Scientists said initial data sent from the robot lab Philae showed traces of organic molecules and a surface much harder than imagined.
Philae, released from its mothership Rosetta, has gone into hibernation on comet 67P/Churyumov-Gerasimenko, having used its onboard battery power after 60 hours of prodding and probing.
ANALYSIS: Success! Hayabusa Captured Asteroid Dust
JAXA aims to bring 100 milligrams (1/286th of an ounce) of samples to Earth after a round trip of more than five billion kilometers.
The probe is the successor to JAXA's first asteroid explorer, Hayabusa -- the Japanese term for falcon -- which returned to Earth in 2010 with dust samples after a trouble-plagued seven-year mission.
The spherical 1999JU3 asteroid, which is around a kilometer (half a mile) across, is believed to contain significantly more organic matter and water than the potato-shaped rock studied by the original Hayabusa.
Despite various setbacks during its epic seven-year odyssey, including intermittent loss of communication and damage to its motors, the first Hayabusa was hailed as a scientific triumph when it returned to Earth.