Voyager 3: A Concept Mission to Interstellar Medium

In 1973 and 1979 Pioneer 10 and Voyager 1 and 2 were the first three spacecraft to visit the gas giant. Voyager 1 and 2 discovered Jupiter’s faint rings and several new moons and volcanic activity on Io’s surface. Between 1995 and 2003, the Galileo spacecraft dropped a probe into Jupiter’s atmosphere and conducted extended observations of Jupiter and its moons and rings. In 2000, Cassini took a highly detailed true-color mosaic photo of the gas giant. New Horizons passed Jupiter on Feb. 28, 2007, while it used the planet’s gravity assist to increase its speed, it also tested its instruments in flight. Although it was the eighth spacecraft to visit Jupiter and its moons, it made discoveries. Accordingly, it recorded lightning, shedding new perspectives on Jupiter’s atmospheric storms, recording aurorae in Jupiter’s atmosphere, volcanic eruptions on the moon Io, the life cycle of fresh ammonia cloud, boulder-sized clumps speeding through the planet’s faint rings, and the pulsing of Jupiter’s magnetosphere. Since 2016, Juno spacecraft has arrived at Jupiter, conducting an in-depth investigation of the planet’s atmosphere, deep structure, and magnetosphere analysis for clues to its origin and evolution. The goal of Voyager 3 mission at Jupiter is to follow up on the previous and current missions (especially the Juno mission) during its flyby of this planet.

The heliosphere is often defined as a “bubble-like” region of space surrounding the sun and planetary system that arose due to the solar wind; more importantly, it is one the main reasons life exists within our solar system as it shields the planets from harmful galactic cosmic radiation. However, there are several features of the heliosphere that are still not very well understood, most stemming from the debates concerning the structure of the heliosphere itself [1]. The magnetic field is one of the key elements for better understanding the structure of the heliosphere and the physical processes that take place within it; thus, the first science objective of Phase I of the Voyager 3 mission examines these magnetic field lines to determine how they influence the overall shape and structure of the heliosphere. The complex fundamental physical processes that govern how energy and matter are transported within our solar system, can also play a role in other astrophysical settings. In that sense, the Sun, the heliosphere, and Earth’s magnetosphere and ionosphere serve as cosmic laboratories for studying universal plasma phenomena, with applications to laboratory plasma physics, fusion research, and plasma astrophysics. The last mission to measure the magnetic fields directly via in-situ observations was Voyager 1 and 2. Measurements from Voyager 2 confirmed the asymmetric structure of the heliosphere, due to the titled interstellar magnetic field [3]. Another observation from Voyager was that magnetic flux in the heliosheath is not conserved, with the reason still unknown [4]. While the Voyagers were able to obtain measurements of the magnetic fields as they traveled throughout the solar system, the equipment was not sensitive enough for the regions of space where the magnetic fields were weak. Voyager 3 is the perfect opportunity to follow up on these measurements and survey the unknown field ahead in the ISM. Here we should note that SO1.2 addresses key science goals 3 and 4 from NASA Decadal Strategy for Solar and Space Physics [26].

The outer heliosphere is divided into different regions: the termination shock, heliosheath, and heliopause. In front of the heliosphere, a bow shock is believed to form as the solar system travels through the ISM, although the presence is still being debated [1]. Another important component is the pick-up ion (PUI) population, which are solar wind protons that interact with the interstellar hydrogen atoms. In fact, $80\%$ of the energy in termination shock is dominated by these PUIs, which were not measured by Voyager, leading to an energy gap in the data. While this energy gap may also be due to energetic electrons, only a revisit to this region with proper instrumentation helps to fill this energy gap [5]. Some of the ongoing debates concerning the nature and structure of the heliosphere include the degree to which the heliosphere influences the local ISM, how the ISM magnetic field drapes around the heliosphere, the physical phenomena responsible for the observed “tailless” two-lobe shape of the HP, and other arguments summarized by the works of Ed Stone, et al. [1]. Many of these conflicts can only be resolved by performing more in-situ measurements of the magnetic fields that would be possible along the trajectory of Voyager 3. Here we should note that SO1.3, addresses key science goals 3 and 4 from NASA Decadal Strategy for Solar and Space Physics [26].

