Scientific Background and Objectives

2.1 The Interaction of Pluto with the Solar Wind

The interaction of the various planets of the solar system with the solar wind goes back to the initial forays outside of the Earth's magnetosphere in the early 1960s. As part of the studies that eventually led to the Voyager program (Dryer et al. 1973) made an initial scoping study of what a solar wind interaction with Pluto might look like. They noted that for the anticipated scale lengths a kinetic approach was more proper for Pluto as for Mercury and that for a vanishingly small ionospheric scale height at Pluto, a long, induced magnetotail is, nonetheless, expected. With plans for a Pluto mission using grand-tour-like trajectories eventually canceled, the then-future Voyager encounter with Neptune and its large moon Triton (Morrison et al. 1982) would potentially provide the best insight into Pluto's interaction with the solar wind (McNutt 1982).

Interest in Pluto's interaction with the solar wind was reignited by the stellar occultation of 1988 (Elliot et al. 1989) and its interpretation of showing significant atmospheric escape (Bagenal and McNutt 1989; McNutt 1989). Current thinking is that the interaction of Pluto

Fig. 2 Schematic of expected

Fig. 2 Schematic of expected interaction of Pluto with the solar

interaction of Pluto with the solar wind for the strong-interaction limit. Based upon scaling from cometary interactions, the first detection of pickup ions from Pluto are expected as early as

20 hours before closest approach with the solar wind is something between that of an un-magnetized planet, such as Venus and that of a comet, depending upon the strength of the atmospheric outflow. If Pluto were to have even a weak intrinsic magnetization, then the interaction would be more akin to that of the magnetized planets due to the weak solar wind ram pressure at ~30 AU and beyond. Estimates of the overall outgassing rate of the atmosphere Q0 are between ~1027 and 1028 molecules s-1. Such rates have effects ranging from just shielding the surface from the solar wind to producing a well-formed magnetosphere encompassing the orbit of Charon (Fig. 2). Detailed discussions of past and current thinking about the interaction is given by (Bagenal et al. 1997) and (McComas et al. 2008), respectively, and more detailed simulations have been carried out as well (Delamere and Bagenal 2004; Harnett et al. 2005).

2.2 PEPSSI Science Objectives

2.2.1 PEPSSI Objectives at Pluto

The PEPSSI sensor is designed to perform in situ measurements of the mass, energy spectra, and distributions of moderately energetic particles in the near-Pluto environment and in the Pluto-interaction region. The instrument measures particle velocity and energy, and derives particle mass. It discriminates between electrons, protons, alphas, and carbon-nitrogen-oxygen (CNO—taken as a closely-spaced group in atomic weight), and heavier ions. The direction of particles is also determined within one of six sectors. PEPSSI objectives, within the context of New Horizons science-mission-group objectives include:

Group 1 Objective. A group 1 objective is characterization of the neutral atmosphere of Pluto and its escape rate. To support this objective, PEPSSI will detect heavy ions and measure associated energy spectra and spatial variation along the trajectory. By analogy with cometary measurements, these measurements will be used to determine the neutral particle escape rate, which along with UV spectral measurements made in the upper atmosphere (Stern et al. 2008), will be used to put together a fully self-consistent model of Pluto's upper atmosphere to satisfy this group 1 objective.

Group 2 Objectives. A group 2 science objective is characterization of Pluto's ionosphere and interaction with the solar wind. This characterization will be aided by PEPSSI measurements of the spatial extent and composition of pickup ions; these measurements are complementary to those that will be made by SWAP (McComas et al. 2008).

Group 3 Objectives. A group 3 science objective is characterization of the energetic particle environment of Pluto and Charon. This will require measurement of the spatial extent and velocity-space distributions of energetic ions (e.g., H+, N+, and N+). The PEPSSI instrument will make the required energetic ion measurements.

2.2.2 Science at Jupiter and During Cruise

Initial scientific results from PEPSSI obtained during the New Horizons flyby of Jupiter show evidence for periodic bursts of energetic, Iogenic particles down the magnetotail of the planet (McNutt et al. 2007). In addition, this magnetospheric passage provided an opportunity for significant in-flight calibration activities that have been used to inform ongoing rehearsal activities for the Pluto flyby.

There is no official "cruise science" on the New Horizons mission. This is by design to help save the instruments for the Pluto flyby as well as minimize operational costs by putting the spacecraft in hibernation for the majority of the cruise from Jupiter to Pluto. Data on the interplanetary medium will be collected as activity during the annual checkouts allows. The general trajectory of New Horizons into the direction toward the incoming interstellar wind will allow for important comparisons of energetic particles and transients in the interplanetary medium as they propagate toward and into the inner heliosheath, now being explored by Voyagers 1 and 2.

