Who is the presiding officer of the senate when the vice president is not there?
This list includes all Presidents of the Senate (the Vice Presidents of the United States), those Presidents pro tempore of the Senate who presided during intra–term vacancies in the vice presidency or when the Vice President was acting as President of the United States, and those Chief Justices who presided during presidential impeachment trials. It does not include Presidents pro tempore who presided over sessions temporarily during an absence of the Senate President, or junior senators designated by the President pro tempore to preside temporarily.
Who is the presiding officer of the senate if the vice president is not there?
President pro tempore
Presiding Officer of the United States Senate
No context provided
Who is the presiding officer of the senate when it's an impeachment trial?
Chief Justice of the Supreme Court
Presiding Officer of the United States Senate
https://en.wikipedia.org/wiki/Presiding%20Officer%20of%20the%20United%20States%20Senate
The Presiding Officer of the United States Senate is the person who presides over the United States Senate and is charged with maintaining order and decorum, recognizing members to speak, and interpreting the Senate's rules, practices, and precedents. Senate presiding officer is a role, not an actual office. The actual role is usually performed by one of three officials: the Vice President; an elected United States Senator; or, in special cases, the Chief Justice. 
Presiding Officer of the United States Senate
The Constitution also provides for the selection of one of the elected senators to serve as President pro tempore. 
Presiding Officer of the United States Senate
The Presiding Officer of the United States Senate is the person who presides over the United States Senate and is charged with maintaining order and decorum, recognizing members to speak, and interpreting the Senate's rules, practices, and precedents. Senate presiding officer is a role, not an actual office, the actual role is usually performed by one of three officials, the Vice President, an elected United States Senator, or, in special cases, the Chief Justice. The Constitution also provides for the selection of one of the elected senators to serve as President pro tempore, and the presiding officer of the senate when it's an impeachment trial is the Chief Justice of the Supreme Court.    