The overall goal of the Zodiacal Dust Cloud study is to understand the initial stages, conditions, and processes of solar system formation, and the nature of interstellar matter that was incorporated through the investigation of the zodiacal light, which would consequently result in the understanding of the dust distribution within the solar system. The current model of the interplanetary dust cloud (IPDC) [6] assumes a smooth cloud, with 3 dust bands, and a circumsolar dust ring. Recent observations suggest the presence of another circumsolar dust ring near Mercury orbit [7]. Near-Earth observations of zodiacal light are unable to produce a unique physical model of IPD and observations from outside the IPDC are necessary. Moreover, it would serve as a model for the dust and the zodiacal light pollution for the exoplanetary systems. Modeling the zodiacal light in the exoplanetary systems is motivated by the observation of the Very Large Telescopic Interferometer (VLTI), which has discovered that the exozodiacal light (zodiacal light around other star systems) poses a challenging obstacle to the direct imaging of Earth-like planets.

The goal of Extragalactic Background Light part of the mission is to address the question of: “What were the first objects to light up the universe, and when did they do it?” by making a precise measurement of the extragalactic background light (EBL). The EBL spectrum captures cosmological backgrounds associated with photons emitted by stars, galaxies, and active galactic nuclei (AGN) due to nucleosynthesis or other radiative processes, including dust scattering, absorption and reradiation or primordial phenomena, such as the cosmic microwave background (CMB), at microns to mm-wave radio wavelengths. The EBL may also contain signals that are diffuse and extended, including high energy photons associated with dark matter particle decays or annihilation [8]. The precise measurement of EBL could be used to constrain models of galaxy formation and evolution. With a precise measurement of the EBL intensity spectrum, a window into new discoveries could be opened with profound implications for astronomy ranging from recombination signatures during reionization and diffuse photons associated with dark matter annihilation and their products (see review in [9]). A precise direct measurement of the cosmic optical background (COB) can be achieved using a small aperture telescope observing at distances of 5AU or more [9].

Due to the expansion of the universe, light emitted from the creation and evolution of early galactic structures is expected to be redshifted into the near-IR and IR wavelengths [11]. Therefore, it is of great interest to obtain accurate measurements of the extragalactic background light (EBL) in these wavelengths in order to probe the history of early sources as well as the nature of early galactic evolution such as star formation rates and initial mass functions. The objective of the Voyager 3 mission is to directly measure the EBL intensity at 1.75, 2.5, and 5.3$\mu$m, the nominal wavelengths of the HgCdTe detectors that would be used on the spacecraft. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light. The constraints on the cosmic optical background (COB) obtained from an analysis of the Pioneer 10/11 Imaging Photopolarimeter (IPP) data with accurate starlight subtraction was achieved by referring to all-sky star catalogs and a Galactic stellar population synthesis model down to 32.0 mag [Matsuoka2011].

Within the range of wavelengths we are interested in, there have been calculations of the EBL using measurements from DIRBE and subtracting the stellar flux from galactic stars using the 2MASS All-Sky Point Source Catalogue that set the intensity of the EBL at 14.69$\pm$4.49kJysr^-1 at 2.2 $\mu$m, 15.62$\pm$3.34kJysr^-1 at 2.5 $\mu$m, and 8.88$\pm$6.26kJysr^-1 at 1.25 $\mu$m [13]. Because the EBL measurements quoted here are residuals after subtracting stellar flux and foreground light, there is little room for improvement without another large sky survey, which Voyager 3 is not capable of. The best measurements of COB come from an indirect technique involving $\gamma$-ray spectra of bright blazars with an absorption feature resulting from pair-production off of COB photons. COB has large uncertainties involving direct measurements due to uncertain removal of the zodiacal light foreground. At optical and near-IR wavelengths, the EBL intensity is predominantly due to stellar emission from nucleosynthesis throughout cosmic history (see [14] for a review). The COB spectrum also includes radiative information from the reionization epoch. The dominant limitation for direct EBL intensity spectrum at these wavelengths is the zodiacal light associated with scattered solar light from micrometer-size interplanetary dust (IPD) particles near the Earth’s orbit. Current measurements with DIRBE on COBE make use of a model to remove zodiacal light or slight variations [97, 27, 9].

