A discarded SpaceX Falcon 9 rocket booster, launched in 2015, is on an unavoidable collision course with the Moon, slated for impact on March 4th. While unintended, this cosmic fender-bender is being hailed by astronomers as a potentially invaluable opportunity to peer beneath the lunar surface.
Astronomers have been tracking the roughly four-tonne booster, initially used to launch a deep space climate observatory, for weeks. It is expected to strike the far side of the Moon at a velocity of around 2.58 kilometres per second, or just over 9,200 kilometres per hour, creating a new crater on the ancient lunar landscape.
A Fortuitous Scientific Accident
Unlike controlled impacts, such as those made by NASA's LCROSS mission in 2009 to search for water ice, this upcoming crash is entirely accidental. However, scientists are seizing the moment. "For 50 years, the Moon has been bombarded by human-made debris, but this is the first time we’ve had good coverage to be able to observe it from Earth," explained Bill Gray, the astronomer who first identified the booster's trajectory, to BBC World.
Observatories on Earth and spacecraft currently orbiting the Moon will be eagerly awaiting the impact. The sudden release of energy will generate seismic waves that could reveal details about the Moon's interior structure, including the composition of its mantle and core. Scientists are particularly interested in analysing the ejecta plume – the debris kicked up by the impact – for clues about lunar geology and the presence of any volatile materials beneath the surface.
Tracing the Trajectory
The Falcon 9's journey began on February 11, 2015, when it launched NASA's Deep Space Climate Observatory (DSCOVR) into an orbit around the L1 Lagrange point, a gravitationally stable position between the Earth and the Sun. After completing its primary mission, the booster was left in a chaotic, high-energy orbit, buffeted by the gravitational pulls of the Earth, Moon, and Sun.
Over the years, the booster's trajectory became increasingly complex and unpredictable. It wasn't until late last year that Mr. Gray and other space debris trackers realised its path was converging with the Moon. Initial predictions of the impact point were refined over weeks, with the latest estimates placing it near the Hertzsprung crater on the far side of the Moon. This location, away from the Earth-facing side, means ground-based telescopes won't be able to directly observe the impact itself, but its effects will be detectable.
Lessons from Previous Lunar Impacts
The Moon's surface is a testament to billions of years of cosmic bombardment, a natural laboratory for studying impact events. However, controlled impacts by human-made objects have provided invaluable data. The Apollo missions, for instance, deliberately crashed upper stages of their rockets into the Moon, using seismometers left behind by astronauts to study the resulting tremors and map the Moon's internal structure.
This unplanned SpaceX impact will offer a fresh perspective. Researchers hope the unique velocity and angle of impact, combined with advanced observational capabilities unavailable during the Apollo era, will yield new insights. The data gathered could also help scientists better understand the risks posed by space debris, a growing concern for future space missions.
The Commercial Space Race and Orbital Debris
The impending collision underscores a broader issue: the increasing volume of space debris generated by a burgeoning commercial space industry. While this particular booster has been in space for some time, the rapid expansion of satellite constellations and lunar missions means more objects are entering complex orbital environments.
While this specific event is unlikely to pose a direct threat to other spacecraft or the Moon itself, it serves as a stark reminder of the long-term consequences of leaving hardware in space. The Australian Space Agency, like its international counterparts, is actively involved in discussions and initiatives aimed at developing sustainable practices for space operations and mitigating the risks of orbital debris. The data from this accidental lunar impact could inadvertently contribute to these ongoing efforts, providing real-world insights into the behaviour of defunct space hardware over extended periods.