used. Beam-powered propulsion is another method of propulsion without reaction mass. Beamed propulsion includes sails pushed by laser, microwave, or particle beams. Advanced propulsion technology Advanced, and in some cases theoretical, propulsion technologies may use chemical or nonchemical physics to produce thrust, but are generally considered to be of lower technical maturity with challenges that have not been overcome. For both human and robotic exploration, traversing the solar system is a struggle against time and distance. The most distant planets are 4.5–6 billion kilometers from the Sun and to reach them in any reasonable time requires much more capable propulsion systems than conventional chemical rockets. Rapid inner solar system missions with flexible launch dates are difficult, requiring propulsion systems that are beyond today's current state of the art. The logistics, and therefore the total system mass required to support sustained human exploration beyond Earth to destinations such as the Moon, Mars or Near Earth Objects, are daunting unless more efficient in-space propulsion technologies are developed and fielded. A variety of hypothetical propulsion techniques have been considered that require a deeper understanding of the properties of space, particularly inertial frames and the vacuum state. To date, such methods are highly speculative and include: Black hole starship Differential sail Gravitational shielding Field propulsion Diametric drive Disjunction drive Pitch drive Bias drive Photon rocket Quantum vacuum thruster Nano electrokinetic thruster Reactionless drive Abraham—Minkowski drive Alcubierre drive Dean drive EmDrive Heim theory Woodward effect Thornson Inertial Engine (TIE) Gyroscopic Inertial Thruster (GIT) A NASA assessment of its Breakthrough Propulsion Physics Program divides such proposals into those that are non-viable for propulsion purposes, those that are of uncertain potential, and those that are not impossible according to current theories. Table of methods Below is a summary of some of the more popular, proven technologies, followed by increasingly speculative methods. Four numbers are shown. The first is the effective exhaust velocity: the equivalent speed that the propellant leaves the vehicle. This is not necessarily the most important characteristic of the propulsion method; thrust and power consumption and other factors can be. However: if the delta-v is much more than the exhaust velocity, then exorbitant amounts of fuel are necessary (see the section on calculations, above) if it is much more than the delta-v, then, proportionally more energy is needed; if the power is limited, as with solar energy, this means that the journey takes a proportionally