The Cassini Spacecraft
Staff posted on October 16, 2006 |
The Cassini Spacecraft
Huygens Probe The Huygens probe, supplied by the European Space Agency (ESA), will 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. The Huygens probe system consists of the probe itself, which will descend to Titan, and the probe support equipment (PSE), which will remain attached to the orbiting spacecraft. The PSE includes 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 remains dormant throughout the 6.7-year interplanetary cruise, except for bi-annual health checks. These checkouts follow preprogrammed descent scenario sequences as closely as possible, and the results are relayed to Earth for examination by system and payload experts.

Prior to the probe's separation from the orbiter, a final health check will be performed. The "coast" timer will be loaded with the precise time necessary to turn on the probe systems (15 minutes before the encounter with Titan's atmosphere), and then the probe will separate from the orbiter and coast to Titan for 22 days with no systems active except for its wake-up timer.

The main mission phase will be the parachute descent through Titan's atmosphere. The batteries and all other resources are 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 will be activated early in the descent phase, and the orbiter will "listen" to the probe for the next 3 hours, which includes the descent plus 30 minutes after impact. Not long after the end of this 3-hour communication window, Cassini's high-gain antenna (HGA) will be turned away from Titan and toward Earth.
The Probe as a System
Probe System

Huygens Probe

The probe system is made up of a number of engineering subsystems, some of which are distributed between the probe and the PSE. The Huygens payload consists of a complement of six scientific instruments. The engineering subsystems are briefly described below. For more detailed information, see the Huygens Probe Home Page in the Netherlands.

 
Probe Subsystems
Probe Engineering Subsystems The probe engineering subsystems are the Entry Subsystem, the Inner Structure Subsystem, the Thermal Control Subsystem, the Electrical Power Subsystem, the Command and Data Management Subsystem, and the Probe Data Relay Subsystem.
 
Entry Subsystem

This subsystem functions only during the release of the probe from the orbiter and its subsequent entry into the Titan atmosphere. It consists of three main elements: (1) the spin-eject device that propels the probe away from the orbiter; (2) a front shield, covered with special thermal protection material, that protects the probe from the heat generated during atmospheric entry; and (3) an aft cover, also covered with thermal protection material, to reflect away heat from the wake of the probe during entry.

The probe remains clamped to the orbiter by pyrotechnic release bolts that bypass the spin-and-eject device. These explosive bolts will be "fired" during the approach to Titan, releasing the probe from the orbiter under the action of three springs, each exerting a force of 500 newtons (112 lbs). A curved track and roller system will ensure that the probe spins at approximately 7 rpm (for stabilization) and that it leaves the orbiter with a relative velocity of 0.3 meter/sec (m/s).

The probe will be released from the orbiter on November 6, 2004 and will be targeted for a high-latitude landing site on the "day" side of Titan. The probe's encounter with Titan is planned for November 27, when it will enter the atmosphere at a velocity of 6.1 km/s (13,725 mph). The entry phase will last about 3 minutes, during which the probe's velocity will fall to about 400 m/s (0.4 km/s or 895 mph).

Probe's Descent


Probes's Parachute

Engineers

Three parachutes will be used during the probe's descent. When the onboard accelerometers detect a speed of Mach 1.5 near the end of the deceleration phase, the 2-m (6.6-ft)-diameter pilot chute will deploy, pulling off the aft cover. This will be followed immediately by deployment of the 8.3-m (27-ft) main parachute. The pilot chute's ejection device, which induces a load of 13,500 newtons (1.5 tons) on the probe's upper platform for about 5 milliseconds, is a primary "driver" of the probe structural design.

About 30 seconds after deployment of the main chute, the probe's velocity will drop from Mach 1.5 to Mach 0.6. The front heat shield will then be released, and the probe will descend slowly below the main parachute for about 15 minutes while initial scientific measurements are made. Then the main parachute will separate from the probe and release a smaller 3-m (9.8-ft) drogue chute, which will allow the probe to descend faster. It will arrive at the surface in 2.5 hours, with an impact velocity of about 7 m/s (15 mph).

