In mid-January the New Horizons spacecraft entered its scientific data gathering phase. This “encounter” period will last till January 2016, and is divided into three approach phases (AP1, AP2 and AP3), the near Pluto encounter (NEP), and three departure phases (DP1, DP2 and DP3). This post summarizes the approach phases so that my postings here, during the approach, will have a context; I’ll describe the near Pluto encounter and departure phases later.
AP1 started in mid-January 2015, 180 days before New Horizons’ closest approach to Pluto. It is focused mostly on navigation, though two plasma instruments will be measuring the ambient plasma to characterize any differences near Pluto. The first AP1 optical navigation (OpNav) images were taken on January 25th and 27th, and released to public view on Clyde Tombaugh’s birthday, February 4th. These images taken by the Long Range Reconnaissance Imager (LORRI), a high resolution telescope, a range of 200 million Kms.
In July 2014, New Horizons executed its first seven day OpNav campaign. The AP1 OpNav campaign will start by repeating this observation, then continue observations every other day for five more weeks, the four brightest objects in the Pluto system (Pluto, Charon, Nix, and Hydra) will be well separated from one another in LORRI images allowing their orbits to be improved and any variations in brightness to be observed. Pluto will be be barely resolved (about 3 pixels in diameter) by the end of this, the second, OpNav campaign.
For the rest of AP1, New Horizons will change its mode of operation. The imaging requires LORRI to be pointing at Pluto which significantly reduces the ability to communicate to home so, for the last month of AP1, New Horizons will turn to point its large antenna towards Earth, spin slowly to maintain that pointing, and use two transmitters at full power to dump the accumulated navigational data to the ground.
AP2 starts on April 4th, 100 days before closest approach, resuming the plasma measurements and starting the third OpNav campaign. By April 25th, Pluto will be five pixels in diameter in LORRI images, and a second camera, with a quarter the resolution of LORRI but imaging in color, will join the campaign. Sometime in the first week of May, New Horizons will pass a major milestone called “BTH” when Pluto reaches eight pixel in LORRI images; from here on LORRI’s images of Pluto will better than Hubble!
The Pluto system is full of surprises; five moons were not expected. Like the Earth, Pluto has a large moon, and large moons tend to clear anything smaller out of orbit. The four little moons were all discovered in the last ten years, after New Horizons’ launch, right at the limit of visibility from Earth. Concern has grown that there may be more small moons or a ring system around Pluto. When New Horizons can see the Pluto system better than Hubble, it will start searching for previously unseen objects, so that they can be added to the observation plan for flyby, and the spacecraft trajectory can be adjusted, if necessary, to avoid a collision bringing a sudden end to the mission.
AP3 starts 21 days before closest approach. Imaging goals are no longer related to navigation; New Horizons will now focus on getting the best global maps of Pluto and Charon during the three planetary rotations before closest approach. The plasma instruments may detect pickup ions and bow shock. Other instruments will start to look for variability in IR and UV radiation. Imaging will search for, and track, any clouds or hazes and other atmospheric features. At 3.2 days out, long-range imaging will include 40Km mapping of Pluto and Charon. This is half the rotation period of Pluto-Charon and will allow imaging of the side of both bodies that will be facing away from the spacecraft at closest approach. Coverage will repeat twice per day, to search for changes due to snows or cryo-vulcanism. Perhaps most importantly, this phase allows global maps of Pluto and Charon at longitudes other than those seen near closest approach.
There will be particularly photogenic opportunities during AP3, such as images taken of the entire system just filling a LORRI field of view.
There will be particularly photogenic opportunities during AP3, such as images taken of the entire system just filling a LORRI field of view.
