So this spring, Cook's quest for one of the two final spots will have to go through Chacon, whom the Rockies believe has a future if he and curbs a nasty bases-on-balls habit on the mound and develops workout habits to satisfy manager Clint Hurdle off it; right-hander Scott Elarton, who missed last season because of shoulder surgery but was a 17-game winner in Houston in 1999; possibly left-hander Darren Oliver, who could be eyed for a relief job; and right-handed prospects Jason Young and Chin-hui Tsao.

The Rockies believe Cook can handle it.

"Obviously, he has a good arm and pitched well for us last year, but it's more important that he's an excellent young man," Rockies general manager Dan O'Dowd said. "Of course, we're going to have him compete for a job. But he's a very competitive young man who has battled, and last year helped him much the same way Jason Jennings helped himself with a few starts during '01."

What Cook did best after his call-up was demonstrate the resilience and the knack for timely pitches that help a pitcher survive at Coors Field.

The first batter he faced, the Chicago Cubs' Moises Alou, homered on an 0-2 pitch at Coors, but Cook retired seven of his next eight hitters and eventually moved into the rotation. His first start was a topsy-turvy, no-decision against San Francisco that saw him allow three runs, nine hits and four walks in six innings, but limited damage with four double-play grounders. His only loss came when San Diego shut out the Rockies, and he finished by forcing three ground balls to every fly ball.

"The first couple of times out he was nervous, but once he settled down and could focus on just pitching you could see the ability that Aaron has," said Jim Wright, the Rockies' pitching coach last year and now the team's minor-league roving pitching coordinator. "He let his stuff work, and showed that he was someone special."

The Rockies knew that when they picked Cook in the second round of the 1997 draft out of Hamilton (Ohio) High. Then a skinny youngster, Cook made all the prospect lists early in his pro career but by the end of 1999 had a 10-23 record. That included a 1999 that saw him put on weight before he knew much about weight training, and finish 4-12, 6.44 at Single-A Asheville.

"I learned by screwing up," Cook said.

But by 2000, Cook began simply learning.

"In his own mind, he started to smell it. 'Hey, I can be pretty good,'" said Rick Matthews, who worked with minor-league pitchers and this year will serve as Colorado's bench coach. "It wasn't us telling him he had a chance. "

Cook also put in the mental and physical work. In 2000 and 2001, Bob McClure, Cook's pitching coach at Salem, helped him develop a more compact windup by pretending to throw in a narrow hallway.

By 2002, Cook had become a pitcher that managers rated the top prospect in the Double-A Southern League. He was promoted to Colorado Springs in mid-August, where McClure had him build confidence in the change-up. Ten starts later, the Rockies called Cook and saw what they wanted.

"He's a power sinker guy, a lot like Jason Jennings only he throws harder," McClure said. "He's tall, has a good build, has good character and is very astute. He's never complacent. Once he got it, there was no stopping him."

Now Cook, who followed up his big-league debut with a wedding to his new wife, Holly, on Oct. 26, wants to make sure the competition in camp doesn't stop him.

"I'm a very level person ... I really don't show emotions or bring them to the surface that much," Cook said. "But I'm really excited going into camp being a guy considered for being in the rotation. That's what I've wanted to do my whole life."

Thomas Harding is a reporter for MLB.com. This story is not subject to the approval of Major League Baseball or its clubs.