Bad astronomy |  Chinese rocket leaves double crater on moon

Bad astronomy | Chinese rocket leaves double crater on moon

On March 4, 2022, an upper stage rocket booster slammed into the Moon.

We know that for sure. But as soon as you dig into this story, it gets weird.

The central character in this saga is Bill Gray, a software designer who wrote Guide, sophisticated programming used by professional and amateur astronomers to calculate the orbits and positions of asteroids in the sky. He wrote about it as events unfolded.

The object was discovered on March 14, 2015 by the Catalina Sky Survey, which takes images of the sky to search for near-Earth asteroids. It was first thought to be such an asteroid and given the temporary designation WE0913A, but soon after was identified as something orbiting the Earth, not the Sun , implying that it was more likely to be space junk, some propellant rocket of some sort that was left in high orbit around Earth after its fuel ran out.

Returning to orbit, it turned out to have passed by the Moon just two days after SpaceX launched the Deep Space Climate Observatory (or DSCOVR) mission on a Falcon 9 rocket, so naturally and naturally, it was assumed that this object was in fact the upper stage of this F9. However, shortly afterwards it became clear that this was a coincidence and that the object was in a very different orbit than the DSCOVR spacecraft – Gray has an updated version of his page with more information.

The orbit corresponded to that of a Chinese lunar mission, the Chang’e 5-T1, launched in October 2014, which was intended to test the atmospheric re-entry of a capsule sent to the Moon. He used a three-stage Long March 3C, and in fact some students at the University of Arizona were able to show that the spectra taken from the object matched those of the third stage of the Long March rocket much better than to those of the Falcon 9.

But another twist: the Chinese government claimed that the booster had burned up in the Earth’s atmosphere “safely” shortly after the mission. Gray points out that it may have been confusion on their part as to the mission being discussed, and is very confident that it was actually the 5-T1 booster.

It was expected to impact the far side of the Moon on March 4, 2022, and on May 22 NASA’s Lunar Reconnaissance Orbiter (or LRO) took images of the area. He found a new impact in the right area, but this story took another turn: it’s a double crater.

The two craters overlap; one is 16 meters and the other 18 meters wide, together creating a crater about 28 meters in diameter. A tennis court is about 24 meters long for comparison.

A single object hitting the Moon should generally leave a single crater. There are many known double craters, likely due to binary asteroids – two rocks orbiting each other hitting each other almost simultaneously – or a single object breaking up shortly before impact .

Dozens of man-made objects, including rocket boosters, hit the Moon, and many were seen; the Saturn S-IVB third-stage boosters that put humans on the Moon orbited for a while before finally impacting, and although the remaining craters were somewhat irregular or elongated in shape, none made a double crater.

So why did this object do this? It’s still a mystery. A booster is basically a long tube with very heavy rocket motors and nozzles at one end and fuel tanks along its length. Once the fuel is used up, these tanks are quite light. Most of the impact force and sculpting of the crater comes from the engines, and I think the rest of the thruster itself would perhaps modify that crater at best, lengthening it.

I first wondered if the rocket was coming from a low angle. The top of the thruster hitting first could create a crater, then the engines reaching about 10 meters downstream slicing through the second, but Gray notes that the angle of impact was only 15° from vertical, so this cannot be the explanation. If the booster still had a payload attached to the top, you might expect two craters because it would have a heavy mass on each end, but the 5-T1 successfully deployed its payload and the top should have been empty.

At this point, we just don’t know why there are two craters. LRO images have sufficient resolution to see the craters, but their exact characteristics are still difficult to disentangle. High-velocity impacts – several thousand kilometers per hour – can be difficult to understand, especially when the lunar surface is rough and irregular, which can affect crater shapes and structures.

It’s an interesting story because of the mystery and the detectives involved – Gray did yeoman work here – but it’s also important. NASA, China and other space agencies are stepping up their exploration of the Moon, and as more material is sent there, more space junk and more impacts are inevitable. These objects need to be better tracked, better predicted, and in general, governments and companies need to ensure that their discarded debris poses no threat to other lunar missions. Space traffic is becoming a very big problem, and it must be taken very seriously.

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