Artist's Concept of the Atlas V-401 Rocket

Interior Exploration using Seismic Investigations, Geodesy and Heat Transport, or InSight, is scheduled to launch from Vandenberg Air Force Base on California's Pacific coast between May 5 and June 8, 2018. The lander will launch to Mars aboard an Atlas V-401 launch vehicle, one of the biggest rockets available for interplanetary flight.

It stands 188 feet (57.3 meters) tall, or about as tall as a 19-story building. Fully stacked, with the spacecraft, the Atlas V-401 weighs about 730,000 pounds (333,000 kilograms). That's about 14 big rigs, fully loaded with cargo!

The three numbers in the 401 designation signify:

4: a payload fairing -- or nose cone -- that is about 13 feet (4 meters) in diameter

0: solid-rocket boosters supplementing the main booster

1: the upper stage, which has one engine

https://photojournal.jpl.nasa.gov/catalog/PIA22231