European Space Agency

The Huygens Probe: Science, Payload and Mission Overview*

J.-P. Lebreton

Huygens Project Scientist, Space Science Department, ESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The Netherlands

D.L. Matson

Cassini Project Scientist, Jet Propulsion Laboratory, Pasadena, California, USA

* An extended version of this article can be found in Huygens: Science, Payload and Mission, ESA SP-1177, August 1997

The Huygens Probe is ESA's element of the joint Cassini/Huygens mission with NASA to the Saturnian system. Huygens will be carried on NASA's Cassini Orbiter to Saturn, where it will be released to enter the atmosphere of Titan, the planet's largest satellite. The Probe's primary scientific phase occurs during the 2-2.5 h parachute descent, when the six onboard instruments execute a complex series of measurements to study the atmosphere's chemical and physical properties. Measurements will also be conducted during the 3 min entry phase, and possibly on Titan's surface if Huygens survives impact. This article provides an overview of the mission and a concise description of the payload.


principal features cassini/huygens

Huygens is designed to study the atmosphere and surface of Titan, Saturn's largest moon, by conducting detailed in-situ measurements of the physical properties, chemical composition and dynamics of the atmosphere and local characterisation of the surface. It is a highly sophisticated robotic laboratory carrying six scientific instruments. Huygens is the element contributed by ESA to Cassini/Huygens, the joint NASA/ESA dual-craft mission to the Saturnian system. NASA has provided the Saturn Orbiter. The overall mission is named after the French/Italian astronomer Jean-Dominique Cassini, who discovered several Saturnian satellites and ring features (the Cassini division) in 1671-1685. The Probe is named after Dutch astronomer Christiaan Huygens, who discovered Titan in 1655.

Launch occured from Cape Canaveral, Florida aboard a Titan-4B/Centaur rocket on 15 October 1997 during the primary launch window. After an interplanetary voyage of 6.7 years, the spacecraft will arrive at Saturn in late June 2004, when a manoeuvre will place it in orbit around the planet. The Probe's mission will be executed in November 2004, at the end of the first of the many orbits around Saturn. Following the Huygens mission, the Orbiter will begin its intensive 4-year exploration of the Saturnian system.

The exploration of Titan is at the very heart of the Cassini/Huygens mission. The Orbiter will make repeated targeted close flybys of Titan, gathering data about the moon and making gravity-assisted orbit changes that will allow it to make a tour of the satellites, reconnoitre the magnetosphere and obtain views of Saturn's higher latitudes. During its 4-year nominal mission, Cassini will make detailed observations of Saturn's atmosphere, magnetosphere, rings, icy satellites and Titan. The detailed in-situ data set acquired by the Probe and the global data set from the Orbiter's tour will undoubtedly provide a unique wealth of information that will substantially increase our knowledge of Titan, a fascinating planet-sized moon shrouded by a thick, hazy and chemically active atmosphere.

The mission's development

The development of such a complex and ambitious venture between NASA and ESA required substantial scientific, technical and programmatic planning efforts over several years. Several scenarios for a mission to Saturn were studied within NASA from the late 1970s as the next natural step, after the Galileo orbiter/probe mission to Jupiter, in the detailed exploration of the giant planets.

The Cassini mission, in its present form, was originally proposed to ESA as a collaborative venture with NASA in response to a regular call for mission ideas released by ESA's Directorate of Scientific Programmes. The mission was proposed in November 1982 by a team of European and US scientists lead by D. Gautier and W. Ip. After an initial assessment, it was then subjected to a joint 1-year ESA/NASA assessment study starting in mid-1984. Very early in that study phase, the Titan Probe was identified as ESA's potential contribution, within its financial constraints and the technical capabilities of the European space industry. It was subsequently selected by ESA for a competitive Phase-A study in 1986, but the start was delayed by a year to allow programmatic adjustment with NASA. The Phase-A study therefore ran from November 1987 to September 1988.

The Titan Probe was selected by ESA's Science Programme Committee in November 1988 as the first medium-size mission ('M1') of the Horizon 2000 long-term space science plan. During that process it was named Huygens, in honour of the discoverer of Titan. Within NASA, Cassini was part of the CRAF (Comet Rendezvous and Asteroid Flyby)/Cassini programme, which was approved in the 1989 budget.

CRAF was cancelled by NASA for budgetary reasons in January 1992 and Cassini was greatly restructured, leaving the modified Cassini-alone programme to be authorised in May 1992. As a result of the restructuring, the two articulated orbiter science platforms and the articulated dedicated Huygens antenna were deleted. The Orbiter instruments became body-mounted, but several instruments added their own articulation to temper the losses of the platforms. The Huygens receivers were directly interfaced with the Orbiter's main antenna. Huygens was essentially unhurt by the restructuring process.

