The mission is divided into phases. Dawn has now completed the Launch (September 27, 2007), Checkout (September 28-December 17, 2007), and Interplanetary Cruise (December 17, 2007-May 3, 2011) Phases of the mission. It has just entered the Vesta Phase, which will last fifteen Earth months (about a third of a Vesta year). The Vesta Phase is divided into sub-phases: Approach, Survey, HAMO, LAMO, and HAMO2. Here's what's going to happen in each phase.
The Approach Phase began today, May 3, 2011, with Dawn still about 1.2 million kilometers away from Vesta, and will last about three months. Dawn is not yet in Vesta orbit; the principal activity of the Approach Phase is to continue the ion thrusting required to enter orbit, and to gather data that will aid in spacecraft navigation and trajectory planning. The Framing Camera will be used to acquire optical navigation images in 24 sessions throughout this period. The first optical navigation images should already have been taken, and should show Vesta as just a few pixels across against a background of stars. Eight of the optical navigation sessions will also include spectrometric observations with Dawn's Visual and Infrared Spectrometer (VIR). By early June, Dawn's images will be higher-resolution than anything achieved from Hubble.
As Dawn gets closer to Vesta it will perform three "rotation characterizations," where it will take images throughout a Vestian day (which lasts about 5 hours and 20 minutes) to see all longitudes of the asteroid. These will take place on July 1, July 10, and between July 23 and 25 (see the next paragraph for why this date is not yet scheduled exactly). During the second rotation characterization, Dawn will also point away from Vesta and systematically cover the nearby sky in order to search for moons. At the time of Dawn's approach, it will be winter in Vesta's northern hemisphere, so some of the northern polar regions will not get any sunlight and therefore will not be visible to the camera or spectrometer.
Radio tracking during this period will help refine estimates of Vesta's mass, a parameter that's crucially important in the planning of Dawn's orbital mission. The mission has been planned out using a reasonable range of possible masses; determining the mass more definitely will pin down the dates of later events and may also result in slight changes to the altitudes I'll mention below. On July 16, Dawn will be close enough to Vesta to be captured into orbit. However, "capture" is probably too strong a term. Dawn will still be continuing to thrust in order to lower its orbit.
Dawn in Orbit
Survey Orbit will begin when Dawn has reached its planned survey altitude of 2,700 kilometers, which will be on August 8 at the earliest, August 11 at the latest. Dawn will be in a near-polar orbit with a period of 2.5 to 3 days, which means Vesta will rotate under the spacecraft and present all longitudes in sunlight several times on each orbit. This is useful for mapping. The camera will achieve about 250 meters per pixel on Vesta's surface, and the spectrometer about 700 meters per pixel. Dawn's Gamma Ray and Neutron Detector (GRaND) will be on but the data acquired during this phase will be inferior to that acquired during LAMO. Since the cameras can only be used on the dayside leg of the orbit, the basic plan is to acquire data on the dayside leg and relay it to Earth on the nightside leg, though there will be a few dayside communications sessions.
Dawn will complete seven of these survey orbits over about 20 days. Then it will continue thrusting to lower its orbit over a period of approximately a month. The rest of the dates in this summary are very approximate.
Once Dawn has reached an altitude of 660 kilometers, somewhen in the vicinity of October 1, it will be in High Altitude Mapping Orbit (HAMO). The HAMO has a 12-hour period; this combined with Vesta's 5 hour and 20 minute rotation period means that Dawn will see all of Vesta's surface after about 10 orbits or five Earth days. Plans for HAMO are organized into 10-orbit cycles, and there will be six total HAMO cycles. The primary goal of HAMO is, again, surface mapping. For two complete cycles Dawn will be pointed straight down in order to map the whole surface; two complete cycles provide redundancy to fill any gaps in coverage that might arise from glitches in data acquisition, transmission, or reception. The other four cycles involve off-nadir observations, slanting views to provide stereo coverage, which will be used to generate topographic maps of Vesta. Then Dawn will again fire its thrusters to reduce its altitude. It will probably take six to eight weeks to arrive at the next phase.
When Dawn has reached an altitude of 180 kilometers and an orbital period of four hours, sometime around or after the end of November, it will have arrived at Low Altitude Mapping Orbit (LAMO). One primary science goal of LAMO is the acquisition of data by GRaND, which will map out the abundance of elements across Vesta's surface. GRaND operates equally well on dayside and nightside, and it will be operated continuously for the two months of LAMO to improve the quality of its map. (Actually, it will be operated continuously throughout the 15-month Vesta phase, but its best data will be acquired during LAMO when it's closest to Vesta.)
The other major goal is to determine Vesta's gravity field through radio science, which will give clues as to the distribution of mass in Vesta's interior. The camera and spectrometer will also be used at times, but the GRaND and radio science observations are the priority. Once LAMO is complete, Dawn will start raising its orbit again.
Dawn will stop thrusting once it has reached the same orbital altitude it had in HAMO for a period called HAMO2. This will be about eight months after HAMO, or about June 2012. Eight months is most of a Vesta season, so the north polar terrain that was invisible in winter darkness to HAMO will be sunlit during HAMO2. The HAMO surveys will be repeated in this different season, filling in the northern polar regions and imaging the rest of Vesta under slightly different illumination conditions. Finally, in July 2012, it will depart Vesta for Ceres, which it will reach around February 2015.