DOUBT

'That' is a relative pronoun. Plural verb is used after relative pronouns (that, who, whose, which whom). Example:

(1) One of the students 'is' tired

Here, 'is' agrees to that one student.

(2) One of the students who 'are' studying **is **tired.

Here, 'are' agrees to 'students' (plural) because it is telling information about what the **students **are doing. Then again, 'is' is telling information about that 'one student'. (singular)

Similarly in this question, 'have' tells us information about the 'telescopes' that have been launched and whatever. But, the 'are' before 'currently' is wrong because it is telling information about Hubble Space Telescope(singular) , not every telescope(plural). Thus, 'is' will be used.