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The line following the title of BBC's Elon Musk SpaceX rocket on collision course with moon reads:

A rocket launched by Elon Musk's space exploration company is on course to crash into the Moon and explode.

and later:

Falcon 9's projected demise was identified by journalist Eric Berger on the space website Ars Technica and by data analyst Bill Gray in his blog.

The collision is due to happen on 4 March when the rocket will explode as it makes contact.

"It's basically a four-tonne empty metal tank, with a rocket engine on the back. And so if you imagine throwing that at a rock at 5,000 miles an hour, it's not going to be happy," Prof McDowell says.

It will leave a small artificial crater on the Moon's surface.

Bill Gray, who uses software to track near-Earth space objects, projects that it made a close fly-by on 5 January. On 4 March it's likely to hit the Moon's far side, he says.

In 2009 Prof McDowell and other astronomers performed an experiment in which a similar-sized rocket was crashed into the Moon. Sensors gathered evidence of the collision so they could study the crater.

That means scientists are unlikely to learn anything new from this crash, Prof McDowell explains.

Question: Will the seven year old spent upper stage rocket body really explode upon impact? Was it not passivated for some reason?


BBC-related question in Physics SE:

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    $\begingroup$ Another in the long line of candidates for the hyperbole tag? $\endgroup$ Jan 27 at 20:32
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    $\begingroup$ @OrganicMarble I'm all in! $\endgroup$
    – uhoh
    Jan 27 at 20:34
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    $\begingroup$ Passivate? What is that? How do you do it to an empty fuel tank out in space? $\endgroup$
    – RedSonja
    Jan 28 at 7:58
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    $\begingroup$ Most of the impact energy will come from the kinetical energy and not from the little chemical energy of the rest fuel. On this reason, I think "impact" would be a better word for that, although the result would look like an explosion. But, for example, we do not call "explosion" a meteorite impact on the Earth. $\endgroup$
    – peterh
    Jan 28 at 7:59

5 Answers 5

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It depends on how you define "explosion". Most generally, it merely describes something that breaks into pieces violently. Astronomical impact events can generate explosions simply from their kinetic energy, as the impacting body violently breaks apart. But there's no hard threshold for how big an impactor needs to be to generate an "explosion" versus a "collision".

There won't be a fireball or mushroom cloud or anything like that, but the rocket will undergo a very precipitous loss of structural integrity and redistribution of kinetic energy to its constituent parts, which you could call an explosion. The end result may be quite similar to other types of explosions, like if you had landed the rocket gently and blown it up with a chemical bomb - the energy in this case is simply stored as kinetic energy, rather than chemical energy. In the end, the rocket is going to be reduced to tiny pieces with bits of rocket or rock from the impact site flying off in every direction in an uncontrolled manner. The impact will be sudden, destructive, and spread debris widely, which fits the bill for an explosion in my book.

You may or may not consider that the rocket itself will "explode", but the impact will result in an explosion by most reasonable definitions. I certainly wouldn't want to be anywhere near the impact site.

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    $\begingroup$ @uhoh Good question. I think you're right, some portion of the rocket becoming embedded seems likely. I've clarified the wording a little that the ejecta may consist of both rock and rocket, but I'd be surprised if no part of the rocket is ejected from the collision. $\endgroup$ Jan 27 at 22:20
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    $\begingroup$ I believe the correct wording is "excessive lithobraking will lead to rapid unplanned disassembly". $\endgroup$ Jan 28 at 10:04
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    $\begingroup$ Back when cannons and cannonballs were used, when the solid metal shots are fired, although there is no explosive in the round itself, the dirt, rocks, and trees being "blown apart" are powerful enough to cut a person in half. $\endgroup$
    – Nelson
    Jan 28 at 10:33
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    $\begingroup$ @AndréLFSBacci You'll certainly get a lot of dust and rock kicked up from the impact, but I might not call it a "plume". Without an atmosphere, you won't get the convection currents responsible for typical cloud shapes on Earth. You'll get a shower of debris which flies off in parabolic trajectories. $\endgroup$ Jan 28 at 14:56
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    $\begingroup$ "the rocket will undergo a very precipitous loss of structural integrity and redistribution of kinetic energy" - so much more colorful than "explode". If only news publications wrote like this. Love it! $\endgroup$
    – Seth R
    Jan 28 at 15:58
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I think it's worth looking at how much energy that rocket is going to be carrying. Plugging the 4 tonnes and 5000 mph figures into an Online calculator we find that the impact will carry close to 10 gigajoules of energy. To put that in perspective using a popular comparator: a tonne of TNT releases 4.2 gigajoules, so the energy released in the impact will be equivalent to around 2.4 tonnes of TNT or nearly 5 Tomahawk Cruise Missiles.

I think that will produce a blast that more than qualifies as an explosion!

