WHAT'S NEXT
• Spacecraft A will arrive at the moon on New Year's Eve and spacecraft B will arrive on New Year's Day.
• Beginning in March, once the spacecraft are orbiting just 34 miles above the moon's surface, scientists will monitor the slight variations in distance between the two to map the moon's entire gravitational field. The measurements will continue through May.
• NASA's grand finale this year will be the launch of the biggest Mars rover ever the day after Thanksgiving.
