Spacewalking astronauts have plugged a leak in a cosmic ray detector outside the International Space Station, bringing it another step closer to new life.
It was the fourth spacewalk since November for Nasa’s Andrew Morgan and Italy’s Luca Parmitano to fix the Alpha Magnetic Spectrometer.
They installed new coolant pumps last month to revive the instrument’s crippled cooling system and needed to check for any leaks in the plumbing.
Mr Parmitano quickly discovered a leak in one of the eight coolant lines – the first one he tested – and tightened the fitting.
The line still leaked after a mandatory one-hour wait, and Mr Parmitano tightened it again – which fixed the problem.
“Let us all take a breath,” Mission Control urged. By then, the astronauts were already halfway into their planned six-hour spacewalk.
Mission Control acknowledged the leak added some unwanted “drama” to the spacewalk. “Everybody’s hearts stopped,” it told the astronauts.
Mr Parmitano wondered aloud what the flight surgeon in Houston saw when the leak erupted – he said his heart rate “either flat-lined or spiked, one of the two”.
Barring any further problems, the 2 billion US dollar (£1.5 billion) spectrometer – launched to the space station in 2011 – could resume its hunt for elusive anti-matter and dark matter next week, according to Nasa.
Nasa has described the spectrometer spacewalks as the most complicated since the Hubble Space Telescope repair missions a few decades ago.
Unlike Hubble, thE spectrometer was never intended for astronaut handling in orbit, and it took Nasa years to devise a repair plan.
Despite their complexity, the first three spacewalks went well.
Mr Morgan and Mr Parmitano had to cut into stainless steel pipes to bypass the spectrometer’s old, degraded coolant pumps, and then spliced the tubes into the four new pumps – no easy job when working in bulky gloves.
The system uses carbon dioxide as the coolant.
As well as checking for leaks, the astronauts had to cover the spectrometer with thermal insulation.
The massive 7.5-ton spectrometer was launched to the space station on Nasa’s penultimate shuttle flight.
Until it was shut down late last year for the repair work, it had studied more than 148 billion charged cosmic rays.
The project is led by Samuel Ting, a Nobel laureate at the Massachusetts Institute of Technology.
The repairs should allow the spectrometer to continue working for the rest of the life of the space station, or another five to 10 years.
It was designed to operate for three years and so has already surpassed its expected lifetime.