DALLAS (REUTERS).- Shortly after Apollo 13 astronauts reported, "Houston, we have had a problem," Commander James Lovell jotted down handwritten calculations in hopes of guiding his crew safely home.

The notebook with those calculations from the aborted 1970 NASA mission to the moon fetched $388,375 at auction on Wednesday in Dallas when it was sold to an American collector who was not identified.

The notebook, the main attraction of a space memorabilia auction, was part of retired NASA commander Lovell's personal collection of artifacts. Lovell, 83, said he had forgotten about the notebook until recently.

"I was cleaning out some old stuff on a bookcase and found it," said Lovell, who now has homes in Horseshoe Bay, Texas, and the Chicago suburb of Lake Forest. "My kids took all they wanted and I donated a lot of my collection to museums."

"I decided to put this up for auction so that someone who is really interested in this piece of history can enjoy it," he told Reuters.

People around the world watched as Lovell and astronauts Jack Swigert and Fred Haise aborted their mission to the moon and maneuvered quickly in space to survive after encountering a critical operating problem aboard the spacecraft's command module. They had to shift to the lunar module to return home.

Lovell's calculations in the notebook were critical to identifying the crew's position in space.

"We didn't have the technology back then that we have now," Lovell said. "I didn't even have a calculator to do the arithmetic. I had to ask the people in Houston to double-check my numbers."

If the calculations had not been correct, the result could have been different, said Michael Riley, a senior historian at Heritage Auctions.

"That's why this is a very, very important piece of space memorabilia and American history," Riley said.

The drama of that moment in history was captured in Ron Howard's 1995 film "Apollo 13," starring Tom Hanks as Lovell.

Also sold at auction on Wednesday was an American flag from the Apollo 11 mission in 1969, which was bought for $33,460. The overall training jacket with tags and patches worn by retired astronaut John Young sold for $17,925.

(Editing by Corrie MacLaggan and Cynthia Johnston)