ESA and NASA released a joint Announcement of Opportunity in October 1989 calling for investigations on the Probe and Orbiter, respectively. Both payloads were selected in close coordination between the two agencies and with the European national agencies that provided funding for specific hardware contributions. The Probe and Orbiter payload selections were announced by ESA and NASA, respectively, in September and November 1990. In addition to hardware investigations, ESA and NASA respectively selected three and seven Interdisciplinary Scientist Investigations.

During the investigation selection process, the Italian Space Agency (ASI) initiated a bilateral collaboration with NASA that provided for significant augmentation of the Orbiter payload capabilities beyond what NASA alone could fund, in areas of prime interest to the Italian scientific community. This bilateral effort also included the provision by ASI of a major Orbiter element: the four-band (S, X, Ku, Ka) High-Gain Antenna (HGA).

During the Phase-A study, the need for using a gravity assist to inject the spacecraft towards Saturn was recognised. Three launch opportunities were identified that included a Jupiter flyby in addition to Venus and Earth flybys. Jupiter is required to reach Saturn in a reasonable time: 6-7 years, instead of 9-10 years. At the time of the joint CRAF/Cassini programme, Cassini was scheduled for launch during the second opportunity, in April 1996. After CRAF's cancellation, the possibility of accelerating the programme and launching in December 1995 was looked at, but the October 1997 launch opportunity was eventually selected as it was the only one of the three compatible with NASA's budget profile for developing the Cassini spacecraft.

Overview of the Cassini/Huygens mission

The Cassini mission is designed to explore the Saturnian system and all its elements: the planet and its atmosphere, rings, magnetosphere and a large number of its moons, namely Titan and the icy satellites. The mission will pay special attention to Titan, Saturn's largest moon and the Solar System's second largest after Jupiter's Ganymede. Cassini's broad scientific aims are to:

An important aspect of the Cassini mission is studying the interaction and interrelation of the system's elements. Studying the interrelation between the rings and the icy satellites, and the interaction of the satellites and of Titan's ionosphere with Saturn's magnetosphere is a key objective.

The Cassini/Huygens spacecraft (Figs. 1 & 2) was launched at 08.43 on 15 October 1997 by a Titan-4B/Centaur from Cape Canaveral Air Station in Florida. At a launch mass of 5548 kg, it is too heavy for direct injection to Saturn. Instead, it requires gravity assists from several planets: Venus (April 1998 and June 1999), Earth (August 1999) and Jupiter (December 2000). This launch opportunity allows Saturn to be reached in 6.7 years. The primary window extended from 6 October to 4 November, with contingency days available to 15 November. There were later opportunities (which add 2 years to the total flight time to Saturn because they do not include a Jupiter flyby) in December 1997 and March 1999, but they were less favourable from the launch performance and science points of view. This is particularly true for the ring science as the solar and Earth-viewing phase angle of the rings will be much less favourable in 2008-2012 than in 2004-2008. The maximum ring opening angle occurs in 2002.

cassini/huygens spacecraft
Figure 1. The Cassini / Huygens spacecraft

Huygens probe descent
Figure 2. The Huygens Probe descent

Cassini/Huygens will arrive in the vicinity of Saturn in late June 2004. The date has been calculated to allow a flyby of distant moon Phoebe during the approach phase to Saturn. The most critical phase of the mission after launch is Saturn Orbit Insertion (SOI), on 1 July 2004. Not only is it a crucial manoeuvre, but also a period of unique Orbiter science activity as, at that time, the spacecraft is as close as it ever will be to the planet (at 0.3 RS about 2 h before and 2 h after ring-plane crossing). Ring-plane crossing occurs in the gap between the F and G rings at a distance of about 2.66 RS. The SOI part of the trajectory provides a unique observation geometry for the rings.

The Huygens Probe is carried to Titan attached to the Saturn Orbiter and released from the Orbiter on 6 November 2004 after SOI at the end of the initial orbit around Saturn, nominally 22 days before Titan encounter (Fig. 3). Shortly (typically two days) after Probe release, the Orbiter will perform a deflection manoeuvre to set up the radio link geometry for the Probe's descent phase. This manoeuvre will also set up the initial conditions for the satellite tour after completion of the Probe mission.

