August 18, 2019 : Will We Ever Walk On The Surface Of Titan ?

On July 21, 1969, Neil Armstrong became the first individual to walk on the Moon. Between 1969 and 1972, twelve astronauts have evolved on the Moon. On July 21, 2019, we celebrated the 50th anniversary of the historical event involving Neil Armstrong, Buzz Aldrin and Michael Collins. Recently, a Chinese probe known as Chang'e 4 landed, for the first time in history, on the surface of the far side or the  Dark Side  of the Moon. That feat appears clearly inspiring for researchers, planetologists or the general public who want to learn more about the geology of the Moon. A growing interest in the exploration of the Moon has been observed in previous weeks or months. Today, several countries, governments or companies are developing projects to explore or even colonize the Moon. For instance, a space station around the Moon or a laboratory on the surface of our natural satellite may take shape in the near future.

The Moon is the only world beyond the Earth where we have walked on the surface. The environment of Venus is too hostile with extremely high environmental temperatures and a particularly high pressure on the surface. The environmental temperature of Mercury which is devoid of any significant atmosphere is also extremely warm on the Sun-facing side and the highly cratered world is farther from the Earth than Venus. Currently, the  Red Planet  Mars is the only planet where we intend to send astronauts to walk on the surface. NASA has developed plans to send astronauts to the surface of the Red Planet in the coming years or decades. Several worlds beyond Mars draw our attention. That's the case for the Dwarf Planet Ceres, for Europa or Io which are moons of the Gas Giant Jupiter, for Enceladus or Titan which are moons of the Ringed Planet Saturn, for Triton which is the largest moon of Neptune or for Pluto which is the largest Dwarf Planet. Can we send spaceships with astronauts beyond the Moon or Mars ?

Let's reflect a little bit on travel time ! It took more than one year and less than two years for the probes Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 to reach Jupiter. It took approximately 6 years for the Galileo spacecraft to reach Jupiter and to enter into orbit around the largest Gas Giant in the Solar System. It took more than 3 years for the Cassini-Huygens probe to reach Jupiter and almost 7 years to reach Saturn and enter into orbit around the Ringed Planet. It took a little bit more than one year for the New Horizons spacecraft to reach Jupiter and more than 9 years to reach the Pluto/Charon System. Thus, with conventional propulsion technologies, it takes a significant amount of time to reach other planets such as Venus, Mars or Jupiter. If we ever walk on the surface of one of the numerous moons of Saturn, we will naturally have to enter into orbit around the Gas Giant. The spaceship will have to slow down in order to be captured by the gravitational field of Saturn and its moons. That process implies a relatively significant amount of fuel and a longer travel from the Earth to the Saturn System compared to the configuration of a typical flyby.

We have walked on the surface of our natural satellite during six missions between 1969 and 1972 but the trip was very fast. It only took a few days to reach the Moon and to land on the Moon. But a trip beyond the Moon, toward the Outer Solar System, would last an incredible amount of time. And humans would have to evolve without any gravity, in the void of space for several years or for more than a decade if they embark on a conventional rocket or shuttle. Would they be in a position to withstand such a journey ? They would have to be particularly strong mentally. But what would happen to their bones, their muscles or their physiology ? They would receive relatively significant amounts of noxious solar radiations during the interplanetary journey. During the trip in the void of space, the risk of cancer related to noxious solar radiations such as X-rays or gamma rays can significantly increase. Researchers fear the risk of a bombardment of solar wind related to a solar flare or a Coronal Mass Ejection (CME) during the interplanetary journey.

Today, with conventional propulsion technologies, if we plan to send humans to the system of Jupiter or to the system of Saturn, we have to envisage extremely long journeys because we will not perform a simple flyby of Jupiter or a simple flyby of Saturn. The hypothetical spacecraft or shuttle will have to enter into orbit around the system of Jupiter and its moons or around the system of Saturn and its moons. The orbital insertion implies a lot of fuel to slow down the spacecraft or to correctly orientate it so that the spacecraft is captured by the gravitational field of the Gas Giant. The Cassini-Huygens spacecraft as well as the Juno spacecraft had been captured by the gravitational field of the Gas Giant. The Cassini spacecraft obtained a huge amount of scientific data of Saturn and its numerous moons from the Saturn-Orbit Insertion in 2004 to the crash into Saturn performed in 2017. The Juno spacecraft, in orbit around the largest planet in the Solar System, is still exploring Jupiter today. The trip for flyby missions to Gas Giants with conventional propulsion systems appears clearly faster than the trip for orbital missions. Obviously, the hypothetical explorers will have to perform an orbital insertion of the Gas Giant prior to the landing attempt on any moon.

