Saturday, July 10, 2010

Visiting Europa

Jupiter has over twice the mass of all the other planets in our Solar System combined and it is the archetype of large gas giant planets, especially so for the countless giant planets now known to orbit other stars. Orbiting Jupiter are 4 large moons named Io, Europa, Ganymede and Callisto. Additionally, Jupiter also has a few dozen small irregular satellites and a ring system in orbit around it. The 4 large moons of Jupiter, also known as the Galilean satellites, are particularly fascinating. Io is by far the most volcanically active world in the Solar System while Europa has a huge global ocean of water hidden beneath just a thin layer of ice. Ganymede and Callisto are large moons that are believed to also harbor internal oceans. Ganymede is the largest moon in the Solar System and it is even larger than the planet Mercury.

A paper by K. Clark, et al. (2010) entitled “Return to Europa: Overview of the Jupiter Europa Orbiter Mission” describes a mission to explore Jupiter’s ocean moon Europa. The Europa Jupiter System Mission (EJSM) is a proposed mission by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) to explore Jupiter and its moons. The Jupiter Europa Orbiter (JEO) will make up the NASA-led portion of the EJSM and it is a satellite that will be placed into orbit around Europa.

Europa is a particularly interesting and intriguing moon of Jupiter because its subsurface ocean is in direct contact with the rocky interior of Europa, enabling the water to be infused with minerals and energy that is necessary for life through features such as hydrothermal vents. Thus, conditions at the bottom of Europa’s ocean could be very similar to the Earth’s ocean floor. Europa’s subsurface ocean is estimated to contain far more water than all the oceans on the Earth combined and the aquatic environment of Europa’s ocean is likely to be within the constraints of known life on Earth.

The Jupiter Europa Orbiter (JEO) is expected to be launched onboard an Atlas V 551 launch vehicle in the first quarter of 2020 and it will use a Venus-Earth-Earth gravity assist interplanetary trajectory to get to Jupiter. It will take approximately 6 years for JEO to get to Jupiter where it is expected to arrive at the end of 2025 or the beginning of 2026. Upon reaching Jupiter, JEO will perform a series of gravity assist with the moons Io, Europa, Ganymede and Callisto over a 30 month period to reduce its orbital energy with respect to Europa. This mission phase provides a unique opportunity to explore the Jovian system as it also includes several flybys of each of the 4 large moons of Jupiter.

In the middle of 2028, the Europa Orbit Insertion (EOI) will occur whereby a main engine burn will decelerate JEO into a low circular orbit around Europa. JEO will then begin its nominal Europa science campaign which is expected to last for 9 months and a mission extension beyond 9 months is very likely because the compounding effect of applying worst-case assumptions at every level in the design of the spacecraft tends to severely underestimate the mission lifetime.

JEO will be powered by Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs) where the radiogenic heat from the decay of Plutonium-238 will be used to power the onboard systems. The huge amount of radiation that JEO will be subjected to throughout its mission poses a unique technical challenge. The 4 main sources of radiation are solar radiation, galactic cosmic rays, high energy particles trapped within the Jovian magnetosphere and neutrons and gamma ray photons from the onboard MMRTG nuclear power source. The original paper briefly addresses the radiation risks and various mitigation methods for the JEO mission.

The JEO mission science objectives, as defined by the international EJSM Science Definition Team are:
1. Europa’s Ocean: Characterize the extent of the ocean and its relation to the deeper interior.
2. Europa’s Ice Shell: Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface-ice-ocean exchange.
3. Europa’s Chemistry: Determine global surface compositions and chemistry, especially as related to habitability.
4. Europa’s Geology: Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in situ exploration.
5. Jupiter System: Understand Europa in the context of the Jupiter system.

If you want to read more about the Europa Jupiter System Mission (EJSM), you can download the final report from