Huygens release orbit
Figure 3. The Huygens release orbit. Huygens will be released from the Orbiter at the end of the first revolution around Saturn, with the second orbit as a backup possibility

Huygens' encounter with Titan is planned for 27 November 2004. The celestial mechanics do not allow much freedom in the arrival date at Saturn, but Huygens' Titan encounter date, dictated by the duration of the initial orbit around Saturn, is adjustable by multiples of 16 days, corresponding to Titan's 15.95 day orbital period around its parent.

Huygens scientific objectives

The scientific objectives of the Cassini/Huygens mission at Titan are to:

Huygens' goals are to make a detailed in-situ study of Titan's atmosphere and to characterise the satellite's surface along the descent ground track and near the landing site. Following the entry phase, at the start of the descent phase and after deployment of the parachute at about 165 km altitude, all instruments will have direct access to the atmosphere. The objectives are to make detailed in-situ measurements of atmospheric structure, composition and dynamics. Images and other remote-sensing measurements of the surface will also be made during the atmosphere descent. After a descent of about 137 min, the Probe will impact the surface at 5-6 m/s. As it is hoped that Huygens will survive after impact for at least a few minutes, the payload includes the capability for making in-situ measurements for a direct characterisation of the landing-site surface. If everything functions nominally, the Probe batteries can provide 30-45 min of electrical energy for an extended surface science phase that would be the bonus of the mission. The current mission scenario foresees the Orbiter listening to the Probe for a full 3 h, which includes at least a 30 min surface phase, as the maximum descent time is expected to be 2.5 h. A surface phase of only a few minutes would allow a quick characterisation of the state and composition of the landing site. An extended surface phase would allow a detailed analysis of a surface sample and meteorological studies of the surface weathering and atmosphere dynamics.


General characteristics
Titan is the second largest moon in the Solar System and it is the only one with a thick atmosphere (Table 1). That atmosphere was discovered in 1907 by Spanish astronomer José Comas Solá, who observed disc edge darkening features and suggested that they were due to an atmosphere, although its existence was not confirmed until 1944 when Gerard Kuiper discovered gaseous methane spectroscopically. Molecular nitrogen is the major constituent, with the surface pressure 1.5 bar (compared to Earth's 1 bar). Until the mid-1970s, methane was believed to be the major constituent but the Voyager-1 measurements in November 1980 replaced it with nitrogen, as was already suspected from late 1970s models. The presence of nitrogen makes Titan's atmosphere more similar to Earth's than any other Solar System body. However, it is much colder: the surface temperature is 94 K and the tropopause temperature is about 70 K at an altitude of 45 km. Other major constituents are CH4 (a few %) and H2 (0.2%). It is speculated that argon could also be present in quantities up to 6%. The presence of methane makes Titan's atmosphere most interesting.

Table 1. Physical properties of Titan

   Surface radius          2575±0.5 km

   Mass                    1.346x1023 kg
                             (2.2% MEarth)

   GM                      8978.1 km3/s2

   Surface gravity         1.345 m/s2
   Mean density            1.881 g/cm3
   Distance from Saturn    1.226x106 km
                             (20.3 RS)

   Orbital period          15.95 d
   Rotation period         15.95 d
   Surface temperature     94 K
   Surface pressure        1496±20 mbar

The photodissociation of CH4 and N2 in Titan's atmosphere, driven by solar UV radiation, cosmic rays and precipitating energetic particles from Saturn's magnetosphere, gives rise to a complex organic chemistry. Titan orbits Saturn at 20.3 RS, which occasionally brings it outside the large Kronian magnetosphere when solar-wind pressure pushes the magnetopause inside the orbit. Most of the time, however, Titan is inside Saturn's magnetosphere, which underlines the importance of the energetic electrons as an energy source for its upper-atmosphere photochemistry. As a result of this complex photochemistry, the atmosphere also contains ethane, acetylene and more complex hydro-carbon molecules. Chemical reactions in the continuously evolving atmosphere provide possible analogues for the prebiotic chemistry that was at work within the atmosphere of the primitive Earth a few thousand million years ago, before the appearance of life. Titan's atmosphere is too cold for life to evolve in it, but the mission does offer the opportunity to study prebiotic chemistry on a planetary scale (see T. Owen's article in this issue).

The nature of the surface is Titan's main mystery. Like Earth, it could be partially covered by lakes or even oceans, but in this case a mixture of liquid methane and ethane. However, it may be a dry surface, with underground liquid-methane reservoirs continuously resupplying the atmosphere's gaseous methane.

Atmospheric thermal profile
The most reliable 'engineering' model of Titan's atmosphere was established in 1986 by Lellouch and Hunten during the Phase-A study (Fig. 4). Subsequently, an improved model was established by Yelle. This did not disagree significantly in the altitude range of prime concern, so the original was retained as the reference model for designing the Huygens entry heat shield and the parachute system.

