Christiaan Huygens, Lord of Zeelhem (April 14, 1629 – July 8, 1695), was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the most important figures in the Scientific Revolution. In physics, Huygens made seminal contributions to optics and mechanics, while as an astronomer he is chiefly known for his studies of the rings of Saturn and the discovery of its moon Titan.
Huygens was an atmospheric entry robotic space probe that landed successfully on Saturn’s moon Titan in 2005. Built and operated by the European Space Agency (ESA)[1], and launched by NASA, it was part of the Cassini–Huygens mission[2] and became the first spacecraft to land on Titan and the farthest landing from Earth a spacecraft has ever made. The probe was named after the 17th-century Dutch astronomer Christiaan Huygens.
Launched aboard a Titan IVB/Centaur on October 15, 1997, from Launch Complex 40, Cape Canaveral AFS, Cassini was active in space for nearly 20 years, with 13 years spent orbiting Saturn and studying the planet and its system after entering orbit on July 1, 2004.
The Huygens Probe was about 9 feet wide and weighed roughly 700 pounds. It was built like a shellfish: a hard shell protected its delicate interior from high temperatures during the two-hour and 27-minute descent through the atmosphere of Saturn’s giant moon Titan.
The probe had two parts: the Entry Assembly Module and the Descent Module. The Entry Assembly Module carried the equipment to control Huygens after separation from Cassini, and a heat shield that acted as a brake and as thermal protection.
The Descent Module contained the scientific instruments and three different parachutes that were deployed in sequence to control Huygens’ descent to the surface of Titan.
The Huygens probe payload also consisted of six scientific instruments, each designed to perform a different function as the probe descended through Titan’s murky atmosphere. The instruments onboard Huygens was designed to find out what aerosols, and therefore chemicals, were present in the atmosphere.
Click on each instrument for more detail
Huygens Atmospheric Structure Instrument (HASI)
This instrument contained a suite of sensors that measured Titan’s atmosphere’s physical and electrical properties. Accelerometers measured forces in all three axes as the probe descended through the atmosphere. Since the aerodynamic properties of the probe were already known, it was possible to determine the density of Titan’s atmosphere and detect wind gusts. Had the probe landed on a liquid surface, this instrument would have been able to measure the probe motion due to waves. Temperature and pressure sensors also measured the thermal properties of the atmosphere. The Permittivity and Electromagnetic Wave Analyzer component measured the electron and ion (i.e., positively charged particle) conductivities of the atmosphere and searched 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 were measured.
Doppler Wind Experiment (DWE)
The intent of this experiment was to measure the wind speed during Huygens’ descent through Titan’s atmosphere by observing changes in the carrier frequency of the probe due to the Doppler effect. This measurement could not be done from space because of a configuration problem with one of Cassini’s receivers. However, scientists were able to measure the speed of these winds using a global network of radio telescopes.
Descent Imager/Spectral Radiometer (DISR)
This instrument made a range of imaging and spectral observations using several sensors and fields of view. The radiation balance (or imbalance) of the thick Titan atmosphere was measured by measuring the upward and downward flow of radiation. Solar sensors measure the light intensity around the Sun due to scattering by aerosols in the atmosphere. This permitted the calculation of the size and number density of the suspended particles. Two imagers (one visible, one infrared) observed the surface during the latter stages of the descent and built up a mosaic of pictures around the landing site as the probe slowly rotated. There was also a side-view visible imager that obtained a horizontal view of the horizon and the underside of the cloud deck. For spectral measurements of the surface, a lamp switched on shortly before landing which augmented the weak sunlight.
Gas Chromatograph Mass Spectrometer (GCMS)
This instrument was a versatile gas chemical analyzer that identified and measured chemicals in Titan’s atmosphere. It was equipped with samplers that were filled at high altitudes for analysis. The mass spectrometer built a model of the molecular masses of each gas, and a more powerful separation of molecular and isotopic species was accomplished by the gas chromatograph. During descent, the GCMS analyzed pyrolysis products (i.e., samples altered by heating) passed to it from the Aerosol Collector Pyrolyser. Finally, the GCMS measured the composition of Titan’s surface in the event of a safe landing. This investigation was made possible by heating the GCMS instrument just prior to impact in order to vaporize the surface material upon contact.
Aerosol Collector and Pyrolyser (ACP)
This experiment drew in aerosol particles from the atmosphere through filters, then heated the trapped samples in ovens (the process of pyrolysis) to vaporize volatiles and decompose the complex organic materials. The products were then flushed along a pipe to the GCMS instrument for analysis. Two filters were provided to collect samples at different altitudes.
