Exploring the Heliosphere

The Solar Wind at Pluto (SWAP) and the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instruments have made unique heliophysics measurements — starting at about 21 astronomical units (AU) — that the Voyager spacecraft were not equipped to make. Additionally, New Horizons’ ultraviolet (UV) spectrograph, Alice, is enabling unprecedented remote measurements of Interplanetary Hydrogen and all-sky mapping of the heliospheric boundary and interstellar clouds. Over the next two decades of operational lifetime, New Horizons will be the only spacecraft exploring the outer heliosphere and will have sufficient power to continue its unique investigations through the heliospheric boundary.

Credit: NASA SHIELD Center

As our magnetized Sun plows through interstellar space at about 26 kilometers per second, it carves out an enormous bubble — the heliosphere — that harbors almost the entire solar system out to hundreds of astronomical units. Only Voyager 1 and Voyager 2 have traversed the boundaries of the heliosphere, leaving behind a range of remarkable discoveries that we do not fully understand.

Heading for the Boundary of the Heliosphere

New Horizons is the only mission in the outer heliosphere and will have sufficient power to operate until 2050. Early in the next decade, or even late in this decade, New Horizons is expected to cross the so-called termination shock, which marks an abrupt slowdown of the solar wind and the beginning of the thick boundary called the heliosheath.

The current thinking is that in the heliosheath, the pickup ions that New Horizons can measure will dominate the pressure and play a critical role in upholding the "shield" against the interstellar medium. Heliosheath thickness as measured by Voyager 1 and Voyager 2 is about 30 AU, which is only half of the best theoretical predictions. Therefore, New Horizons’ unique measurements will provide critical insight into this mystery.

The Voyager mission crossed the heliopause into the very local interstellar medium (VLISM) at around 120 AU. New Horizons will be the third spacecraft to cross into the VLISM, and with skillful power management, we will likely be able to observe this transition.

Upholding the Heliosphere Against the Flow of the Interstellar Medium

Voyager 1 and Voyager 2 finally crossed into the VLISM at 122 and 119 AU, respectively, but left scientists with a new mystery of how the heliosphere stands off this interstellar wind: The pressure inside the heliosphere measured by the Voyager spacecraft simply did not match the pressure found in the interstellar medium.

It turns out that the Voyagers were not equipped to measure the particular energy range of ions created when gas from interstellar space penetrates the solar system, becomes ionized as it gets close to the Sun, and is carried back out by the solar wind. With their unique measurements, the SWAP and PEPSSI instruments on New Horizons have revealed that these so-called "pickup ions" (or PUIs, mostly hydrogen and helium ions) dominate the pressure in the outer heliosphere and therefore play a central role in the force balance of the heliosphere against the relative flow of interstellar material. As New Horizons approaches the boundary of the heliosphere, these measurements will become increasingly important for solving the decades-long mystery left behind from the Voyager missions.

Schematic of the heliosphere and currently operating spacecraft near its boundary. With both Voyagers now in the very local interstellar medium (VLISM) outside our heliosphere, New Horizons is the only operating spacecraft in the outer heliosphere. In less than a decade, New Horizons could encounter the termination shock and begin its traversal of the heliosheath. Adapted from Krimigis et al., 2019, https://doi.org/10.1038/s41550-019-0927-4

Trace Solar Wind and Shock Evolution Across the Heliosphere

With their high-resolution trigger modes, the SWAP and PEPSSI instruments on New Horizons study how the complex interactions in propagating shocks accelerate particles to high energies.

As the expanding solar wind "picks up" more interstellar neutral gas in the form of PUIs, it loses momentum and slows down — an effect that is clearly visible in SWAP data. On its way, the solar wind experiences turbulence and shock waves propagating from the Sun all the way out to the edge of the heliosphere, where they affect the dynamics of the global heliosphere. With their high-resolution trigger modes, the SWAP and PEPSSI instruments study how these complex interactions in propagating shocks accelerate particles to high energies.

