A “Bussard ramjet,” a type of starship popularized in science fiction, is technically feasible, but the engineering challenges are overwhelming, according to recent research.

A Bussard ramjet is a type of starship hypothesized in 1960, which became a staple of science fiction. The ramjet operates by sweeping interstellar hydrogen as fuel using a magnetic field.

Researchers now say the magnetic net would need to be at least 4,000 kilometers wide.