Sources cited

Who is who

Cassini is the orbiter, built by NASA and JPL. It never lands: it enters orbit around Saturn and stays there to observe.

Huygens is the atmospheric entry probe, built by the European Space Agency. It was a disc bolted to the side of Cassini. It detached, entered Titan's atmosphere and landed there.

The name of the whole mission is Cassini-Huygens. Launched on 15 October 1997 from Cape Canaveral on a Titan IVB/Centaur, it arrived at Saturn in 2004.

The separation of Huygens

On 25 December 2004 the mechanism called SED (Spin/Eject Device) separated Huygens from Cassini at a relative speed of 0.35 meters per second and with a spin of 7.5 revolutions per minute. The whole operation lasted about 0.15 seconds.

The mechanism: pyrotechnic bolts release three loaded springs that push the probe away; ramps and rollers give it its spin; and at the same time the three 19-pin electrical connectors that linked probe and orbiter come apart.

The spin stabilizes the probe along its trajectory, like a football thrown in a spiral. Huygens was aimed at a steep atmospheric entry, with an angle of -65° (±3°).

Huygens landed on Titan on 14 January 2005: the first and only landing ever achieved in the outer solar system.

What the mission found

From 2004 to 2017 Cassini completed more than 200 orbits around Saturn, traveling more than 3 billion kilometers in the Saturn system, with more than 100 close flybys of Titan and 23 of Enceladus. It sent back more than 300,000 images.

Enceladus. Cassini discovered that this icy moon has an ocean of liquid water beneath its crust, and that it sprays it into space: at least 101 eruption sites were identified, concentrated along the fractures at the south pole known as the “tiger stripes.” Cassini flew through those plumes and analyzed them in place, finding water, salts, organic molecules and signs of hydrothermal activity on the ocean floor.

Titan. More than 1.6 million km² of the surface (almost 2%) is covered in liquid: seas and lakes of methane and ethane. Cassini followed their seasonal changes.

The rings. They turned out to be young: between 10 and 100 million years old. Cassini also discovered 6 new moons and 2 new rings.

The Grand Finale

On 15 September 2017 Cassini was deliberately sent plunging into Saturn's atmosphere, where it disintegrated, transmitting data until the very end.

The reason was planetary protection: the spacecraft was running out of fuel and would soon no longer be controllable. Destroying it prevented it from eventually crashing onto one of the moons, in particular Enceladus, with its subsurface ocean, which scientists want to keep uncontaminated for future exploration.

The final phase consisted of 22 orbits that passed between the planet and the rings, through a gap no spacecraft had ever crossed, providing the first measurements of Saturn's core, its gravitational field and the mass of the rings.

The choice between two concepts

Two approaches had been considered for the landing: the traditional one of propulsive descent, as Viking had done in 1976, and a risky idea: using giant airbags to cushion the impact, then letting the lander bounce and roll until it stopped.

At NASA they said that propulsion was “the old way of doing things” and that with it “you will never finish the job: it costs too much.”

Rockets and airbags, not rockets or airbags

Pathfinder did not abandon retrorockets. The landing system was made up of a set of retrorockets mounted on a bridle plus the airbag impact attenuation system: the rockets slow the descent, and the airbags protect the lander from local irregularities in the terrain (bumps, craters, rocks, hills).

The system's margins

The impact could reach up to 25 meters per second at a grazing angle of 30 degrees to the ground. The lander could bounce up to 12 meters high and travel 100-200 meters between one bounce and the next. Testing verified that the system could withstand even stronger impacts and longer bounces.

The airbags were built with the help of a Delaware supplier better known for spacesuits than for interplanetary landings.

Where they were tested

The airbag system was tested at Plum Brook Station in Sandusky, Ohio, part of NASA's Glenn Research Center. It is the same facility Adam returns to years later, in chapter 7, for the MER airbag tests in the largest vacuum chamber in the world, using Pathfinder's spare airbags, which were brought back from the Smithsonian.

The facts

Second mission of the Discovery program. Launched on 4 December 1996, it landed on 4 July 1997 in Ares Vallis, an ancient flood plain chosen because it was relatively safe and rich in rocks to analyze.

The lander was later renamed the Carl Sagan Memorial Station.

Sojourner, the rover: 10.6 kg, the first vehicle to operate outside the Earth-Moon system. It traveled about 100 meters in 83 days.

The mandate

The team had been asked to send a lander and a rover to Mars in half the time and with one-fourteenth of the budget compared with previous standards.

JPL's mandate: three years of development and about 150 million dollars to reach the December 1996 launch. Final cost: about 265 million, including launch and operations.

For comparison: the Pathfinder lander cost less than the life-detection experiment alone that flew on Viking.

The results

The lander was designed for 30 days, the rover for 7. They worked for 83 days: the first for almost three times its planned lifetime, the second twelve times as long.

They sent back 2.3 billion bits of information: more than 16,500 images from the lander and 550 from the rover, more than 15 chemical analyses of rocks and soil, and extensive data on winds and other weather factors.

The three phases

• Entry: entering the Martian atmosphere at more than 21,000 km/h, with the heat shield exposed to temperatures of about 2,100 °C
• Descent: opening the supersonic parachute, jettisoning the heat shield, firing the retrorockets
• Landing: contact with the surface: airbags for Pathfinder and MER, Sky Crane for Curiosity

The aeroshell

The rigid shell that protects the spacecraft during entry is made of two halves:

• the heat shield, the front half, which absorbs the heat of entry and is jettisoned after peak heating
• the backshell, the rear half, which contains the parachute and, on Pathfinder, the retrorockets

The lander travels inside. The shell never lands: it is discarded piece by piece during the descent.

How they were built

Two twin spacecraft, Viking 1 and Viking 2, launched in 1975. Each was made up of an orbiter and a lander: once in Martian orbit they separated, the lander descended and the orbiter stayed to observe the planet from above.

The landers were stationary. They had no wheels and never moved from their landing spot: they took photographs and dug around themselves with a robotic arm.

The goals

Three: obtain high-resolution images of the surface, characterize the structure and composition of the atmosphere and the soil, and search for evidence of life on Mars.

The biology experiments

Each lander carried three experiments designed to look for signs of life. In the Labeled Release experiment, the arm collected Martian soil and mixed it with a drop of water containing nutrients and radioactive carbon atoms. The hypothesis: if there were microbes in the soil, they would metabolize the nutrients and release radioactive carbon dioxide or methane, measurable by a detector on board.

The result remained ambiguous. The experiments revealed unexpected and puzzling chemical activity in the Martian soil, but no definitive evidence of living microorganisms near the landing sites. It is still debated today.

Dates, sites and duration

• Viking 1: landed on 20 July 1976 in Chryse Planitia, “the golden plain,” in the northern equatorial region
• Viking 2: landed on 3 September 1976 in Utopia Planitia, a large impact basin north of the equator

The last data from Viking 2 reached Earth on 11 April 1980; Viking 1 ended its mission on 13 November 1982.

The cost

The project's costs have been estimated at up to 2 billion 1970s dollars. The figure most often used in comparisons is 935 million 1974 dollars, equivalent to about 3.5 billion 1997 dollars.