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    $\begingroup$ You and I were thinking along the same lines, I see. $\endgroup$
    – codeMonkey
    Jan 28 at 13:16
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    $\begingroup$ @codeMonkey: Yeah, but I was faster! ;) $\endgroup$ Jan 28 at 13:45
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    $\begingroup$ but I included equations! ;-) $\endgroup$
    – codeMonkey
    Jan 28 at 19:44
  • $\begingroup$ Or 0.0028 Gigawatt (hour)s! Great Scott! $\endgroup$ Jan 30 at 2:59
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Energy-wise, I'd call it an explosion

Kinetic energy (KE) = 1/2 mv2

The article quotes the mass at 4 tons, and the speed as 5,000 mph. That gives:

KE = 0.5 * 3628 kg * 2235 m/s * 2235 m/s = 9 gigajoules

One metric ton of TNT (the base unit that's used when talking about "X kiloton bombs") releases 4.1 GJ.

So this collision is more energetic than detonating 2 tons of TNT. I'd probably call that an explosion.

Per Jack's comment, the article's spelling of "tonnes" implies metric tons, which raises the mass from 3628 kg to 4000 kg, and raises the total energy to ~ 9.99 GJ.

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    $\begingroup$ Small point, but the professors states four tonnes, and that spelling implies a metric ton = 1000kg which I think then brings your calculation in line with mine. $\endgroup$ Jan 28 at 13:46
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    $\begingroup$ @NPSF3000 2235m/s corresponds to an energy of 2.5 kJ/g. Heat of vaporization of things like metal and rock is about 5 kJ/g. So, the energy here is sufficient to vaporize a fair fraction of the impactor and the material it hits, while reducing the rest to tiny pieces. It would certainly look like an explosion if you were nearby! $\endgroup$
    – John Doty
    Jan 28 at 18:29
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    $\begingroup$ @NPSF3000 The caps in a cap gun explode, is that even contentious? The amount of energy evolved in an event is irrelevant, if it's a rapid expansion caused by the release of chemical, mechanical or nuclear energy, it's an explosion. $\endgroup$
    – JeffUK
    Jan 28 at 19:38
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    $\begingroup$ @NPSF3000 - I would define an explosion as a "large, violent release of energy". Large is certainly a fuzzy value, but a there are several orders of magnitude between a big game rifle round (~5,000 J) and the collision observed here (10,000,000,000 J). The fact that the energy can be reasonably converted to units of "equivalent tons of TNT" seems like a good threshold for "large". $\endgroup$
    – codeMonkey
    Jan 28 at 19:43
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    $\begingroup$ @stef it wouldn't change much. At these velocities materials on impact behave very much like liquids, indeed the thermal energy of even light contact is enough to liquefy a lot of material surrounding the contact point. Any possible "rebounding" action won't come from elasticity of solids, but from the liquefied materials passing boiling point and expanding rapidly as superheated gas and blasting whatever happens to be above them upwards. $\endgroup$
    – SF.
    Jan 30 at 13:23
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There are astronomical, volcanic, chemical, electrical, high pressure or nuclear explosions. None of these kind of explosions may happen.

No supernovae, no very large meteoroid or an asteroid impacting the moon. There is no left fuel and oxidizer for a chemical reaction. There is no shorted high voltage high power electric transformer. High pressure helium gas tanks will be nearly empty. There is no hot steam boiler under high pressure. No nuclear fission or fusion possible.

There is only an empty rocket stage weighing some tons moving at high speed impacting the moon. So there is a lot of kinetic energy set free in a very short time. A spectacular view for an hypothetical observer in a distance of some km, but nothing we call an explosion.

There were lunar seismometers placed by the Apollo missions, but they were switched of in 1977 and they are out of the necessary electric power from the RTGs for several decades. So there will be no seismic measurement data from this impact.

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    $\begingroup$ Your first source (wikipedia) literally lists impact events as a cause for explosions. $\endgroup$ Jan 28 at 5:56
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    $\begingroup$ Some km seems a little close for comfort. Don't want rocket shrapnel piercing my spacesuit… $\endgroup$
    – gerrit
    Jan 28 at 8:11
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    $\begingroup$ @DavidMulder the source lists large impacts. $\endgroup$
    – Uwe
    Jan 28 at 12:26
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    $\begingroup$ From that list, It will be a high pressure explosion, the force of the rocket hitting the ground will exert plenty of pressure on the rock it hits. $\endgroup$
    – JeffUK
    Jan 28 at 16:03
  • $\begingroup$ I guess flywheels and bolides can't explode either, then. $\endgroup$
    – Will Chen
    Jan 31 at 2:04
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Somehow my paper napkin math came out slightly different:

Both rockets have velocity of approximately 9,000 kph. 2,300 kg (LCROSS) will deliver an energy of approximately 1.7 tons of TNT. 4,000 kg (SpaceX) will deliver an energy of approximately 3.9 tons of TNT.

Regardless, I reckon the moon ground zero experiences an explosion.

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