Big science is finally headed to the international space station.

As the space shuttle Endeavour stands ready for its 25th and final flight, scheduled to begin Friday afternoon, the spotlight shines on commander Mark E. Kelly and his wife, Rep. Gabrielle Giffords (D-Ariz.).

Giffords arrived in Florida on Wednesday to observe the launch, her first trip away from the Houston rehabilitation hospital where she is being treated for injuries suffered in a shooting at a public event in Tucson in January.

President Obama also plans to watch the launch with his family after touring the storm-stricken South.

But to scientists, the real star is tucked away in Endeavour’s cargo bay: A $2 billion, seven-ton cosmic experiment about 17 years in the making.

The brainchild of Nobel Prize-winning physicist Samuel C.C. Ting, the experiment will sniff space for cosmic rays, antimatter, dark matter and other exotic and poorly understood phenomena.

If the mission goes as planned, the device will deliver marquee science to the space station, which has had its science mission repeatedly trimmed since the $100 billion orbiting outpost was proposed in the 1980s. Although the station boasts three laboratory modules, maintenance duties preclude each of the station’s six astronauts from spending more than an hour a day on science experiments, according to a 2009 report from the Government Accountability Office.

“I think the AMS will be a great uplift for American particle physics,” said Ulrich Becker, Ting’s colleague at the Massachusetts Institute of Technology. Becker is one of 600 physicists involved in the experiment, called the alpha magnetic spectrometer, or AMS.

But not every physicist is happy about the AMS. A vocal contingent complains that Ting circumvented the usual scientific review processes at NASA and the Department of Energy. They also say the expensive experiment will offer limited information.

“This kind of science is not worth billions of dollars,” said Gregory Tarle, an experimental physicist at the University of Michigan at Ann Arbor. In some physicist circles, Tarle said, “people are shaking their heads that Sam could do this. He did it by bullying his way through.”

Ting, in a phone interview, steered clear of the controversy, saying that it is “very hard to predict” what exotic phenomena the device might find.

The Energy Department says it contributed $50 million to the experiment, and NASA will spend an additional $104 million through 2020, not including the expense of the shuttle flight. Fifteen other countries picked up the rest of the tab.

The 2003 destruction of the shuttle Columbia pushed NASA to cancel the experiment’s launch. Only a prolonged lobbying campaign by the famously strong-willed Ting returned the device to NASA’s launch manifest. In 2008, Congress passed a law forcing NASA to extend the shuttle program and fly the experiment.

Then, last year, Ting ordered a redesign of the AMS, delaying its scheduled November launch. Concerned that liquid helium needed to supercool the device’s central magnet would boil off into space, Ting swapped in an older, weaker magnet that needs no such cooling. The experiment is now rated to function until at least 2020, the space station’s scheduled destruction date. But the weaker magnet made the experiment less sensitive, and so Ting led other major modifications to the device’s sensitive particle detectors.

The redesign delayed the launch by several months, and the Giffords shooting prompted speculation that Kelly would give up the commander’s chair and force NASA to choose a backup. In February, though, Kelly said he would fly the mission.

If Endeavour launches Friday as planned, on Monday astronauts will use the shuttle’s robotic arm to pluck the ring-shaped detector from the craft’s cargo bay. In a delicate three- to four-hour maneuver involving four astronauts on the shuttle and the station, the shuttle’s robotic arm will hand the AMS to the station’s robotic arm, which will then perch the device atop a space-station truss.

There, the experiment will silently sift space for high-energy particles that Ting and his supporters say will provide clues about some of the universe’s deepest mysteries.

Chief among them: What happened to the antimatter that prevailing theories say the big bang was supposed to create. The big-bang theory of cosmic creation sketches an explosion that was supposed to create equal amounts of matter and its strange twin, antimatter. But no evidence of this primordial antimatter has been detected. So Ting designed the AMS to find antimatter from distant, theoretical “anti-stars” and “anti-galaxies.”

As these theoretical particles fly through the detector’s magnet, their paths will bend in the opposite direction of normal matter.

However, many physicists say that other experiments have ruled out the existence of exotic antimatter stars or galaxies. “I doubt it’s going to see any fundamental antimatter,” said Lawrence M. Krauss, an experimental physicist at Arizona State University.

Krauss and others are more hopeful that the experiment will catch a whiff of another elusive substance: dark matter. The fundamental nature of this weird stuff is unknown, although experiments have shown that it makes up about 23 percent of the mass of the universe. Without it, the Milky Way would fly apart. Theoretical collisions of dark matter should produce positrons — the inverted, positively charged twins of electrons. The AMS is designed to sense such particles.

Two independent teams will sift the AMS data for signals of antimatter and positrons. “If we make a mistake, it will feed critics for a very long time,” Ting said.