
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packs a suite of instruments designed to unlock the secrets of Titan’s
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atmosphere and surface. Cameras will take pictures of Titan’s cloud
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cover and surface, and a microphone will listen to the moon’s wind,
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while other instruments will report back on the chemical, thermal, and
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electrical properties of the atmosphere. After touchdown, a small bundle
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of sensors will examine the surface.
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Scientists believe the information gathered during the descent will open
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not only a window onto a mysterious world at the far end of the solar
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system but one onto the past as well, since Titan’s atmosphere is
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believed to be similar to that of the primordial Earth.
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Getting Huygens’s once-in-a-lifetime readings and observations back to
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Earth is a two-stage process. Huygens is too small to be equipped with a
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radio transmitter powerful enough to reach Earth, so instead a receiver
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onboard Cassini will pick up Huygens’s transmissions. With its powerful
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4-meter main antenna, Cassini will then relay the data back to a small
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army of researchers, some of whom have been waiting decades for the
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insights they hope Huygens will provide.
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**When the Cassini-Huygens mission blasted** off from Cape Canaveral in
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October 1997, no one suspected that a critical design flaw was lurking
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deep within the telemetry system onboard Cassini that was dedicated to
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harvesting Huygens’s broadcast. Uncorrected, the flaw meant the data
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flowing from the hardy lander was in danger of being hopelessly
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scrambled, its seven-year odyssey across the solar system in vain.
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“We have a technical term for what went wrong here,” one of Huygens’s
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principal investigators, John Zarnecki of Britain’s Open University,
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would later explain to reporters: “It’s called a cock-up.”
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But back in 1998, as Cassini was swinging past Venus and the Earth to
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build up speed for its run out to Saturn, Zarnecki and the other
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scientists and engineers at ESA and NASA were still blissfully unaware
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of any problem.
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In fact, everything was working fine. The mission builders felt
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confident in their work: both the Cassini orbiter and the Huygens lander
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had been extensively tested on the ground, both separately and together.
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However, a proposal for a so-called full-up high-fidelity test of the
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radio link between the probes (where every system is subjected to a
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simulation of the exact signals and conditions it will experience during
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flight) had been rejected because it would have required disassembly of
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some of the communications components.
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“Budget was a key part” of this decision, explained Robert Mitchell,
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program manager for the Cassini-Huygens Mission at NASA’s Jet Propulsion
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Laboratory (JPL) in Pasadena, Calif. The reassembled spacecraft would
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then have had to undergo exhaustive and expensive recertification. In
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hindsight, these testing failures were embarrassing. "We had three
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safety nets set up to catch things like Cassini-Huygens’s communications
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problem," said John Credland, head of ESA’s scientific projects,“and it
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now appears that we fell through all three.”
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Fortunately, Claudio Sollazzo, Huygens’s ground operations manager at
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ESA’s European Space Operation Centre (ESOC) in Darmstadt, Germany, had
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a nagging worry about the lack of a full-up communications systems test.
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Sollazzo knew there was time to run some tests during Cassini’s long,
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uneventful stretches between the planets. So he approached Smeds in
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January 1998 with an unusual request: design a test to send a signal
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from Earth toward Cassini that would mimic a radio transmission from
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Huygens during its landing.
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Smeds normally works on the communications links between ESA’s global
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ground antenna network and its 11 active science spacecraft. Most of
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these are satellites that never stray more than tens of thousands of
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kilometers from Earth, a far cry from the Huygens probe, which was
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designed to plunge into an atmosphere 1.2 billion km away. But Smeds’s
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experience with ground antennas was just what Sollazzo needed.
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It was impossible to test the Huygens-to-Cassini radio link during the
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cruise using the spacecraft themselves: they were firmly mated together,
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communicating not by radio but via a cable. And even if Huygens could be
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made to transmit to Cassini, successfully sending a radio signal a few
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centimeters would hardly inspire confidence for the difficult Titan
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descent.
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The Cassini-Huygens mission plan had Cassini jettisoning Huygens toward
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Titan before Cassini began a low-altitude, high-velocity fly-by of the
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mysterious moon. Huygens would reach Titan well in advance of Cassini,
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and as Cassini streaked along at some 21 000 km/h relative to Titan,
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Huygens would be descending on parachutes through the moon’s soupy
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atmosphere at a comparatively leisurely 18 to 22 km/h. The relative
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velocity of Huygens to Cassini was expected to be about 5.5 kilometers
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per second, increasing the frequency of Huygens’s transmitter by about
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38 kilohertz as seen by Cassini because of Doppler shift.
