The Moon's pockmarked surface will gain a fresh crater in about five weeks, when a hefty chunk of a SpaceX Falcon 9 rocket crashes into its far side in a fiery explosion.
The wayward projectile's whereabouts is being tracked by US data analyst Bill Gray, who developed software that keeps tabs on objects whizzing around inside our Solar System, and a troupe of observers around the world.
They predict it will crash at 11.25:58am AEDT (give or take a few seconds) on March 4.
When the rocket stage makes catastrophic contact with the Moon's surface, it will join the ranks of a handful of artificial objects that have ploughed into the Moon.
For instance, back in the Apollo era of Moon exploration, spent bits of rocket were deliberately aimed at the lunar surface and used to calibrate moonquake-sensing instruments, Mr Gray wrote in his blog.
"And, more recently, [NASA's Lunar Crater Observation and Sensing Satellite] LCROSS hit the Moon (quite deliberately) in 2009 without causing trouble."
But the Falcon 9 rocket booster is the first artificial object, one that wasn't sent as part of a mission to the Moon to unintentionally hit it, that astronomers have noticed.
So how did a 4-tonne piece of rocket accidentally end up on track to smash on the lunar surface — and is it anything to worry about?
What normally happens to spent rockets?
The rockets on which satellites are ferried into space tend to have multiple stages, comprising engines and fuel, which propel the payload until they run out of propellant and drop off.
The first stage of a launch includes the blazing blast you see on lift-off. It gets the payload off the ground and up a couple of hundred kilometres.
Once out of fuel, the first stage typically falls back to Earth quickly, simply because it's not reached the speed or altitude that will keep it in stable orbit around the planet, says Brett Carter, a space scientist at RMIT University.
As of December 2015, SpaceX (owned by Elon Musk) has successfully landed the first stage of Falcon 9 rockets to be reused.
The later or upper stages are a different story. Their job is to insert the satellite to where it's needed.
And there are plenty of depleted upper rocket stages circling the planet as space junk.
"For objects launched [under around 800km] into low-Earth orbit, those upper-stage rocket bodies … slowly degrade in altitude, and eventually re-enter the atmosphere and burn up," Dr Carter says.
For launches bound for high-Earth orbit — we're talking around 36,000km — anything that needs disposing of gets shoved out another few hundred kilometres into what's known as the "graveyard orbit".
But even that is nowhere near the distance to the Moon, which hangs out around 384,000km away.
So how did the SpaceX rocket get all the way to the Moon?
The Falcon 9 engine that's currently en route to the Moon has travelled much further than most other satellite-delivering rockets.
In 2015, it helped ferry the Deep Space Climate Observatory, or DSCOVR, to its spot in deep space, about 1.5 million kilometres from Earth, between us and the Sun.
DSCOVR's job is to monitor ebbs and flows in the stream of charged particles that spew from the Sun — called the solar wind —- and give space weather forecasters warning that a damaging surge might be on its way.
The SpaceX Falcon 9 rocket that delivered DSCOVR had two stages. When the first stage ran out of fuel, it dropped off and headed back to Earth (but could not be reused because it crashed into the ocean).
The second stage provided the extra oomph needed to steer DSCOVR closer to its operational spot.
Its job done, the second stage was jettisoned and left to tumble through space, more than a million kilometres from Earth.
When it relinquished its DSCOVR payload, the rocket stage was already out way past the Moon.
But in the seven years since, the rocket stage has been tugged and yanked by the Moon, Sun and Earth's gravity, wrangling it in an unpredictable, or "chaotic", orbit.
Until January this year, that is, when a close flyby with the Moon sent the rocket stage on a long, looping course that will, on March 4, see it crash into the Moon at about 8,000 kph.
What will happen when the SpaceX rocket hits the Moon?
The rocket stage is tipped to crash into a 520km-wide crater called Hertzsprung.
