today, multiple hiking trails exist, including the mauna kea trail, and by 2007 over 100,000 tourists and 32,000 vehicles were going each year to the visitor information station (vis) adjacent to the onizuka center for international astronomy. the mauna kea access road is paved up to the center at . one study reported that around a third of visitors and two thirds of professional astronomers working on the mountain have experienced symptoms of acute altitude sickness; visitors traveling up the volcano's flanks are advised to stop for at least half an hour and preferably longer at the visitor center to acclimate to the higher elevation. it is strongly recommended to use a four-wheel drive vehicle to drive all the way to the top. brakes often overheat on the way down and there is no fuel available on mauna kea. a free star gazing program is held at the vis every night from 6-10 pm. between 5,000 and 6,000 people visit the summit of mauna kea each year, and to help ensure safety, and protect the integrity of the mountain, a ranger program was implemented in 2001.