Another clip:

Filmmakers in the 1950s and 1960s... had to rely on propane tanks to mimic the exhaust coming off a rocket in space. When gas leaves the tank it “curls” as the atmosphere causes it to form vortices. In a vacuum gas does not behave in this manner, so these films were inaccurate in this respect. During production for Deep Impact, Chris Luchini explained this to the propmakers regarding the rocket exhaust as well as the comet ’ s outgassing. Liquid nitrogen helped them get around this problem for the rocket exhaust, but for safety reasons they were unable to utilize this for outgassing jets. When Luchini saw a rough cut of the film he noticed the curling of the gas off these jets. He mentioned this error to a special effects technician who used a CGI wipe effect to remove the curling days before the film’s premiere. Such a fix would have been impossible prior to the development of CGI technologies. Although CGI work can be expensive and difficult, it is often easier and cheaper to fix scientific inaccuracies during postproduction than it would be to struggle with them during production. In this case, they were able to rectify an error days before the release of the film.