Learn French: L'évaluation de l'objectif scientifique est très rigoureuse. - The evaluation of the scientific objective is very rigorous.

Rigoureuse describes l'évaluation (the evaluation), not l'objectif (the objective). Because évaluation is feminine, the adjective must agree with it, changing from the masculine rigoureux to the feminine rigoureuse.

Is there a reliable way to remember the gender of évaluation?

Yes! In French, almost all nouns that end in -tion are feminine. Remembering this rule will save you a lot of time when trying to figure out whether to use le or la.

Does objectif translate exactly to the English word "objective"?

It is very close. Just like in English, it can mean a "goal" or "aim" (as it does in this sentence), or it can describe someone who is unbiased. Watch out though, because in French, un objectif is also the word for a camera lens!

How could I use this lesson's grammar topic (afin de) to add more detail to this sentence?

You can easily attach afin de followed by an infinitive verb to explain the purpose of this strict evaluation. For example: L'évaluation de l'objectif scientifique est très rigoureuse afin de garantir le succès (The evaluation of the scientific objective is very rigorous in order to guarantee success).