The Jovian system has long attracted the interest of human exploration. However, Jupiter and its four Galilean moons form a unique and complex multi-body dynamical environment that greatly challenges trajectory design and optimization.

Moreover, the extremely strong radiation environment of Jupiter and the low available fuel of spacecraft further increase the difficulty of trajectory design. In order to satisfy the requirements of diverse missions of the Jovian system exploration, develop new mission concepts, and obtain higher merit with lower cost, a variety of theories and methodologies of trajectory design and optimization were proposed or developed in the past two decades.

There is a lack of comprehensive review of these methodologies, which is unfavorable for further developing new design techniques and proposing new mission schemes.

In a review article recently published in Space: Science & Technology, scholars from Nanjing University of Aeronautics and Astronautics and Rutgers, The State University of New Jersey provide a systematic summarization of the past and state-of-art methodologies for four main exploration phases, including Jupiter capture, the tour of the Galilean moons, Jupiter global mapping, and orbiting around and landing on a target moon.

French novelist Jules Verne delighted 19th-century readers with the tantalizing notion that a journey to the center of the Earth was actually plausible.

Since then, scientists have long acknowledged that Verne's literary journey was only science fiction. The extreme temperatures of the Earth's interior—around 10,000 degrees Fahrenheit (5,537 Celsius) at the core—and the accompanying crushing pressure, which is millions of times more than at the surface, prevent people from venturing down very far.

Still, there are a few things known about the Earth's interior. For example, geophysicists discovered that the core consists of a solid sphere of iron and nickel that comprises 20% of the Earth's radius, surrounded by a shell of molten iron and nickel that spans an additional 15% of Earth's radius.

That, and the rest of our knowledge about our world's interior, was learned indirectly—either by studying Earth's magnetic field or the way earthquake waves bounce off different layers below the Earth's surface.

But indirect discovery has its limitations. How can scientists find out more about our planet's deep interior?