The Zodiacal light and its pollution in our inner solar system have long been a challenge for the study of the early Universe for the cosmologists. There is much to be gained from simply performing the same observations from outside of the interplanetary dust cloud, where measurements aren’t dominated by the zodiacal light leading to large uncertainties. For this reason, a team at NASA, JPL and Caltech has been looking into the possibility of hitching an optical telescope to a survey spacecraft on a mission to the outer solar system, such as in the ZEBRA concept mission [10]. Voyager 3, investigates the EBL and takes on further missions.

We set the requirement that Voyager 3 must be able to measure the intensity of the EBL at or below 8.0 kJysr^-1, and expect a significant reduction in uncertainty by completely eliminating the contribution from the IPD cloud which was previously estimated using models and found to contribute up to $90\%$ of the total light emitted in the wavelength regime of interest [15].

Studies of primitive bodies encompass asteroids, comets, Kuiper belt objects (KBOs), the moons of Mars, and samples, meteorites and IPD particles, derived from them [2]. Primitive bodies are thought to have formed earlier than the planets in the hierarchical assembly of solar system bodies, have witnessed, or participated in, many of the formative processes in the early solar nebula. Therefore, these objects provide unique information on the solar system’s origin and early history and help researchers to interpret observations of debris disks around other stars [2]. There are more than 100,000 KBOs accounted for in this region and many more yet to be discovered beyond the orbit of Neptune (30AU) to approximately 50AU. The study of primitive bodies over the past decade has been accomplished as a result of several space missions such as Deep Impact, Stardust, EPOXI, Cassini, and the Japan Aerospace Exploration Agency’s (JAXA’s). Hayabusa has provided a sample of a near-Earth asteroid; determining that small asteroids can be rubble piles [2]. Ground-and space-based telescopes and radar studies have discovered that binary objects are common among near-Earth, main-belt asteroids, and KBOs [2]. Cassini had come to the first encounter with a possible former trans-Neptune object in the form of Phoebe, Saturn’s satellite. Deep Space has determined the density of a comet nucleus via the first controlled cratering experiment on a primitive body. However, important questions remain unanswered [2]. Necessary steps have been accomplished in constraining the nature and mixing during the formation of primitive bodies, as well as formation chronology [16, 17]. Diverse components in comet samples returned by the Stardust spacecraft and the composition of the Sun constrained by the detailed analysis of the solar wind samples from Genesis spacecraft illustrate a dynamical nebular mixing [18]. However, the size distributions of KBOs, now moderately well known for bodies greater than 100 km, are serving as the best tracer of the accretion phase in the outer solar nebula [19]. In addition, the structure of the Kuiper belt provides one of the best constraints on the dynamical rearrangement of the giant planets, and some recent KBO studies have revised scenarios for the early orbital history of the solar system [20]. Ground-based telescopes continue the discovery and characterization of a significant portion of KBOs. The 3-meter NASA Infrared Telescope Facility (IRTF) has provided significant data for studies of primitive bodies. The Large Synoptic Survey Telescope (LSST) project was a top-rated priority for ground-based telescopes for the years 2011-2021 by the 2010 Astronomy and Astrophysics Decadal Survey [21]. Even though ground-based telescopes continue to be relevant to the study of larger or closer objects, Voyager 3 would provide a unique close view and extended observation time to KBOs.

The study of Interplanetary Dust & Plasma has the goal to understand what it was about the Sun’s birth environment or its star formation process that determined the final properties of our solar system versus that of other planetary systems; in addition, Voyager 3 would attempt to determine how much gas and dust was left over for the planet formation in our solar system. Dust grains are a very significant component for the understanding of the heating and cooling of the ISM. Moreover, dust is also important in the study of the interstellar chemistry which is essential in the formation of most organic and inorganic molecules [22, 23, 1]. The Voyagers 1 and 2, and the New Horizons missions investigated interplanetary dust and plasma. However, a follow-up study with Voyagers 3 new instruments would address many of the yet open questions as mentioned in Table 1.

