Huygens (spacecraft)
 | This article documents a current event. Information may change rapidly as the event progresses, and initial news reports may be unreliable. The latest updates to this article may not reflect the most current information. |
The Huygens probe, supplied by the European Space Agency (ESA) and named after the Dutch 17th century astronomer Christiaan Huygens, is an atmospheric entry probe carried to Saturn's moon Titan as part of the Cassini-Huygens mission. The combined Cassini-Huygens spacecraft was launched from Earth on October 15, 1997. Huygens separated from the the Cassini orbiter on December 25, 2004, and landed on Titan on January 14, 2005 on the Xanadu Regio site.
Overview
When the mission was planned, it was not yet certain whether the landing site would be a mountain range, a flat plain, an ocean, or something else, and it was hoped that analysis of data from Cassini would help to answer these questions. Huygens was planned to scrutinize the clouds, atmosphere, and surface of Saturn's moon Titan. It was designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to the surface.
Based on pictures taken by Cassini about 745 miles (1200 kilometers) away from Titan, the landing site appeared to be "for lack of a better word, shoreline. Assuming the landing site would be non-solid, the Huygens probe was designed to survive the impact and splash-down with Titan's liquid surface for several minutes and send back data on the conditions there. If that occurred it was expected to be the first time a human probe would land in an extraterrestrial (i.e. non-Earth) ocean. The spacecraft had no more than three hours of battery life, a majority of which was planned to be taken up by the descent. Engineers only expected to get at best 30 minutes of data from the surface.
The Huygens probe system consists of the probe itself, which descended to Titan, and the probe support equipment (PSE), which remained attached to the orbiting spacecraft. The PSE included the electronics necessary to track the probe, to recover the data gathered during its descent, and to process and deliver the data to the orbiter, from which it will be transmitted or "downlinked" to the ground.
The probe remained dormant throughout the 6.7-year interplanetary cruise, except for bi-annual health checks. These checkouts followed preprogrammed descent scenario sequences as closely as possible, and the results were relayed to Earth for examination by system and payload experts.
Prior to the probe's separation from the orbiter on December 25 2004, a final health check was performed. The "coast" timer was loaded with the precise time necessary to turn on the probe systems (15 minutes before its encounter with Titan's atmosphere), then the probe detached from the orbiter and coasted in free space to Titan in 22 days with no systems active except for its wake-up timer.
The main mission phase was a parachute descent through Titan's atmosphere. The batteries and all other resources were sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus at least 3 additional minutes (and possibly a half hour or more) on Titan's surface. The probe's radio link was activated early in the descent phase, and the orbiter "listened" to the probe for the next 3 hours, including the descent phase, and the first thirty minutes after touchdown. Not long after the end of this three-hour communication window, Cassini's high-gain antenna (HGA) was turned away from Titan and toward Earth.
Very large radio telescopes on Earth were also listening to Huygens's 10-watt transmission using the technique of very long baseline interferometry and aperture synthesis mode. At 11:25 CET on January 14, the Robert C. Byrd Green Bank Telescope (GBT) in West Virginia detected the carrier signal from the Huygens probe. The GBT continued to detect the carrier signal well after Cassini stopped listening to the incoming data stream. In addition to the GBT, eight of the ten telescopes of the continent-wide VLBA, located at Pie Town and Los Alamos, NM, Fort Davis, TX, North Liberty, IA, Kitt Peak, AZ, Brewster, WA, Owens Valley, CA, and Mauna Kea, HI, also listened for the Huygens signal.
