CAPE CANAVERAL, Fla. — When space shuttle Discovery thunders off the launch pad Tuesday afternoon, it will be a cleaner, tougher and vastly improved spaceship compared to the one that rolled off an assembly line in California in October 1983.

For its coming mission to the International Space Station, Discovery will sport 33 new and improved heat-protection tiles over critical areas of the ship and new ceramic bolt covers that won’t come undone in flight. It is also carrying new instruments installed on its belly to measure the heat buildup on the area when the orbiter makes its fiery dive back home.

What makes all these upgrades remarkable is that they were all installed for the craft’s final flight. After more than 26 years and 143 million miles circling the Earth, Discovery will be the first of NASA’s remaining three orbiters to be decommissioned — and ultimately sent to a museum — when it lands in about two weeks.

So why would NASA keep on spending tens of millions of dollars — the agency couldn’t come up with a precise figure — on an orbiter destined to become a roadside attraction? Isn’t it a bit like installing new brakes and tires on your car before bringing it to the scrapyard?

Not according to John Shannon, NASA’s shuttle program manager and the man overseeing the end of perhaps the most successful space vehicle ever to have flown.

“”We have had a mantra throughout the program that even though the program is ending, we are not going to stop trying to improve the vehicles,”” he said. For a program that’s twice seen tragedy strike — when Challenger exploded minutes after launch in 1986 and Columbia disintegrated while returning in 2003 — staying alert is critical, he said.

“”I have to have the team focused, paying attention and saluting the fact that these are still experimental vehicles,”” the bespectacled engineer said in a recent interview. “”We have to do everything we possibly can do to make sure we are flying the best vehicle we possibly can, and going through with these modifications and not taking our foot off the gas all the way to the last flight keeps that attitude.””

The possible consequences of letting slide the drive for continuous improvement are his biggest worry, he said.

“”If we said, ‘Well, don’t worry, we only have one flight left. Let’s not do that upgrade or safely improvement,’ then nobody would know where to draw the line,”” Shannon said. “”‘Is it OK to fly with this crack? Do I even need to report it because, for goodness sake, it’s just the next-to-last flight.'””

It’s a lesson that was hard learned.

A culture of ignoring concerns and not encouraging technicians to report problems was blamed in large part for the series of events that led to the Columbia tragedy, when a suitcase-size chunk of foam from the external fuel tank punched a hole in the shuttle’s wing. Foam had been coming off the tank during launches for decades, and engineers had learned to ignore it.

NASA’s mission management team decided there was nothing to worry about — and Columbia broke apart, killing all seven astronauts on board, during its fiery re-entry into the atmosphere.

Since then, NASA has made improvements on the shuttles before every flight, including installing harder heat-protection tiles in critical areas of the orbiter’s underside. The covering, known as “”Boeing replacement insulation,”” or BRI, tile, toughens areas around landing-gear and external doors — areas thought to be more vulnerable to flying bits of ice and foam on takeoff, or micrometeoroids in orbit.

Replacing older tile with BRI in strategic areas was one of the recommendations of the Columbia Accident Investigation Board to keep the orbiters flying. The 33 new BRI tiles bring Discovery’s total to more than 200.

Discovery was the first orbiter to test a lot of the post-Columbia thinking. In 2005, it was the first shuttle to return to flight.

Named after the vessel used by Henry Hudson in the early 1600s to explore Hudson Bay and search for a northwest passage to the Pacific, Discovery was the third shuttle to join the NASA fleet and first flew on Aug. 30, 1984. It’s currently the oldest orbiter in service.

Over the years, NASA has spent hundreds of millions of dollars for improvements in all three shuttles, to upgrade tires, landing gears, hydraulic pumps and even the cockpits, with old-fashioned “”steam gauges”” swapped out for digital displays.

All that now will fly to the space station one final time, to deliver spare parts and a cylindrical module, named Leonardo, that will be used for storage on the station.

If all goes according to plan, it will land 11 days later at Kennedy Space Center, where workers will begin removing toxic chemicals and readying it for a new life as a museum exhibit, most likely at the Smithsonian Air and Space Museum in Washington.

“”We are prepared to send Discovery out on a very high note,”” Shannon said.