When most people think of the ships in NASA's fleet, they think of the space shuttles that pierce the sky as they carry astronauts toward space. But NASA has two seagoing ships, Liberty Star and Freedom Star, which also stand ready on shuttle launch day. Their crews' mission is heading to sea to retrieve the two solid rocket boosters that power the shuttle's ascent.

"A typical crew that we carry is 24 people. We've got ten crew, ten diver specialists, and retrieval operations personnel," says Freedom Star Captain Mike Nicholas, a 24-year booster retrieval veteran who works for United Space Alliance. "We depart the port 24 hours in advance. It takes us roughly 12 to 15 hours to get offshore to our SRB impact area.

Then we'll stand by, and do surveillance work to keep other vessels out of the area so that when the launch goes, we have a window that the boosters can come in safely without any traffic being around."

The crews, divers and ships are prepared long before the solid boosters ignite at the launch pad.

"We're ready to go before launch," says Larry Collins, Manager of Dive Operations for United Space Alliance. "We'll have all the diver gear ready, all of our camera gear ready, all the retrieval equipment ready to go. The reels will have all the lines on them, everything that we use for retrieval will be ready."

But diving isn't their only function during their mission at sea.

"The divers are also the retrieval team," explains Collins, "so the divers operate the cranes, they operate the small boats, they operate the reels. They are multi- functional. They're the diver medics that operate the recompression chambers, they fill the tanks, they lead the dives, they do the diving. Everybody is doing everything."

While most eyes are still glued to the shuttle's climb, the crews and divers aboard these two vessels are on alert, ready to power toward the boosters' impact zone in the Atlantic Ocean to begin their work. When the boosters are spent, they are jettisoned and fall to the sea as the shuttle's main engines finish the job of lifting the spacecraft out of Earth's atmosphere and into orbit.

"Once the launch goes off, we can actually visually see the boosters, or we have our radar equipment that we can see them coming through the air," says Nicholas, who adds that the boosters make a sonic boom during their descent, much like the returning shuttle does before landing. "Generally they're as close as four miles from us, which is very close, and as far as 15 to 20 miles away."

"We are not allowed in until after it's cleared," explains Collins. "It can take us about an hour to get to the boosters from where we are."

During the operation, each ship is assigned to retrieve one booster. First the pilot chutes and main parachutes are brought aboard. They're followed by the drogue parachute and the 5,000-pound frustum that houses the chutes at the top of the booster. With those elements secured onboard, attention turns to the booster itself, as a team of divers boards two small boats and first conducts a visual inspection of the booster.

"After that we go into the recovery of the booster itself. The next operation would be to launch the diver operated plug, which is about 1,400 pounds. We use four divers to put this plug in," says Collins.

"When we're diving deep around the booster, we usually have at least one, sometimes two safety divers, and we try to do video as well. So it's not unusual for us to have seven divers on that dive to put the plug in."

"We dive it down, put the plug into the nozzle, attach an inch-and-a-half air line to that, and start blowing air into the booster that forces the water out of the booster," Collins explains. "The booster rises up out of the water and falls over on its side. At that point we attach an inch-and-a-half steel cable to the booster once it's dewatered and proceed to tow it back to port."

Retrieving the boosters is a potentially dangerous operation and requires that the ship's crew and divers closely follow safety procedures as they work with the booster components, staying constantly aware of the ever-changing conditions around them.

"Weather conditions are a major factor because a lot of times you have to pay attention to what the wind and the waves are doing and they'll dictate where you can actually position the vessel, putting people in the water, putting boats in the water. You have to give the ship a lee so that the boats can be launched in the water so that the personnel goes in safely," says Nicholas.

"The other particular challenge is when it's windy and seas are high, there's usually a lot of current, meaning that the maneuvering of the vessel takes a lot of concentration to make the vessel do what you want it to do, or stay where you want it to stay."

The memory of their toughest retrieval brings back the same mission for both captain and diver.

"The roughest retrieval was probably STS-63. The conditions were so bad that the boosters were badly damaged, the nozzles were broken loose, the forward skirts were damaged. In fact one of the forward skirts fell off on the way back in. The conditions were so bad that for many, many days we couldn't even attempt a recovery," remembers Collins.

Both ship crews and dive teams are cross-trained, with divers serving in a variety of positions between launches. Training keeps them sharp for each mission.

"We do at least four training missions a year where we go out and train on the same type of operations as much as we can to a retrieval. We also do diver training, and we do some operations with NOAA and the Navy where we actually perform diving ops," says Collins.

In addition, these ships and their crews have participated in full-scale drills to prepare them to assist in rescuing astronauts in the event of a mishap over the ocean.

While these ships were specifically designed for booster retrieval when they were built in 1981, they took on added duties over the years. Beginning in 1998, they were put into service as "tow boats" providing the transportation for the barges that carried the shuttle's external fuel tanks.

The ships transported the tanks from the Michoud Assembly Facility in Louisiana to NASA's Kennedy Space Center on Florida's east coast, entering through Port Canaveral. This 12-day roundtrip required that the ships be retrofitted for towing the heavier barges with the tanks onboard.

The Freedom Star helped deliver the last external tank to Kennedy in September 2010, and with the approach of the final shuttle mission, the crew and divers will be as keenly focused on the work at hand as ever.

"I haven't really dwelled on what that last shuttle mission will feel like," says Collins. "You're so busy doing the job, trying to make sure that the job is done correctly, and that everyone gets home safely, that you don't really have time to think about what those feelings are. That will come later."

"My thoughts about the very last space shuttle launch, just for myself, is that I'm very, very fortunate to have been a part of something that's very unique and very special," says Nicholas.

"I think the mood on the ship on the way back after recovery is going to be probably somber, maybe very reflective. We always have a victory dinner on the way home, and I think that when we all sit down, there'll be a lot of talk about past trips, who's still here and who's gone, and where everybody's going to go. It should be pretty interesting and probably very humbling."

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