Learn English – How should I understand “beckon” in this sentence

It's not two verbs, cracked and open. It's one phrasal verb, crack open. Open is the particle (open can be an adjective and an adverb, as well as a verb), and crack is the verb.

To crack means to make a crack, or opening, in something, which can be literal, like crack an egg, or figurative, like crack the books (which means study) or crack another bottle of scotch, which means to open a sealed bottle.

The phrasal verb crack open means to open something sealed (like a door) a very small amount ("only a crack"), so that one can see inside. It refers to the very beginning of an opening.

In this case, it's metaphoric, and refers to the first "opening" of scientific knowledge of the atom, which was due to Curie's work.

It's a simple Extraposition.

However, it's not correct to say that the boldfaced that refers to it in the sentence. In fact, neither that nor it has any reference at all. They're both function words; that is a complementizer which introduces a tensed subject Complement clause, while it is a dummy subject introduced by Extraposition to keep the subject slot open.

This sentence comes from something like the following, with all pronouns expanded:

• That while humbug has some of the distinguishing characteristics of lies, there are other characteristics that humbug lacks must be part of the point of saying that humbug is "short of lying".

This is even less clear, which is why Extraposition moves the entire subject

• that while humbug has some of the distinguishing characteristics of lies, there are other characteristics that humbug lacks

(leaving it behind as the the dummy subject) to the end of the verb phrase

• must be part of the point of saying that humbug is "short of lying"

where it can be parsed more easily. Well, somewhat more easily, anyway. This is a good place to use the fact that, and to emphasize the contrast between some and others to make the parallels clear.

• The fact that humbug has some of the distinguishing characteristics of lies, but lacks others, must be part of the point of saying that humbug is "short of lying".