The celestial mechanics of space junk are set to deliver a dramatic, if unobserved, spectacle as a defunct segment of a SpaceX Falcon 9 rocket is on a collision course with the Moon. Scientists predict the booster’s upper stage will slam into the lunar surface at an astonishing speed exceeding 8,000 kilometres per hour (5,000 miles per hour) within the coming weeks.
First launched in 2015, the Falcon 9’s second stage, a roughly 15-metre-long cylinder, completed its primary mission of deploying NASA’s Deep Space Climate Observatory (DSCOVR) satellite. However, with insufficient fuel to return to Earth’s atmosphere or escape the Earth-Moon system, it became a derelict object, trapped in a chaotic orbit. This accidental journey culminates in an impact event that, while unplanned, offers a unique opportunity for lunar observation.
Eight Years Adrift in the Void
The rocket segment has been adrift in deep space for nearly eight years, subjected to gravitational pulls from both Earth and the Moon, as well as the subtle, yet persistent, influence of solar radiation pressure. This complex interplay of forces made its trajectory difficult to predict precisely, but recent observations have solidified the impending lunar impact. Space tracking experts, including Bill Gray, developer of the Project Pluto software, have been meticulously charting its course, ultimately pinpointing a likely impact on the far side of the Moon, out of direct view from Earth-based telescopes.
While this particular piece of space junk is relatively small on a cosmic scale, its high velocity means the impact will be significant. The energy released will be comparable to several tonnes of TNT, excavating a new crater on the lunar surface. Scientists anticipate the crater could be tens of metres wide, offering a fresh geological feature for future lunar orbiters to study in detail.
A Boon for Lunar Science
Far from being a concern, this unintended impact is being viewed by some scientists as a potential scientific boon. Controlled impacts of spacecraft, like those from NASA's LCROSS mission in 2009, have been deliberately orchestrated to study the composition of the lunar regolith and search for water ice. While this Falcon 9 impact is uncontrolled, the exact time and location, once refined, could allow for opportunistic observations by spacecraft already in lunar orbit, such as NASA's Lunar Reconnaissance Orbiter (LRO) or India's Chandrayaan-2.
Data gathered from observing the new crater could provide valuable insights into the Moon's subsurface structure and the behaviour of impacts on airless bodies. The unique aspect of this event is that it's an object of known origin and composition hitting the Moon, which can help calibrate impact models. NPR National reported that astronomers are eager to analyse any post-impact plumes or debris fields, which could reveal information about the lunar crust in the impact area.
The Growing Problem of Space Debris
While this specific incident offers scientific opportunities, it also serves as a stark reminder of the ever-growing problem of space debris. Thousands of defunct satellites, spent rocket stages, and fragments from collisions are currently orbiting Earth, posing a threat to active spacecraft and future missions. Although this particular piece of debris was destined for deep space rather than low Earth orbit, the incident underscores the need for sustainable space practices.
Space agencies worldwide are increasingly focused on developing strategies to mitigate space junk, including de-orbiting technologies and design principles for more responsible end-of-life management of spacecraft. The Falcon 9’s lunar impact, while a fascinating astronomical occurrence, highlights the long-term consequences of leaving objects in orbit once their primary mission is complete, even far from our home planet.