The signal strength received at Earth from Huygens was comparable to that from the Galileo probe (the atmospheric descent probe) as recieved by the VLA, and was therefore too weak to detect in real time because of the signal modulation by the (then) unknown telemetry. Instead, wide-band recordings of the probe signal were made throughout the three-hour descent. After the Probe telemetry is finished being relayed from Cassini to Earth, the recorded signal is processed against a telemetry template, enabling signal integration over several seconds for determining the probe frequency. It is expected that through analysis of the Doppler shifting of Huygens's signal as it descends through the atmosphere of Titan, wind speed and direction will be able to be determined with some degree of accuracy. Through interferometry, it is also expected that the radio telescopes will allow determination of Huygens's landing site on Titan with exquisite precision, measuring its position to within 1 km (about two-thirds of a mile) at a distance from Earth of about 1.2 Tm (750 million miles). This represents an angular resolution of approximately 170 microarcseconds. A similar technique was used to determine the landing site of the Mars exploration rovers by listening to their telemetry alone.
Detailed Huygens activity timeline
- Huygens probe separated from Cassini orbiter at 02:00 UTC on December 25, 2004 in SCET.
- Huygens probe is scheduled to enter Titan's atmosphere at 09:06 UTC on January 14, 2005 in SCET.
- The probe will land on the surface of the moon at ~163.1775 degrees east and ~10.2936 degrees south around 11:24 UTC in SCET.
There will be a transit of the Earth/Moon across the Sun as seen from Saturn/Titan just hours before the landing. Huygens probe will enter the upper layer of Titan's atomsphere 2.7 hours after the end of the transit of the Earth, or only one or two minutes after the end of the transit of the Moon. However, the transit will not interfere with Cassini orbiter or Huygens probe. First, although they cannot receive any signal from us because we are in front of the Sun, we can still listen to them. Second, Huygens does not send any readable data to the Earth; it transmits data to Cassini orbiter, which relays the data received to the Earth later. For details about transits of the Earth as seen from Saturn, see also Transit of Earth from Saturn.
See also Detailed timeline of Huygens mission.
Instrumentation
The Huygens probe has six complex instruments aboard that will take in a wide range of scientific data after the probe descends into Titan's atmosphere. The six instruments are:
Huygens Atmospheric Structure Instrument (HASI)
This instrument contains a suite of sensors that will measure the physical and electrical properties of Titan's atmosphere. Accelerometers will measure forces in all three axes as the probe descends through the atmosphere. With the aerodynamic properties of the probe already known, it will be possible to determine the density of Titan's atmosphere and to detect wind gusts. In the event of a landing on a liquid surface, the probe motion due to waves will also be measurable. Temperature and pressure sensors will also measure the thermal properties of the atmosphere. The Permittivity and Electromagnetic Wave Analyzer component will measure the electron and ion (i.e., positively charged particle) conductivities of the atmosphere and search for electromagnetic wave activity. On the surface of Titan, the conductivity and permittivity (i.e., the ratio of electric flux density produced to the strength of the electric field producing the flux) of the surface material will be measured. The HASI subsystem also contains a microphone, which will be used to record any acoustic events during probe descent and landing. If the Huygens mission succeeds, it will be only the second time in history (a Venera-13 recording being the first) that audible sounds from another planetary body have been recorded.
Doppler Wind Experiment (DWE)
This experiment will use an ultra-stable oscillator to improve communication with the probe by giving it a very stable carrier frequency. The probe drift caused by winds in Titan's atmosphere will induce a measurable Doppler shift in the carrier signal. The swinging motion of the probe beneath its parachute due to atmospheric properties may also be detected.
Descent Imager/Spectral Radiometer (DISR)
This instrument will make a range of imaging and spectral observations using several sensors and fields of view. By measuring the upward and downward flow of radiation, the radiation balance (or imbalance) of the thick Titan atmosphere will be measured. Solar sensors will measure the light intensity around the Sun due to scattering by aerosols in the atmosphere. This will permit the calculation of the size and number density of the suspended particles. Two imagers (one visible, one infrared) will observe the surface during the latter stages of the descent and, as the probe slowly spins, build up a mosaic of pictures around the landing site. There will also be a side-view visible imager to get a horizontal view of the horizon and the underside of the cloud deck. For spectral measurements of the surface, a lamp that will switch on shortly before landing will augment the weak sunlight.
