Plans for and initial results from the exploration of the Kuiper belt by New Horizons

C H A P T E R

17 Plans for and initial results from the exploration of the Kuiper belt by New Horizons S. Alan Sterna, John R. Spencera, Anne Verbiscerb, Heather E. Elliottc, Simon P. Portera a Southwest

Research Institute, Boulder, CO, United States b University of Virginia, Charlottesville, VA, United States c Southwest Research Institute, San Antonio, TX, United States

17.1 New Horizons Kuiper belt mission background New Horizons is NASA’s mission to explore the Pluto system and the Kuiper belt (KB). It was launched in 2006 as the first mission in NASA’s New Frontiers program and conducted a successful Jupiter system gravity assist and science flyby in 2007, and the first exploration of the Pluto system in mid-2015 (Stern et al., 2015). New Horizons has a payload consisting of seven scientific instruments. As described in Weaver et al. (2008), they are: Ralph—a visible/IR remote sensing suite comprised of panchromatic and color imagers and an IR mapping spectrograph; Alice—an ultraviolet mapping spectrograph; LORRI (LOng Range Reconnaissance Imager)—a long focal-length panchromatic visible imager; REX (Radio Experiment)—a radio science receiver; SWAP (Solar Wind Around Pluto)—a low-energy (∼25–7500 eV) particle spectrometer; PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation)—a high-energy (1–1000 keV) particle spectrometer; and Venetia Burney SDC (Student Dust Counter)—a student-built dust impact counter. Together this instrument suite provides these primary capabilities: (i) medium- and high-resolution panchromatic visible wavelength mapping, (ii) medium-resolution visible wavelength color mapping, (iii) IR surface composition mapping, (iv) stereo imaging for terrain height mapping, (v) ultraviolet spectroscopy for atmospheric composition and vertical The Trans-Neptunian Solar System. https://doi.org/10.1016/B978-0-12-816490-7.00017-5

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structure studies, (vi) plasma spectroscopy to measure the atmospheric escape rate and the composition of ionized gases escaping from Pluto’s atmosphere and the interaction of the atmosphere with the solar wind, (vii) radio science to measure the brightness temperature of Pluto’s surface, to determine the vertical temperature-pressure profile of Pluto’s lower atmosphere, and to make bistatic radar measurements at 4.2 cm wavelengths, and (viii) a dust detector to search for particulates in orbit around Pluto. Following its exploration of the Pluto system, New Horizons was approved to conduct an extended mission from 2016 to 2021 to study the KB and KB objects (KBOs) out to a heliocentric distance of 50 AU (Stern et al., 2018). No other operating or planned space mission or groundbased work is capable of performing this science. The KB is a rich scientific and intellectual frontier. Its exploration has important implications for better understanding comets, the origin of small planets, the solar system as a whole, the solar nebula, and extrasolar disks, as well as for studying thermally primitive material from the planet formation era. This exploration will transform KB and KBO science from a purely astronomical regime, as it is today, to a geological and geophysical regime, which radically changed paradigms when the same happened to asteroids and comets in past decades. The centerpiece of the New Horizons KB Extended Mission (KEM) is the close flyby of the “cold classical” KBO 2014 MU69 (hereafter MU69 ). This flyby will occur on January 1, 2019. The planned flyby will approach MU69 to ∼3500 km, almost four times closer than New Horizons flew past Pluto; consequently, imaging and compositional mapping spectroscopy resolutions will be significantly enhanced. MU69 is a ∼25-km-diameter KBO that is ∼103 times more massive than comet 67P that the European Space Agency (ESA) Rosetta mission orbited, but ∼5×105 times less massive than Pluto. This places MU69 in a key intermediate size regime to better understand the accretion processes that operated in the KB. Given its undisturbed orbit, and 4+ Gyr existence in storage at ∼35 K, MU69 will be the most primitive body ever studied by spacecraft. The flyby of MU69 will obtain the first and only planned high-resolution geological and compositional studies of a small KBO and will make sensitive searches for coma, activity, and satellites/rings at MU69 . However, KEM involves more than just the close flyby of MU69 . As described below, it also exploits the unique resource of New Horizons to act as an observatory in the KB, in order to: (i) study numerous other KBOs during 2016–2020 in multiple and otherwise unachievable ways; (ii) make measurements of the KB/heliospheric dust and plasma environment across the KB to 50 AU; and (iii) study the frequency of KBO dust rings using high-phase angle photometry that cannot otherwise be practically achieved.

17.2 Kuiper belt mission detailed objectives As described above, the scientific investigations of the first New Horizons KEM fall into three distinct categories. Here we provide additional details on each, in turn.

