Speed is always measured relative to a frame of reference. In space, like in any other context, speed is determined by comparing the motion of an object to a chosen reference point or another object.
In the context of space exploration, scientists and engineers use various reference frames to measure the speed and motion of spacecraft and celestial bodies. Some common reference frames include:
Earth-centered frames: Earth-centered reference frames are commonly used for spacecraft orbiting the Earth or missions involving the Earth. The International Celestial Reference Frame (ICRF) is an example of a reference frame that uses distant quasars to define its coordinates.
Heliocentric frame: This reference frame centers on the Sun and is often used for objects within the solar system, such as planets, asteroids, and comets.
Barycentric frame: The barycenter is the center of mass of a system of celestial bodies. The barycentric reference frame centers on this point and is used for interplanetary or interstellar missions to measure velocities with respect to the entire solar system.
Galactic frame: This frame centers on the center of the Milky Way galaxy and is used for studying objects and phenomena on a galactic scale.
Cosmic Microwave Background (CMB) frame: The CMB frame is a reference frame that takes into account the motion of the solar system relative to the afterglow of the Big Bang. It is used for cosmological measurements.
Each of these reference frames provides a unique perspective for measuring speed and motion. By choosing an appropriate reference frame, scientists can accurately describe the movement of objects in space. Speed measurements in space are essential for navigation, rendezvous and docking procedures, planning spacecraft trajectories, and understanding the dynamics of celestial bodies and their interactions.