Bussard Ramjets: Possible but Impracticable

The Bussard ramjet won’t work as well as you think it could. A new study does the math on John Fishback’s contribution to the ramjet—gathering protons from the interstellar medium for the ramjet’s fusion drive via a magnetic scoop—and concludes that while it’s physically possible, there are substantial constraints: the cut-off speed is lower than expected, limiting the effects of time dilation, and the magnetic field would have to be something like 4,000 km wide and 150 million km long in order to work. In other words: for certain very preposterous values of physically possible. [Universe Today]