Fact-Checking a Space Film: Moons, Dwarf Planets and Distances

The Science Check for "Ice Moon Run"

1The film club at Brookhaven Middle School is shooting a short science-fiction film, "Ice Moon Run," for the district's spring festival. The club has one rule: the story can be made up, but the science has to hold up. Club president Zuri Alcántara handed each member a page of the script to fact-check against a data table that their science teacher, Mr. Dmitri Osei, put together from spacecraft measurements. In the table, each world's mass is given compared with the mass of our Moon, so a mass of 2.0 means twice the Moon's mass.

2In the opening scene, the crew's ship leaves Earth on a Friday night and reaches Jupiter "by Monday morning." The script gives the ship a top speed of 20 km/s and says Jupiter is 630 million km from Earth on the day they leave. On the way, the ship crosses the asteroid belt, and the pilot dodges rocks "close enough to touch" every few seconds. Mr. Osei's notes say the belt holds millions of rocky objects, but they are spread through a huge ring of space, and neighboring large asteroids are often about a million kilometers apart. All of them together have only about 3% of the Moon's mass.

3At Jupiter, the crew lands on Ganymede. One character calls it "a little moon, about half the size of our Moon." Later the crew flies on to Saturn's moon Titan and makes a live video call home, talking back and forth with no delay. Radio signals travel at the speed of light, and light takes about 8.3 minutes to cross 1 AU, Earth's average distance from the Sun. In the final scene, the crew reaches Pluto, which the narrator calls "the smallest world in the solar system."

4Mr. Osei explained how the worlds are grouped. A planet orbits the Sun and has cleared most other objects from its path. A dwarf planet, such as Ceres or Pluto, also orbits the Sun but shares its path with many other small objects. A moon orbits a planet. Zuri also noticed something she did not expect: some of the moons in the table are wider than a planet. She asked the club to check whether "wider" always means "more mass" before anyone wrote that line into the script.