The explosion, a gamma ray burst older than Earth itself, was monitored by the National Aeronautics and Space Administration's Swift satellite and shattered the record for the most distant object seen without visual aid.

"No other known object or type of explosion could be seen by the naked eye at such an immense distance," said Swift team member Stephen Holland of NASA's Goddard Space Flight Center in the eastern state of Maryland.

"If someone just happened to be looking at the right place at the right time, they saw the most distant object ever seen by human eyes without optical aid."

Gamma ray bursts are among the most violent phenomenon produced in the universe. NASA described them as the most luminous explosions since the "Big Bang."

The satellite's burst alert telescope discovered the explosion on Wednesday and located it in the Bootes constellation, with telescopes on Earth adjusting to witness the afterglow.

NASA measured the explosion as having occured 7.5 billion years ago, before Earth was formed and more than halfway across the visible universe.

Until now the most luminous object visible with the naked eye was galaxy M33, a "relatively short" 2.9 million lightyears from Earth.

The explosion seen Wednesday "blows away every gamma ray burst we've seen so far," said Neil Gehrels of Goddard Space Flight Center.

Gamma ray bursts occur when huge stars use up all their fuel and their core collapses, forming black holes or neutron stars that release bursts of gamma rays, ejecting particles into space at nearly the speed of light and generating afterglows.

The burst, named GRB 080319B, was among a record four bursts detected by Swift on Wednesday, the same day of the death of prolific science fiction writer Arthur C. Clarke who wrote "2001: A Space Odyssey."

"Coincidentally, the passing of Arthur C. Clarke seems to have set the universe ablaze with gamma ray bursts," said Swift team member Judith Racusin of Penn State University.

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