A stray rocket body will crash onto the far side of the moon on Friday in an unparalleled exhibition of cosmic trash, marking the first time that a piece of space debris has mistakenly touched the lunar surface.
The spent rocket booster is expected to bash into the Hertzsprung crater at 12.25pm GMT, however the exact time and position are unknown. It is likely to be part of the Chinese Chang'e 5-T1 mission, which swung around the moon in 2014.
The plummeting 4 tonne rocket body will slam into the surface at a shallow angle at more than 5,500mph (2.5km per second), scattering debris and carving a crater of its own that will be 20 to 30 meters across (65ft to 100ft).
Dr. Vishnu Reddy, an associate professor at the University of Arizona whose team assisted in the discovery of the item, said it demonstrates how space trash has already spread beyond Earth, where the US already tracks over 27,000 pieces of orbital debris.
"Things have hit the moon in the past," Reddy explained, "but those were mostly purposeful impacts or we tried to land and crashed on the surface." "This is an unintended impact of a rocket body."
Although astronomers will not be able to see the impact firsthand, scientists are hoping that photographs of the crash site will be captured soon after by Nasa's lunar reconnaissance orbiter or India's Chandrayaan-2 probe, both of which are now orbiting the moon. The Chang'e 4 lander from China, which arrived on the far side of the moon in 2019, is too far away to see the event.
The moon has been pocked with half a billion craters the same size or greater than the one the rocket would make due to natural meteorite impact. However, the lunar surface displays the scars of purposefully crashed rocket stages and moon missions that landed in the dust rather than on the surface.
Huge Saturn V rocket bodies were driven into the moon's surface during the Apollo era so that devices on the surface could monitor the accompanying shock waves and analyze the moon's interior.
More strange material has now been discovered among the debris left behind by the US astronauts. In 1999, a private lunar mission buried the ashes of astrogeologist Eugene Shoemaker near the moon's south pole in a "crater of endless darkness." The Israeli Beresheet spacecraft fell on the surface three years ago, scattering thousands of microscopic tardigrades. Despite their best efforts, many experts believe they were converted to mush.
Reddy's team has discovered around 200 objects in "cislunar space," the area between the Earth and the moon. The booster was initially thought to be part of a SpaceX Falcon 9 rocket that launched in 2015, but Reddy and his students discovered that its optical spectra – the wavelengths of light it reflects – more closely matched the Chinese Long March 3C rocket that launched the Chang'e 5-T1 mission a year before.
According to Reddy, the inadvertent impact on the moon should draw attention to the rising problem of space junk. "People realize it's horrible in Earth orbit," he continued, "but we're now progressively pushing garbage into cislunar space." "We've already fouled up around the Earth; let's not do it on the moon."
However, there is more at stake than cosmic littering. Given how difficult it is to see and monitor things so distant from Earth, there are national security ramifications, according to Reddy. "What can our adversaries do in cislunar space that we don't know about?" he wondered. "Tracking something 4in across in Earth orbit is possible, but tracking something so small in cislunar space is impossible." "Forget about it."
Given plans to return humans to the moon, Chris Newman, a professor of space law and policy at Northumbria University, called the potential impact a "warning flag." "It's evident that this is something we'll have to think about as we start placing humans on the moon more permanently," he said.
Professor of planetary geosciences at the Open University, David Rothery, said that anyone concerned about the rocket body colliding with the moon should be more concerned about biological contamination, but he cautioned that this is a small possibility. "It's likely that very few of the bacteria it brought with it survived or will survive the impact," he said. "It's pointless to get outraged over one more crater on the moon."
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