Closest Approach Sequencing for New Horizons
Alice Bowman, Mission Operations Manager for New Horizons, spoke Sunday about some of the nuts and bolts of the spacecraft's closest approach to Pluto.
With signals sent between spacecraft and Earth taking four and a half hours to reach their destination, it's worth keeping in mind whether the perspective for a given event is that of the spacecraft, or the Mission Operations Center here at the Johns Hopkins Applied Physics Laboratory in Laurel, MD.
From our Earth-bound perspective, the last telemetry we will receive before closest approach operations begin will be on 11:17 EDT tomorrow, 13 July. This will include any critical data sets that have already been collected and are residing on the craft's 64 gigabit data recorder. By the time we receive that, the spacecraft will already have begun its close approach science observations.
From that point forward, there will be no more contact with the spacecraft while it carries out its mission. New Horizons has very few moving parts; it's designed to be as redundant and resilient for its long duration mission as possible. In order to communicate with Earth, it has to physically rotate itself to point its high gain antenna towards us. This will not be possible while it's carrying out its primary sequence of science observations leading up to and following closest approach.
So the spacecraft will be silent while it carries out a very precisely-timed and complicated sequence of commands that were uploaded to the spacecraft on Monday the 6th. This sequence was many years in the design process, crafted to accomplish as much science as possible as the probe speeds by Pluto. It was so long in the making, in fact, that it was mostly designed before Kerberos and Styx, Pluto's outermost moons, were discovered. Consequently, most of the observations focus on Charon, Nix, and Hydra.
The commands are carefully prioritized, with "Group 1" observations -- those that are necessary to fulfill NASA's required science for the mission -- grabbing the highest billing. The entire sequence is loaded into the primary flash memory of the spacecraft as well as the backup computer's memory, so that if any unexpected power loss or restart occurs, the sequence will reload and continue to unfold. Most of the observations have a tolerance of 450 seconds; that is, if the spacecraft is where it needs to be to perform an observation within 7 minutes of when it was supposed to be performed, it can generally still successfully perform the observation. But if all goes nominally, the margin won't be needed.
From the spacecraft's perspective, the actual closest approach to Pluto occurs at 7:50 am EDT on 14 July. It will not attempt to contact Earth until 8 hours later, for a brief 15 minute period. This is a concession by the science team to the engineering team. One of the observations called for the probe to point in a direction that placed the high-gain antenna roughly facing Earth. This was tweaked slightly to allow a short burst of 3 or 4 cycles of what's called "real-time telemetry" -- continuous "housekeeping" data that will tell the engineering team if the spacecraft is in good health, and whether it has recorded the amount of data they would expect given the observations completed up to that point. It should be received at Earth starting at 8:53 pm EDT and ending at 9:09 pm EDT. This is more a "phone home" signal then any scientific bounty, and we won't receive any new images at this point.
The first LORRI images from the closest approach will not come down until sometime Wednesday evening, when the primary approach science is complete and the spacecraft has time to perform a lengthier downlink to Earth. Actual playback of the full dataset from the spacecraft's recorder will take months, due to a few factors: the distance of the spacecraft from Earth, which causes the 4.5 delay in signal transmission, the low rate of transmission due to the distance and power requirements of the spacecraft, and the need for the raw data on the 64 gigabit recorder to be compressed prior to downlink to Earth. Some investigators will be waiting until the end of 2015 for playback of the data that is relevant to their discipline.