2.3 Measurement Requirements

2.3.1 Measurement Ranges

Energy thresholds and energy ranges depend upon the energy measurement mode, i.e., TOF-only, energy (SSD) only, or coincidence measurements through the entire system (Table 1).

2.3.2 Derived Instrument Specifications

2.3.2.1 Mass Resolution (Mass Uncertainty) Particle mass is derived from energy and TOF measurements. The uncertainty in the derived mass, i.e., the mass resolution, is determined by (a) energy measurement resolution, (b) TOF measurement resolution, (c) particle mass, and (d) calibration accuracy. For Energy-plus-TOF measurements, the mass resolution for three species of particles (spanning light, medium, and heavy mass) is specified as <2

Table 1 Energy measurement ranges for PEPSSI Species Energy measurement range

Energy + TOF measure Energy-only measure TOF-only measure

Energetic electrons

Protons

Not applicable 25 keV to l MeV 60 keV to 1 MeV

25 keV to 500 keV

Not applicable

15 keV to ~l MeV/nucleon

Not applicable Not applicable

atomic mass units (AMU) for H+ (25 keV to 1 MeV), <5 AMU for C+/N+ /O+ (60 keV to 1 MeV), and < 15 AMU for Fe+ (60 keV to 1 MeV).

For TOF-Only measurements, the means to ascertain particle mass is less precise, and this requirement, with respect to mass resolution, is to distinguish between H+ and CNO group particles. To support derivation of species mass, for particle energies in the ~ 1 keV to 1 MeV range, PEPSSI was specified to be capable of measuring particle TOF over a range of 1 to 320 ns. As TOF measurements respond to the energy-per-mass of an incident particle, these measurements can, in principle, respond to heavy ions up to ~30 MeV total energy (~1 MeV/nucleon). However, such extremely high energies are not expected in the Pluto environment and the current capability of the instrument is only to deal with events with up to ~ 1 MeV total energy.

2.3.2.2 Species Mass Range The PEPSSI instrument is constrained in downlink capability from Pluto as well as in the mass and power available for the instrument. Hence, prudent choices had to be made to meet all of the constraints while still enabling the collection of appropriate data from the vicinity of Pluto and its transmission to Earth following the flyby. Species resolution for the various energy spectra is limited by the counting statistics and the physical size of the detector that limits the TOF drift space. To enable the discrimination of solar wind particles (primarily protons and alpha particles, i.e. doubly-ionized helium nuclei) from pickup particles from Pluto, including atomic "debris" as well as ionized molecules of nitrogen, methane, and carbon monoxide, and allow for discovery science within the confines of the requirements, energy spectra are output for proton events, electron events, CNO events, and heavy particle events (>24 AMU, typified by Fe).

2.3.2.3 Sensitivity and Geometric Factor Requirements With PEPSSI mounted on the spacecraft, including installation of the RTG power source, and with the PEPPSI covers closed, the background ion particle count rate was specified not to exceed one particle per second. Expected fluxes at Pluto are relatively low (~ 100 events per second), so to stay within low power limits of operation, the PEPSSI instrument was specified to be capable of processing at least 103 particle events per second, where this event rate is applicable to the total of all classes of measurements, i.e., Energy-plus-TOF, Energy-Only, and TOF-Only. The PEPSSI instrument has the potential to measure particle events at a much higher rate; this rate should be established once the analysis of data from the Jupiter flyby is fully analyzed.

2.3.2.4 Geometric Factor Expected count rates at Pluto are unknown but expected to be low. Hence, the geometric factor was required to be as large as possible, consistent with the targeted low mass of the instrument of ~1.5 kg.

On the basis of these trades, the PEPSSI geometric factors, for electron and ion detection, were specified to meet or exceed the values given in Table 2. The geometric factors for electron and ion detection are different because of the difference in numbers of ion and electron detectors. The values that follow apply to the entire aperture acceptance angle of 160° by 12°, i.e. the geometric factor per "pixel" is less.

2.3.2.5 Integration Interval Nominally, energy-plus-TOF measurements, used to determine particle species and associated energy spectra, are integrated over as low as a 1-second interval (based upon consideration of spacecraft speed, and hence spatial resolution, telemetry rates and data volume playback during the Pluto encounter). TOF measurements, used to determine particle velocity distribution, are integrated over the identical time interval. By command, the integration interval may be adjusted from 1 to 65,535 seconds.

Table 2 PEPSSI sensor specifications

Ion detection geometric factor Electron detection geometric factor Acceptance angle

Aperture area TOF length

Number of Detectors Per Sector Detector Area Number of Ion Detectors Number of Electron Detectors

2.3.3 Measurement Resolution Requirements

>0.1 cm2steradian >0.033 cm2 steradian

160° by 12°, 6 sectors of 25° by 12° each, 2° gaps between sectors

6 cm nominal between entry, exit foils

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