interplanetary

Interplanetary is an adjective describing space, travel, or phenomena that occur between planets within the same star system. It is used to refer to missions or trajectories that move from one planetary body to another, as well as to the space environment between planets. The term is often contrasted with interstellar, which refers to the space between stars.

Interplanetary space lies outside a planet's atmosphere but inside the heliosphere, the region influenced by the Sun. It contains the solar wind, energetic particles, dust, and occasional micrometeoroids. Conditions vary with distance from the Sun and with planetary alignments, and communication delays with Earth increase with distance.

Interplanetary missions rely on orbital mechanics to depart a planet, cruise through interplanetary space, and arrive at another planetary body. The most efficient two-phase trajectories between circular, coplanar orbits are known as Hohmann transfers; gravity assists from planets can further reduce propulsive requirements. Propulsion options include chemical rockets for launch, upper stages for transplanetary legs, and electric propulsion or other advanced concepts for long-duration cruise. Notable interplanetary missions include the Mariner and Voyager flybys, and the Mars orbiters and rovers that have traveled from Earth to Mars.

Human interplanetary travel presents significant challenges, including long mission durations, exposure to cosmic radiation, communication delays, and complex surface operations on distant worlds. Ongoing research covers propulsion improvements, power and life-support systems, radiation shielding, and planetary protection as part of planning for future crewed missions beyond Earth orbit. In science fiction and space policy, “interplanetary” is commonly used to describe practical or exploratory activity between planets within a solar system.