CAPE CANAVERAL, Fla. - 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. It was launched this week aboard Endeavour on the second-to-last flight.

Two astronaut teams were assigned to attach the Alpha Magnetic Spectrometer to the outside of the space station 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 would justify the scientific purpose of the space station.

"It's in the same scale of importance" as the 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 was part of a 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 seven-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.

Also Wednesday, NASA said it was taking a close look at damage to the black thermal tiles on Endeavour's belly.

The gouges and nicks were spotted in photos taken right before Endeavour docked with the space station. The shuttle did a slow backflip so space-station cameras could capture any signs of launch damage.

NASA official LeRoy Cain said there was no cause for concern. If needed, the astronauts will use a boom to make a closer inspection this weekend.

A Skull Implant for Rep. Giffords

Doctors used a hard plastic implant to cover a hole in Rep. Gabrielle Giffords' skull Wednesday, the latest milestone in her recovery from an assassination attempt and a procedure that experts say will improve her quality of life.

The hospital said in a statement that doctors and members of Giffords' staff would hold a briefing Thursday to give an update and discuss the next steps.

A gunman shot her in the head more than four months ago in Tucson, Ariz., and doctors had to remove a portion of her skull to relieve pressure on her brain.

The implant - or bone flap, as doctors call it - will protect the brain and the skull, said Robert Friedlander, chair of neurosurgery at the University of Pittsburgh Medical Center.

It will allow Giffords to freely move about without her helmet, adorned with the Arizona state flag, for the first time since she began therapy in late January.

Her astronaut husband, Mark Kelly, is orbiting Earth on the space shuttle Endeavour and is getting updates, NASA said.

- APEndText