Note: Launch has been rescheduled for Feb. 3. The following story was written prior to the delay.

CAPE CANAVERAL, Fla. - The first probe to orbit another planet. A craft that gave mankind its only up-close look at Mercury. A robotic explorer that will encounter a star four million years from now. Dozens of satellites that expanded communications around the globe and bolstered American national security.

These diverse missions are linked in history by launching from the same Cape Canaveral launch pad, which hosts its last Atlas rocket liftoff on Thursday, January 27.

Complex 36 will see its final Atlas rocket launch on Thursday. Photo: Pat Corkery/Lockheed Martin

Built in the early 1960s, launch pad 36B has supported 75 Atlas missions. Its finale launch will be a middle-of-the-night affair, with the exact time to be concealed until next week.

The secrecy has been imposed by the launch's customer -- the U.S. National Reconnaissance Office. The NRO is the government agency responsible for operating the nation's fleet of spy satellites.

A countdown dress rehearsal was accomplished last Friday and the hush-hush payload was mounted atop the rocket Monday.

The upcoming mission represents not only the last use of Complex 36 by Lockheed Martin but also retirement of the rocket-maker's Atlas 3 booster. It'll be the sixth flight for the rocket, which debuted in May 2000 and boasts a flawless success record.

Atlas 3 was conceived as a short-lived, stepping-stone program between the older Atlas 2 series of rockets and the next-generation Atlas 5 vehicles. It safely introduced the Russian-designed RD-180 engine and later a stretched version of the Centaur upper stage that are vital parts of Atlas 5.

Lockheed Martin retired the Atlas 2 family last August and now the Atlas 3 era's conclusion leaves the company with the various Atlas 5 models -- each able to launch cargos of different sizes and weights -- to carry on the Atlas name that dates back a half-century. Atlas 5s are launched from the Cape's Complex 41.

The first Atlas 3 rocket blasts away from pad 36B in 2000. Photo: Pat Corkery/Lockheed Martin

The modern Atlas 5 is greatly different from the Atlas boosters of the early days. The first stage represents the biggest change with its rigid structure that replaces the "balloon" pressure-stabilized design that fades into history with Atlas 3.

Atlas 5 is assembled in a nearby building atop a mobile launching platform and rolled to its pad at Complex 41 just hours before liftoff unlike any other Atlas. West Coast military launches of Atlas 5 from California's Vandenberg Air Force Base, however, will use the more conventional method of putting the rocket stages together on the pad.

Development of Atlas 5, spurred by the Air Force's call for an Evolved Expendable Launch Vehicle, was aimed at creating a family of rockets that would be cheaper, easier to operate and offer higher reliability. So far, four Atlas 5 launches have occurred with successful outcomes since 2002.

For the past two-and-a-half years, Atlas workers at Cape Canaveral have shuttled between Complex 36 and 41 as they prepped and launched the heritage boosters as well as Atlas 5. They will call Complex 41 their exclusive home after next week's mission. The program's Atlas 5-only era begins in March when a commercial communications satellite is set for launch.

Pending a safety and environmental review, a new commercial rocket family will be using Complex 36. Photo: Pat Corkery/Lockheed Martin

Once the Atlas 3 flies over the horizon, Lockheed Martin begins an extensive safing and deactivation process at Complex 36 after 145 launches from the two-pad site. Pad A was secured following the final Atlas 2AS rocket launch on August 31.

Pad 36B, where the 18-story Atlas 3 currently stands, saw its first launch in August 1965 as an Atlas booster lofted a dynamic model of NASA's Surveyor program. This very pad has served as the starting point for several historic exploration missions:

• Several Surveyors: These lunar explorers were designed to make soft landings on the moon as precursors to Apollo, including Surveyor 3 that was visited by the Apollo 12 crew two years later.
  
  
• Mariner 6: A robotic probe that flew past Mars in 1969, returning 75 pictures of the planet.
  
  
• Mariner 9: Became the first spacecraft to orbit another planet when it arrived at Mars in 1971. The probe mapped the Red Planet's surface and captured the first sharp pictures of Mars' polar caps, Valles Marineris, volcanoes and the moons Phobos and Deimos.
  
  
• Mariner 10: Launched in 1973, this was the first space probe to visit two planets. It flew past Venus - where it discovered evidence of rotating clouds - en route to the solar system's innermost planet, Mercury. Mariner 10 gave the first and so far only up-close examination of Mercury when it made three encounters that found a thin atmosphere and magnetic field.
  
  
• Pioneer 11: After sailing past Jupiter, the probe became the first to explore Saturn and its majestic rings in 1979. Although communications with Pioneer 11 ceased in 1995 after leaving the solar system, the spacecraft is traveling toward the constellation of Aquila where NASA says it will pass near one of the stars in about four million years.

More recent historical footnotes for pad 36B include the first flights of Atlas 1, 2, 2AS and 3 rockets since 1990, launches of civilian weather satellites that provide images and information that Americans see every day on TV, the premier sun-watching probe called SOHO, commercial and military communications satellites linking users around the world and several hush-hush spy spacecraft.

The secret payload, already enclosed in the rocket's nose cone, is lifted into the pad 36B tower for mating to the Atlas 3 vehicle. Photo: ILS

What the Atlas 3 will be launching Thursday is shrouded in mystery. The NRO won't comment on its satellites given the classified nature of the spacecraft.

"Everything we put up is uniquely built for a particular mission, and this is the same," NRO spokesman Rick Oborn said.

It will take about 78 minutes for the Atlas 3B and its single-engine Centaur upper stage to haul the payload into the desired orbit. The actual details of the orbit's altitude are being kept under wraps, too. But officials acknowledge the rocket will head northeastward from the Cape to a highly inclined orbit extending more than 60 degrees north and south of the equator.

The payload will be deployed from the rocket over the South Pacific. If all goes smoothly, this would be the record-extending 75th consecutive time an Atlas has achieved success since 1993.

The Atlas mission is managed by International Launch Services. The firm markets the American Atlas and Russian Proton rocket families.