The success in producing proton collisions represents a remarkable comeback for CERN, but the lab is still only halfway back to where it wanted to be. Only a year and a half ago, the first attempt to start the collider ended with an explosion that left part of its tunnel enveloped in frigid helium gas and soot when an electrical connection between two of the powerful magnets that steer the protons vaporized.

Image Scientists in the CERN control room outside Geneva reacted Tuesday to the start of the $10 billion Large Hadron Collider. Credit... Anja Niedringhaus/Associated Press

A subsequent investigation revealed that the collider was riddled with thousands of such joints, a result of what Lucio Rossi, head of magnets at CERN, said was a “lack of adequate risk analysis,” in a recent report in the online journal Superconductor Science and Technology. As a result, the collider, which was designed to accelerate protons to seven trillion electron volts, then smash them together to reveal particles and forces that reigned during the first trillionth of a second of time as we know it, can only be safely run for now at half power.

CERN physicists say that operating the collider for a year and a half at this energy level should allow them to gather enough data to start catching up with its American rival, the trillion-electron-volt Tevatron at the Fermi National Accelerator Laboratory in Illinois. The Tevatron is smaller but has been running for years and thus has a head start in data. After that, the CERN machine will be shut down for a year so that the connections can be rebuilt.

Particle colliders get their oomph from Einstein’s equation of mass and energy. The more energy — denoted in the physicists’ currency of choice, electron volts — that these machines can pack into their little fireballs, the farther back in time they can go, closer and closer to the Big Bang, and the smaller and smaller are the things they can see.

The first modern accelerator was the cyclotron, built by Ernest Lawrence at the University of California, Berkeley, in the 1930s. An early version was a foot in diameter and accelerated protons to energies of 1.25 million electron volts.

Over the last century, universities and then nations leapfrogged each other, building bigger machines to peer deeper into the origins of the universe. But the race ended in 1993, when Congress canceled the Superconducting Supercollider, a 54-mile, 20 trillion-electron-volt machine being built underneath Waxahachie, Tex., after its projected cost ballooned to $11 billion.