Here's why:

* Orbital Velocity and Distance: A planet's orbital velocity is determined by its distance from the sun. The closer a planet is to the sun, the stronger the gravitational pull, and the faster it needs to travel to maintain its orbit.

* Mercury's Proximity: Mercury is the closest planet to the sun.

* Mercury's Speed: Mercury orbits the sun at an average speed of 47.87 kilometers per second (29.78 miles per second).

Let me know if you have any other space questions!