With the fleet of spacecraft in the inner heliosphere and Voyager 1 and Voyager 2 in the VLISM outside our heliosphere, New Horizons is uniquely positioned to help us understand how shocks and solar disturbances propagate across the heliosphere and out to the VLISM. Solar activity is on the upswing, with New Horizons seeing an increasing number of shocks, marking the last opportunity to study the propagation out to the Voyagers before the spacecraft’s dwindling power supplies run out.

Comparing remote energetic neutral atom (ENA) observations from IBEX and IMAP with the insitu measurements from New Horizons will provide new important global context regarding how the solar wind disturbances affect the global heliospheric dynamics.

Study Galactic Cosmic Rays

Very-high-energy Galactic Cosmic Rays (GCRs) are produced in the blasts of supernovae and permeate the galaxy — including some into our protective heliosphere, where scientists believe they affect atmospheric chemistry and even induce mutations in biological systems. Innovative analysis of the PEPSSI instrument data can also detect the very-high-energy particles that manage to penetrate the instrument. Studying GCRs from PEPSSI will yield important information on their interactions with the heliosphere.

Map Interplanetary Hydrogen

New Horizons’ Alice UV spectrographAlice UV spectrograph has been mapping the distribution of so-called Lyman-alpha emissions across the sky that mostly result from the resonantly backscattered solar Lyman-alpha of the interstellar hydrogen flowing through the heliosphere. Estimates of the amount of hydrogen between New Horizons and other spacecraft in the inner solar system can be made. Estimates have been made by taking sightline measurements toward New Horizons from the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft and then differentiating those from Alice measurements along outward-looking sightlines. Several other missions, such as Lunar Reconnaissance Orbiter, Jupiter Icy Moons Explorer (JUICE) and Europa Clipper can or could obtain further sightline estimates to New Horizons that scientists can use to derive the hydrogen density distribution over large portions of the solar system.

The Sun has spent its last 60,000 years traversing the local interstellar cloud (LIC) and appears now to be in the transition region to the new and largely unknown environment of the G-cloud that could drastically alter the heliospheric shape and size. Our current knowledge of these clouds is derived from Hubble Space Telescope spectral observations toward nearby stars, and these observations have provided only basic information on their possible large-scale distribution.
Credit: Adler Planetarium, Frisch, Redfield, Linsky

New Horizons scientists are analyzing the first full Lyman-alpha sky map compiled by the spacecraft’s Alice ultraviolet spectrometer. The map shown here expands on the earlier proof-of-concept maps and consists of six great-circle scans and one 30-degree-wide contiguous sector. The smooth background model includes multiple scatterings of solar Lyman-alpha and isotropic 43-R galactic background (Gladstone et al., 2021). New Horizons is uniquely positioned to reveal the large-scale structure of the hydrogen wall and local interstellar medium (LISM) cloud signatures (colored outlines of LIC, G, Blue, Aql, mapped by Redfield et al., 2008)

Detect Interstellar Dust

The entire galaxy is full of dust grains of different sizes and compositions. This "interstellar dust" (ISD) is produced from a range of processes, including condensation of enormous stellar blowoffs. As the heliosphere plows through the galaxy, such grains encounter the heliosphere at about 26 kilometers per second. While ISD grains larger than a micrometer mostly penetrate the heliosphere, sub-micrometer-sized ISD grains are strongly modified by solar gravity, radiation pressure, interplanetary magnetic fields, and the heliospheric boundary, depending on their mass-to-charge ratio. New Horizons is heading nearly upwind into the incoming ISD flow.

The Student Dust Counter (SDC) onboard New Horizons is currently returning surprisingly high interplanetary dust fluxes that may be indicative of an extended Kuiper Belt or other mechanisms. Once New Horizons approaches the boundary of the heliosphere, the interplanetary dust flux may decrease to levels where the ISD flux is uncovered and presents an important opportunity to investigate how ISD grains interact with the heliosphere.

Simulated 0.3-micrometer interstellar dust (ISD) densities within the heliosphere during contrasting heliospheric conditions. (a) Each second solar minimum, the interplanetary magnetic field polarity defocuses the trajectories of charged ISD grains. (b) During the focusing solar minima, the interplanetary magnetic field polarity focuses the trajectories of charged grains. (Model of 0.316-micrometer grains adapted from Slavin et al., 2012)