Surface-Science Package (SSP)
The Surface-Science Package contained a number of sensors designed to determine the physical properties of Titan’s surface at the point of impact. These sensors also determined whether the surface was solid or liquid. An acoustic sounder, activated during the last 328 feet of the descent, continuously determined the distance to the surface, measuring the rate of descent and the surface roughness (e.g., due to waves). During descent, measurements of the speed of sound provided information on atmospheric composition and temperature, and an accelerometer accurately recorded the deceleration profile at impact, providing information on the hardness and structure of the surface. A tilt sensor measured any pendulum motion during the descent and indicated the probe attitude after landing
The above scientific instrument descriptions were taken directly from NASA.gov
The Huygens probe system consists of the 701 lb probe itself, which descended to Titan, and the probe support equipment (PSE), which remained attached to the orbiting spacecraft.
Huygens’ heat shield was 8.9 ft in diameter. After ejecting the shield, the probe was 4.3 ft in diameter. The PSE included the electronics necessary to track the probe, recover the data gathered during its descent, and process and deliver the data to the orbiter, from where it was transmitted or “downlinked” to the Earth.
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 by a spring-loaded mechanism 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.
Huygens landed at around 12:43 UTC on January 14, 2005, with an impact speed similar to dropping a ball on Earth from a height of about 3 ft. It made a dent 4.7 in deep, before bouncing onto a flat surface and sliding 12 to 16 in across the surface. It slowed due to friction with the surface and, upon coming to its final resting place, wobbled back and forth five times. Huygens’ sensors continued to detect small vibrations for another two seconds until motion subsided about ten seconds after the touchdown. The probe kicked up a cloud of dust (most likely organic aerosols that drizzle out of the atmosphere) which remained suspended in the atmosphere for about four seconds by the impact.
Titan has a hazy thick atmosphere, which Huygens found to be even hazier than expected due to the presence of aerosols or dust particles. Scientists were also surprised to find that the two noble gases, Xenon and Krypton, were not found.
Measurements of the atmosphere confirmed that complex organic compounds, the building blocks of the amino acids necessary for life, were present in both gas and solid phases. The first images of the surface showed a world that resembled the Earth in many ways with evidence that a liquid, possibly methane, had flowed on the surface causing erosion. At the landing site of the probe, the surface was found to have the consistency of loose wet sand that was mostly made up of dirty water-ice pebbles.
The surface material detected gases that had not been found higher up in the Titan atmosphere, including carbon dioxide. The Huygens probe also measured the weather conditions at the surface, detecting light winds, a temperature of -274 degrees Fahrenheit, and an atmospheric pressure slightly higher than Earth’s. On Titan, temperatures remain hundreds of degrees below zero. But even so, scientists can’t help but wonder if life swims in the methane seas of the alien moon—life fundamentally different from anything found on Earth.
Data that Huygens transmitted during its final descent and for 72 minutes from the surface included 350 pictures that showed a shoreline with erosion features and a river delta.
Footnotes
- The European Space Agency is an intergovernmental organization of 22 member states dedicated to the exploration of space. Established in 1975 and headquartered in Paris, ESA has a worldwide staff of about 2,200 in 2018 and an annual budget of about €4.9 billion in 2023. ESA’s space flight program includes human spaceflight (mainly through participation in the International Space Station program); the launch and operation of unmanned exploration missions to other planets and the Moon; Earth observation, science, and telecommunication; designing launch vehicles; and maintaining a major spaceport, the Guiana Space Centre at Kourou (French Guiana), France. The main European launch vehicle Ariane 5 is operated through Arianespace with ESA sharing in the costs of launching and further developing this launch vehicle. The agency is also working with NASA to manufacture the Orion spacecraft service module that will fly on the Space Launch System. [Back]
- Cassini–Huygens, commonly called Cassini, was a space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, including its rings and natural satellites. The Flagship-class robotic spacecraft comprised both NASA’s Cassini space probe and ESA’s Huygens lander, which landed on Saturn’s largest moon, Titan. Cassini was the fourth space probe to visit Saturn and the first to enter its orbit, where it stayed from 2004 to 2017. The two craft took their names from the astronomers Giovanni Cassini and Christiaan Huygens. [Back]
Further Reading
Sources
NASA Jet Propulsion Laboratory
Universe Today
Popular Mechanics
Britannica
ESA
Space Foundation Discovery Center
Aerospace-Technology
Wikipedia