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If you’ve ever heard a screaming ambulance or whistling train pass by,
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you’re familiar with Doppler shift. When an acoustic or radio wave is
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emitted by a moving object, an observer in front of the object will
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notice an increase in the wave’s frequency as the wave’s peaks and
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troughs are compressed by the object’s motion, and an observer behind it
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will notice a decrease in the wave’s frequency as the wave is
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stretched—hence the familiar rise and fall in the pitch of an
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ambulance’s siren as the vehicle speeds by.
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In the case of Huygens, its signal will vary not only in frequency but
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also in strength as the probe is buffeted by the atmosphere, changing
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the orientation of its transmitter. When Smeds was brought into the
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picture, the plan to test Cassini’s receivers was to transmit a signal
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from Earth that would duplicate Huygens’s carrier signal without
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modulating it with any simulated telemetry from the lander’s
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instruments. If the Cassini receiver could pick up a fluctuating,
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Doppler-shifted carrier wave, all should be well. But Smeds wanted to do
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better. “If I do a test like this, I want to do it properly and simulate
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everything, not just a part of it,” he told IEEE Spectrum.
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Smeds used ESOC’s engineering model of Huygens—an exact duplicate of the
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lander down to the last bolt and transistor—to generate a stream of
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typical telemetry. Then he developed a test signal pattern on his office
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computer that could modulate a carrier wave with telemetry as Huygens
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would. His plan was to broadcast the simulated Huygens telemetry from
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Earth to Cassini and have Cassini echo what it received back to Earth.
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![1004titanf3](/img/1004titanf3-1395852701517.jpg) Illustration: Armand
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Veneziano **Going Through a Phase:** Huygens’s telemetry is sent to
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Cassini using a technique known as binary phase-shift keying. In the
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simple two-phase example above, a stream of bits \[top\] is encoded onto
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a carrier wave \[middle\] by modulating the phase of the wave
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\[bottom\]. To represent a 1, the modulated signal is in phase with the
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unmodulated carrier wave, and to represent a 0, the modulated wave is
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180 degrees out of phase with the unmodulated wave. Decoding the
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modulated signal requires precise timing, as the incoming wave is
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compared with an unmodulated wave at precise intervals to determine each
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bit’s phase and whether the bit is a 1 or a 0.
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Huygens is designed to generate telemetry at a rate of 8192 bits per
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second. Using a common modulation technique known as binary phase-shift
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keying, Huygens’s transmission system represents 1s and 0s by varying
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the phase of the outgoing carrier wave. Recovering these bits requires
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precise timing: in simple terms, Cassini’s receiver is designed to break
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the incoming signal into 8192 chunks every second. It determines the
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phase of each chunk compared with an unmodulated wave and outputs a 0 or
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a 1 accordingly \[see chart, above\].
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Smeds’s scheme required that his test signal pattern be broadcast from
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Earth in a sequence of varying power levels to simulate the effect of
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Huygens and its transmitter’s being swung around in Titan’s atmosphere.
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The test signal’s frequency would also be adjusted at broadcast so that
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when it arrived at Cassini, it would match the Doppler-shifted signal
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expected from Huygens. The echoed signal could then be decoded and
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verified by matching it against the original telemetry used to create
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the test signal.
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In proposing this more complex test with simulated telemetry, Smeds “had
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to argue with those who didn’t think it was necessary,” recalled JPL’s
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Mitchell. Smeds was persistent and continued championing the test even
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after it was initially rejected. In the end, with the backing of
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Sollazzo and Huygens’s project scientist, Jean-Pierre Lebreton, Smeds’s
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plan was accepted because it was easy to do, even though hardly anybody
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seemed to think it was worth doing. On such seeming trivia US $300
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million missions can turn: the simpler carrier-signal-only test,
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Mitchell noted, would never have uncovered any problems.
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![1004titanf4](/img/10CassiniAntenna-1395852747785.jpg) Photo: Richard
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Ross/Corbis **Desert Dish:** A 34-meter antenna, like this one at NASA’s
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Deep Space Network facility at Goldstone in California’s Mojave Desert,
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was used to transmit the test signal that revealed Cassini-Huygens’s
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communications problem in early 2000.
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So it was that in early February 2000, a jet-lagged Smeds found himself
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sitting in a windowless, fluorescent-lit, concrete basement below one of
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NASA’s Deep Space Network (DSN) 34-meter dish antennas in Goldstone,
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Calif. He had been scheduled for two test sequences during consecutive
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days, when Cassini would be above the horizon and in view of the dish.
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The test signal Smeds had devised on his office computer was loaded into
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Goldstone’s signal-processing center, located at the far end of the
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sprawling Mojave Desert complex, which would adjust the frequency to
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simulate Huygens’s Doppler shift.
