Cosmic rays can have energies going into the $10^{20}$ eV domain. Asteroids and meteoroids originating in the solar system are probably limited in their speed because they all started out from the same lump of matter having more or less the same speed, but what about rocks from other galaxies ? Could they reach relativistic speeds in relation to the solar system? Are there astronomical events that could be interpreted as a collision between such an asteroid and a planet or star?

Just a mole of hydrogen atoms with that kind of energy would have three times the energy of the Chicxulub impact event, so I hope there's a reason for them not existing!

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    $\begingroup$ The asteroids from other galaxies would have to escape first; $v_{esc}\sim500$ km/s velocities are an order of magnitude greater than normal asteroid velocities of $v_{ast}\sim50$ km/s. $\endgroup$
    – Kyle Kanos
    Aug 9, 2013 at 13:40
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    $\begingroup$ @Dimension10 The energy is out there - Big Bang, Black Holes, Super-Nova etc. Why doesn' that energy end up in macroscopic objects hurtling through the universe? $\endgroup$
    – yippy_yay
    Aug 9, 2013 at 13:45
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    $\begingroup$ @SebastianHenckel - at such energies rocks are pulverized first. A relativistic rock is a dangerous, planet-killing weapon. Thus, there are high chances the first relativistic rock we encounter has been launched by an angry ET civilization. $\endgroup$ Aug 9, 2013 at 14:18
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    $\begingroup$ A super-relativistic asteroid would be radiating gamma rays due to collisions with ISM particles. It would also bleed kinetic energy during its whole travel due to collisions with these particles. Even though deep space is very diffuse, this effect would add up--you encounter roughly 5 kg of ISM particles every light year you travel. If we're talking about extra-galactic sources, this begins to get to be the order of the mass of an asteroid. $\endgroup$ Aug 9, 2013 at 16:14
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    $\begingroup$ @EugeneSeidel: yes. And there is matter no matter how deep in space you go. A significant fraction of the total mass of the universe is stored as "loose" hydrogen between galaxies. $\endgroup$ Aug 9, 2013 at 16:46

4 Answers 4


First, the speed of other galaxies isn't too helpful. For example, the radial velocity of the Andromeda galaxy relatively to us is 300 km/s, i.e. 0.1% of the speed of light only. Moreover, internally, everything in that galaxy moves by pretty much the same speed and is confined to the vicinity of that galaxy which makes us pretty sure that no piece will reach us before Andromeda galaxy will.

More importantly, macroscopic systems in outer space aren't moving with the same huge speeds near the speed of light as the cosmic rays essentially because of the second law of thermodynamics.

When cosmic rays are accelerated to high speeds, we may treat them statistically and the high energy of these elementary particles may be assigned a high temperature. But the physical systems with many degrees of freedom prefer to evolve to the most likely, high-entropy configurations. That's why the excess energy (e.g. in a supernova) tends to divide between the elementary particles chaotically.

In particular, the individual particles' kinetic energy results from velocities that have a random direction. At these huge temperatures (kinetic energies per particle), the atoms are unbound (well above the ionization energy) and macroscopic matter doesn't exist. So the likelihood that a large object will move towards the Earth at a near-luminal velocity is as unlikely as the possibility that the numerous atoms in the large objects are assigned velocities with the exactly equal direction although the directions are being chosen from an isotropic, random distribution.

After some time for "thermalization" (interactions between atoms are allowed to change the system with the conservation laws' being the only absolutely constraints), the greater object you consider, the less likely it is that all the atoms in that object will be doing the same thing. This is a form of the second law of thermodynamics.

The previous paragraph prevents the creation of "coherent macroscopic cosmic rays", macroscopic objects that would move in the same direction, from the thermal chaos of hot environments such as the supernova. But even if some astrophysical process managed to eject a chunk of matter at these high speeds, the previous paragraph will still guarantee that we won't receive it on Earth. Instead, the individual atom of that speedy object would still have some residual mutual velocities so the object would split into individual atoms and we would observe cosmic rays only once again.

I could summarize the situation in this way: to shoot a large object by a huge speed from a very distant celestial body to the Earth requires one to have the same and huge radial velocities of all atoms but almost vanishing relative transverse velocities. The likelihood of that goes to zero exponentially in any thermal environment.

If one assumes that the source of the initial speed is not thermal, then one must accept that the projectiles will derive their speed from their broader environments – speeds of astrophysical bodies that already exist – and those are simply of order 0.1 percent of the speed of light as in the Andromeda example or lower. These modest speeds boil down to the inhomogeneities during the structure formation and, ultimately, to inflation, or to the planetary speeds derivable for a star. Whenever you want to locally violate the modesty of the speeds, e.g. by a gravitational collapse, you don't "shoot" any new particles before the collision and when the collision happens, the excess energy of the collision also inevitably creates a high temperature and we're back to the previous paragraph.

So near-luminal macroscopic bodies aren't observed. I would even go further and despite my being a believer that life in the Universe is extremely rare, I would say that an object moving by a near-luminal speed would prove the existence of an advanced civilization. I should be a bit careful: the gravitational slingshot is a process that allows the speed to be enhanced even naturally. But even if the source of the speed were a gravitational slingshot and the speed would be really high, like 99.9999% of the speed of light, chances would be high that some intelligence was behind the optimization of the gravitational slingshot because it's extremely unlikely for such an outcome to occur naturally.

