LOS ANGELES — NASA was reeling yesterday night after one of its top astronauts was arrested and accused of hatching a bizarre plot to kidnap and murder a love rival.

Lisa Nowak, 43, a U.S. Navy captain and NASA robotics specialist who flew on a shuttle mission to the International Space Station last July, is facing trial for attempted murder.

She is alleged to have plotted to snatch Colleen Shipman, 30, a U.S. Air Force captain whom Ms. Nowak believed to be romantically involved with a space shuttle pilot.

The married mother-of-three was said to have driven 950 miles from her home in Houston, Texas, to Orlando International Airport in Florida, to intercept Ms. Shipman. During the drive, she reportedly wore a diaper — as astronauts do during launch and re-entry —so she would not have to make an unscheduled stop en route to Orlando.

Once at the airport, disguised in a long coat, black wig, and glasses, Ms. Nowak allegedly followed the younger woman to the car park where she confronted her, spraying pepper spray in her face. But Ms. Shipman managed to drive off and alert police, who found Ms. Nowak still in the parking lot.

Police said a bag she was carrying contained an air rifle, a steel mallet, a 4-inch folding knife, rubber tubing, and bin bags.

In her car, left nearby, authorities found a pepper spray package, an unused BB-gun cartridge, latex gloves, and e-mails between Ms. Shipman and William Oefelein, 41. Mr. Oefelein, a graduate of the U.S. Navy’s “Top Gun” fighter school, made his space debut in December as pilot for Discovery on the most recent shuttle mission with British-born astronaut Nicholas Patrick.

According to the arrest affidavit, a letter “that indicated how much Ms. Nowak loved Mr. Oefelein” was also recovered along with Ms. Shipman’s address near Cape Canaveral and hand-written directions.

Ms. Nowak appeared before a judge at Orange County Jail in Florida yesterday. Looking worn and a world away from a model NASA astronaut, she faced charges of attempted kidnapping, attempted vehicle burglary, and battery. She was to be freed on $15,000 bail, subject to being tagged and not contacting her alleged victim, not even “to send flowers to say ‘I’m sorry,'” the judge said.

Police said they were planning to add attempted murder to the charge sheet.

Ms. Nowak will remain in protective custody until her first appearance before a judge on the new charge, expected today. Chris Ferguson, a pilot on Ms. Nowak’s Discovery mission, said when asked for his reaction to his colleague’s behavior: “Perplexed is the word I’m sticking with.”

Ms. Nowak trained with Mr. Oefelein, but the pair never flew together. She told police her relationship with Mr. Oefelein, who is not married but a father of two, was “more than a working relationship but less than a romantic one.”

NASA, which is famously straight-laced and subjects all astronauts to rigorous psychological and physical testing, does not bar them from having relationships with one another. The organization said it was unaware of any previous incidents in which an astronaut had been arrested.

James Hartsfield, a NASA spokesman, said Ms. Nowak’s status with the astronaut corps remained unchanged. “What will happen beyond that, I will not speculate,” he added. He said Ms. Nowak, who was scheduled to play a major part in the next shuttle flight, “did an excellent job” during the July mission. “There is a feeling that it is all very unfortunate for the families involved,” he said.

In her statement to police, Ms. Nowak, who has a teenage son and younger twin girls, said she had not intended to harm her victim but “only wanted to scare Ms. Shipman into talking with her.”