Thursday

Raven spreads a printed table across the picnic table. "NASA's Planetary Fact Sheet," she says. "Every planet and the Moon."

Rocket reads the gravity row. "Earth 9.8. Moon 1.6. Jupiter 23.1!"

"What do you notice about Jupiter's mass?" Raven asks.

"Huge," Rocket says. "More mass, more gravity. Like NASA said."

Nova hovers over the Saturn column. "Then look here. Saturn has far more mass than Earth. What is its gravity?"

Rocket squints. "Nine point zero. Less than Earth? That makes no sense."

Raven taps the diameter row. "Unless distance matters too. Saturn is very, very wide."

NASA's Planetary Fact Sheet lists facts for every planet and the Moon. Here are four rows for nine worlds.

Mass is given in units of a million million million million kilograms, so Earth's 5.97 is a very large number.

Surface gravity tells how fast a dropped object speeds up at that world's surface, in metres per second each second.

The last column is from NASA's ratio table. It compares each world's surface gravity with Earth's.

Jupiter has the most mass in the table and the strongest gravity. More mass, more gravity, just as NASA Space Place says.

Saturn is the puzzle. Its mass is 95 times Earth's, yet its surface gravity is a little less than Earth's.

NASA Space Place gives the missing piece. Gravity also gets weaker with distance.

Saturn is so wide that its surface is very far from its center. That distance weakens the pull you would feel standing there.

NASA Glenn says the same about Earth. Gravity decreases with the square of the distance from Earth's center.

So two things set a world's surface gravity: how much mass it has, and how far its surface is from its center.

Excellent data work. Tomorrow you will see gravity in every game, and review the week.