Fly-Out Direction

4. Science Operations Concept

The first stage of this study focused on identifying science targets and their preliminary trade-offs.

A more detailed analysis of operations concepts given these targets will be performed in the sec-ond stage of the study.

4.1. Fly-Out Direction

The orbital position of Jupiter opens solar system escape in a specific direction every 12 years.

Ultimately, the fly-out direction is driven by science, but is therefore a trade with launch date if the trajectory is kept in the ecliptic plane. Because of the slingshot motion around Jupiter (gravity assist) planned for two of the mission architecture options, any ecliptic inclination would decrease the asymptotic speed for the same amount of propulsion and mass. For a solar Oberth maneuver, this decrease would be less significant. The following science drivers have been considered and discussed during the 1st Interstellar Probe Exploration Workshop and would need to be traded against each other to select an optimal fly-out direction for a final mission architecture.

4.1.1. Heliospheric and Interstellar Science

The science questions relevant to the heliosphere and local interstellar medium (LISM) are the highest priority that should drive the primary operational scenario. The global shape of the helio-sphere is one of the most outstanding questions that could be uniquely answered from an external vantage point. Given that the scientific debate revolves around a bubble shape versus a bifurcated tail structure, a vantage point displaced sufficiently in ecliptic longitude from the nose direction would be a good option.

Reaching the LISM as fast as possible has been expressed to be of high scientific importance. This would allow time for long-term investigations and increase the chances of surviving instruments reaching the unperturbed LISM. The shortest distance to the heliopause (HP) is in the general direction of the nose.

IBEX and Cassini (and IMAP in several years) observations clearly identify directions that would be extremely valuable to measure in situ. Foremost, the intriguing “ribbon” or “belt” appears as a great circle with a cone angle of ~70° around the interstellar medium (ISM) magnetic field direc-tion, and so intersects the ecliptic plane in two places (Figure 4-1). Neither of the Voyager space-craft are flying through these regions. Also, the region around the heliospheric nose may contain significant plasma pressure in the heliosheath that may move gradually in ecliptic latitude, as has been indicated by IBEX observations (Schwadron et al., 2014). Voyager 2 measured flows that were consistent with directions away from a pressure maximum and not the nose direction (McComas & Schwadron, 2014). Energetic neutral atom (ENA) imaging during the traversal of the heliosphere phase would offer a changing vantage point and viewing angles against the ENA sources, which would be crucial to constrain, for example, the radial location of the ribbon and its generation mechanism. This would also guide the future science operations.

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Figure 4-1. The ultimate fly-out direction of an interstellar probe is dictated by the primary heliophysics science goal traded with the speed and launch year. The image shows the sky in ecliptic coordinates with the contours of 1-keV ENA emissions as measured by the IBEX mission and also shows the ribbon. The locations of planets, dwarf planets, and Kuiper Belt Objects (KBOs) are shown for the 2030–2040 period.

A trajectory through the ribbon in the ecliptic plane at ~295° ecliptic longitude would also enable an external imaging of the heliospheric shape and offer a potential flyby of Quaoar.

The existence and exact shape of a heliotail is still debated, as discussed above. Flying out down a potential heliotail would definitively determine the HP distance in that direction. However, doing so would come at the expense of a much longer flight time to reach the pristine LISM. Further-more, an imaging vantage point from directly down the tail would not be optimal for discerning the shape of the heliosphere. Flying along one of the directions of two theorized jets would pro-vide direction measurements of the turbulent scale sizes of that region, which could be a very important constraint on the nature of the global heliospheric interaction with the LISM.

4.1.2. Kuiper Belt Object (KBO) Targets

There are a vast number of KBOs, so any direction governed by heliophysics would offer at least one opportunity for a flyby of a compelling object (Figure 4-1). Targeting larger worlds, such as dwarf planets, would allow the fascinating geology and potential habitability of these small planets to be investigated while providing context for understanding the other Kuiper planets of Pluto, Charon, and Triton. A flyby of Quaoar, at a heliocentric distance of 44 astronomical units (au), lies in a direction that intersects the ribbon and achieves an external vantage point to image the heli-ospheric shape. Secondary flybys of sub-planetary KBOs (typically <400 km in diameter) could oc-cur as opportunistic alignments allow, but even small trajectory corrections (tenths of degrees) require significant amount of propellant.