Lellouch-hunten Titan atmosphere model
Figure 4. The Lellouch-Hunten Titan atmosphere model used as the 'engineering model' for designing the Huygens heat shield and parachutes. This model comprises three profiles taking account of all thermal uncertainties

Upper-atmosphere composition
The possible presence of argon in Titan's atmosphere was a major design constraint for the heat shield, as it would significantly contribute to the radiative heat flux during entry. The shield was thus designed to be compatible with the maximum argon content identified by the Lellouch-Hunten model (21%). The upper limit was subsequently reduced to 14% and then to 6%, to the growing satisfaction of the heat shield designers as their performance margin increased.

Wind model
The presence of a zonal wind will affect the Probe's parachute-descent trajectory. A proper estimation of the zonal wind was of paramount importance for designing the Probe-to-Orbiter radio relay link geometry, and hence the Orbiter trajectory during the Probe descent. The wind model used was derived by Flasar et al. from the measured latitudinal thermal gradients. This model provides the amplitude of the zonal wind versus altitude, but it cannot predict whether the wind blows west-to-east or east-to-west. Both directions have been assumed to be of equal probability for designing the radio link.

Other models
Other models were established for designing the Probe, including:

All of the models used in designing the Probe are documented in ESA SP-1177

The Huygens payload consists of six instruments provided by the Principal Investigators. The principal instrument characteristics are listed in Table 2, and a brief description of each instrument is provided below. More detailed descriptions are provided in ESA SP-1177, which also includes papers on the interdisciplinary science investigations.

principal characteristics huygens payload
Table 2. The principal characteristics of the Huygens payload

The Huygens payload

The Gas Chromatograph and Mass Spectrometer (GCMS)
GCMS is a highly versatile gas chemical analyser designed to identify and quantify the abundance of the various atmospheric constituents. It can analyse argon and other noble gases and make isotopic measurements. The GCMS inlet system is located near the apex in front of the Probe, where the dynamic pressure drives the gas into the instrument. GCMS works either in direct mass-spectrometer mode, or in the more powerful mode in which the gas sample is passed through gas-chromatograph columns to separate components of similar mass before analysis with the mass spectrometer. The instrument is also equipped with gas samplers for filling at high altitude, for analysis later in the descent when there is more time available. The instrument is equipped with a separate ionisation chamber for analysis of the aerosol pyrolyser products fed by the ACP, the instrument described next. Thanks to its heated inlet, GCMS can also measure the composition of a vaporised surface sample in the event that a safe landing allows the collection and transmission of data for several minutes.

The Aerosol Collector and Pyrolyser (ACP)
ACP is designed to collect aerosols for GCMS to analyse their chemical compositions. It is equipped with a deployable sampling device that will be operated twice in order to collect the aerosols from two atmospheric layers: the first from the top of the atmosphere down to about 40 km, and the second in the cloud layer from about 23 km down to 17 km. After extension of the sampling device, a pump draws the atmosphere and its aerosols through filters in order to capture the aerosols. At the end of each sampling, the filter is retracted into an oven where the aerosols are heated to three, increasing, temperatures in order to conduct a step pyrolysis. The volatiles are vaporised first at the lowest temperature, then the more complex less volatile organic material, and finally the core of the particles. The products are flushed to GCMS for analysis, thereby providing spectra for each analysis step.

Descent Imager/Spectral Radiometer (DISR)
DISR is a multi-sensor optical instrument capable of imaging and making spectral measurements over a wide range of the optical spectrum (ultraviolet- infrared, 0.3-1.64 µm).

An important feature of Titan is its aerosols and thick atmosphere, where the temperature structure is determined by the radiative and convective heat-transport processes. DISR measures the upward and downward heat fluxes. An aureole sensor measures the intensity of the Sun's halo, yielding the degree of sunlight scattering caused primarily by the column density of aerosols along the line of sight. This in turn allows deduction of the aerosols' physical properties. DISR is also equipped with a side-looking horizon instrument to image the clouds.

DISR also has the ability to address one of Huygens' prime objectives: investigating the nature and composition of the surface. Two cameras (one visible, one infrared) looking downwards and sideways image the surface and, as Huygens spins slowly, build up mosaic panoramas. By recording several panoramas during the last part of the descent, it may be possible to infer the Probe's drift (if the surface is not featureless) and contribute to the wind measurements.

Titan's daytime surface brightness is about 350 times that of nighttime on Earth with a full Moon. While the surface illumination is adequate for imaging, a surface lamp will be activated a few hundred metres up to provide enough light in the methane absorption bands for spectral-reflectance measurements. These will provide unique information on the composition of the surface material.

