These backup encounter sequence plans are called SHBOTs, an acronym for Safe Haven By Other Trajectory.
Our first SHBOT is called GIS, for Generic Inner SHBOT. It continues on the nominal trajectory and aim point, but for three hours near closest approach, we repoint the spacecraft so its dish antenna can shield it from impacts. This pointing attitude, called Antenna to Ram (or ATR), would cost us some science because we won’t be as free to point the science instruments toward Pluto system targets during those three hours. But tests and modeling show this provides a factor of three to four times increase in success probability, and reduces the estimated loss of mission probability to about 1 in 1,000.
If necessary, the high-gain (“dish”) antenna on New Horizons can be used to shield most of the spacecraft from dust particle impacts during the Pluto encounter.
Our second SHBOT is called DIS, for Deep Inner SHBOT. DIS also uses the ATR attitude. It also directs the spacecraft toward a much closer encounter with Pluto – just inside 3,000 kilometers from Pluto’s surface, compared to the nominal encounter close approach of about 12,500 kilometers from the surface. Why go closer, not farther, to avoid hazards? Because if we go close enough, we can benefit from the fortuitous “drag clearing” of debris particles from Pluto’s extended upper atmosphere! DIS has more severe science impacts than does GIS, but there is a strong consensus among the team that it’s both the best choice if late-breaking news tells us the nominal trajectory is unexpectedly riskier than we’re comfortable with, and losing some science to execute Deep Inner SHBOT is far better than losing the mission to a lethal impact.
What I’ve briefly summarized here resulted from thousands of hours of work by many people on New Horizons and in the scientific community. They’ve been carefully scrutinized by the independent technical panel NASA set up at our request to peer-review the hazards analysis and mitigation plans we’ve worked on since late 2011, and approved by NASA Headquarters.
We’re excited to have these plans on the books and to begin testing both SHBOTs before the encounter begins so that they are ready to use if needed. When we launched we never imagined we’d be planning three separate encounters with Pluto, but that's what we’re doing!