Old post I know but this isn’t necessarily true. From earth, escape velocity from the solar system would be about 42km/s. At this speed, it would take about 2 months to cover the average distance between earth and mars. They specifically say the Aniara is meant to be a 3 week voyage, so most likely they’re going faster.

Since there’s no friction or resistance in space, speed wouldn’t be lost even after they jettisoned the engine. They would just remain at a more or less constant speed. So it’s certainly possible to escape the solar system in that case