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Smeds and a DSN technician couldn’t stay in the relative comfort of the
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processing center. They had to be present there in the bowels with the
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noisy signal generators to adjust the power of the outgoing transmission
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during the test. Smeds and the technician set up shop, ready to swap in
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and out a series of laptop-controlled attenuators to simulate the signal
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strength fluctuations that were expected from Huygens.
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When Cassini appeared over the horizon, the test sequence began. Smeds’s
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test signal was transmitted to Cassini at a given power level for 5
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minutes at a time before moving on to another power level. Cassini was
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now so far from Earth—430 million km away, somewhere in the asteroid
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belt—that it took 48 minutes for the signal to reach the probe and be
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relayed back to Goldstone. The signal from Cassini was then sent to ESOC
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in Darmstadt for decoding and verification; the center kept in touch
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with Smeds during the test by fax and phone.
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Soon, it became obvious that something was very wrong. Darmstadt
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reported that it was picking up the carrier signal, but none of the
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simulated telemetry was coming through. The data in the decoded signal
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was a mess. As Smeds worked through his test sequence, the situation
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grew even more puzzling, as Darmstadt would occasionally get short
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bursts of good data. “Specific things were very confusing. When you
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increase the power, you expect the signal to get better. Initially it
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did, but then when I increased the power even more, the data was
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corrupted again,” Smeds told Spectrum.
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After the day’s test sequence, Smeds kept thinking about the scrambled
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data during the hour-long drive back to Barstow, Calif., the nearest
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town to Goldstone with a motel. He started to get a hunch that the
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problem didn’t have anything to do with signal strength but with Doppler
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shift. He was running out of time, however, to test theories—he had only
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a few hours the next day at Goldstone before the communications pass
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would be over. It would be months before another test could be arranged,
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because other investigators were in line to communicate with their
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equipment onboard Cassini.
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Smeds decided to carve out some more time for himself. The next day he
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cut each step in the official test sequence from 5 minutes to 2,
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allowing him to finish early.
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Now he could act on his intuition. He called up Goldstone’s
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signal-processing center and had it reduce the simulated Doppler shift
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of the signal reaching Cassini to zero. Forty-eight minutes later—light
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speed to the asteroid belt and back—Smeds’s hunch paid off. “Suddenly I
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got better results. I knew then that there was something wrong in the
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data-detection system and that it was sensitive to Doppler shift,” said
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Smeds.
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Even with the test results in hand, Smeds was greeted with some
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skepticism on his return to Darmstadt. “Some people didn’t believe me,”
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he chuckles. They thought that “something was wrong with the test setup.
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But I had the engineering model, and I continued doing tests on the
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ground and doing more investigations. I could demonstrate the effect of
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the Doppler shift and the effect it had on the data reception.”
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By September 2000, Smeds and his allies had managed to convince ESA that
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the problem was real and that it was time to tell NASA. “Without Smeds,
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we wouldn’t have known we had a problem,” says JPL’s Mitchell. Adds
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Zarnecki, “The guys who pushed the original test through are heroes.”
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But what had gone wrong?
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**ESA immediately convened an inquiry board,** with two NASA observers.
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One of them was Richard Horttor, who was then JPL’s telecommunications
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system engineer for the Cassini project. He recalls, “We worked our way
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out by being totally candid from top to bottom once we detected the
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problem. There was no hesitancy or lack of resources. Nor was there any
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‘nation-to-nation finger-pointing.’ ”
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The board discovered that Alenia Spazio SpA, the Rome-based company that
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built the radio link, had properly anticipated the need to make the
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receiver sensitive over a wide enough range of frequencies to detect
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Huygens’s carrier signal even when Doppler shifted. But it had
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overlooked another subtle consequence: Doppler shift would affect not
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just the frequency of the carrier wave that the probe’s vital
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observations would be transmitted on but also the digitally encoded
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signal itself. In effect, the shift would push the signal out of synch
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with the timing scheme used to recover data from the phase-modulated
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carrier.
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Because of Doppler shift, the frequency at which bits would be arriving
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from Huygens would be significantly different from the nominal data rate
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of 8192 bits per second. As the radio wave from the lander was
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compressed by Doppler shift, the data rate would increase as the length
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of each bit was reduced.
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Although the receiver’s decoder could accommodate small shifts in the
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received data rate, it was completely out of its league here. The
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incoming signal was doomed to be chopped up into chunks that didn’t
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correspond to the actual data being sent, and as a result the signal
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decoder would produce a stream of binary junk. The situation would be
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like trying to watch a scrambled TV channel—the TV’s tuned in fine, but
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you still can’t make out the picture.