17.2.1 Science objectives of the close flyby cold classical KBO target 2014 MU69 As noted above, the centerpiece of the KEM is the January 1, 2019, close flyby of the cold classical KBO 2014 MU69 . Approach observations begin with LORRI images that are used

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to provide optical navigation data and but also to search for satellites and rings. Detected satellites will be tracked to determine their orbits. Throughout the 4-month long approach to MU69 spanning over 1 AU of approach distance, the SWAP, PEPSSI, and SDC instruments will observe the heliospheric plasma (solar wind, pick-up ions, and energetic particles) and the dust environment in the KB near MU69 , and the LORRI imager will obtain a rotational light curve for MU69 . MU69 will only reach a resolved scale of 2 LORRI pixels some 2–3 days before closest approach. From this point inward New Horizons will study MU69 ’s rotationally resolved photometric, color, and geological properties. Starting about 1 day before closest approach, Ralph instrument color images, Ralph near-IR composition spectroscopy, and Alice ultraviolet spectroscopy will be included with the LORRI imaging to obtain multiwavelength rotational coverage. Deep searches for satellites and rings will also intensify, allowing the detection of satellites as small as 0.1 km diameter for an assumed visible albedo of 0.1. During the final hours of approach these observations will be supplemented by Alice UV spectrograph integrations to search for airglow emissions from any gaseous coma surrounding the KBO. Color imaging, panchromatic imaging, and near-IR spectroscopy will be obtained through closest approach, with the best resolution being 300 m/pixel, 140 m/pixel, and 1.8 km/pixel, respectively. LORRI will obtain images with the best resolution at 35 m/pixel. Imaging at various phase angles up to 165 degree will be used to characterize surface photometric properties and to obtain digital elevation maps using stereo techniques. The flyby will also obtain disk-integrated observations of the day- and night-side microwave thermal emission and UV surface reflectance measurements. New Horizons will also attempt an uplink X-band bistatic radar detection reflected from MU69 ’s surface. Plasma and dust observations with SWAP, PEPSSI, and SDC will be conducted during the close approach to search for MU69 dust and plasma interactions. After closest approach, the Alice ultraviolet spectrograph will observe the Sun and a star to search for absorption due to possible coma gases. Departure imaging observations will conduct deep searches for forward-scattering rings around MU69 . Table 17.1 more completely describes the complete set of scientific objectives for this flyby.

TABLE 17.1 New Horizons first extended mission primary science and measurement objectives. Science objective

Best achieved at MU69 in prime sequence (green: better than Pluto; blue: comparable to Pluto; red: not as good as Pluto)

Measurement objective Group 1

Characterize the global geology and morphology

Panchromatic full-disk close approach imaging

Full disk 0.13 km/pixel, partial coverage at 0.035 km/pix

Panchromatic rotational coverage imaging

1.5 km/pix or better, all longitudesa

Topography, digital elevation models (DEM)

0.14 km/pixel Continued

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TABLE 17.1 New Horizons first extended mission primary science and measurement objectives —cont’d Science objective Map surface composition

Search for satellites and rings

Measurement objective

Best achieved at MU69 in prime sequence (green: better than Pluto; blue: comparable to Pluto; red: not as good as Pluto)

Close approach full-disk IR maps

1.8 km/pix

IR compositional spectroscopic rotational coverage

19 km/pix or better, all longitudesa

Color full-disk close approach imaging

0.33 km/pix

Color rotational coverage imaging

6 km/pix or better, ail longitudesa

UV reflectance spectroscopy

Disk integrated

Deep high and low phase imaging for satellite and ring searches

Nested observations starting with full Hill sphere; most sensitive satellite detection threshold ∼0.2 km diam to 5000 km distance (∼5% of Hill sphere), ring detection threshold I/F ∼5e−7

Group 2 Characterize composition and magnitude of any volatile or dust escape

Surface properties of MU69

UV stellar occultation coma search

Alice stellar appulse

UV solar occultation coma search

Alice solar appulse (0.5 h integration)

UV coma airglow search

Several Alice scans of potential coma, high and low phase

Heliospheric Ly-alpha coma absorption search

Alice scans near C/A

High-phase imaging dust coma search

Multiple high-phase coma dust searches

volatile escape detection via plasma interaction

Near-continuous SWAP and PEPSSI measurements near MU69 , including plasma rolls

4 cm day and night brightness temperature

Both day and night, hemispheric unresolved

Near-IR spectroscopic temperature measurements via band shifts

1.8 km/pix

Range of phase angles for MU69 to determine photometric properties

Global color and pan imaging at phase angles 1.5–169 degrees

Photometric properties of any satellites and rings

Global color and pan imaging at phase angles 4.5–169 degrees Continued

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TABLE 17.1 New Horizons first extended mission primary science and measurement objectives —cont’d Science objective

Measurement objective

Best achieved at MU69 in prime sequence (green: better than Pluto; blue: comparable to Pluto; red: not as good as Pluto)

Crater size/frequency distributions

High-resolution imaging

Full disk 0.13 km/pix, partial coverage at 0.035 km/pix

Characterize any satellites and rings

Sizes, shapes, rotation periods of any satellites

0.6 km/plx to 1000 km distance, dozens of visits for lightcurves

Geology of any satellites

0.6 km/pix to 1000 km distance

Color of any satellites and rings

2.5 km/pix to 10,000 km distance

Surface composition of any satellites and rings

6 km/pix to 1200 km distance

a For typical rotation period of 5 h.

Note: These quantitative measurements refer to the prime trajectory and flyby sequence.

17.2.2 Science objectives for other KBO studies A major aspect of the KEM mission is that it uses the LORRI telescope/imager, which can make observations down to an apparent visual magnitude of V ∼ 21 , to observe ∼25 KBOs, Centaurs, and dwarf planets (DPs) to make the following types of survey measurements: (1) light curves at multiple aspect (i.e., viewing) angles to determine shapes, rotation rates, and pole positions that cannot practically be obtained from the Earth; (2) high-phase photometric searches for ring and dust material; (3) high-parallax astrometry to refine their orbits; (4) photometry at moderate and high-phase angles to determine phase functions and photometric/regolith microphysical properties that cannot be obtained from the Earth; and (5) searches for both smaller satellites and closer binaries than can be achieved using groundbased systems, Hubble Space Telescope (HST), or James Webb Space Telescope (JWST). From its measurements of small KBOs, New Horizons will yield a statistical sample of their surface properties, shapes, and binarity to place its flyby target, 2014 MU69 , in context. From its measurements of DPs, New Horizons will better place Pluto and Charon in context with similar-sized bodies in the KB.

