Editorâ€™s note: ExpertsÂ expect the crash did occur as predicted, but are still waiting for visual verification.
This article was originally published atÂ The Conversation.Â The publication contributed the article to Space.com’sÂ Expert Voices: Op-Ed & Insights.
On March 4, a lonely, spent rocket boosterÂ smacked into the surface of the moonÂ at nearly 6,000 mph. Once the dust has settled,Â NASA’s Lunar Reconnaissance OrbiterÂ will move into position to get an up-close view of the smoldering crater and hopefully shed some light on the mysterious physics of planetary impacts.
As aÂ planetary scientist who studies the moon, I view this unplanned impact as an exciting opportunity. The Moon has been a steadfast witness to solar system history, its heavily cratered surface recording innumerable collisions over the last 4 billion years. However, scientists rarely get a glimpse of the projectiles â€“ usually asteroids or comets â€“ thatÂ form these craters. Without knowing the specifics of what created a crater, there is only so much scientists can learn by studying one.
The rocket impact will provide a fortuitous experiment that could reveal a lot about how natural collisions pummel and scour planetary surfaces. A deeper understanding of impact physics will go a long way in helping researchers interpret the barren landscape of the moon and also the effects impacts have on Earth and other planets.
When a rocket crashes on the moon
There hasÂ been some debateÂ over the exactÂ identity of the tumbling objectÂ currently on a collision course with the Moon. Astronomers know that the object is an upper stage booster discarded from a high-altitude satellite launch. It is roughly 40 feet (12 meters) long and weighs nearly 10,000 pounds (4,500 kilograms). Evidence suggests that it isÂ likely either a SpaceX rocket launched in 2015Â or aÂ Chinese rocket launched in 2014, but both partiesÂ have denied ownership.
An instant after the rocket touched the lunar surface, a shock wave would have traveled up the length of the projectile at several miles per second. Within milliseconds, theÂ back end of the rocket hull will be obliteratedÂ with bits of metal exploding in all directions.
A twin shock wave will travel downward into the powdery top layer of theÂ moon’s surface called the regolith. The compression of the impact will heat up the dust and rocks andÂ generate a white-hot flashÂ that would be visible from space if there happened to be a craft in the area at the time. A cloud of vaporized rock and metal will expand from the impact point as dust, and sand-sized particles will be thrown skyward. Over the course of several minutes, the ejected material will rain back down to the surface around the now-smoldering crater. Virtually nothing will remain of the ill-fated rocket.
If you are a fan of space, you may have experienced some dÃ©jÃ vu reading that description â€” NASA performed a similar experiment in 2009 when it intentionallyÂ crashed the Lunar Crater Observation and Sensing Satellite, or LCROSS, into a permanently shadowed crater near the lunar south pole. I was a part of theÂ LCROSS mission, and it was a smashing success. By studying the composition of the dust plume lofted into the sunlight, scientists were able to find signs ofÂ a few hundred pounds of water iceÂ that had been liberated from the Moonâ€™s surface by the impact. This was a crucial piece of evidence to support the idea that for billions of years,Â comets have been delivering water and organic compoundsÂ to the moon when they crash on its surface.
However, because the LCROSS rocket’s crater is permanently obscured by shadows, my colleagues and I have struggled for a decade to determine the depth of this buried ice-rich layer.
Observing with the Lunar Reconnaissance Orbiter
The accidental experiment of the March 4 crash will give planetary scientists the chance to observe a very similar crater in the light of day. It will be like seeing the LCROSS crater in full detail for the first time.
Since the impact occurred on the far side of the moon, it was out of view for Earth-based telescopes. But about two weeks after the impact, NASAâ€™s Lunar Reconnaissance Orbiter was expected to begin to get glimpses of the crater as its orbit takes it above the impact zone. Once conditions are right, theÂ lunar orbiter’s cameraÂ will start taking photos of the impact site with a resolution of about a 3 feet (1 meter) per pixel. Lunar orbiters from other space agencies may also trainÂ their camerasÂ on the crater.
The shape of the crater and ejected dust and rocks will hopefully reveal how the rocket was oriented at the moment of impact. A vertical orientation will produce a more circular feature, whereas an asymmetric debris pattern might indicate more of a belly flop. Models suggest that the crater could be anywhere from aroundÂ 30 to 100 feet (10 to 30 meters) in diameter and about 6 to 10 feet (2 to 3 meters) deep.
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The amount of heat generated from the impact will also be valuable information. If observations can be made quickly enough, thereâ€™s a possibility theÂ lunar orbiterâ€™s infrared instrumentÂ will be able to detect glowing-hot material inside the crater. This could be used to calculate the total amount of heat from the impact. If the orbiter canâ€™t get a view fast enough, high-resolution images could be used to estimate the amount of melted material in the crater and debris field.
By comparingÂ before and after imagesÂ from the orbiterâ€™’ camera and heat sensor, scientists will look for any other subtle changes to the surface. Some of these effects canÂ extend for hundreds of times the radius of the crater.
Why this is important
Impacts and crater formation are aÂ pervasive phenomenonÂ in the solar system. Craters shatter and fragment planetary crusts, gradually forming the loose, granular top layer common onÂ most airless worlds. However, the overall physics of this process are poorly understood despite how common it is.
Observing the upcoming rocket impact and resulting crater could help planetary scientists better interpret the data from the 2009 LCROSS experiment andÂ produce better impact simulations. With a veritableÂ phalanx of missionsÂ planned to visit the Moon in the coming years, knowledge of lunar surface properties â€“ especially the quantity and depth of buried ice â€“ is in high demand.
Regardless of this wayward rocketâ€™s identity, this rare impact event will provide new insights that may prove critical to the success of future missions to the Moon and beyond.