Mercury’s axis has the smallest tilt of any of the Solar System’s planets (about 1⁄30 degree). Its orbital eccentricity is the largest of all known planets in the Solar System;[b] at perihelion, Mercury’s distance from the Sun is only about two-thirds (or 66%) of its distance at aphelion. Mercury’s surface appears heavily cratered and is similar in appearance to the Moon’s, indicating that it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day across the equatorial regions. The polar regions are constantly below 180 K (−93 °C; −136 °F). The planet has no known natural satellites.
The prime crew members selected for actual missions are here grouped by their NASA astronaut selection groups, and within each group in the order selected for flight. Two versions of the Apollo Command/Service Module (CSM) spacecraft were developed: Block I intended for preliminary low Earth orbit testing, and Block II which was designed for the lunar landing. The Block I crew position titles were Command Pilot, Senior Pilot (second seat), and Pilot (third seat). The corresponding Block II titles were: Commander, Command Module Pilot (second seat), and Lunar Module Pilot (third seat). The second seat pilot was given secondary responsibility for celestial navigation to keep the CSM’s guidance computer accurately calibrated with the spacecraft’s true position, and the third seat pilot served as a flight engineer, monitoring the health of the spacecraft systems.