Evaluation of the gas flow around the Descent Module during the 1 min phase of back-cover separation and heat-shield release showed there was a small risk of contaminating DISR's optical windows. A cover was later added for safety and it will be ejected shortly after the heat shield is released. Should its release mechanism fail, the cover is provided with optical windows that would still allow measurements to be made with it still in place.

Huygens Atmosphere Structure Instrument (HASI)
HASI is also a multi-sensor instrument, intended to measure the atmosphere's physical properties, including its electrical properties. Its set of sensors comprises a three-axis accelerometer, a redundant set of a coarse and a fine temperature sensor, a multi-range pressure sensor and an electric-field sensor array.

The set of accelerometers is specifically optimised to measure entry deceleration for inferring the atmosphere thermal profile during entry.

The electric-field sensor comprises a relaxation probe to measure the atmosphere's ion conductivity and a quadrupolar array of electrodes for measuring, by using the mutual impedance probe technique, atmosphere permittivity and surface-material permittivity after and possibly just before impact, when the Probe is still a few metres above the surface. Two electrodes of the quadrupolar array are also used as an electric antenna to detect atmospheric electromagnetic waves, such as those produced by lightning.

Several of HASI's sensors require accommodation on booms. The temperature and pressure sensors are mounted on a fixed stub, which is long enough to protrude into the free flow. The electrical sensors are mounted on a pair of deployable booms in order to minimise the shielding effects of the Probe body.

The capability for processing the surface-reflected signal of the radar altimeter (the altitude sensor is provided as part of the Probe system, as described later) was added to HASI late in the programme. This additional function allows it to return important information about the surface topography and radar properties below the Probe along its descent track.

Doppler Wind Experiment (DWE)
DWE uses one of the two redundant chains of the Probe-Orbiter radio link. It required the addition of two ultra-stable oscillators (USOs) to one chain of the Probe data-relay subsystem. The Probe Transmitter USO (TUSO) provides a very stable carrier frequency to the Probe-to-Orbiter radio link; the Receiver USO (RUSO) aboard the Orbiter provides an accurate reference signal for Doppler processing of the received carrier signal. The Probe wind drift will induce a measurable Doppler shift in the carrier signal, and that signature will be extracted aboard the Orbiter and merged into the Probe data stream recorded on the Orbiter solid-state recorders. It is expected that the Doppler measurements will be so sensitive that, by having the Probe transmit antennas offset from the spin axis by a few centimetres, the Probe spin rate and spin phase will also be determined. The Probe's swinging motion under the parachute and other radio-signal perturbing effects, such as atmospheric attenuation, may also be detectable from the signal.

The chain provided with the TUSO and RUSO is also equipped with the same standard oscillators that equip the other radio relay link chain. Selecting between the DWE USOs (the default configuration) and the standard oscillators will be done during the Probe configuration activity before its release from the Orbiter.

Surface Science Package (SSP)
SSP comprises a suite of rather simple sensors for determining the surface physical properties at the impact site and for providing unique information on the composition of the surface material. The SSP package includes a force transducer for measuring the impact deceleration, and other sensors to measure the refractive index, temperature, thermal conductivity, heat capacity, speed of sound and dielectric constant of the (liquid) material at the impact site. The SSP also includes an acoustic sounder to be activated a few hundred metres up for sounding the atmosphere's bottom layer and the surface's physical characteristics before impact. If Huygens lands in a liquid, the acoustic sounder will be used in a sonar mode to probe the liquid's depth. A tilt sensor is included to indicate the Probe's attitude after impact. Although SSP's objectives are mainly to investigate the surface, several sensors will contribute significantly to the studies of atmospheric properties during the whole descent phase.

The Huygens mission
The Huygens Probe is carried to Titan attached to the Saturn Orbiter. It is released after Saturn Orbit Insertion (SOI) during the initial orbit around Saturn, nominally 22 days before Titan encounter, as shown in Figure 3. Shortly after release, the Orbiter executes a deflection manoeuvre to establish the proper radio communication geometry with Huygens during the Probe's descent phase, and also to set the initial conditions for the satellite tour after completion of the Probe mission.

Huygens separates from the Orbiter at 30 cm/s and a spin rate of 7 rpm for stability during the coast and entry phases. The entry subsystem consists of the 2.75 m-diameter front heat shield and the aft cover, both protected against the radiative and convective heat fluxes generated during the entry phase at 350-220 km altitude, where Huygens decelerates from about 6 km/s to 400 m/s (Mach 1.5) in less than 2 min.