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Alenia Spazio wasn’t alone in missing the impact Doppler shift would
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have on the decoder. All the design reviews of the communications link,
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including those conducted with NASA participation, also failed to notice
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the error that would threaten to turn Huygens’s moment of glory into an
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embarrassing failure.
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Alenia Spazio’s insistence on confidentiality may have played a role in
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this oversight. NASA reviewers were never given the specs of the
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receiver. As JPL’s Mitchell explained to Spectrum, “Alenia Spazio
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considered JPL to be a competitor and treated the radio design as
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proprietary data.” JPL’s Horttor admitted that NASA probably could have
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insisted on seeing the design if it had agreed to sign standard
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nondisclosure agreements, but NASA didn’t consider the effort
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worthwhile, automatically assuming Alenia Spazio would compensate for
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the changing data rate.
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Horttor never got an explanation of why Alenia Spazio’s telemetry system
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was built with a timing system that couldn’t accommodate the Doppler
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shift in Huygens’s telemetry. “It is a design feature of another
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application in Earth orbit, and they just reused it,” he told Spectrum,
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adding, “I don’t know why anyone would ever want to build it that way.”
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(An Alenia Spazio spokeswoman said that none of the company’s officials
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were available to comment because of a company-wide summer vacation
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period.)
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Frustratingly, engineers discovered that the timing scheme was
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implemented by firmware loaded in Cassini’s receiver; a trivial change
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to some operating parameters would have fixed Cassini’s comprehension
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problem. But the firmware could not be altered after launch.
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Now, the question remained: how to save Huygens’s mission?
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**From a variety of proposed fixes,** the Cassini team crafted a
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response plan that centered on reducing the Doppler shift sufficiently
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to keep the data signal within the recognition range of the receiver.
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They accomplished this trick by altering the planned trajectory of
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Cassini. Now, Cassini will be much farther from Titan when Huygens
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enters its atmosphere. As a result of this geometrical rearrangement,
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the probe’s major deceleration component will be perpendicular to the
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Huygens-Cassini line of sight rather than mostly along it. This simple
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change literally sidesteps the Doppler shift problem, as the radio waves
|
||
coming out perpendicular to Huygens’s direction of motion will be
|
||
neither stretched nor compressed.
|
||
|
||
By the time NASA and ESA realized a rearrangement was needed,
|
||
interplanetary navigation experts had already laboriously developed
|
||
Cassini’s multiyear flight plan to maximize the number of visits to
|
||
Saturn’s moons. There were to be 44 close fly-by passes of Titan, 8
|
||
close passes of smaller moons, and between 50 and 100 more distant
|
||
passes of these other moons. Reconstructing this celestial ballet from
|
||
scratch would have been prohibitively expensive.
|
||
|
||
So the navigators designed a trajectory in which Cassini initially
|
||
enters a lower and faster orbit around Saturn, drops off Huygens, and
|
||
then hits a specific point in space that coincides with a point on the
|
||
previously planned path. There Cassini fires its rocket engine again to
|
||
get back on the original course. During this altered period, it will
|
||
make three orbits of Saturn instead of the original two, but the extra
|
||
rocket fuel needed to make the changes is available because Cassini’s
|
||
navigation has been so precise that a lot of fuel allocated to course
|
||
corrections has not been used.
|
||
|
||
The upshot of this maneuvering is that instead of landing on Titan in
|
||
November 2004, Huygens will now be deployed on 24 December 2004 for a 14
|
||
January 2005 landing. The lander still faces enormous engineering
|
||
challenges as it ventures into the unknown conditions of Titan’s
|
||
atmosphere and surface. But at least now it has a fighting chance to
|
||
transmit its findings back to Earth.
|
||
|
||
As for Smeds, ESA’s staff association awarded him and some of his
|
||
colleagues a plaque and a small cash prize for their role in saving the
|
||
$300 million mission, though Smeds told Spectrum that he is still
|
||
looking forward to his real reward: “I hope to sit in Darmstadt and see
|
||
the data coming in on the screen in January.”
|
||
|
||
## About the author
|
||
|
||
James Oberg is a 22-year veteran of NASA mission control. He is now a
|
||
writer and consultant in Houston. His last article for IEEE Spectrum was
|
||
in August, about the first private suborbital spacecraft, SpaceshipOne.
|
||
|
||
Stephen Cass contributed additional reporting for this article.
|
||
|
||
## To Probe Further
|
||
|
||
For more information on the Cassini-Huygens Mission, go to
|
||
<http://saturn.jpl.nasa.gov/home/index.cfm>.
|
||
|
||
[Download a PDF version of the article](/ns/pdfs/1004titan.pdf).
|