CAPE CANAVERAL, Florida: A mammoth cosmic ray detector arrived at the International Space Station on Wednesday, a $2 billion experiment that will search the invisible universe and help explain how everything came to be.

It’s the most expensive cargo ever carried by a space shuttle and almost didn’t make it to orbit before the fleet retires this summer. Endeavour, making the second-to-last flight, docked at the space station after doing a slow backflip so cameras could capture any signs of launch damage. NASA said it was taking a closer look at some gouges and nicks on the shuttle’s belly but there was no cause for concern.

Two astronaut teams were assigned to attach the Alpha Magnetic Spectrometer to the outside of the space station on Thursday, using a pair of robot arms, where it will stay for the life of the outpost.

The scientists on Endeavour’s crew said it will justify the scientific purpose of space station.

“It’s in the same scale of importance as Hubble (Space Telescope),” astronaut Gregory Chamitoff said before the flight. “And it is going to be by far the biggest, most expensive and perhaps the most fundamentally valuable science apparatus we have on the space station.”

Physicist Phil Schewe said the experiment is part of a centuries-long tradition of scientists exploring the building blocks of matter, from elements to atoms to subatomic particles.

“This is just a grand extension of trying to answer the question, why we have matter and what it is,” said Schewe, a spokesman for the American Institute of Physics.

“Is this a big deal? Especially if they find something, yes it is.”

The 7-ton instrument, known by its acronym AMS, has been 17 years in the making, and involves 600 scientists from 16 countries. The heart of the experiment is a magnet ring 3 feet across.

The made-in-China magnet will bend the path of charged cosmic particles as they pass through eight detectors, enabling scientists to identify their properties.

Nobel Laureate Samuel Ting, the team leader, said the greatest discovery may well be what he and others aren’t anticipating. On Wednesday, he said he was excited but not nervous.

“It’s a bit too late to think about that now,” Ting said from NASA’s Johnson Space Center in Houston.

Endeavour – making its final journey and the next-to-last flight of NASA’s shuttle era – arrived at the space station Wednesday. Commander Mark Kelly, the husband of wounded Rep. Gabrielle Giffords, expertly steered the shuttle in for the 220-mile (354-kilometer)-high linkup, after doing the backflip for cameras.

The space agency has checked for damage to the shuttle’s delicate heat shield ever since the 2003 Columbia disaster. The shuttle is also now equipped with tile repair kits. A slab of fuel tank foam knocked a hole in Columbia’s wing which led to its destruction during re-entry.

At an afternoon news conference, NASA deputy shuttle program manager LeRoy Cain said they will continue to assess the damage to the protective thermal tiles. If needed, the astronauts will use a boom to make a closer inspection this weekend.

The docking was overshadowed by the continuing Kelly-Giffords saga, with the flight director’s news conference earlier in the day dominated by questions about the Arizona congresswoman. Giffords received a plastic skull implant during surgery in Houston on Wednesday, two days after witnessing her husband’s launch. She was shot in the head on Jan. 8 at a political event in Tucson, Arizona, her hometown.

Lead flight director Gary Horlacher said Kelly performed “unbelievably” well during the linkup and noted that no one would know all he’s been through in recent months.

“I know folks are kind of focused on Mark and his situation,” he told reporters. “But AMS is going to be around talking to us for a long, long time. So I’m very much looking forward to the results over the years.”

Ting, a deliberate, soft-spoken physicist at the Massachusetts Institute of Technology, said recently that he wasn’t concerned that the attention was on Kelly and Giffords, rather than the spectrometer.

“All success or failure depends on what physics we find. That’s the only thing that’s important,” he said.

Hooman Davoudiasl, a physicist at Brookhaven National Laboratory in New York said the experiment is “going to look at some fundamental questions ... that would be important in our overall understanding of the makeup of the universe and some of its basic characteristics,”

One question is why the universe is dominated by matter rather than sharing the stage with so-called “antimatter.” While matter is made up of familiar particles like protons and electrons, antimatter is a collection of particles that are identical except for carrying the opposite charge. Standard theories have trouble explaining why matter is so dominant, Davoudiasl said. By studying cosmic rays, the spectrometer can help determine whether the universe actually has significant sources of antimatter. he said.