A dead SpaceX rocket hit the moon and joined 209 tons of junk

A discarded piece of a SpaceX rocket hit the moon early on Wednesday, August 5, at 5,400 miles an hour, punching a crater that scientists estimate could be 100 feet across and 16 feet deep. Nobody aimed it.

The object was the upper stage of a Falcon 9, about 45 feet long, 12 feet wide and weighing 8,800 pounds. It struck at around 2:35 in the morning Eastern time with a force of nearly three tons of TNT, on the lit side of the moon near the Einstein and Bell craters.

It joins a great deal of company. Around 3,000 human made objects now sit on the lunar surface, weighing about 209 tons between them, according to figures compiled by the BBC and cited by Forbes. They have been accumulating since the Soviet Union's Luna 2 arrived in 1959.

How a rocket reaches the moon by accident

The stage did its job in January last year. It carried two lunar landers off Earth and released them, one from the American company Firefly and one from the Japanese company ispace. Firefly's Blue Ghost landed successfully. The Japanese lander crashed.

The spent stage kept going. SpaceX says it followed the rules for disposing of the hardware, and that what happened afterwards was a matter of physics rather than neglect.

In this case, over time, solar activity and gravity led the second stage toward the Moon.

Independent astronomers spotted the trajectory first, working from publicly available data. NASA's Center for Near Earth Object Studies at the Jet Propulsion Laboratory later confirmed the stage had, in its phrasing, a 100 percent chance of impacting the moon.

Almost nobody saw it. The impact was invisible to the naked eye and, because it fell on the sunlit side, probably invisible even to large telescopes. Researchers note that no impact flash, natural or artificial, has ever been observed on the lit face of the moon.

What was detected was chemistry. Carl Schmidt, a planetary scientist at Boston University's Center for Space Physics, said a plume of sodium and lithium gas was picked up by the Very Large Telescope in Chile using spectroscopy, and another scientist recorded a sodium emission from Sudbury, Massachusetts.

Why scientists wanted to watch

There's only so many times where events like this will occur, and it gives us a chance to understand how the atmosphere of the moon works. It's happened twice in my lifetime.

The moon has an atmosphere so thin it is barely worth the name, and impacts throw material up into it. Schmidt hopes the data will let him work out how much water ice was hidden below the surface at the impact site, though the site sits near the equator, where the study of the collision expects little to no water ice.

It is also something close to a controlled experiment, which researchers value because the energy and timing are known. NASA offered a comparison that puts the scale of it in proportion.

By that measure the impact was unremarkable. The moon absorbs a strike of this size roughly every six days without anyone noticing. What has changed is not the damage but the number of countries and companies putting things there.

Who has hit the moon before

Most of what is up there arrived on purpose. The Soviet Union put the first human object on the moon in 1959. American spacecraft stages were deliberately crashed during the Apollo missions between 1969 and 1972, to be used as seismic sources. A Japanese robotic probe hit in 1993, and a European Space Agency satellite in 2006.

The 2009 case is the one worth remembering. NASA deliberately crashed a rocket stage into a permanently shadowed crater at the lunar south pole to see what came up. The plume it raised confirmed that water ice is there, in material that may not have seen sunlight for billions of years.

That finding is part of why the south pole matters now. Water ice can be turned into drinking water, breathable air or rocket fuel, and CBS News reports it as one of the major reasons countries including the United States and China want to build bases there.

The traffic is not hypothetical for long. Forbes, citing Space.com, notes that more than 100 lunar missions are planned over the next decade, and NASA's own base is intended for the south pole, the same region the water ice makes valuable.

The part with no clear rules

Which brings the story back to the debris. There is law here, but it is old and general. The 1967 Outer Space Treaty makes a launching state responsible for the objects it sends up and liable for damage they cause, and the 2020 Artemis Accords add commitments on limiting debris and preserving heritage sites.

What is missing is the practical machinery. CNN reports there is no formalised tracking of debris beyond Earth, and Forbes reports that scientists want better tracking and international rules on disposing of spacecraft, warning of collision risk, contamination of scientifically valuable ground and disturbance of historic sites such as the Apollo landings.

Nothing in this episode appears to have broken any rule. Julianna Scheiman, SpaceX's director of NASA science and Dragon programs, told reporters on August 3 the company had followed the appropriate rules and regulations for disposing of the stage, and that NASA had worked with it on where the stage should end up.

That is the point rather than an exception to it. A stage disposed of by the book still cratered the moon 19 months after launch, because solar activity and gravity moved it, and the compliance question and the outcome turned out to be separate things.

What comes next

Pictures are pending. NASA's Lunar Reconnaissance Orbiter was to photograph the site before and after, though the agency says the images will probably take a week or two to appear. The before and after comparison is how the crater's real size will be established.

The agency also says its scientists plan to use the event to refine techniques for tracking objects in space, which is the practical response to a problem that announced itself by accident.

SpaceX said in its written statement on August 5 that it works to be as responsible as possible with hardware left in space. Scheiman, asked about the impact, said that as a scientist she was very excited to see the observation.

The number underneath it

The crater figures will firm up once the orbiter's images arrive. The study of the collision gives a range of roughly 65 to 98 feet across and about 16 feet deep, which is where both published numbers come from. What will not move is the arithmetic underneath: around 3,000 objects on the moon, one of them arrived by accident, and more on the way.