The scientific goals of the New Horizons mission were established before it was built and a decade before it was launched. The specific objectives placed demands and constraints on the spacecraft's activity at Pluto, and the development of the timeline for events before, during and after at New Horizons' encounter at Pluto was agreed upon a long time ago.
In the 150 days of the approach, Pluto and Charon would be imaged, every 12 hours for about six and a half days, each time the image resolution improved by 50% (at 150 [Approach Phase 1], 100 [Approach Phase 2], 66, 44, 28, 19, 12 and 6 days before Pluto closest approach. This sequence would provide twelve images, 30 degrees, over one revolution of the Pluto/Charon system. Early approach imaging will record one complete 30 day orbit of Hydra; imaging another complete orbit will start about 100 days out.
These observations will address time variability, provide airglow spectra, obtain a series of maps at increasingly higher resolution maps, and refine the orbits and (hence the masses) of Pluto, Charon, Nix, and Hydra. The returned imagery will also be searched for small satellites that had previously escaped detection from Earth. The final set of observations, starting at 6 days out, will only cover half a rotation of Pluto and Charon, and will give the highest spatial resolution images and spectra of the portions of the surface away from the closest-approach hemisphere (including the far-side hemisphere at 3.2 days before closest approach).
When Approach Phase 1 was done, New Horizons performed a 90 second thruster firing for a trajectory correction maneuver, to slow the spacecraft’s velocity by just 1.14 meters per second. This shifted its July 14 arrival time by 14 minutes and 30 seconds, back on schedule; and shifted the course “sideways” (looking from Earth) by 3,442 kilometers, sending the spacecraft toward a desired flyby close-approach target point. New Horizons then turned its antenna to Earth to download all the data it has gathered during AP1.
Observations resumed on April 5th more intensive Approach Phase 2, with a repeat of the imaging of one Pluto/Charon revolution and one Hydra orbit. AP2 adds numerous new and significant observations of the Pluto system, including the first color and spectral observations of Pluto and its moons, and series of long-exposure images that will help the team spot additional moons or rings in the Pluto system. The spacecraft will also make its first ultraviolet observation to study the surface and atmosphere of Pluto and the surface of Pluto's largest moon, Charon, and the spacecraft will conduct a major test of flyby radio science observations. These various activities are critical to developing a fuller picture of that system, and in assessing any hazards New Horizons could face as the spacecraft passes between Pluto and Charon.
Although New Horizons cannot yet match Hubble's resolution on the Pluto system, New Horizons can now do what Hubble can't: perform continuous observations. By measuring the positions of Nix and Hydra with respect to background stars in these images, scientists will be able to predict the future paths of the moons with much greater precision, and navigators will be able to target New Horizons with less uncertainty. New Horizons' lower resolution, color camera will also start imaging as a backup for navigation in case LORRI fails. At the end of the forty days of AP2, the spacecraft will again cease observing Pluto, turning to Earth in a spin-stabilized mode to completely empty its data recorders in preparation for the flyby. The team is evaluating new tracking data to decide whether they'll need to carry out a course correction on May 15; a decision is expected about May 1.
