Aluminum shields fashioned from the remains of the Twin Towers have been on Mars with the rovers Spirit and Opportunity since 2004 and will likely stay there for at least a few million years, says NASA.

The U.S. space agency and scientists at a New York-based robotics company said this week both rovers include a component -- a cuff that protects cable on their rock grinding tools -- made from aluminum salvaged from Ground Zero.

The employees of Honeybee Robotics were working on the rovers just over a kilometer away when the terrorist attacks occurred, but because of their tight work schedule couldn't spend much time helping in the immediate aftermath.

"I felt totally helpless," said Honeybee founder Stephen Gorevan in a statement, recalling that grim morning ten years ago. "We watched the rest of the sad events of that day from the roof of our facility."

Employees at Honeybee and NASA's Jet Propulsion Laboratory came up with the idea of the cable shields, which both bear the image of the American flag, as a tribute to the victims. They got help from the New York mayor's office, a metal shop in Texas and the rover missions' science leader, Steve Squyres at Cornell University.

"It's gratifying knowing that a piece of the World Trade Center is up there on Mars. That shield on Mars, to me, contrasts the destructive nature of the attackers with the ingenuity and hopeful attitude of Americans," said Gorevan.

Spirit was launched in June 2003, followed by Opportunity on July 7. Both rovers landed in January 2004 and have long since completed their primary missions.

The significance of the cable shields was kept quiet at the time and was first revealed later that year to the New York Times. NASA formally drew attention to the tributes this week.

Both rovers have made important discoveries about environmental conditions on ancient Mars that suggest the existence of microscopic life. Spirit has since shut down but Opportunity continues to function and will soon use its rock grinder to explore a large crater it reached last month.

Opportunity too will eventually go quiet, say scientists, but the now-dry environment on the Red Planet is expected to preserve their 9-11 tribute for millions of years.