At Mach 1.5, the parachute deployment sequence initiates, starting with a mortar pulling out a pilot chute, which in turn pulls away the aft cover. After inflation of the 8.3 m-diameter main parachute, the front heat shield is released to fall from the Descent Module (DM). Then, after a 30 s delay built into the sequence to ensure that the shield is sufficiently far below the DM to avoid instrument contamination, the GCMS and ACP inlet ports are opened and the HASI booms deployed. The main parachute is sized to pull the DM safely out of the front shield; it is jettisoned after 15 min to avoid a protracted descent, and a smaller 2.5 m diameter parachute is deployed.

The major events of the entry and descent sequence are illustrated in Figure 5. The altitude profile is shown in Figure 6, where the middle curve indicates the nominal profiles and the two other curves define its envelope, taking into account the Lellouch-Hunten atmospheric model uncertainties and all other descent calculation uncertainties.

Huygens Probe entry and descent sequence
Figure 5. The Huygens Probe entry and descent sequence

profiles of the Lellouch-Hunten model
Figure 6. The altitude descent profile for the three atmospheric profiles of the Lellouch-Hunten model

After separation from the Orbiter, the only energy source is from primary batteries with a total capacity of 1800 Wh. The batteries and all other resources are sized, with a comfortable margin, for a maximum mission duration of 153 min, corresponding to a maximum descent time of 2.5 h and at least 3 min on the surface. Instrument operations are based either on time in the top part of the descent or on measured altitude (from the system-provided radar altimeter) in the bottom part.

Huygens transmits its data at a constant 8 kbit/s to the overflying Orbiter; which points its HGA to a pre-defined location on Titan for a full 3 h to allow for data reception after landing for 43 min for a nominal descent time of 137 min. The Probe data are stored onboard the Orbiter in the two solid-state recorders for later transmission to Earth as soon as the HGA can be redirected after Huygens has completed its mission.

Payload accommodation

Mechanical accommodation
All the payload elements described above are accommodated on the payload platform, as shown in Figure 7. ACP and GCMS are both single-box instruments with their inlets below Huygens for direct access to the gas flow. Each also has an exhaust tube projecting through the top platform. ACP and GCMS are linked by a temperature-controlled pneumatic line to transfer ACP's pyrolyser products to GCMS for analysis. A serial link between the two instruments synchronises their operations.

accomodation of the payload and major subsystemes accomodation of the payload and major subsystemes accomodation of the payload and major subsystemes accomodation of the payload and major subsystemes
Figure 7. Accommodation of the payload and the major subsystems on the top/bottom of the experiment platform
ACP: Aerosol Collector Pyrolyser; BAT-1/5: Batteries; CASU: Central Acceleration Sensor Unit; CDMU-A/B: Command and Data Management Unit; DISR: Descent Imager/Spectral Radiometer; DISR-E: DISR Electronics Box; DISR-S: DISR Sensor Head; GCMS: Gas Chromatograph Mass Spectrometer; HASI: Huygens Atmospheric Structure Instrument; MTU: Mission Timer Unit; PCDU: Power Conditioning and Distribution Unit; PYRO: Pyro Unit; RASU: Radial Acceleration Sensor Unit; RUSO: Receiver Ultra Stable Oscillator; RX-A/B: Receiver Antennas for Radar Altimeter A/B; SEPS: Separation Subsystem; SSP: Surface Science Package; SSP-E: SSP Electronics Box; TUSO: Transmitter Ultra Stable Oscillator; TX-A/B: Transmit Antennas for Radar Altimeter A/B.

DISR consists of two boxes: the Sensor Head (DISR-S) and the Electronics box (DISR-E). DISR-S is mounted on the platform's periphery to accommodate the field-of-view and scanning requirements. DISR-E is mounted on the platform's inner area and connected to the DISR-S via a short harness.

HASI's sensors, with the exception of the accelerometers, are mounted either on a fixed stub (HASI STUB) or on deployable booms (HASI boom 1 and boom 2). This satisfies post-deployment requirements for access to the gas flow for pressure and temperature measurements, while minimising Probe-induced perturbations to the electric-charge distribution at the electric-field sensors. The accelerometers are located near the Probe's centre of gravity in its entry configuration. All HASI sensors are connected to the central electronics box (HASI-DPU), which contains the conditioning pre-amplifiers and the central processing functions. The electric antenna preamplifiers are housed in two small boxes located as close as possible to the sensors, but still inside the Descent Module, in order to minimise the cable length.