17.2.3 Science objectives for heliospheric studies The primary goal of the KEM heliospheric observations is to create a set of transect measurements of the KB plasma, dust, and neutral gas environment from 33 to 50 AU. The 50 AU distance was chosen for KEM to correspond to the aphelion of Pluto, so that these heliospheric observations also inform the space-weathering environment of Pluto and KBOs in similar orbits. Throughout the KEM, PEPSSI, SWAP, and SDC combine to create a transect record of plasma and dust across the inner and classical KB out to 50 AU. Additionally, the REX radio science instrument makes integral, total electron content measurements along the

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path back to Earth approximately monthly, and the Alice UV spectrometer makes a series of sets of 6 great circle neutral H brightness maps approximately twice per year to tomograph the neutral H density across the KB.

17.3 Sample results to date 17.3.1 Distant KBO results From its unique vantage point in the outer solar system, New Horizons has observed Kuiper belt objects (KBOs) from distances ranging between 0.1 and 70 AU and at solar phase angles (α) far larger than those attainable from Earth. While the size of the Earth’s orbit limits Earth-based KBO observations to phase angles α < 2 degree, New Horizons’ LORRI can observe KBOs at nearly the full range of solar phase angles, with its viewing geometry limited only by flight rules that prohibit pointing LORRI close to the Sun at α > 165 degrees. The sensitivity of LORRI is such that distant KBOs (DKBOs) must also have apparent magnitude V < 21 in order to be detected. The first DKBO to be observed by New Horizons’ LORRI at phase angles α > 10 degrees was the Plutino (15810) Arawn (1994 JR1). By combining low-phase, Earth-based KBO observations from sources including the HST and the University of Hawaii 2.2-m telescope with those obtained at higher phase angles by LORRI, Porter et al. (2016) produced the first rotational lightcurve (Fig. 17.1) and solar phase curve (Fig. 17.2) for this target. Fitting Arawn’s disk-integrated solar phase curve to the Hapke (2012) photometric model makes it possible to determine physical characteristics, such as macroscopic roughness and directional scattering properties, and compare those characteristics to other outer solar system bodies. Arawn has a rough surface with a 37◦ ± 5◦ mean topographic slope and is strongly backscattering. Its rotation period is a relatively rapid 5.47 ± 0.33 h. These New Horizons observations of Arawn laid the groundwork for subsequent observations of nearly two-dozen distant KBOs by LORRI during the New Horizons KEM.

FIG. 17.1 From Porter et al. (2016), the first rotational lightcurve from a distant Kuiper belt object, (15810) Arawn (1994 JR1), seen from New Horizons LORRI at an average phase angle of 58.3 degree. The median V magnitude is 16.9 and the peak-to-peak amplitude is 0.8 magnitudes. This lightcurve demonstrates that Arawn rotates once every 5.47 h. V. Prospects for the future

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FIG. 17.2 From Porter et al. (2016), the first solar phase curve of a distant Kuiper belt object using observations acquired by New Horizons LORRI at phase angles unattainable from Earth. Solid line is the best fit to the Hapke (2012) photometric model, and the dashed line is simply a constant phase coefficient (slope) of β = 0.048 mag/degree. The low phase angle data are supplied by Green et al. (1997) and Benecchi et al. (2011) and D. Tholen, personal communication. R-band photometry was converted to V with V-R = 0.76 (Porter et al., 2016).

New Horizons’ distant KBO targets successfully observed to date include DPs Haumea, Makemake, Quaoar, and 2002 MS4 ; cold classical KBO 2011 HJ103 and hot classical 2012 HZ84 ; resonant KBO 2012 HE85 ; and Plutino Arawn. In order to combine these high-phase observations with those acquired from Earth at smaller phase angles to produce complete phase curves, accurate transformations between different photometric systems must be performed. Only following such transformations will it be possible to construct the first full KBO solar phase curves with substantial phase angle coverage using multiple observations at phase angles ranging from α = 0.06 to 74 degrees. The production of a disk-integrated phase curve for each distant KBO enables the evaluation of the phase integral q (Verbiscer and Veverka, 1988). The spherical, or Bond albedo is then A = pq, the ratio between the total flux radiated in all directions to the total incident solar flux, a measure of the energy balance on the surface of the KBO as a whole. The full characterization of complete phase curves for all New Horizons distant KBO targets is beyond the scope of this chapter; however, we can compare the phase curve contributions from LORRI alone by evaluating observations of all distant KBOs observed to date at multiple phase angles and compare their phase coefficients β or slopes, in magnitudes/degree. Fig. 17.3 compares the phase functions of distant KBOs from LORRI alone for all objects observed to date by New Horizons at multiple phase angles (Table 17.2). Evaluating the phase coefficients (slopes) over these ranges of phase angles illustrates clearly that the scattering properties of these distant KBOs are distinct. Arawn has the steepest slope while resonant KBO 2012 HE85 and hot classical KBO 2012 HZ84 have intermediate slopes between that of Arawn and the DPs. Given the Hapke photometric model derived from the disk-integrated analysis of Arawn observations, surface particles on the hot classical and resonant (nonplutino) DKBOs observed so far are more opaque than those on Arawn and they are more backscattering. The DPs exhibit shallow phase coefficients, meaning their surfaces are less backscattering and not as rough as that observed on Arawn. The full analysis of the distant KBO phase curves will