Unfortunately, it's on the non-Earth-facing side of the Moon, so we won't see it happen.
And it will probably be a spectacular sight. The Moon has no atmosphere, so the stage won't burn up on its way down.
Instead, it will smash straight into the rock and scatter bits over the lunar surface, Jonathan McDowell, an astrophysicist at Harvard University, told Radio National.
"There's going to be a huge spray of moon dust going many miles into the sky above the Moon," Dr McDowell said.
But, he added, the chances of the collision being a problem for current Moon missions is pretty small.
"I'm a bit worried that some of the moon dust that gets knocked up could interfere if a satellite going around the Moon was unlucky enough to be flying straight through it, but that's super unlikely," Dr McDowell said.
"The Moon's a big world.
Alice Gorman, a space archaeologist at Flinders University, agrees.
The predicted crash zone is far enough away from missions such as China's Yutu-2 probe that it shouldn't be an issue, and the Hertzsprung crater walls might limit the dust cloud's reach anyway.
Dr Gorman sees the crash as a prime opportunity to learn more about that patch of the Moon.
"It will cause a crater inside that crater, so you'll get to see the subsurface a little bit, and we'll learn about its geological structure by looking at the features of the [new] crater," she says.
"Even though we won't be directly imaging the collision because there are no instruments to watch it happen, the Lunar Reconnaissance Orbiter will probably pass over at some point, so we will actually get some science out of it."
The fact that the Falcon 9 stage is crashing into the Moon is, in a sense, a good thing for people tracking such objects, she adds.
"So any risks of it floating around in these very unstable and unpredictable orbits are removed."
It's likely that other objects abandoned in deep space have smashed into the Moon too, Dr McDowell wrote on his website.
"There are about 30 to 50 lost deep space objects like this that have been missing for years — 50 years in some cases — that haven't been picked up by asteroid searches, and probably some of them hit the Moon without us noticing."
Who deals with space junk?
It's been standard practice since the dawn of space exploration to leave equipment in space once they've run out of fuel or power, or if they stop working.
That's because it takes fuel — and thus costs money — to provide the thrust needed to bring gear down to Earth from higher orbits, Dr Gorman says.
"Get rid of all the fuel and you can't manoeuvre [an object] or give it instructions or tell it to go somewhere. But keep the fuel, and you have a heavier and a more expensive mission."
Besides, if you leave fuel on a spacecraft, there's a chance it will explode and create loads of bits of space junk instead of one large trackable object, she added.
Generally, old rockets and satellites way out in high-Earth orbit and beyond aren't much cause for concern. That part of space is less densely populated, so the chances of collision with a satellite are extremely low.
It's the stuff circling much closer to home that has space organisations and astronomers worried.
Literally tonnes of space junk, from decommissioned satellites launched half a century ago to empty rocket stages down to minuscule flecks of paint, are concentrated in low-Earth orbit.
And the threat of space junk collisions poses a risk not just to current satellite missions, but future launches too, Dr Carter said.
"At the moment, some may argue that space debris is not that big of a problem, because there's lots of room up there.
"But with an exponential increase in the number of launches into orbit, this is going to show up at our front door, and we haven't solved the issue of space sustainability yet."
The UN released Guidelines for the Long-term Sustainability of Outer Space Activities in 2018, but there's no international regulator when it comes to issues such as space junk, Dr Gorman says.
"The guidelines are saying nations have to take responsibility at that national level for making sure that the companies and space activities under their watch are actually doing the right thing and being responsible," she said.
"At the moment, there are no penalties for wrongdoing in space, unless your spacecraft damages someone else's under the liability convention. That's it."
Dr McDowell is adamant that it's time to do something about the problem of space junk.
"We're now at the point where we really have to clear up the mess we've been making in the past 60 years of the space age."
Editor's note (14/02/21): Since publication of this article, reanalysis of the rocket stage showed it appears to be a part of a Chinese mission, not a SpaceX launch.)