The probe's entry into Titan's atmosphere — which is mostly nitrogen, with some methane and argon will cause a shock wave to form in front of the 2.7-m (8.9-ft)-diameter front heat shield. The plasma in the shock, just forward of the shield, will reach a temperature of around 12,000 deg C (21,632 deg F), which is approximately twice the surface temperature of the Sun. Simultaneously, the deceleration force on the probe will reach its maximum of around 16 g. The high temperature and deceleration pressure are design drivers for most of the probe structure. The outer shell of the probe must also be able to withstand the extreme cold (-200 deg C or -392 deg F) of Titan's atmosphere without buckling.


Inner Structure Subsystem
. The inner structure of the probe consists of two aluminum honeycomb platforms and an aluminum shell. It is linked to the front heat shield and the aft cover by fiberglass struts and pyrotechnically operated release mechanisms. The central equipment platform carries, on both its upper and lower surfaces, the boxes containing the electrical subsystems and the science experiments. The upper platform carries the parachute (when stowed) and the antennas for communication with the orbiter.

Thermal Control Subsystem. At different times during the mission the probe will be subjected to extreme thermal environments requiring a variety of passive controls to maintain the required temperature conditions. For instance, during the two Venus flybys the solar heat input is high. The probe gets some protection from the shadow of the high-gain antenna (HGA), and when the orbiter (and thus the HGA) is off Sun-point for maneuvers or communication, the probe is protected by multilayer insulation that will burn off during the later atmospheric entry.

 

Probe - esa Photo

The probe will be at its coldest just after it separates from the orbiter. To ensure that none of the equipment falls below its storage-temperature limits, the probe carries a number of radioisotope (i.e., radioactive) heater units (RHUs) that each generate 1 watt (thermal).

As described above under the Entry Subsystem, the front heat shield will protect the probe during initial atmospheric entry. The front shield is covered with Space Shuttle-like tiles made of a material known as AQ60, developed by Aerospatiale. This material is essentially a low-density "mat" of silica fibers. The tile thickness on the front shield is calculated to ensure that the structure will not exceed 150 deg C (302 deg F), which is below the melting temperature of lead. The rear side of the probe will reach much lower temperatures, so a spray-on layer of "Prosial" silica foam material is used on the rear shield. The overall mass of the Thermal Protection System is more than 100 kg (220 lbs), or almost one third of the entire probe mass.

Electrical Power Subsystem

Electrical Power Subsystem. During probe checkout activities, the probe obtains power from the orbiter via the umbilical cable. After separation, the orbiter will continue to supply power to the probe support equipment (PSE), but power for the probe itself will be provided by five lithium sulphur-dioxide (LiSO2) batteries. Much of the battery power will be used to power the timer for the 22 days of "coasting" to Titan. The higher current needed for probe mission operations is only required for the descent duration of 2.5 hours. The Electrical Power Subsystem is designed to survive the loss of one of its batteries and still support a complete mission.

 

Command and Data Management Subsystem (CDMS). This subsystem provides the monitoring and control of all probe subsystem and payload (i.e., science instrument) activities. Specifically, the CDMS performs the following functions:

  • Times the 22-day "coast" phase to Titan and switches the probe "on" just prior to atmospheric entry.
  • Controls the activation of deployment mechanisms during the descent to Titan's surface.
  • Distributes telecommands to the engineering subsystems and science instruments.
  • Distributes to the science instruments a Descent Data Broadcast providing a timeline of conditions on which the instruments can base the scheduling of mode changes and other operations.
  • Collects scientific and housekeeping data and forwards the data to the orbiter via the umbilical cable (during the cruise phase) or the Probe Data Relay Subsystem (during descent).
 
Probe Data Relay Subsystem Probe Data Relay Subsystem (PDRS). The PDRS provides the one-way probe-to-orbiter communications link and includes equipment on both the probe and the orbiter. The elements that are part of the probe support equipment (i.e., on the orbiter) are the probe system avionics (PSA) and the radio frequency electronics (RFE), the latter including an ultra-stable oscillator (USO) and a low-noise amplifier. The probe carries two redundant S-band transmitters, each with its own antenna. The telemetry in one link is delayed by about four seconds with respect to the other link in order to avoid data loss if there are brief transmission outages. Reacquisition of the probe signal would normally occur within this interval.
 
Page: 7 Of 11First  Previous  1  2  3  4  5  6  7  8  9  10  11  Next  Last  

Recommended For You