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TABLE 17.2 Selected phase coefficients measured from New Horizons LORRI images. KBO

Class

Phase angle range (degrees)

Phase coefficient β (mag/degree)

Makemake

Dwarf planet

8.4–32.0

0.0152

Quaoar

Dwarf planet

51.2–65.8

0.0199

Haumea

Dwarf planet

8.3–39.4

0.0243

2002 MS4

Dwarf planet

37.8–51.4

0.0270

Arawn

Plutino

26.7–58.3

0.0425

2012 HE85

Resonant

19.7–64.5

0.0328

2012 HZ84

Hot class.

28.8–73.1

0.0326

FIG. 17.3 Solar phase functions for all distant KBOs observed to date by New Horizons LORRI at multiple phase angles. All have been distance corrected to the magnitude at 1 AU from the observer and the Sun and displaced vertically as needed for clarity. Observations of Plutino Arawn (magenta) and all dwarf planets have been corrected for rotational lightcurve variation except 2002 MS4 . Dwarf planets Haumea (green), Quaoar (blue) and 2002 MS4 (red) have similar phase coefficients (slopes). Dwarf planet Makemake (cyan) has the shallowest phase coefficient among the KBOs shown here. Plutino Arawn has the steepest slope (highest phase coefficient). The resonant KBO 2012 HE85 (solid black circles) and hot classical KBO 2012 HZ84 (open black circles) observations are averages of all LORRI observations at a given phase angle. They have similar phase coefficients intermediate between the Plutino Arawn and the dwarf planets.

only be possible following transformation from LORRI to each of the photometric systems at which these KBOs were observed from Earth at smaller phase angles.

17.3.2 Heliospheric results The KEM New Horizons particle and UV observations are highly exploratory, extend the particle and UV observations of the successful Pioneer and Voyager missions, and provide new information for better understanding KB dust environment and the space weathering of objects.

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Few missions have explored the outer heliosphere beyond 30 AU, and some key charged particle populations have been inaccessible owing to gaps in the energy coverage (e.g., Belcher et al., 1980) and sensitivity limitations. SWAP and PEPSSI data provide this missing, crucial energy coverage with much higher sensitivities. The only other missions to explore the solar wind and particle environment beyond 30 AU are the Pioneer and Voyager missions. The Pioneer 10 and 11 observations do not extend beyond 63.0 and 35.6 AU respectively, and the solar wind plasma instrument on Voyager 1 stopped working near Saturn at ∼9.74 AU. Voyager 2 has the most extensive observations of the solar wind observations in the outer heliosphere, and the undisturbed solar wind observations extend to ∼82.94 AU when Voyager 2 began to observe the foreshock of the termination shock and eventually crossed the termination shock at ∼83.65 AU (Richardson and Stone, 2009). As the solar wind moves away from the Sun, it encounters interstellar neutral material. The interstellar material is ionized through charge exchange and photoionization, and these ions begin to spiral around the Interplanetary Magnetic Field (IMF) carried away from the Sun by the solar wind. Consequently, the ionized interstellar material is referred to as interstellar “pickup ions” (PUIs). The pickup process mass loads the solar wind causing the solar wind to slow and eventually become subsonic at the termination shock. The SWAP instrument simultaneously measures the slowing and heating of the solar wind owing to the interaction with the interstellar material, and unlike other missions also measures the interstellar pickup ions. The necessary analysis techniques to determine the solar wind (Elliott et al., 2016) and the interstellar H+ PUIs (Randol et al., 2012, 2013; McComas et al., 2017) properties are already in place. Fig. 17.4 shows an example distribution with distinct H+ and He++ solar wind peaks, a clear H+ interstellar pickup ion cutoff and a portion of the He+ interstellar pickup distribution (McComas et al., 2017). McComas et al. (2017) also found that by ∼20 AU, the solar wind internal pressure is dominated by the interstellar pickup ions. Elliott et al. (2016) concluded that many solar wind structures have been worn down and merged beyond 20 AU, and the average solar wind speed at New Horizons was starting to

FIG. 17.4 A count rate versus energy per charge distribution averaged over 1-day with error bars taken at ∼25.7 AU. Labels indicate the: solar wind (SW) or interstellar pickup ions (PUI) populations. The average solar wind conditions are shown in the upper left corner. Percentages in brackets on the solar wind parameters are normalized root mean square (RMS) variations of hourly values. From McComas, D.J., Zirnstein, E.J., Bzowski, M., Elliott, H.A., Randol, B., Schwadron, N.A., 2017. Interstellar pickup ion observations to 38 AU. Astrophys. J. Supp. Series 233 (1), 8–12.

