Question: Who is the presiding officer of the senate when the vice president is not there?
Disambiguated questions and answers: question: Who is the presiding officer of the senate if the vice president is not there?
Who is the presiding officer of the senate when it's an impeachment trial?
answer: President pro tempore
Chief Justice of the Supreme Court
Natural Questions answer: President pro tempore (from the Wikipedia article "Presiding Officer of the United States Senate")