The goal of Parallax Science is to address the question: “At what rate is the universe expanding?” Dark energy is hypothesized to explain the tending to accelerate the expansion of the universe. As stated in the Astronomy and Astrophysics Decadal Survey (2010) [21], the only way moving forward in understanding this mysterious component of our universe is to use the universe at large to infer the properties of dark energy by measuring its effects on the expansion rate and the growth of structure, (addressing the Frontier of Knowledge, Expansion of the Universe). Therefore, precision measurements of the expansion of the universe with time and of the rate at which cosmic structure grows are required. There are two distinct methods for the measurement of the Hubble constant, which gives us the current rate of the expansion of the universe, and these methods provide two very different values for the constant. The discrepancy between the results of these two methods implies that there could be new physics underlying the foundations of the universe. Possibilities include the interaction strength of dark matter, dark energy being even more exotic than previously thought, or an unknown new particle in the tapestry of space [24]. With the second data release of the Gaia mission, the parallax measurement of hundreds of Cepheid variables has been determined with a precision on the order of $10\mu as$ [25], allowing the most precise measurement of $H_{0}$ to date. However, Gaia is still limited by its measurement baseline distance of $\sim$1AU. The Voyager 3 can extend the measurement baseline to 550 AU. Although the actual measurement quality could be worse than Gaia.

According to the general theory of relativity, rays of light passing in the vicinity of a massive object are deflected from their initial direction by the amount of $\theta=4GM/c^{2}b$, where $G$ is the gravitational constant, $M$ is the mass of the object, $c$ is the speed of light, and $b$ is the rays impact parameter. This phenomenon is called gravitational lensing. Gravitational lensing is a well-known effect and has been observed where relatively nearby galaxies, or clusters of galaxies, act as gravitational lenses for background galaxies, and in our Galaxy where micro-lensing of stars in the Galactic bulge or in the Magellanic clouds are caused by intervening bodies. Astronomers have used it to measure the shape of stars, look for exoplanets, measure dark matter in distant galaxies, and measure the size and age of the Universe. In our solar system, the effect was originally observed by Eddington in 1919 [27]. The gravitational field of the sun acts as a spherical lens to magnify the intensity of radiation from a distant source along a semi-infinite focal line, creating a powerful instrument, solar gravitational lens (SGL). Depending on the impact parameter of the incoming ray, electromagnetic (EM) waves traveling from distant sources in the close proximity of the Sun are focused by the solar gravitational field at heliocentric distances beyond $b^{2}/(2r_{g})\leq 547.6(b/R_{\odot})^{2}$AU, where $b$ is a light ray’s impact parameter, $r_{g}=2GM_{\odot}/c^{2}$ is the Schwarzschild radius of the Sun and $R_{\odot}$ is its radius. The focus of the SGL starts at $\sim$547 AU and going beyond 2,500 AU [28, 29], forming a focal line. One advantage of the gravitational lens is the amplification of the power received at the telescope. A mission to the gravitational focus of the Sun was listed as a precursor goal in the 1998 Jet Propulsion Laboratory (JPL) workshop “Robotic Interstellar Exploration in the Next Century” [30]. A mission utilizing SGL could provide direct multi-pixel high-resolution images and spectroscopy of a potentially habitable Earth-like exoplanet.

To construct an image using the solar gravity lens, Voyager 3 must travel to approximately 550AU and maintain a precise alignment along the focal line of the Sun’s gravity lens. Due to the precise alignment, Voyager 3 would not be used to search for habitable exoplanets; the extreme magnification of the solar gravity lens means that we would simply be unable to cover enough sky space to reliably locate exoplanets. Rather, Voyager 3 focuses on the characterization of a previously discovered potentially habitable exoplanet, selected before the mission is launched. It is unlikely that the trajectory of the spacecraft allows for imaging of more than one system, so the target must be chosen carefully. Factors affecting the target choice include host star spectral type, distance to the host star, and the number of exoplanets around the host star, and the type of exoplanets around the star. The ideal candidate would be a nearby, Sun-like star hosting multiple rocky exoplanets in or near the star’s habitable zone.