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decrease owing to the interaction with the interstellar material. More recent New Horizons observations indicate the speed decrease has become distinct (∼6%) between 30 and 40 AU. Further, Zirnstein et al. (2018) analyzed the solar wind and interstellar pickup ions using SWAP data and found that an interplanetary shock at 34 AU is modified by the presence of the interstellar PUIs. Upstream of the shock, the solar wind is slowed and heated and the pickup ions downstream of the shock have six times the energy flux of the solar wind ions. Elliott et al. (2018) then found that many Interplanetary Coronal Mass Ejections (ICMEs), large releases of mass sporadically emitted from the Sun, can reliably be identified beyond 30 AU in the NH solar wind observations using an estimate of the solar wind alpha particle (He++ ) to proton density ratio derived from the coarse energy sweep. Understanding the particle environment in the outer heliosphere has already proven useful for understanding how space weathering could affect the coloring of Charon Grundy et al. (2016). The Alice UV spectrograph provides the first Lyman-α (Lyα) observations in the outer solar system since Voyager. These Lyα observations agree well with the prior Voyager results when the Voyager brightness is reduced by a factor of 2.4 as Quémerais et al. (2013) recommends in their reanalysis of the Voyager data (Gladstone et al., 2018). Fig. 17.5 shows the Lyα brightness in the upwind direction from Voyager and New Horizons. All the observations are scaled by a factor of (3 × 1011 photons/cm2 /s)/π FSun , where π FSun is the estimated subspacecraft 6-day average solar Lyα flux at 1 AU to remove the contribution from the solar flux. The fits have a 1/r functional form, but an additional 40 Rayleighs (R) is added to reproduce the brightness beyond 25 AU. Gladstone et al. (2018) concluded this additional brightness is consistent with Lyα emissions from the hydrogen wall. As the interstellar wind encounters the heliosphere, a bow shock is formed upstream in interstellar space. The hydrogen wall is a region of hot, interstellar material consisting mostly of hydrogen upstream, immediately outside the outer boundary of the heliosphere called the heliopause, but inside the bow shock in interstellar space.

FIG. 17.5 The Lyα brightness viewed in the upwind direction measured by the UVS from Voyager 1 (red crosses) and Voyager 2 (blue crosses) (Hall, 1992), scaled downward by a factor of 2.4 as recommended by Quémerais et al. (2013), and from New Horizons Alice (black asterisks, with 3-σ error bars). All data are scaled by a factor of (3 × 10 11 photons/cm2 /s)/π FSun , where π FSun is the estimated subspacecraft 6-day average solar Lyα flux at 1 AU. The brightness data are fit and reveal the expected 1/r brightness dependence, but the dashed curve has additional distant upstream brightness of 40 R to match the brightness beyond 25 AU. From Gladstone, G.R., Pryor, W.R., Stern, S.A., Ennico, K., Olkin, C.B., Spencer, J.R., et al., 2018. The Lyman-α sky background as observed by New Horizons. Geophys. Res. Lett. (in press) https://arxiv.org/abs/1808.00400. V. Prospects for the future

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The Student Dust Counter (SDC) is the first instrument dedicated to the study of dust in the KB. These interplanetary dust observations provide clues about the overall formation and evolution of the solar system. The only other dust observations beyond 18 AU are indirect dust observations from the Voyagers where they inferred the dust flux from plasma clouds created as dust grains impacted the spacecraft and caused a voltage pulse in the antenna Gurnett et al. (1997). Those observations indicate a nearly constant flux of about 4–5 detections per hour (Gurnett et al., 2005); however, estimating the true flux from such an indirect measure is challenging. SDC directly measures dust impacts for masses between 10−12 and 10−9 g and can determine the dust flux (Horanyi et al., 2008; Piquette et al., 2018a,b). The KB has been suggested to be a primary source of Interplanetary Dust Particles (IDPs; Landgraf et al., 2002), which are produced by collisions between KBOs (e.g., Stern, 1996) and by the bombardment of KBOs by IDPs and interstellar dust particles (ISDs; e.g., Yamamoto and Mukai, 1998). Recent models of KB dust production and loss use Pioneer 10/11 and NH observations to estimate the strengths of the IDP/ISD sources and sinks, and follow dust transport in the outer solar system Vitense et al. (2014); Poppe (2016). However, the two models predicted dust fluxes which dramatically diverge beyond ∼35 AU (Fig. 17.6) (Poppe, 2016; Piquette et al., 2018a,b). Both models have similar parent distributions for the classical, scattered, and resonant populations of KBOs, but Poppe (2016) included an additional, currently undetectable population of “outer/detached” KBOs as described by Petit et al. (2011), yielding significant dust fluxes beyond 40 AU. By measuring the dust environment at distances beyond 40 AU, SDC will determine if such an outer source of extended dust exists, and in the process helps to constrain the collisional environment of the KB.

FIG. 17.6 Comparison of SDC observed dust fluxes and recent models, which dramatically diverge beyond ∼35 AU with interplanetary dust flux onto SDC for grains with radii >0.6 μm. The blue (Vitense et al., 2014) and red (Poppe, 2016) curves represent two different models of the predicted dust flux. New Horizons measurements by SDC will resolve which model is correct as the spacecraft flies outward; it will reach late 70 AU in the late 2020s. From Piquette, M., Poppe, A.R., Bernardoni, E., Szalay, J.R., James, D., Horányi, M., et al., 2018a. Student dust counter: status report at 38 AU. In: Lunar and Planet. Sci. Conf., vol. 49; Piquette, M., Poppe, A.R., Bernardoni, E., Szalay, J.R., James, D., Horányi, M., et al., 2018b. Student dust counter: status report at 38 AU. Icarus (submitted for publication).

