Traveling to Jupiter to find water

Opening words

How do astronauts live in space? What new discoveries have been made in the lunar and Mars exploration missions? Have we found evidence of dark matter? With the successful implementation of my country's aerospace projects, popularizing space knowledge is more likely to attract public attention, especially to stimulate the curiosity and thirst for knowledge of young people.

In order to better popularize scientific knowledge in the field of space, Popular Science Times and the China Space Science Society have jointly launched a space science column to popularize the latest research progress and achievements in space science. For new space missions, large-scale equipment and facilities related to the space field, future plans, etc., experts will be regularly invited to interpret the scientific background knowledge. At the same time, the column will also report on space science activities and invite relevant experts to share their experiences in space science. The column is hosted by Liu Yong, director of the Science Popularization Working Committee of the China Space Science Society and China's space science ambassador.

The European Space Agency launched the Jupiter Icy Moons Explorer (Juice) into orbit on an Ariane 5 rocket from French Guiana on April 14. The main goal of the probe is to explore the liquid water ocean hidden under the outer ice shells of Jupiter and its three largest icy moons - Europa, Ganymede and Callisto.

Is there an ocean beneath the crust of an icy satellite?

The Jupiter Icy Moons Explorer will travel in interplanetary space for eight years and reach Jupiter in 2031. After entering Jupiter's orbit, the probe will pass by three icy moons at a distance of 200 to 1,000 kilometers from the satellites, and deeply characterize the structural characteristics and potential habitability of Jupiter's satellites in unprecedented detail. The probe will fly over Europa twice, and conduct 12 and 21 flybys of Ganymede and Callisto respectively, and finally orbit Ganymede for at least nine months. This is the first time that humans have sent a spacecraft to orbit the satellite of another planet. Before this, there were only spacecraft orbiting the moon.

Previously, scientists have conducted several deep space exploration missions to Jupiter, such as Voyager 1 and Voyager 2, Galileo, Cassini, and Juno, which is currently carrying out missions in Jupiter's orbit.

People once thought that these icy moons were just dead lumps of ice and rock. But through the exploration of Jupiter's icy moons by several satellite probes, we know that these distant moons are actually planet-like and have a rich history of geological activity, which is of great significance to human exploration of possible extraterrestrial life in the universe. Before this, when searching for celestial bodies that may be habitable, we often focused on terrestrial planets such as Mars, believing that terrestrial planets have a history similar to that of the Earth and may have once had an atmosphere and oceans.

On Earth, which we are most familiar with, people have known that even in the darkest abyss, as long as there is a little heat from the hydrothermal vents, life can bloom tenaciously. The three largest icy satellites of Jupiter have revealed an extremely important clue to us: Under their icy shells, there may be oceans in the form of liquid water. So it is natural for us to ask: Is there life at the bottom of the oceans of distant icy satellites of Jupiter?

The mission of the Jupiter Icy Moons Explorer was born for this purpose. Based on the experience gained from life on Earth, we know that there are three basic elements for the emergence of life, namely liquid water, heat source and organic matter, and these three elements need to exist stably for a long enough time so that life can arise. Compared with terrestrial planets such as Mars, the light and heat provided by the sun to the very distant Jupiter and Saturn are extremely limited. Therefore, people have long believed that these giant planets and their satellites cannot have liquid water and life. However, through the observations of Galileo and Cassini, people realized that although the outermost layers of Jupiter's three icy satellites are covered by thick ice shells, the inner core of the satellite may still generate heat, and this heat source may produce and maintain liquid oceans under the ice shell, making them a habitat for existing or extinct life.

Using gravity to help get a closer look at icy satellites

Since the main goal of the Jupiter Icy Satellite Probe is to explore the three icy satellites at close range, in the early stage of the mission around Jupiter, the probe needs to fly past the satellites multiple times by constantly changing its orbit, which requires a large amount of fuel to propel and brake the probe.

The Jupiter Icy Moon Probe weighs about 6 tons, of which about 3 tons are fuel, while the total weight of the 10 most advanced scientific instruments on board is only about 100 kilograms. It is precisely because a large amount of fuel must be used to supply the complex orbit change after entering Jupiter's orbit that the probe needs to sacrifice the timeliness of the Earth to Jupiter - the spacecraft cannot carry enough fuel for rapid interplanetary navigation. The compromise solution adopted is: if there is no dry food for a long journey, then rely on the weather all the way - use the gravity of the celestial bodies in the solar system for boosting. Therefore, during the 8-year interplanetary voyage, the probe will return to the vicinity of the Earth in August 2024 and perform a gravity boost of the Earth-Moon system, then perform a gravity boost of Venus in August 2025, and finally return to Earth twice in September 2026 and January 2029, respectively, for two gravity boosts from the Earth. In other words, the probe will spend nearly 6 years circling the Sun with the Earth and Venus, patiently accelerating gradually with the help of the gravity of the Earth and Venus. During a final gravity assist from Earth in 2029, the probe will finally gain enough speed to quickly fly to Jupiter in two years.
Withstand the test of high-energy radiation "bullets and rain"

After an arduous journey into Jupiter's orbit, the Jupiter Icy Satellite Probe will face another important test - the harsh high-energy charged particle radiation environment in the space near Jupiter.

Jupiter's magnetic field is the strongest among all the planets in the solar system, so it can capture a large number of ultra-high-energy charged particles with speeds very close to the speed of light, forming a high-energy particle radiation environment that is much more severe than that of the Earth. If the particle radiation environment in near-Earth space is like the dust in Beijing in spring, then Jupiter's particle radiation is like a sandstorm in the Sahara.

As the main targets of the probe, Europa and Ganymede are closer to Jupiter, so the orbits of these two satellites are deeply buried in the "hay of bullets" of strong radiation. In order to survive in the harsh radiation environment, the probe can only make two flybys of Europa, which is closest to Jupiter, and 12 and 21 flybys of Ganymede and the farthest Callisto, respectively. During this process, the magnetometer and particle detectors carried by the probe will conduct a comprehensive measurement of Jupiter's magnetic field and high-energy particle environment near the icy satellites, and further analyze the impact of Jupiter's strong magnetic field and particle radiation environment on the surface of the icy satellites. This is crucial for exploring possible life hidden under the icy crust of the satellite, because high-energy particles can not only penetrate the spacecraft and instrument payloads and cause damage, but also pose a threat to life forms including humans. This is also one of the major problems that must be overcome for manned round trips to Mars.

Compared with its predecessors who explored Jupiter, the name of this probe (Juice) suddenly became sweet and light. But this is not because it is ignorant of the difficulties ahead. On the contrary, the Jupiter Icy Moon Probe, after seeing the dangers of the exploration, shed the heavy and bitter peel and offered Jupiter a glass of sweet water. It actually holds the same belief as its predecessors to explore the possibility of life in the extreme environment of Jupiter, because people firmly believe that even in the darkest abyss, as long as a little light and heat are given, life can bloom tenaciously.

Cheers, Juice! Cheers to the king of the solar system's planets, to the indomitable deep space explorers, and to life in the vast universe.

(The author is an associate researcher at the Institute of Geology and Geophysics, Chinese Academy of Sciences)