The disappearance of a renowned computer scientist has sparked a massive online manhunt, with thousands of volunteers poring over satellite images in search of clues.

James Gray, 63, a Microsoft researcher and winner of the prestigious Turing Award, failed to return home from a sailing trip on Sunday, January 28.

He left from San Francisco Bay aboard a 40-foot sailing boat, and had intended to scatter his mother's ashes at the nearby Farallon Islands.

Several days of intensive searching by the US Coast Guard and private planes revealed no sign of Dr Gray or his boat, so desperate friends and colleagues turned to the internet for help.

On Friday, engineers from NASA, online retailer Amazon and technology companies such as Google and Microsoft organised a satellite and high-altitude aircraft to photograph the area where he was believed to be located.

The photographs were then split into smaller tiles and uploaded to Amazon's Mechanical Turk website, allowing virtually anyone to take part in the search effort.

"We need your help in reviewing these images to see whether you can locate Jim's boat in any of these images," Amazon's chief technology officer, Werner Vogels, wrote in his blog late on Friday.

"The weather conditions were not ideal as some areas were cloudy, but we can still look for him in those places where there is a somewhat clear view."

On Sunday, Mr Vogels updated his blog with news that almost 100,000 tiles had been reviewed, a number of which the online volunteers had marked for further inspection.

He said "satellite image inspection experts" would then examine the marked tiles to determine which should be forwarded to the Coast Guard for action.

Captain David Swatland, a deputy commander with the US Coast Guard, told The New York Times: "This is the largest strictly civilian, privately sponsored search effort I have ever seen."

But this is not the first time the technology community has banded together to search for a missing person.

Late last year, high-profile US technology journalist James Kim and his family went missing while on a road trip in south-west Oregon over the Thanksgiving holiday.

The disappearance attracted huge interest online, with a plethora of bloggers and technology websites covering the search as it unfolded.

Other concerned volunteers published videos on YouTube to alert users to the missing family, and Google Maps was used to create annotated maps of the search area.

After being stranded for several days on a remote logging road in treacherous weather conditions, Kim's wife and two children were found alive and well by rescue helicopters, but Kim was not with them as he had gone to seek help.

The remote sensing operator GeoEye said it would volunteer one of its satellites to help online volunteers continue the search, but Kim's body was found by rescuers soon after.