Panel #1 (Title)
• This is Outer Space Pals #5. In this installment, we talk a bit about how the Moon became "locked" with Earth, and now shows us only one of its sides. Astronomers call this process "tidal locking."

Panel #2
• Right after the Moon formed, it orbited only about 10 percent as far away as it is now.

Panel #3
• Both characters are right. One of the properties of the gravitational pull of one object on any other is that the strength of the pull decreases as the distance between the two objects increases. In fact, mathematicians call this an "inverse-square" law: If you double the distance between two bodies, the gravitational pull becomes one-fourth as strong. Here's a table to help you see how that works:

Panel #4
• This panel shows that meteors (often called "shooting stars") can happen as tiny bits of rock in space get so close to Earth that our planet's gravity captures them.

Panel #5
• When two bodies, like Earth and the Moon, become locked, two things happen: 1) The larger body's spin (rotation) slows down; and 2) The smaller body starts to move farther away from its companion. We know how much farther the Moon moves from Earth to an astounding degree of precision. Each year, our satellite's distance increases by 38.247 millimeters, plus or minus 0.004 millimeter. How do we know this so precisely? We bounce laser beams off mirrors left by the Apollo astronauts during their visits to the Moon from 1969 to 1972. By carefully measuring the travel time of the light, astronomers can calculate the Moon's distance to an accuracy better than a thousandth of a millimeter.

Panel #6
• Indeed the force that caused the Moon to be locked to Earth was strongest when the gravitational attraction between them was the greatest. That happened when the Moon was closest, or right after it formed. So, it wouldn't have taken long for the Moon to become locked. Well, not long in astronomical terms. That probably happened between 10 million and 15 million years after it formed.

Panel #7
• Most of the moons in our solar system are locked to their planets. And until 1965, scientists also thought Mercury was tidally locked in its orbit around the Sun. In that year, however, radar observations showed that the innermost planet rotates three times for each two orbits of the Sun. We call this a spin-orbit resonance, where the ratio of the body's spin to its orbit is some simple fraction. For Mercury, it's 3 to 2, written 3:2. But other ratios, such as 2:1 or 5:2, also happen.

Panel #9
• Hmm. So, when we see the dark part of the Moon, that's not its farside? That leads to the question, "What causes the Moon's face to change?" We'll find out in Outer Space Pals #6.