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    $\begingroup$ projectrho.com/public_html/rocket/spacegunexotic.php#rbomb $\endgroup$ Aug 9, 2013 at 14:28
  • $\begingroup$ so in other words most asteroids that receive that kind of energy would break up due to the heat and energy propelling its atoms & molecules in various directions? $\endgroup$
    – Tomcat
    Aug 10, 2013 at 0:30
  • $\begingroup$ yup, @Snipergirl. $\endgroup$ Aug 10, 2013 at 4:48
  • $\begingroup$ @LubošMotl cool! (not for the asteroid though :) ) $\endgroup$
    – Tomcat
    Aug 10, 2013 at 13:43
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    $\begingroup$ There are Some pretty fast moving neutron stars, I believe. Couldn't one of those slingshot an iron asteroid that crossed its path just right, close to the limit of tidal forces not breaking the asteroid up, to a ridiculously high speed? Chances of such encounter are tiny of course, but some must exist in the Universe, perhaps even in our galaxy. $\endgroup$
    – hyde
    Sep 4, 2013 at 20:12

Why don't we observe any relativistic asteroids?

The answer to this question would not be complete without mentioning the virial theorem.

Considering our galaxy as a system of $N$ gravitating objects, according to the virial theorem, twice the average total kinetic energy of all objects, plus the average total potential energy of these objects, adds up to zero. In simple terms this means that on average all objects tend to move with speeds of about 0.7 times their escape velocity.

As we, and any objects in our proximity, are bound to the Milky Way with an escape velocity of about 550 km/s, typical kinetic velocities have values of roughly 390 km/s. That is much smaller than the speed of light that is close to 300,000 km/s. Moreover, everything around us moves roughly in the same direction, following a circular flow pattern around the core of the Milky Way. So relative velocities between objects that encounter each other are typically smaller than the viral theorem suggests. Asteroids tend to hit earth's atmosphere with speeds of about 20 km/s. The fastest impact ever observed occurred at less than 30 km/s, or less than 0.01 % of the speed of light.

In principle you can consider the Milky Way and galaxies like Andromeda that are gravitationally bound to each other (the local group of galaxies) also as one system to which the viral theorem can be applied. This gives only slight changes in the gravitational binding, and hence slight changes in the average kinetic energy.

Beyond the local group, galaxies and all objects therein are speeding away from us due to the Hubble expansion.

  • $\begingroup$ But the question specifically asked about "rocks from other galaxies". Well, rocks in other galaxies are moving at relativistic speeds in relation to us... the farther away from us they are, the faster their relative speeds are... but they are receding, not approaching. So we're safe. $\endgroup$ Aug 9, 2013 at 15:15
  • $\begingroup$ @Eugene - good point. Have added a short paragraph. $\endgroup$
    – Johannes
    Aug 9, 2013 at 15:20

Relativistic rocks from other galaxies are in all probability out there. They'll never hit us, though, because they're far away and heading in the wrong direction, just like everything else we can see that moves at those velocities.

  • $\begingroup$ I believe that there aren't any known or unknown macroscopic objects near the speed of light out there, as has been pointed out in the answers above. $\endgroup$
    – yippy_yay
    Aug 9, 2013 at 18:57
  • $\begingroup$ @SebastianHenckel I interpret this FAQ (Prof. Ned Wright, UCLA) as saying that yes, very distant (highly redshifted) objects can and do move away from us at close to or even faster than c: "The time and distance used in the Hubble law are not the same as the x and t used in special relativity, and this often leads to confusion. In particular, galaxies that are far enough away from us necessarily have velocities greater than the speed of light [...]". Hence +1 from me for this Answer. $\endgroup$ Aug 9, 2013 at 19:28
  • $\begingroup$ @SebastianHenckel Eugene got it--I'm talking about the very distant galaxies that are moving away from us at relativistic velocity. There are no doubt bits that escape from them and would be called a meteoroid if they hit us. They're just going the other way. $\endgroup$ Aug 10, 2013 at 16:36
  • $\begingroup$ Yes, I saw the link. I guess you're right. The link talks about galaxies moving away from us faster than light. I guess there's some principle that makes it impossible for any matter from there to come over here or else we would be having close encounters with FTL objects. It must be a principle, not just 'we got lucky they're moving away from us'. I didn't understand the faster-than-light aspect. $\endgroup$
    – yippy_yay
    Aug 10, 2013 at 16:50
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    $\begingroup$ @SebastianHenckel The basic thing is that the velocity is obtained from the expansion of the universe rather than local forces accelerating the rock. $\endgroup$ Aug 10, 2013 at 17:08

A macroscopic body at over 900 km/s is a very unlikely event in the Solar system (may I skip explanations?), but an astronomic body “happy” enough to collide with a neutron star will certainly impact the surface at very high speed (values depend on mass of the star). BTW, relativistic collisions between neutron stars, or of a neutron star and a black hole, are a long-researched topic in numerical general relativity.

In spite of neutron star’s strong gravity, I do not expect so many bodies with high chances to crash into the star to make such impacts a frequent event. There should be few “relativistic asteroids” in the system of a neutron star at all. Planetary systems around neutron stars are known (see https://en.wikipedia.org/wiki/PSR_B1257%2B12 ) although rare, but even if there are many rocky bodies orbiting the star, most of them should have pericenters too far from the star, whereas a close encounter is necessary to gain a relativistic speed. I do not know typical conditions to infer how quickly can such orbit decay to a lower orbit with higher speed.


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