The explorers who leave the Earth to try to explore one of the moons of Jupiter or one of the moons of Saturn would have to keep in mind that the mission may last more than a decade. One can easily imagine that a trip to Europa or Ganymede would probably last about 10 years and that a trip to Enceladus, Iapetus or Titan would probably last about 14 years. First, we must reach the orbit of the Gas Giant and it may imply several gravity assists of Terrestrial planets like Venus, the Earth or Mars. That configuration is not a straight line from the Earth to the Gas Giant ! Then, we must be in a position to leave the orbit of the Gas Giant in order to return home. A mission with a team of explorers could be envisaged with several rockets, with an orbiter and with a shuttle. We had walked on the moon thanks to a giant rocket, an orbiter, a lander and a small rocket to leave the Moon. Therefore, we have demonstrated that evolving or walking on an airless world is possible in a relatively safe way. However, a mission with astronauts sent to the surface of one of Jupiter's moons or Saturn's moons implies much more fuel than the manned missions to the Moon.

In fact, an enormous amount of fuel is needed to reach the Gas Giant, to enter into orbit around the Gas Giant and to leave the Gas Giant. Saturn V, the famous rocket that sent humans to the Moon, was 363 feet high or almost 111 meters high, had a mass at launch of 6,540,000 pounds or 2,970,000 kilograms and was mostly composed of fuel. About 94 percent of the gross mass of the rocket represented fuel. The first challenge is to leave the Earth and to reach the escape velocity for the trip to the Outer Solar System. If the explorers try to land on Europa, Ganymede, Callisto, Enceladus or Rhea, they will have to resort to retrorockets since those worlds are devoid of any atmosphere making any parachute useless. On the other hand, they may resort to parachutes if they try to land on Titan because Saturn's largest moon is covered with a particularly dense atmosphere. The density of Titan's atmosphere at the level of the ground is much higher than that of the Earth's atmosphere at sea level. The Huygens probe which had performed a soft landing on Titan via a system of parachutes, on January 14, 2005, recorded an atmospheric pressure on the surface of 1467 hPa which is much higher than the mean atmospheric pressure encountered on the surface of the Earth at sea level.

The shuttle or the rocket which may land one day onto the surface of the Hazy Moon must have enough fuel to leave the giant moon of Saturn and to reach the orbiter like the small rocket used by the astronauts of the Apollo program to leave the Moon and to reach the orbiter or the orbital rocket. The astronauts of Titan will have to face air drag during the launch or the take off from the surface of Titan. Therefore, that's a different configuration from that of the Moon since the Moon is devoid of any atmosphere or air. However, the gravity on the surface of Titan is lower than that of the Moon. The gravity on the Moon is approximately 6 times lower than that of the Earth and the gravity on Titan is approximately 7 times lower than that of our planet. A lower gravity implies a lower amount of fuel needed to reach the orbiter or the orbital rocket. The explorers of Titan may use natural fuels on the surface or beneath the surface of Titan since the environment of the giant moon is rich in organics, hydrocarbons, methane or ethane. Artificial light on Titan will be necessary because the moon is about ten times farther from the Sun than the Earth and the opaque atmosphere limits the amount of solar light reaching the surface. The hypothetical explorers will have to take seasonal factors into account prior to the landing process. If they go to Titan during the Summer season in the northern hemisphere, they may choose to explore the area of the lakes and seas found in the high latitudes of the northern hemisphere. On the other hand, if they go to Titan during the Summer season in the southern hemisphere, they may choose to explore Ontario Lacus found in the high latitudes of the southern hemisphere. A manned mission to Titan or even Europa is clearly a giant challenge.

The image above represents a simulated view of a pool of hydrocarbons on Saturn's largest moon Titan. The disk and the rings of the Gas Giant Saturn can be discerned in the opaque atmosphere of Titan. In reality, Saturn may not be visible in the sky in the visible spectrum due to the thick haze of organics or hydrocarbons present in Titan's atmosphere. The apparent diameter of Saturn from the surface of Titan represents about 11 times the apparent diameter of the Moon from the surface of the Earth. The surface of Titan may be rich in tholins and the seas, the lakes or the rivers found in the high latitudes of the northern hemisphere of the giant moon may be dominated by methane. Image credit: Marc Lafferre, 2019.

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