Idea
Efficiency and force are different advantages
An engine can eject a small amount of propellant very quickly and gain a useful change in velocity over time. Ion propulsion uses that tradeoff. Its thrust is gentle compared with a chemical launch engine, but high exhaust speed lets it obtain more change in motion from a given propellant mass.
Dawn's mission used long periods of thrusting to travel between and maneuver around distant bodies. Instead of demanding all the needed change in a brief burn, the trajectory accommodated an engine that worked gradually. The mission design and the propulsion system were parts of the same choice.
A tiny push becomes a large change when it can continue.
Judge an engine against the route and timescale it must serve.
Where it breaks down
Low thrust is unsuitable for lifting a heavy vehicle against Earth's gravity. Electrical power also limits performance, and solar-powered missions receive less sunlight farther from the Sun. Efficient propellant use does not remove the costs of time and power.
How it connects
A long trajectory can exploit efficient acceleration that a short deadline cannot.
This idea appeared in A small push with time to work, the Involves connection for September 27, 2026, which asked: How can a gentle engine make a journey a powerful engine cannot easily afford?
Check yourself
Why can low thrust still produce a large velocity change?
It can act for a long time. Exactly. Velocity change accumulates through sustained acceleration.
What the sources establish
Dawn accumulated trajectory changes through sustained thrust, subject to time and electrical power constraints.
Sources
Dawn FAQ (NASA Science), Ion propulsion, low thrust, propellant economy, and electrical power
Dawn at Ceres Press Kit (NASA Jet Propulsion Laboratory), Ion propulsion system and Vesta-to-Ceres mission profile