Ask Merlin your own question or browse our FAQs for more answers.

What is the Van Allen radiation belt? Is it possible for a manned space vehicle to go through it with no harm to the occupants?

As Earth turns on its axis, its metallic inner core spins at a different rate from the partially molten rock around it. This creates a “dynamo effect,” which produces a magnetic field around our planet. The field traps electrically charged particles from the Sun, forming two doughnut-shaped radiation belts.

Explorer 1 (left), the first American satellite, discovered the radiation belts in 1958. They were named for physicist James Van Allen, who devised Explorer’s radiation detector.

Prolonged exposure could be dangerous. The International Space Station and most space shuttle missions stay below the belts. Apollo astronauts flew through the belts in just a couple of hours, so they were not harmed. Future travelers to Mars shouldn’t have any problems, either.

What is the biggest moon in the solar system?

That is a tricky question, which astronomers didn’t answer correctly until fairly recently.

For a while, they thought the biggest moon was Titan, which orbits Saturn. But a deep atmosphere topped by orange smog surrounds Titan, so scientists couldn’t see its surface. Not until spacecraft began to visit Titan at close range could they measure its true size and find that it’s only the runner-up. The prize for biggest moon goes to (the envelope, please) ... Ganymede, a moon of Jupiter, at a diameter of 3,270 miles (5,262 km) — about 70 miles larger than Titan.

For more information about the largest moons in the solar system, see our Moons Comparison Table.

Why did Comet Shoemaker-Levy 9 make such large impact marks on Jupiter?

When the fragments of Shoemaker-Levy 9 slammed into Jupiter in 1994, the energy released in the collisions far exceeded that of any nuclear weapon on Earth.

The visible scars on Jupiter were especially large because the planet is a big, rapidly spinning ball of gas. Shock waves from the initial explosions produced large, dark circles and rings, like the one at lower left in this Hubble Space Telescope picture. Upper-level winds quickly stretched these structures, producing dark splotches thousands of miles long.

On Earth, such an impact would blast so much material into the atmosphere that it would block out the Sun, turning our planet’s surface dark and cold and killing off much of the plant and animal life. Scientists believe that the effects of an asteroid impact 65 million years ago may have killed the dinosaurs.