After the discussion of the compelling science questions at the 1st Interstellar Probe Exploration Workshop in 2018, a preliminary, example baseline payload was discussed and documented. Re-source requirements and measurement capabilities were drawn from the input of and presentations made by community members at the workshop. It was noted that the stated required resources were optimistic as compared to past flown instrumentation. The intention was to provide resource, spacecraft, and operational requirements to drive the design of a mission architecture. Trade-offs for a lighter, example threshold payload were also discussed but were not analyzed in detail.
6.1. Example Baseline Model Payload
Table 6-1. Example Baseline Model Payload as Discussed and Documented at the 1st Interstellar Probe Exploration Workshop, New York City, 2018, and presented at the AGU Fall Meeting 2018.
Instrument Mass (kg) Power
(W) Bitrate
(bps) Capabilities Spacecraft
Requirements TRL and Heritage Vector Helium
Instrument 6 1.5 100 Includes sensor, wire antennas, shielding,
harness >10-m stacer
Mass and charge state of H-Fe ions 1.4 × 10−3 cm2 sr eV/eV
NASA Task Order NNN06AA01C
(bps) Capabilities Spacecraft
Requirements TRL and Heritage Cosmic-ray
45° × 2° pixels using scanning platform
Scanning
100 m/px at 10,000 km: <5 µrad (baselined
~LORRI optics)
Framing (panchromatic) and pushbroom (multispectral) modes (baselined ~EIS electronics)
Single-pass pushbroom stereo capability Millisecond to multiple second exposures Tolerance needed to observe planet-Sun transits beyond 30 AU as exoplanet analog.
Also could observe moons crossing planets' disks.
NASA Task Order NNN06AA01C
(bps) Capabilities Spacecraft
Requirements TRL and Heritage VISNIR/FIR
6.2. Discussions on an Example Threshold Payload
A threshold payload must be derived from a careful analysis of a threshold mission and its science value. At the 1st Interstellar Probe Exploration Workshop in 2018, a brief discussion was held among the participating community members to analyze and document the science loss of a few example trades. It is important to note that no consensus was reached on any particular threshold science or payload.
Although imaging of the circumsolar dust by an infrared (IR) detector would provide groundbreak-ing information on the large-scale structure, in situ detection by a dust analyzer would still provide unprecedented information on the size and compositional distribution along an outward trajec-tory. The only dust measurements accomplished to date in the outer solar system have been made by the Student Dust Counter on board New Horizons. Even though it did not resolve composition or size, it provided the community with a very important constraint, and the only one thus far.
A dust analyzer provides important elemental and, to some extent, isotopical compositional infor-mation about circumsolar dust and interstellar dusts (ISDs). This inforinfor-mation would be particularly important crossing the heliopause (HP) into the local interstellar medium, providing insights into the properties of ISDs in our galaxy that could be an essential part of the building blocks of our and other solar systems, as well as galactic chemical evolution. Although a dust counter requires much less mass and power, it does not resolve any composition and would provide total number density as a function of distance from the Sun. Dust hits could also be counted by a plasma wave antenna and provide at least a rough relative number density.
NASA Task Order NNN06AA01C
The spectral resolution uniquely provided by a Lyman-alpha spectrograph would be the first to remotely differentiate the interstellar H flow and wall from the Lyman-alpha background and the hotter components believed to exist in the inner and outer heliosheath. Lyman-alpha intensity measurements that would not resolve the line shape would repeat the total column density meas-urements of H that have been performed by the Voyager/Ultraviolet Spectrometer (UVS) and New Horizons/Alice instruments that cannot uniquely resolve the interstellar flow but can be used for atmospheric observations during a potential flyby of a dwarf planet or Kuiper Belt Object.
Given the success of ENA observations from IBEX and Cassini, energetic neutral atom (ENA) imag-ing was generally agreed to be one of the more important and appropriate observations from an interstellar probe. The wide field of view (FOV) of a high-energy (approximately tens of kiloelec-tronvolts) ENA camera (Cassini/INCA and IMAP-Ultra) is required to capture the first unique image of the heliosphere shape from an external vantage point beyond the HP. Minimizing the FOV to a telescope-like configuration would save some mass but would severely impact the ability to derive any information on the global shape. In the medium 0.4- to 10-keV energy range, current instru-ment techniques (IBEX-Hi and IMAP-Hi) revolve around a narrow FOV, which necessitates the use of a scanning platform to achieve wide angular coverage. This is important to discern the source location and mechanisms of the ribbon by imaging from the changing vantage point that a trajec-tory out of the heliosphere would provide. Also, it would ensure that any temporal variations in emission strength could be captured globally. A telescope-like configuration mounted perpendic-ular to the spin axis without a scanning platform would cover a great circle in the sky that would scan the boundary along the outward trajectory, much like an MRI scan. However, this would se-verely limit the capability to discern global structure or ribbon source location and would make it difficult to differentiate between temporal versus spatial variations.
NASA Task Order NNN06AA01C
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