SSP consists of two boxes: the 'Top Hat' structure (SSP-TH) that accommodates all but two of the sensors, and an electronics box (SSP-E). SSP-TH is below the platform, allowing for sensor wetting in case of landing in a liquid. It is connected to SSP-E (on the top of the platform) via a harness through the platform. SSP-TH is also instrumented with a pylon designed for effective transmission of the impact deceleration to the force transducer on the platform. Two sensors are directly mounted on the electronics box: the tilt meter and one of the two accelerometers.

DWE's TUSO is also accommodated on the experiment platform, while RUSO is accommodated in the part of Huygens that remains attached to the Orbiter (Probe Support Equipment, PSE).

The overall accommodation of the payload sensors that require direct access to Titan's atmosphere is illustrated in Figure 8.

Huygens Probe under its parachute
Figure 8. The Huygens Probe under its parachute. The HASI booms are deployed and the DISR sensor head can be seen. At right, the accommodation of the GCMS, ACP and SSP inlets is shown

Probe spin requirements
Huygens is required to spin throughout the descent to provide the azimuth coverage needed by several sensors. The real-time spin information requirements are imposed by DISR and are very stringent for the final part of the descent for imaging the surface, in order to adapt the time delay between consecutive frames during the mosaic image-taking cycle. The spin is induced by a set of 36 vanes mounted on the bottom part of the fore dome. The spin rate is measured by a set a system-provided accelerometers covering the 0-15 rpm range with an accuracy of 0.1 rpm.

Probe altitude measurements
During the early descent, instrument operations are time-based. However, for maximising the science return, the measurement cycle during the last part of the descent is based on the true altitude. Furthermore, as impact survival is not guaranteed by the Probe's design, maximum science return can be achieved from the last few hundred metres and possibly for the crucial first few seconds after impact if the altitude is reliably known. To meet these requirements, altitude is measured by a set of two radio altimeters working in the Ku-band (15.3 and 15.7 GHz). The measurements are processed by a sophisticated algorithm in the Probe's central computer that will fall back on the default time-based altitude table in case of a temporary loss of radar lock, e.g. caused by a higher than nominal pendulum motion.

The Descent Data Broadcast (DDB) pulse
The Probe time, measured spin and processed altitude are broadcast every 2 seconds to all experiments for their real-time use during descent. The DDB altitude information is used by DISR, HASI and SSP to optimise their measurement cycle.

Probe targeting requirements
Targeting requirements are imposed by the payload and certain system design aspects, such as the telecommunications geometry and the design of the heat-shield ablative material, which are affected by the choice of entry point. DISR and DWE impose demanding requirements on the Sun Zenith Angle (SZA), which should lie within 35-65°, and the maximisation of the zonal wind component along the Probe-Orbiter line of sight. As a result of all the targeting trade-offs, made early during Phase-B, an entry angle of -64° was selected. Entry and descent occur over Titan's sunlit hemisphere (Fig. 9). Figure 10 shows the landing ellipse on images obtained by the Hubble Space Telescope. It so happens that the landing site is ideally located - Huygens will fly over the region of highest contrast on Titan.

Probe targeting (B-plane)
Figure 9. Probe targeting (B-plane) and entry geometry

Hubble Space Telescope image of Titan
Figure 10. Hubble Space Telescope image of Titan with the Huygens 200 km ´ 1200 km landing ellipse. The entry location was selected before such HST images became available, but it turns out that Huygens will descend over the region of greatest surface contrast

Entry measurements
Only HASI will perform measurements during the entry phase. These and all data acquired by the other instruments before the Orbiter radio link is established will be buffered within each instrument and interleaved with the real-time data packets that are transmitted by each instrument when the link is made.

Huygens operates autonomously after separation from the Orbiter, the radio link to the Orbiter being one-way for telemetry only. Until separation, telecommands can be sent via an umbilical from the Orbiter (which also provides electrical power to the Probe), but this facility will be used only during the cruise and Saturn-orbit phases for monitoring the health of subsystems, maintaining mechanical devices and routinely calibrating the instruments for the biannual checkouts. There will be no scientific measurements before Titan arrival, and Huygens will be switched off throughout most of the cruise phase. During the 22-day coast phase, after separation from the Orbiter, only a triply-redundant timer will operate to wake up Huygens shortly before the predicted entry into Titan's atmosphere. Loading the value of this timer's duration and depassivation of the batteries that power the Probe after separation will be the last activities initiated by ground command.

