The Astral Queen is a prisoner transport ship. There are currently 1,500 prisoners on board (a figure retconned from 500). The Astral Queen was transporting prisoners from Sagittaron to their parole hearings on Caprica at the time of Cylon attack and is one of the vessels that manage to regroup with President Roslin's refugee fleet. Viewed as another transport ship, albeit one with a slightly undesirable cargo, none of the government personnel that survived the assault knew that it was carrying Tom Zarek, a dangerous political prisoner and anti-government terrorist.
Following an attack on the fleet water supply, Captain Lee "Apollo" Adama is dispatched to the ship in order to enlist the prisoners so they can help transport water from an icy moon that has been found nearby. After greeting the prisoners' spokesman, Tom Zarek, the prisoners break out of their cells and manage to hijack the ship. They later surrender once Apollo agrees to Zarek's demands, which are to hold a fair election for a new president at the end of Roslin's term. In return, Apollo leaves Zarek's men in control of the Astral Queen, against the wishes of both President Roslin and Commander Adama.
The Astral Queen becomes the focus of several incidents within the fleet. First, repair crews made up of the vessel's ex-convicts begin to operate in competition with those organized by the government. For these efforts, Tom Zarek becomes the Sagittaron representative to the reformed Quorum of Twelve. From there, Zarek makes a failed bid at the vice presidency.
Later, after Laura Roslin is stripped of her presidency by Commander Adama, and martial law is declared in the fleet, the Astral Queen becomes the flagship of a splinter group departing the fleet for Kobol, made up of 24 ships. The crews and passengers of these ships are supporters of Laura Roslin's presidency, and she is aboard the Astral Queen when it jumps away from the fleet. The splinter fleet rejoins Galactica and the fleet after Commander Adama and President Roslin resolve their differences.
The same ship design appears in the original series, where it is known simply as the "Prison Barge", but serves the same basic function as it does in the re-imagining. Count Baltar spends most of the latter part of the series imprisoned on the Prison Barge.
That ship was in turn named for the ship which serves as the centrepiece of Isaac Asimov's short story, Marooned off Vesta