The debris field, located about 1,000 miles west of California, is made up of mostly plastic, but also has discarded fishing lines and other trash that can't decompose.
A group from U.C. San Diego left over the weekend and group from the Bay Area left Tuesday.
Scripps Institution of Oceanography researchers left Sunday aboard the 170-foot New Horizon for the nearly three-week expedition to study trash.
The second set of scientists is from Project Kaisei, a non-profit expedition from the Bay Area. They will set sail aboard the 151-foot sailboat called Kaisei.
Scientists say the "Great Pacific Garbage Patch" is trash that made it into the ocean from storm drains and rivers.
The trash is naturally sucked into a vortex out at sea that works like water draining from a sink in a swirling motion.
The Scripps Environmental Accumulation of Plastic Expedition (SEAPLEX) will be the first of its kind, officials said, and will focus on, among other things, how the garbage patch is affecting ocean life and how fast it is accumulating. The exact size of the blob is unknown -- something else the expedition will try to determine.
"There is little scientific information on the composition, extent and effects of the debris," according to Scripps' Web site.
Because it is made of tiny pieces of garbage, the blob can't be detected by planes or space satellites.
This is a research-only expedition. Cleaning it up may not be possible.