It's a good question. Yalkut Yosef says in Brachot 228:16
Contemporary Poskim debated whether the blessing of ose ma'ase bereshit is to be recited over other natural phenomena that inspire wonder and awe, such as volcanic eruptions, caverns of stalactites and stalagmites, natural geysers and waterfalls. In practice, we must apply the principle of safek berakhot lehakel and recite the blessing without pronouncing God's Name.
The footnote:
The Or Letziyon (Vol. II, 46:62) ruled that this blessing is to be recited upon entering a cavern filled with stalagmites and stalactites...Obviously, he was certain that these features have existed since the world's creation... According to the Or Letziyon the list of phenomena in the Mishna - thunder, lightning, shooting stars etc. are only examples of natural awe-inspiring phenomena... Harav Hayim Kanyevski ruled that the blessing can be recited over those phenomena listed in the Mishna only. The Halakha Berura (page 481) concurred with this.
Regarding Rivers, the footnote for (ibid) 9:
Regarding the Tigris and Euphrates rivers, the Mishna Berura (228:5) cited the Eliya Rabba rule that if one has reason to suspect that the section of the river he is viewing was diverted at some time, he must not recite the blessing. In the Sha'ar Hatziyun he cited the Peri Megadim who ruled that unless one knows otherwise, one may assume that it was not diverted, but he added that he is reluctant to rely on that. We must apply the principle of safek berakhot lehakel. This matter needs more research.
As you can see, the most contemporary poskim have not thoroughly investigated even the matter of rivers.
The principle does seem to be the same between rivers and all other natural phenomena this bracha is stated on - it is to do with whether they have existed since creation. The question becomes, does a "diverted shooting star" fall into the "diverted river" camp?
On the one hand, a shooting star that would have been seen before it is diverted, can be considered as still something that has "existed since creation", but on the other, one can make this argument about diverted rivers as well. All we have done is divert it, but it's not like we made the river. Still, the diversion removes it from its status as "existing since creation" and we no longer say a bracha on it.
So we should certainly not feel qualified to give an answer here on Mi Yodeyah, and leave it to the gadolei adar to do "more research" and get back to us on the matter.
In the meantime, it seems safek berakhot lehakel is a very strong principle when there's even the slightest doubt (and it seems there's a strong reason to doubt here) as the Yalkut Yosef does with these "other natural phenomena" even though he is very pro-saying shem u'malchut for the "special brachot" in general (see ibid 224, 226-229), so certainly in this case, until the gadolim do the research, I wouldn't say a bracha with shem u'malchut on a diverted comet.
Remember, Mi Yodeya answers are never intended to be taken as psak.