Flight operations
Probe operation and the collection of telemetered data are controlled from a dedicated control room, the Huygens Probe Operations Centre (HPOC), at ESOC in Darmstadt (D). Here, command sequences are generated and transferred by dedicated communication lines to the Cassini Mission Support Area (MSA) at the Jet Propulsion Laboratory (JPL), Pasadena, California. There, the Probe sequences are merged with commands to be sent to other subsystems and instruments of the Orbiter for uplinking via NASA's Deep Space Network (DSN). Probe telecommands are stored by the Orbiter and forwarded to the Probe Support Equipment (PSE) at specified times (time tags) for immediate execution. Due to the great distance between Earth and Saturn (requiring up to 160 min for round-trip radio communications), real-time operation of Huygens is not possible.

Data collected by the Probe and passed to the PSE via the umbilical (during the attached phase) or the relay link (during the descent phase) are formatted by the PSE and forwarded to the Orbiter's Command and Data Subsystem (CDS). The Orbiter stores the Huygens data in its two solid-state recorders for transmission to Earth when the Orbiter is visible from one of the DSN ground stations. From the ground station, the data are forwarded to the MSA where Probe data are separated from other Orbiter data before being stored in the Cassini Project Database (PDB). Operators in the HPOC access the PDB to retrieve Probe data via a Science Operations and Planning Computer, supplied to ESOC by JPL under the terms of the inter-agency agreement.

Subsystem housekeeping data are used by ESOC to monitor Probe performance, while data from the science instruments are extracted for forwarding to the Investigators. During the cruise phase, these data are shipped to the scientists' home institutes by CD-ROM (the prime medium) and possibly by public data line. After analysing these data, the Investigators meet the Operations Team to assess the health of the payload and to define the activities for the following checkout period.

During the Saturn-orbit and Probe-mission phases, the investigators are located in HPOC to expedite their access to the data and facilitate interaction with their colleagues and the Probe flight-operations team. Accommodation will be provided for the ground-support equipment needed to reduce and interpret their data.

Data analysis and archiving

The raw Huygens data will be provided to the Huygens Principal Investigator (PI) teams on CD-ROM after each checkout and for the descent phase. It is the responsibility of each PI team to process the data and to provide a reduced data set to allow a coordinated analysis of the Huygens data set. The Huygens Science Working Team (HSWT) intends to produce a commonly agreed descent profile within weeks of the event to allow all experimenters to analyse their data and interpret their measurements in the most efficient way. A subgroup of the HSWT, the Descent Trajectory Working Group (DTWG), has been set up to optimise the data analysis that should lead to establishing the Probe's descent profile in Titan's atmosphere, providing the optimum means for coordinating analysis of the data from the six instruments.

The initial uncertainty ellipse of the Probe's landing site may be as large as 200 ´1200 km. The HSWT will work in coordination with the Orbiter teams to reduce the uncertainty in the Probe descent trajectory to allow a proper coordinated analysis of the Probe and Orbiter data set and to help plan the observations of the Probe landing site by the Orbiter's radar and remote-sensing instruments after the Probe mission.

The Huygens data set will be archived as an integral part of the Cassini data archive that is being defined by the Cassini Project Office at JPL. This will provide the optimum approach for synergistic studies using both Probe and Orbiter data.

The Huygens Science Working Team (HSWT)

The HSWT (Table 3) manages the overall Huygens science activities. It advised the Huygens Project on all science-related matters during the Probe's development, and it will meet periodically during the cruise phase to assess the payload's performance and to prepare itself for the Huygens mission and data-analysis phase. Activities will peak during the Huygens mission phase as it coordinates the analysis and interpretation of Probe data. It will also play an important role in planning the post-Huygens observations of Titan by the Orbiter, and it will participate in joint Probe/Orbiter investigations, data analysis and interpretation studies.

Table 3. The Huygens Science Working Team (HSWT)

Chairman: Jean-Pierre Lebreton, ESA/ESTEC, Huygens Project Scientist Vice-Chairman: Dennis Matson, NASA/JPL, Cassini Project Scientist

PI/DWE: Michael Bird, University of Bonn, Germany

PI/HASI: Marcello Fulchignoni, Université Paris 7/Observatoire Paris- Meudon, France

IDS/Aeronomy of Titan: Daniel Gautier, Observatoire Paris-Meudon, France

PI/ACP: Guy Israel, CNRS/SA Verrieres-le-Buisson, France

IDS/Titan Atmosphere-Surface Interaction: Jonathan Lunine, University of -- Arizona, USA

PI/GCMS: Hasso Niemann, NASA Goddard Space Flight Center, USA

IDS/Titan Organic Chemistry & Exobiology: Francois Raulin, LISA, Université Paris 12, Creteil, France

PI/DISR: Martin Tomasko, University of Arizona, USA

PI/SSP: John Zarnecki, University of Kent at Canterbury, UK

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Right Left Up Home ESA Bulletin Nr. 92.
Published November 1997.