Pasadena, Calif. ? NASA’s Opportunity rover landed late Saturday on Mars, arriving on the Red Planet exactly three weeks after its identical twin set down, prompting whoops and cheers of delight from mission scientists who gathered to monitor the touchdown.

“We’re on Mars everybody,” Rob Manning, manager of the entry, descent and landing portion of the Mars mission, shouted as fellow scientists at NASA’s Jet Propulsion Laboratory burst into wild applause.

The unmanned, six-wheeled rover landed at 11:05 p.m. CST in Meridiani Planum, NASA said. The smooth, flat plain lies 6,600 miles and halfway around the planet from where its twin, Spirit, set down Jan. 3.

Together, the twin rovers make up a single $820 million mission to determine if Mars ever was a wetter world capable of sustaining life. NASA launched Spirit on June 10. Opportunity followed on July 7.

Since arriving, Spirit has developed serious problems, cutting off what had been a steady flow of pictures and other scientific data.

Scientists said earlier Saturday they were closing in on the root of the problem that led the Spirit rover to begin spewing gibberish and beeps instead of science and engineering data earlier this week.

They brought stability to the six-wheeled vehicle by disabling its flash memory, which is similar to the memory digital cameras use to store pictures, said Orlando Figueroa, director of NASA’s Mars exploration program.

“We made good progress overnight,” project manager Pete Theisinger said during a news conference. “The rover has been upgraded from critical to serious.”

Spirit had resumed transmitting data Friday in limited batches.