The dwarf planet Ceres taken by NASA's Dawn spacecraft on February 19, 2015 from a distance of nearly 29,000 miles is shown in this handout photo provided by NASA March 2, 2015.[Photo/Agencies]

The double trips are made possible by Dawn's ion propulsion engines, which provide gentle yet constant acceleration and are more efficient than conventional thrusters.

As Dawn approached Ceres, it beamed back puzzling images revealing a pair of shiny patches inside a crater _ signs of possible ice or salt.

Scientists hope to get a better glimpse when the spacecraft spirals closer to the surface to study whether previously spotted plumes of water vapor continue to vent.

Dwarf planets lately have become the focus of exploration.

This summer, another NASA spacecraft _ New Horizons _ is set to make the first visit to Pluto, which was demoted to dwarf planet.

Dawn almost never made it out to the inner solar system. The mission endured funding-related project cancellations and launch delays before it received the green light to fly.