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17.4 Anticipated future results 17.4.1 The MU69 flyby New Horizons will fly closest to MU69 at 05:33 UT on January 1, 2019. The spacecraft will fly to the north (in ecliptic coordinates) of MU69 , with a nominal close approach distance of 3500 km, though an alternative trajectory and flyby sequence, with a close approach distance of 10,000 km, has been developed for use in contingency situations. Planned observations of MU69 extend from the first detection with LORRI on August 16, 2018 through January 3, 2019. Approach LORRI images will be used to provide optical navigation data and to search for both satellites and rings at resolutions and sensitivities not achievable from Earth or Earth orbit. An intensive ring and satellite search campaign between 32 and 19 days before encounter will return deep (limiting visual magnitude ∼21.5) images to search for potentially hazardous dust structures, or satellites capable of generating hazardous material. These data will be used to inform the decision on whether to divert to the alternate trajectory and flyby sequence: this divert maneuver can occur as late as 10 days before closest approach. Detected satellites will be tracked to determine or constrain their orbits, both for more detailed imaging, and to determine the orbit of the MU69 primary around the system barycenter to better target close approach observations of the primary itself. Throughout the approach to MU69 , SWAP, PEPSSI, and SDC will observe the heliospheric plasma (solar wind, pickup ions, and energetic particles) and the dust environment near MU69 . Intense rotational coverage imaging will start approximately 2.5 days before the closest approach when MU69 will subtend about 2 LORRI pixels. Starting about 1 day before closest approach, Ralph color images, Ralph near-IR composition spectroscopy, and Alice ultraviolet spectroscopy will be interleaved with the panchromatic LORRI images to provide multiwavelength rotational coverage. Deep searches for satellites and rings will be able to detect moons as small as ∼100 m in diameter. About 3 h before the closest approach, New Horizons will obtain images and spectra blanketing a region of about 1000 km radius around MU69 , to characterize the shapes and compositions of any satellites or rings in this region. In the final hour before the closest approach, assuming the prime aim point 3500 km from MU69 , color and panchromatic imaging with the MVIC camera inside Ralph and near-IR spectroscopy with the LEISA IR imaging spectrometer, also inside Ralph, will be obtained with the best resolution 300 m/pixel, 140 m/pixel, and 1.8 km/pixel, respectively. LORRI, also operating during these observations, will obtain images with the best resolution 35 m/pixel, though most LORRI images will have greater smear and lower SNR than the MVIC images. Images from multiple viewing directions, with phase angles up to 165 degree, will be used to characterize MU69 ’s solar phase curve and textural surface properties and to obtain digital elevation maps using stereo techniques. New Horizons will also make disk– integrated observations of day and night side microwave thermal emission using REX, and attempt to measure UV surface reflectance using Alice. REX will also attempt an uplink bistatic radar experiment, looking for a signal sent from the Deep Space Network (DSN) reflected from MU69 ’s surface. Plasma and dust observations with SWAP, PEPSSI, and SDC will be conducted during the close approach to search for associated MU69 dust and its plasma interactions.

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Shortly after departure, Alice will observed both the Sun and a star to search for any UV absorption due to possible coma gases. Subsequent departure observations using the imagers will conduct deep searches for forward-scattering rings around MU69 . The alternate flyby sequence will obtain similar observations, but best resolutions will be about a factor of 2 lower than for the prime trajectory. Approximately 50 gigabits of data will be collected during the MU69 flyby. Owing to low data rates from the KB and the shared nature of the DSN, these data will require at least 20 months to be downlinked to Earth. The resulting dataset will provide the first definitive portrait of a small Kuiper belt object. Note added in proof: Because the hazard environment around MU69 was determined to be benign, New Horizons flew its nominal trajectory to a closest approach distance near 3500 km.

17.4.2 Kuiper belt object studies Through the remainder of KEM, more Kuiper belt objects will be observed by LORRI. We expect to obtain phase curves of these objects (including photometric searches for forwardscattering ring material), rotational lightcurves, and imaging searches for companions at better than HST resolution, down to 100 km/pixel for almost all of them.

17.4.3 Heliospheric studies New Horizons can obtain unprecedented detail on the structure and time variability of the heliospheric plasma environment as the solar wind moves away from the Sun and picks up additional interstellar material. Based on Voyager and prior New Horizons experience, we expect increased interplanetary shock modification (Zirnstein et al., 2018) and continued slowing and heating of the solar wind (Richardson and Wang, 2003; Elliott et al., 2016; Richardson and Smith, 2003; McComas et al., 2017). ICMEs can be identified using enhancements in the alpha (He++ ) to proton (H+ ) density ratio (n(He++ )/n(H+ )) in the outer heliosphere (Elliott et al., 2018). It is estimated that New Horizons will be able to operate its particle instruments out to 90–100 AU when power decreases may terminate the mission (Stern et al., 2018); this is likely long enough to observe the termination shock and its foreshock, which were crossed by Voyager 1 (and 2) at ∼94 AU (and 84 AU) (Richardson and Stone, 2009). New Horizons is headed along nearly the same longitude as Voyager 2, but unlike either Voyager it will remain in the ecliptic, heading toward the Energetic Neural Atom ribbon providing valuable constraints for simulations of the Interstellar Boundary Explorer (IBEX) mission Energetic Neutral Atom (ENA) maps. Future Alice Lyα observations will be able to determine if the enhanced brightness of Lyα in the upstream direction is from the hydrogen wall associated with the interstellar boundary (Gladstone et al., 2018). Future measurements of heliospheric dust at greater and greater distances will be able to test models of the distribution of dust parent bodies in the KB (e.g., Piquette et al., 2018a,b), including whether there is an extended dust source from KBOs in the 48 AU to several hundred AU region (Poppe, 2016; Petit et al., 2011; Vitense et al., 2014). These dust observations can thus provide key information on the nature of the outer solar system and are valuable for comparing dust in the KB to dust disks around other stars (Piquette et al., 2018a,b).

