Comparing per departure would give an enormous advantage to cars. The average airliner trip is hundreds or thousands of miles. The typical car trip is single or double digits. Cars thus make many more departures for the same distance traveled, so their denominator would me much larger.
The standard metric is fatalities per passenger mile. In the US, cars account for roughly 10x the number of passenger miles as airlines, so you’d expect 10x the fatalities if they were equally safe. In recent years, cars average around 40,000 fatalities per year, and airliners average around zero.
I wonder if there have been any numbers published for space. Looks like the ISS does the equivalent distance of (only) 134 crossings of North America in one day. Or, stated another way, 134 people crossing it once. Clearly airlines carry way more people than that, and the fatalities are lost in the noise. So it would seem that yeah you're right, airlines are still beating space for safety by a big margin. To make space win you'd have to cheat and count something absurd like the miles our solar system travels as it orbits the galactic center... but then to be fair you'd have to credit the airlines with the same distance... so nevermind.
The standard metric is fatalities per passenger mile. In the US, cars account for roughly 10x the number of passenger miles as airlines, so you’d expect 10x the fatalities if they were equally safe. In recent years, cars average around 40,000 fatalities per year, and airliners average around zero.