The biggest satellite in our solar system is Ganymede, a moon of Jupiter.

Here's why it's considered the biggest:

* Size: Ganymede is larger than the planet Mercury.

* Mass: It's more massive than all the other moons in the solar system combined.

It's important to note that while Ganymede is the biggest moon, it's not the largest celestial body in the solar system. That title belongs to the Sun, of course!