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17.4.4 Potential astrophysical studies While there are no plans for astrophysical (nonsolar-system) observations for the duration of the current extended mission through early 2021, the spacecraft’s unique vantage point makes it potentially capable of unique astrophysical observations in future extended missions once outside the KB at 70 AU and beyond (Zemcov et al., 2018). For example, the location of New Horizons outside almost all our solar system’s zodiacal light may allow sensitive studies of galactic and extragalactic background light. Any such work would, however, take fuel and so would limit future KB science, including flybys and studies of distant objects from New Horizons.

17.5 Future Kuiper belt exploration after New Horizons At the conclusion of the current New Horizons extended mission, the spacecraft will be at 50 AU from the Sun. Power and fuel aboard are expected to be sufficient to operate the spacecraft until the mid-2030s, perhaps longer—which will take the spacecraft out to about 90 AU or more. The New Horizons team plans to propose a second KB extended mission to observe more distant KBOs and to attempt another close flyby of a KBO—likely to be a still smaller one than MU69 , more akin to a pristine comet nucleus, should this prove feasible. The extended mission will also involve continued heliospheric plasma, dust, and neutral gas observations. As such, the first future exploration of the KB is likely to be carried out with New Horizons itself. Once the spacecraft is beyond the KB, it may be used to conduct heliospheric and astronomical observations until its fuel or power declines and terminates the mission. However, in addition to the landmark science that New Horizons is performing in the KB, calls have been made for various new types of missions, both to explore the Pluto system in more detail and to sample the diversity of DPs and smaller bodies in the KB. The primary mission types being explored now in studies for future proposals to US Decadal Surveys and the NASA New Frontiers program include: ➢ A Pluto orbiter. This mission would study Pluto and its system of satellites at higher resolution over time to study temporal changes and with new techniques that New Horizons could not bring to bear such as thermal mappers, radars to sound the subsurface, and mass spectrometry to study atmospheric composition in more detail. ➢ Flybys of other large DPs. Such missions, which can also make additional studies of small (MU69 -like) KBOs (and which can be combined with ice giant flybys) would allow the known, exciting degree of diversity of the KB DP population to be studied at high resolution for the first time. ➢ Missions to study a suite of Centaurs as KBOs. This opportunity allows multiple KBO escapees to be studied in a single mission, which importantly can include mid-sized targets like 2060 Chiron (which also shows activity (e.g., Bus et al., 2001) and may have rings Ortiz et al., 2015); such mid-sized targets are too dilute a population to be likely targets for an orbiter on its way to KBO DPs or to Pluto. No one can say when the next KB mission will be launched, but there are clearly scientifically exciting opportunities ahead for such exploration. V. Prospects for the future

References

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Acknowledgments This work was funded by NASA’s New Horizons project; we thank NASA for this support. We also thank both Ms. C. Conrad and Ms. R. Tedford for editorial support.

