Endeavour's astronauts have installed a two billion US dollar (£1.23bn) cosmic ray detector on the International Space Station to scan the invisible universe for years to come.
They used a pair of robot arms to remove the Alpha Magnetic Spectrometer from the shuttle, then hoist it onto the sprawling framework on the right side of the station.
The instrument, which has a three-foot magnet at its core, is the most expensive piece of equipment at the orbiting lab and certainly the most prominent scientific device.
It will search for antimatter and dark matter for the rest of the life of the space station, and hopefully help explain how the cosmos originated.
Nobel Laureate Samuel Ting, the principal investigator, personally relayed his thanks from Mission Control. He has worked on the project for 17 years and fought to have it placed back on the shuttle when its flight was suspended several years ago.
"This has been a very difficult experiment, and I think in the next 20 to 30 years, nobody will be able to do such a thing again," Dr Ting told the astronauts. "I hope together with you, we will try to make a contribution to a better understanding of our universe."
Back at Mission Control, meanwhile, engineers continued to analyse several areas of damage on Endeavour. Thermal tiles were gouged and nicked during Monday's lift-off, the penultimate one for the shuttle programme. Some of the slashes are six inches long and two inches wide.
Nasa wants to make certain the shuttle is safe to come home in two weeks. The damage was spotted in photographs taken by the space station crew just before Endeavour docked on Wednesday.
The shuttle performed a slow backflip for the cameras, a customary procedure put in place after shuttle Columbia disintegrated as it re-entered Earth's atmosphere in 2003.
Mission Control may ask shuttle commander Mark Kelly and his five crewmates to take a closer look at the gouges this weekend, using a laser-tipped inspection boom.