Follow-up observations should sharpen the accuracy of the forecast, but the approximately 3 ton, 15 m long by 3 m wide upper stage is currently projected to hit at a speed about 2.58 km/s.

Scientifically vital points in space

The European Ariane 5 that recently delivered the James Webb Space Telescope to its observing point flew a mirror trajectory to that of the Falcon 9—but the good news is that its upper stage has already evaded a comparable fate thanks to a specifically developed and qualified maneuver.

Europe's Ariane 5 delivered the James Webb Space Telescope to L2, the second Sun-Earth Lagrange point—'behind' instead of in 'front of' our planet—but after separating from Webb the upper stage used all its remaining fuel to escape the Earth-moon system entirely, putting it into a stable heliocentric orbit.

A brief history of human-made moon impacts

Human-made objects have intentionally impacted the moon before, starting as early as the 1950s, including Apollo upper stages used to induce 'moonquakes' for surface seismometers.

In 2009 NASA crashed its LCROSS mission into the moon, revealing water in the resulting debris plume, with the LADEE spacecraft doing the same on the lunar farside in 2013. ESA's Smart-1 spacecraft was crashed into the moon in 2006, the subject of a worldwide observing campaign.

"This forthcoming Falcon 9 impact is a little beyond our usual area of interest, because we are mainly focused on the debris population in highly-trafficked low-Earth orbits, up to 2000 km altitude, as well as geosynchronous orbits around 35,000 km away," explains Tim Flohrer of ESA's Space Debris Office.

"Our colleagues in the ESA Planetary Defence Office peer further into space, however. They use telescopes around the globe to track Near-Earth asteroids, and sometimes observe human-made objects as well. Extending our own remit into the 'cislunar' space between Earth and the moon has been discussed, due to the increasing use of the scientifically vital Sun-Earth Lagrange points in coming years."

Detlef Koschny, heading ESA's Planetary Defence Office, adds: "We use telescopic observations to pinpoint the orbits, mainly of natural objects in the space surrounding Earth. Occasionally, we also pick up man-made objects far away from the Earth, such as lunar exploration spacecraft remnants, and objects returning from Lagrange points."

For international spacefarers, no clear guidelines exist at the moment to regulate the disposal at end of life for spacecraft or spent upper stages sent to Lagrange points. Potentially crashing into the moon or returning and burning up in Earth's atmosphere have so far been the most straightforward default options.

"The upcoming Falcon 9 lunar impact illustrates well the need for a comprehensive regulatory regime in space, not only for the economically crucial orbits around Earth but also applying to the moon," says Holger Krag, Head of ESA's Space Safety Program.

"It would take international consensus to establish effective regulations, but Europe can certainly lead the way."

All the launchers developed by ESA during the last decade—Vega, Ariane 6 and Vega C—incorporate a built-in reignition capability, which ensures the safe return to Earth for atmospheric burn-up of their upper stages.

Assessing lunar impact risk

Space rocks hit the moon all the time. Researchers are interested in quantifying the frequency of these natural lunar impacts. Using a system developed through an ESA contract, the Greek NELIOTA project (Near-Earth object Lunar Impacts and Optical TrAnsients) detects flashes of light caused by small bodies striking the moon's surface, particularly across its shadowed face. NELIOTA can determine the temperature of these impact flashes as well as their brightness. From this, the impacting mass can be estimated.

ESA's Space Safety program is interested in this research as a way of assessing the number of incoming objects ranging in size from tens of centimeters to meters across. This is useful because the precise number of objects in this range is not known very well.

This research might also be valuable for future lunar colonists. One of the dangers they might face is small meteoroids doing damage to their infrastructure—NELIOTA results are helping to quantify the danger. Without an atmosphere to burn up such bodies, it is likely that future permanent lunar structures will be underground, to provide shielding against impacts as well as space radiation.

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