References Belcher, J.W., et al., 1980. The low energy plasma in the Jovian magnetosphere. Geophys. Res. Lett. 7, 17–20. Benecchi, S.D., Noll, K.S., Stephens, D.C., Grundy, W.M., Rawlins, J., 2011. Optical and infrared colors of transneptunian objects observed with HST. Icarus 213, 693–709. Bus, S.J., A’Hearn, M.F., Bowell, E., Stern, S.A., 2001. Chiron: evidence for activity near Aphelion. Icarus 150, 94–103. Elliott, H.A., McComas, D.J., Valek, P., Nicolaou, G., Weidner, S., Livadiotis, G., 2016. The new Horizons solar wind around Pluto (SWAP) observations of the solar wind from 11-33 AU. Astrophys. J. Supp. Series 223 (2), 19–22. Elliott, H.A., Valek, P., McComas, D.J., Delamere, P.A., Bagenal, F., Gladstone, G.R., et al., 2018. Determining the alpha to proton density ratio for the New Horizons solar wind observations. Astrophys. J. 866, 2. Gladstone, G.R., Pryor, W.R., Stern, S.A., Ennico, K., Olkin, C.B., Spencer, J.R., et al., 2018. The Lyman-α sky background as observed by New Horizons. Geophys. Res. Lett. 45, 8022–8028. Green, S.F., McBride, N., O’Ceallaigh, D.P., et al., 1997. Surface reflectance properties of distant solar system bodies. MNRAS 290, 186–192. Grundy, W.M., Cruikshank, D.P., Gladstone, G.R., Howett, C.J.A., Lauer, T.R., Spencer, J.R., et al., 2016. The formation of Charon’s red poles from seasonally cold-trapped volatiles. Nature 539 (7), 65–68. Gurnett, D.A., Ansher, J.A., Kurth, W.S., Granroth, L.J., 1997. Micron-sized dust particles detected in the outer solar system by the voyager 1 and 2 plasma wave instruments. Geophys. Res. Lett. 24, 3125–3128. Gurnett, D.A., Wang, Z.Z., Persoon, A.M., Kurth, W.S., 2005. In: Dust particles detected in the outer solar system by Voyager 1 and 2, vol. 1280, Dust in Planetary Systems Workshop 2005, 1280, pp. 63. Hall, D.T., 1992. Ultraviolet Resonance Line Radiation and the Structure of the Heliosphere (Doctoral Dissertation). The University of Arizona, Tucson. Hapke, B., 2012. Theory of Reflectance and Emittance Spectroscopy, second ed. Cambridge Univ. Press. 528. https://doi.org/10.1017/CBO9781139025683. Horanyi, M., Hoxie, V., James, D., Poppe, A., Bryant, C., Grogan, et al., 2008. The student dust counter on the New Horizons mission. Space Sci. Rev. 140, 387–402. Landgraf, M., et al., 2002. Origins of solar system dust beyond Jupiter. Astron. J. 123, 2857–2861. McComas, D.J., Zirnstein, E.J., Bzowski, M., Elliott, H.A., Randol, B., Schwadron, N.A., et al., 2017. Interstellar pickup ion observations to 38 AU. Astrophys. J. Supp. Series 233 (1), 8–12. Ortiz, J.L., et al., 2015. Possible ring material around centaur (2060) Chiron. Astron. Astrophys. 576, A18. Petit, J.M., Kavelaars, J.J., Gladman, B.J., Jones, R.L., Parker, J.W., Laerhoven, C.V., et al., 2011. The Canada-France ecliptic plane survey—full data release: the orbital structure of the Kuiper belt. Astron. J. 142, 131–136. Piquette, M., Poppe, A.R., Bernardoni, E., Szalay, J.R., James, D., Horányi, M., et al., 2018a. Student dust counter: status report at 38 AU. In: Lunar and Planet. Sci. Conf., vol. 49. Piquette, M., Poppe, A.R., Bernardoni, E., Szalay, J.R., James, D., Horányi, M., et al., 2018b. Student dust counter: status report at 38 AU Icarus 321, 116–125. Poppe, A.R., 2016. An improved model for interplanetary dust fluxes in the outer solar system. Icarus 264, 369–86. Porter, S.B., et al., 2016. The first high-phase observations of a KBO: New Horizons imaging of (15810) 1994 JR1 from the Kuiper belt. Astrophys. J. Lett. 828, L15–18. Quémerais, E., Sandel, B.R., Izmodenov, V.V., Gladstone, G.R., 2013. Thirty years of interplanetary background data: a global view, cross-calibration of far-UV spectra of solar system objects and the heliosphere, ISSI Sci. Rep. Series. Berne, Switzerland, vol. 13. 141–162. Randol, B.M., et al., 2012. Observations of isotropic interstellar pick-up ions at 11 and 17 AU from New Horizons. Astrophys. J. 755, 75–80. Randol, B.M., McComas, D.J., Schwadron, N.A., 2013. Interstellar pick-up ions observed between 11 and 22 AU by new horizons. Astrophys. J. 768, 120.

V. Prospects for the future

394

17. Plans for and initial results from the exploration of the Kuiper belt by New Horizons

Richardson, J.D., Smith, C.W., 2003. The radial temperature profile of the solar wind. Geophys. Res. Lett. 30 (5), 1206–1208. Richardson, J.D., Stone, E.C., 2009. The solar wind in the outer heliosphere. Space Sci. Rev. 143, 7–20. Richardson, J.D., Wang, C., 2003. The solar wind in the outer heliosphere at solar maximum. In: SOLAR WIND TEN: Proceedings of the Tenth International Solar Wind Conference. AIP Conference Proceedings. vol. 679. pp. 71–74. Stern, S.A., 1996. Signatures of collisions in the Kuiper disk. Astron. Astrophys. 310, 999–1010. Stern, S.A., et al., 2015. The Pluto system: initial results from its exploration by new horizons. Science 350, 292. Stern, S.A., Weaver, H.A., Spencer, J.R., Elliott, H.A., 2018. The New Horizons Kuiper belt extended mission. Space Sci. Rev. 214 (4), 77–92. Verbiscer, A., Veverka, J., 1988. Estimating phase integrals: a generalization of Russell’s law. Icarus 73, 324–329. Vitense, C., Krivov, A.V., Löhne, T., 2014. Will New Horizons see dust clumps in the Edgeworth-Kuiper belt? Astrophys. J. 147, 154–158. Weaver, H.A., et al., 2008. Overview of the New Horizons payload. Space Sci. Rev. 140 (1), 75–82. Yamamoto, S., Mukai, T., 1998. Dust production by impacts of interstellar dust on Edgeworth-Kuiper belt objects. Astron. Astrophys. 329, 785–791. Zemcov, M., Arcavi, I., Arendt, R., et al., 2018. Astrophysics with New Horizons: making the most of a generational opportunity. Proc. Astron. Soc. Pacific 130, 993. Zirnstein, E.J., McComas, D.J., Kumar, R., Elliott, H.A., Szalay, J.R., Olkin, C.B., et al., 2018. In situ observations of preferential pickup ion heating at an interplanetary shock. Phys. Rev. Lett. 121, 075102.

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