As others have already explained well, hydrogen in the air doesn't contribute to the explosion.
But what does make a big difference is the air around the bomb---a nuclear explosion in space is way different from one in atmosphere. X-rays from the nuclear reaction heat surrounding air to many millions of degrees [1] and that's what causes (most of) the fireball you see. In space, the only matter available to make a fireball is the weapon's structure, probably only a few hundred kg, and that dissipates and cools rapidly. Nuclear-armed air intercept missiles were built in the nineteen-fifties and sixties that depended for effectiveness on being in air to generate the necessary blast effects to kill a bomber with relatively inaccurate aiming, but in space, a nuclear explosion almost needs be a contact hit to do much mechanical damage...discounting nuclear radiation effects, of course.
[1] Fahrenheit, Celsius, Kelvin...it makes no difference.
> Third, in the absence of the atmosphere, nuclear radiation will suffer no physical attenuation and the only degradation in intensity will arise from reduction with distance. As a result the range of significant dosages will be many times greater than is the case at sea level. With such weapons the lethal radii (from nuclear radiation) in space may be of the order of hundreds of miles.
Second, thermal radiation, as usually defined, also disappears.
There is no longer any air for the blast wave to heat and much
higher frequency radiation is emitted from the weapon itself.
So more of the energy of the bomb remains in the x-ray or gamma portion of the spectrum---penetrating radiation, hard to shield against---in addition to the longer effective range.
But what does make a big difference is the air around the bomb---a nuclear explosion in space is way different from one in atmosphere. X-rays from the nuclear reaction heat surrounding air to many millions of degrees [1] and that's what causes (most of) the fireball you see. In space, the only matter available to make a fireball is the weapon's structure, probably only a few hundred kg, and that dissipates and cools rapidly. Nuclear-armed air intercept missiles were built in the nineteen-fifties and sixties that depended for effectiveness on being in air to generate the necessary blast effects to kill a bomber with relatively inaccurate aiming, but in space, a nuclear explosion almost needs be a contact hit to do much mechanical damage...discounting nuclear radiation effects, of course.
[1] Fahrenheit, Celsius, Kelvin...it makes no difference.