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4-3

Eris, the most massive known Kuiper Belt planet, will be situated near the heliosphere’s tail, lo-cated ~96 au away. Although exploring Eris would be technically possible for Interstellar Probe, the decision to fly by there would have to be weighed in light of other science priorities.

While it is hard to select exactly what object or system might be most interesting in combination with the logistics of a flyby trajectory, some of the most interesting targets might include one or more objects that are part of the Haumea collisional family. One of the major successes of the Dawn mission was its confirmation, on a variety of levels, of the genetic link between Vesta, the Vestite dynamical family, and the collected Vesta meteorites. Although we do not expect to pick up any Haumea-family meteorites from Earth’s surface, genetic linking of spectroscopic and/or surface properties between proposed Haumea family members and Haumea itself would be clear confirmation of many modeling efforts to understand the physics of collisions, dynamics, and in-teractions of objects in the Kuiper Belt. Similar arguments could be made for other multi-body systems in the Kuiper Belt.

The probability of lining up two or more KBOs is small, in particular if one of the flybys is fixed.

With the high speeds of Interstellar Probe, there is little to no room for course adjustments be-cause they would take a significant amount of fuel. For example, MU69 was ~0.2° off the initial direction of New Horizons, and a significant amount of fuel was required to achieve the desired flyby distance. This implies that all targets preferably should be identified early in the development phase to ensure that any operational requirements are incorporated into the mission design. The closest analog is, of course, the identification of MU69, a task that was not straightforward from the Hubble Space Telescope. By the time Interstellar Probe is in full development, the James Webb Space Telescope will have been operational for at least several years, and there should therefore be improved capabilities to search for suitable targets. Given the known density of small and large KBOs in the outer solar system, one would expect at most two to be sufficiently aligned to be realistic for flybys without expending too much fuel.

4.1.3. Planetary Targets

A Jupiter gravity assist (JGA) is part of all of the mission architecture options in this study. Given the high science return of past planetary flybys, science operations at Jupiter are a high priority, except during the mission-critical kick-stage burn. The approximate traversal of the Jovian system, from entrance to exit (bow shock crossing) of the magnetosphere (including the burn at closest approach) would be on the order of several hours.

Both the Uranus and Neptune systems would be compelling flybys because they would enable exploration of the planets, their moons, and their exotic magnetospheres. For launches in the 2030s, Uranus would lie in the direction of the heliotail and Neptune would lie ~110° off the nose direction and not fall within the ribbon.

Batygin and Morbidelli (2017) hypothesized the existence of a fifth giant planet with a semimajor axis >250 au. If a discovery of such a trans-Neptunian giant planet were confirmed well before the

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launch of an interstellar probe, it might necessitate a reprioritization of science targets and oper-ations of an interstellar probe and could revolutionize our view of the solar system’s structure and evolution.

4.1.4. Circumsolar Debris Disk

An optimal vantage point for the disk would be at high inclination, which would enable direct im-aging of azimuthal structure and gaps in the disk caused by planetary interaction. However, not only would this come at a significant cost of asymptotic speed, but it would prevent in situ sam-pling of the dust in the disk on the way out through the ecliptic. For a trajectory in the ecliptic plane, infrared (IR) imaging could be performed from the changing vantage point, which would provide good information for retrieving the large-scale distribution of the dust in the disk, includ-ing potential gaps associated with planetary interactions. Once at a distance beyond the inner zodiacal cloud, the ecliptic vantage point would provide direct constraints on its vertical scale heights. It would certainly be beneficial to study techniques for optimizing the retrieval of the large-scale structure.

4.1.5. Example Optimal Fly-Out Direction

Although there is a need for several other detailed trade-off studies, providing one example illus-trates some of the acceptable compromises that must be made. An example direction toward Quaoar satisfies flying through the ribbon close to the ecliptic, and sufficiently away from the nose direction to achieve a side view of the heliosphere that could discern its global shape. Quaoar is only ~8° off the ecliptic plane, which would keep the losses in speed at a minimum. As discussed in section 7, this option would require a launch either just before 2030 or in the late 2030s.