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At What Distance Can We Detect a Nuclear Explosion in the Universe?

-- Joe | October 24, 2025

Question:

Hello,

I have a question… taking account of our current technology, do we have the ability to detect a nuclear weapon being detonated in another solar system?

Hypothetically, at what distance would we be able to detect a 150kt weapon being tested in another solar system?

Thanks,
Joe

-- Joe

Answer:

Let’s start with a modest example. A large nuclear bomb (say 1 megaton of TNT) releases roughly:

$E_(bomb)=4.18×10^(15) Joules$

Much of that goes into heat and blast, but suppose for an optimistic upper limit that 1% of this energy, or about 4×10^(13) Jourles, is emitted as visible or near-infrared light. This is an extremely generous estimate, but will allow us to set an upper bound. Next, we need to convert this energy output to a luminosity, which is energy emitted per second (measured in Watts). A 1 megaton nuclear bomb’s fireball will last about 1 second, so the optical luminosity will be approximately:

$L_(bomb)≈4×10^(13) Watts$

Compare this luminosity to that of the Sun, which is $L_(sun) = 3.8×10^(26) Watts:$

L_(bomb)/L_(sun) =~ 10^(-13)

Now, to compare this to what a telescope can detect from an object at a distance of D in space, we need to take account of the fact that the flux F of an object falls as 1/D^2:

F = L/(4*pi*D^2)

Comparing to example detection thresholds:

Naked Eye: F ~ 10^(-10) W/m^2
Hubble-class Telescope: F ~ 10^(-20) W/m^2
JWST; F ~ 10^(-21) W/m^2
10m optical telescope integrating for many hours: F ~ 10^(-22) W/m^2

… and solving for the distance D in the above equation:

D = sqrt(L/4*pi*F)

… allows us to solve for the maximum distance that a one megaton nuclear explosion (L = 4×10^(13) Watts) would be detectable with each measurement system above:

Naked Eye: D ~ 1.8×10^(11) m ~ 1.2 Astronomical Units
Hubble: D ~ 1.8×10^(16) m ~ 2 light years
JWST: D ~ 5.6×10^(16) m ~ 6 light years
10m optical telescope integrating for many hours: D ~ 1.8×10^(17) m ~ 19 light years

This suggests that even the most powerful telescopes could only detect a 1 megaton nuclear explosion out to approximately four-times the distance of the nearest star (about 4 light years), and could not be seen at the nearest cluster of stars (about 153 light years). This also assumes that we caught the blast at just the moment that it exploded and that 1% of its energy was emitted at optical wavelengths. These are very optimistic assumptions, so the actual distances are likely to be much closer than the estimates above.

For your example 150 kiloton nuclear bomb, the estimated luminosity would be 0.15 times the 1 megaton example, which would make the estimated distances above decrease by a factor of sqrt(0.15), or about a factor of 40%.

-- Jeff Mangum

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