After 7 years, a spent Falcon 9 rocket stage is on course to hit the Moon
SpaceX launched its first interplanetary mission almost seven years ago. After the second stage of the Falcon 9 rocket completed a long burn to reach a transfer orbit, NOAA’s Deep Space Climate Observatory began its journey to a LaGrange Sun-Earth point more d a million miles from Earth.
At that time, the Falcon 9 rocket’s second stage was high enough that it didn’t have enough fuel to return to Earth’s atmosphere. It also lacked the energy to escape the gravity of the Earth-Moon system, so it has been following a somewhat chaotic orbit since February 2015.
Now, according to skywatchers, the exhausted second-stage orbit is on course to intersect with the Moon. According to Bill Gray, who writes the Project Pluto software widely used to track near-Earth objects, asteroids, minor planets and comets, such an impact could occur in March.
Earlier this month, Gray appealed to amateur and professional astronomers to make additional observations of the scene, which appears to be tumbling through space. With this new data, Gray now believes Falcon 9’s upper stage will most likely impact the far side of the Moon near the equator on March 4. More information can be found here.
Some uncertainties remain. As the object tumbles, it is difficult to accurately predict the effects of sunlight “pushing” on the rocket stage and thus making slight changes to its orbit. “These unpredictable effects are very small,” writes Gray. But they will accumulate by March 4, and further observations are needed to refine the precise time and place of impact.
This information is important because it will allow satellites currently orbiting the Moon, including NASA’s Lunar Reconnaissance Orbiter and India’s Chandrayaan-2 spacecraft, to collect observations of the impact crater. With the LCROSS mission, NASA deliberately impacted a spent rocket upper stage on the Moon in 2009 for this purpose. Although scientists are more eager to understand the presence of ice at the lunar poles, being able to observe the underground material ejected by the Falcon 9 rocket strike could still provide valuable data.
The dry mass of Falcon 9’s second stage is about 4 metric tons and it is expected to impact the Moon at a speed of about 2.58 km/s.
It is likely that this will be the first time that a piece of space hardware has unintentionally hit the Moon. Typically, on interplanetary missions, the upper stage of a rocket is sent into a heliocentric orbit, keeping it away from Earth and its Moon.
For spacecraft launches intended to orbit Earth, the best practice is to reserve enough fuel in a rocket’s upper stage to return it to Earth’s atmosphere, where it will burn up. That’s what SpaceX and most Western rocket companies usually do to help control debris in low Earth orbit. The Moon, of course, has no atmosphere for the scene to burn.