At the focal line, light from an exoplanet forms an annulus, surrounding the edge of the Sun, which is much dimmer than the Sun. We refer to this as the Einstein ring. The remarkable optical properties of SGL include major brightness amplification ($8\pi^{2}GM/(c^{2}\lambda)$, $\sim 10^{11}$ at $\lambda=1\mu$m, ) and extreme angular resolution of $0.1(\lambda/1\mu\text{m})(650\text{AU}/r_{o})^{1/2}$ nas [34]. The apparent brightness of sun at 550 AU is $\sim 4.5\times 10^{-3}\text{W}/\text{m}^{2}$. However, even with this amount of amplification, the apparent brightness of the planet is $1.85\times 10^{-10}\text{W}/\text{m}^{2}$, for a planet which is 12 parsec from Sun. When averaged over a 1-m telescope, the light amplification is reduced. However, this is still sufficiently bright, even on the solar corona background. redThe contribution of the solar corona is removable, A preliminary coronagraph design was analyzed in [33]. The concept here used for the imaging of the exoplanet is to use a single telescope, but the light from different parts of the Einstein ring is imaged separately. Then, the signals from the different parts of the ring are then deconvoluted using an algorithm to reconstruct the planet [31]. It is also important to note that before the SGL mission is launched, one has to gather information about the target exoplanet and its system, as much as possible. The planet’s orbit would have to be measured using astrometry or radial velocity measurements. Any other information such as the orbital ephemeris, estimates of the rotation rate, and some understanding of cloud and surface properties from Doppler imaging, could pave the way for the further information that would be gained from SGL.

The solar gravitational lens addresses the Astronomy and Astrophysics decadal survey science frontier discovery area of the identification and characterization of nearby habitable exoplanets”. There has been a surge of exoplanet discoveries, especially in the Earth and super-Earth mass regime, which was previously not well-detected. NASA’s Transiting Exoplanet Survey Satellite mission and the future James Webb Space Telescope will lead to thousands more detected exoplanets, many of which could be potentially habitable. Detection of these potentially habitable exoplanets is an issue that is currently being addressed by many parties across the globe; however, characterization of habitable exoplanets is extremely difficult and limited to planets which transit their host star and planets which we can directly image. While direct imaging has been successful for some systems, it is limited to nearby systems with very giant planets due to the optical limits of our current telescopes. Transit spectroscopy has been more successful, and we have been able to characterize the atmospheres of their transit spectra. Unfortunately, transit spectroscopy requires both proper orbital alignment and very precise observation times, making it difficult or impossible for many exoplanet systems. Additionally, while there are many global atmospheric models from which we can make predictions about the planetary climate based on atmospheric parameters, the models are not definitive, and we cannot predict surface conditions with certainty. The resolution of the solar gravitational lens would allow for spatially resolved images of the surface of the exoplanet, addressing the questions of habitability such as the presence of liquid water or vegetation.

The trajectory design and weight of the spacecraft significantly limited the available launch vehicles. Because of the time of flight requirement, the trajectory requires a high launch C3 of 44.89 $\text{km}^{2}/\text{s}^{2}$ ($V_{\infty}$ of 6.70 km/s). It is assumed that the launch vehicle would deliver this entirely in order to save the spacecraft’s propellant and to not have to use a kick stage. Existing Evolved Expendable Launch Vehicles (EELV) like ULA’s Delta IV Heavy and Atlas V 551 were considered for their reliability and payload-carrying capability. SpaceX’s Falcon Heavy in the recoverable and fully expendable configurations were also investigated due to their cost. Still, this vehicle has only launched twice at the time of analysis. Finally, the NASA/Boeing Space Launch System (SLS) Block 1B and 2 were traded as these are heavy-lift, high-reliability vehicles designed for deep space missions with high launch energy requirements. Note that options such as Blue Origin’s New Glenn and SpaceX’s Starship were not considered due to a lack of available data for a high energy launch. ULA’s Vulcan was considered, but is not able to launch enough our design at a C3 of 44.89 $\text{km}^{2}/\text{s}^{2}$. Starship and New Glenn could be prime contenders for this mission as they are intended to compete with the SLS for deep-space heavy-lift missions. At the time of this analysis, both Starship and New Glenn do not have mass-C3 curves available for higher energy departures such as Voyager 3.

The SLS is expected to launch NASA’s flagships in the coming future. Despite the launch cost and development status, the SLS Block 1B was chosen as it is the only physical vehicle able to meet the mass and energy requirements. Voyager 3 can be comfortably accommodated in the B8.4m-27000 fairing due to its large 7.5-meter diameter dynamic envelope and over 18-meter tall fairing [45]. Also, the spacecraft and launch vehicle adapter estimated to be <14,500 kg, allowing for a 3,100 kg mass margin, giving ample room for mass growth over the course of the design process. The SLS Block 1B could launch Voyager 3.