Word for “Essential that it is not”

Question

I'm wondering if there is a word that describes how it is essential that something not be true.

For example, where one might say:

"The President knows it is essential that we make it to the moon first."

The opposite would be

"The President knows it is [...] that the Russians make it to the moon first."

Of course, the negation "unessential/inessential" doesn't mean that, it just means that it doesn't matter.

The whole sentence could be simplified to "The President knows that the Russians must not make it to the moon first." but that's not used like the word essential.

You could possibly replace it with something more emotional like "The President knows that it is inconceivable that the Russians make it to the moon first." But that's not a real truth statement.

Answer 1

unacceptable Unacceptable can be much stronger on the negative side than acceptable is on the positive side. Acceptable frequently means OK. Unacceptable can mean intolerable.

The Free Dictionary says:

2. intolerable: hitting children is unacceptable

There are also milder definitions of unacceptable (TFD, same link)

1. not satisfactory; inadequate: the standard was wholly unacceptable

Note that here, wholly reinforces unacceptable so that the phrase takes on the meaning of the first definition I listed -- intolerable.

The OP's sample sentence becomes:

"The President knows it is unacceptable that the Russians make it to the moon first."

The meaning intolerable is understood from the context.

Answer 2

If you consider 'essential' in a more literal sense than 'absolutely necessary', but rather

pertaining to or constituting the essence of a thing.

it might be clearer why you wont find a perfect antonym in the way you are hoping.
The essence of a thing,

the basic, real, and invariable nature of a thing or its significant individual feature or features:

is what defines it and, in that sense no value judgement is attached to it, it just is.
Therefore, if the essence of the acceptable situation is the US putting men on the moon first then the Russians putting men on the moon first is merely the essence of the unacceptable situation.
So my answer is that there is no word for 'essential that it is not' because the difference is in the state of being (is/is not) rather than in the essentialness of the state of being.
Which doesn't mean that there aren't ways of expressing the idea of it being essential that something is not, only that the word can't function in exactly the way 'essential' does.