Gas Chromatograph Mass Spectrometer (GC/MS)
This instrument will be a versatile gas chemical analyzer designed to identify and measure chemicals in Titan's atmosphere. It will be equipped with samplers that will be filled at high altitude for analysis. The mass spectrometer will build a model of the molecular masses of each gas, and a more powerful separation of molecular and isotopic species will be accomplished by the gas chromatograph. During descent, the GCMS will also analyze pyrolysis products (i.e., samples altered by heating) passed to it from the Aerosol Collector Pyrolyser. Finally, the GCMS will measure the composition of Titan's surface in the event of a safe landing. This investigation will be made possible by heating the GC/MS instrument just prior to impact in order to vaporize the surface material upon contact.
Aerosol Collector and Pyrolyser (ACP)
This experiment will draw in aerosol particles from the atmosphere through filters, then heat the trapped samples in ovens (the process of pyrolysis) to vaporize volatiles and decompose the complex organic materials. The products will then be flushed along a pipe to the GCMS instrument for analysis. Two filters will be provided to collect samples at different altitudes.
Surface-Science Package (SSP)
The SSP contains a number of sensors designed to determine the physical properties of Titan's surface at the point of impact, whether the surface is solid or liquid. An acoustic sounder, activated during the last 100 meters of the descent, will continuously determine the distance to the surface, measuring the rate of descent and the surface roughness (e.g., due to waves). If the surface is liquid, the sounder will measure the speed of sound in the "ocean" and possibly also the subsurface structure (depth). During descent, measurements of the speed of sound will give information on atmospheric composition and temperature, and an accelerometer will accurately record the deceleration profile at impact, indicating the hardness and structure of the surface. A tilt sensor will measure any pendulum motion during the descent and will indicate the probe attitude after landing and show any motion due to waves. If the surface is, indeed, liquid, other sensors will measure its density, temperature and light reflecting properties, thermal conductivity, heat capacity, and electrical permittivity.
A critical design flaw
Long after launch, a few persistent engineers discovered that the communication equipment on Cassini had a fatal design flaw, which would have caused the loss of all data transmitted by the Huygens probe.
As Huygens is too small to transmit directly to Earth, it is designed to radio the telemetry data obtained while descending through Titan's atmosphere to Cassini, which would relay it to Earth using its large 4-meter diameter main antenna. Some engineers, most notably ESA Darmstadt employees Claudio Sollazzo and Boris Smeds, felt uneasy about the fact that, in their opinion, this feature had not been tested before launch under sufficiently realistic conditions. Smeds managed with quite some difficulty to convince superiors to perform additional tests while Cassini was in flight. In early 2000, he sent simulated telemetry data at varying power and Doppler shift levels from Earth to Cassini. It turned out that Cassini was unable to relay the data correctly.
The reason: When Huygens descends to Titan, it will accelerate relative to Cassini, causing its signal to be Doppler shifted. Consequently, the hardware of Cassini's receiver was designed to be able to receive over a range of shifted frequencies. However, the firmware was not: The Doppler shift changes not only the carrier frequency but also the timing of the payload bits, coded by phase-shift keying at 8192 bits per second, and this, the programming of the module fails to take into account.
Reprogramming the firmware was impossible, and as a solution the trajectory had to be changed. Huygens detached a month later (December 2004 instead of November) and approached Titan in such a way that its transmissions travelled perpendicularly to its direction of motion relative to Cassini, greatly reducing the Doppler shift. (See IEEE Spectrum article for the full story.)
The trajectory change overcame the design flaw and data transmission succeeded.
See also
External links
- European Space Agency Saturn page
- National Aeronautics and Space Administration Cassini page
- ESA Huygens Homepage about the probe that will study Titan's atmosphere and possibly the surface
- ESA - Where is Cassini-Huygens now? Interactiv Flash-Animation of Cassini orbits through 2008
- New Scientist - Cassini: Mission to Saturn Online Special Report
- Photo Gallery