Washington — As President Trump delivers his State of the Union address to a joint session of Congress on Tuesday, all his top aides and Cabinet members will be in attendance — except one. This year, it's Interior Secretary David Bernhardt.

The "designated survivor" is the member of Mr. Trump's Cabinet who will be whisked away from D.C. by U.S. Secret Service, just in case catastrophe strikes at the Capitol and decimates the president and the top government officials. The designated survivor would ensure continuity of government by taking over the presidency in that case.

The novel concept inspired a primetime show, "Designated Survivor," in which a fictional Housing and Urban Development secretary, played by Kiefer Sutherland, is the sole survivor when a terrorist attack takes out the president and the rest of his Cabinet during a State of the Union address. The show ran on ABC in 2016, and when it was canceled after two seasons, Netflix picked it up, but canceled it again after one season.

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Here are a couple of other things to know about the designated survivor:

Who is the designated survivor this year?

The White House hasn't told us yet but typically makes the announcement shortly before the speech. Last year, then-Energy Secretary Rick Perry was chosen. Check back for updates.

Why is there a designated survivor?

The Presidential Succession Act of 1947 created a line of succession if the president dies or is incapacitated: first to take office would be the vice president, followed by the speaker of the U.S. House, the president pro tempore of the U.S. Senate, the secretary of state, the treasury secretary, the defense secretary, and so on.

What happens when you're the designated survivor?

In 2017, when then-Veterans Affairs Secretary David Shulkin was selected, White House chief of staff Reince Priebus gave him a call several days before the joint address to Congress to ask him to serve as the designated survivor. Hours before the speech, the Secret Service spirited him away to an undisclosed location, where he watched the address on TV. At the time, Shulkin was 17th in the line of succession to the presidency, next to last, just before the Homeland Security secretary. He remained at a secret location until the president and others in the line of succession had been safely returned to their homes.

Who's invited to the address?

Current and former members of the House and Senate

Vice president

Cabinet (except the designated survivor)

Supreme Court justices

Joint chiefs of staff

Diplomatic corps

The president also invites several guests, and members of Congress are allowed to invite a guest, too.

Mark Knoller contributed to this report.

For complete coverage of the State of the Union address, download the CBS News app.

