The Student Dust Counter: Status report at 23 AU
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 7 November 2012
Accepted: 19 February 2013
Published: 24 October 2013
The Student Dust Counter (SDC) is an impact dust detector on board the New Horizons Mission to Pluto. SDC was designed to resolve the mass of dust grains in the range of 10−12 < m < 10−9 g, covering an approximate size range of 0.5–10 μm in particle radius. The measurements can be directly compared to the prediction of a grain trajectory tracing model of dust originating from Edgeworth-Kuiper Belt (EKB). Comparing our measurements and model prediction is used to estimate the net mass production rate and the ejecta mass distribution power law exponent to be and , respectively. Through April 2012, the New Horizons spacecraft reached approximately 23 AU, enabling SDC to map the dust density distribution along its path.
In situ dust measurements beyond the asteroid belt.
Max Heliocentric Distance [AU]
Detector area [m2]
In Section 2 we briefly review the scientific motivation for SDC, followed by the instrument description in Section 3, and report on the observations to date in Section 4. The estimates on the dust production in the Edgeworth-Kuiper belt based on SDC and Pioneer 10 measurements are discussed in Section 5.
2. Science Background
The interplanetary dust particle (IDP) population reflects a complex balance between sources, transport, and sinks which are all functions of the dust particle sizes and their positions in the solar system. IDPs are created primarily in asteroid-asteroid collisions, from the disintegration of comets near the Sun, from collisions between Kuiper Belt objects (KBOs), and from the continuous meteoroid bombardment of the moons, rings, asteroids and KBOs. Data from near-Earth and deep space missions suggest five distinct IDP populations (Divine, 1993). While this model makes predictions for IDP populations throughout the solar system, it has not been validated by direct measurements beyond 18 AU. The Divine (1993) model was used to set the science and measurement requirements for SDC in 2004. Since then, newer and more complete models became available (Kuchner and Stark, 2010; Vitense et al., 2010, 2012; Han et al., 2011b).
Beginning at about 3 AU from the Sun, the Pioneer 10 and 11 observations of particles larger than about 10 μm in radius showed a constant spatial density (number of particles per unit volume) out to 18 AU (Humes, 1980). At this distance, the argon and nitrogen gas mixture in the Pioneer pressurized detector cells froze and no further measurements could be made.
IDPs which are born on the outskirts of the solar system slowly lose orbital energy due to Poynting-Robertson drag and migrate toward the Sun. A 10 μm radius grain born at 50 AU reaches the inner solar system in about 6.5 My. However, the continuous migration can significantly slow down or come to a halt due to mean motion resonances with the planets (Liou and Zook, 1999; Moro-Mart′ in and Malhotra, 2002, 2003). Grains can also get ejected from the solar system via close encounters with the planets (Liou and Zook, 1999). Orbital integrations find that Neptune prolongs the lifetime of the inwardly migrating dust grains outside its orbit, imprinting its resonance structure on the spatial density of dust in the Edgeworth-Kuiper belt.
Jupiter, on the other hand, acts as a gate-keeper and keeps the inner part of our solar system relatively dust free by ejecting most of the particles that would cross its orbit. The nature of the inner planets effect on dust distribution is largely hidden due to dust production from active comets.
SDC measurements made as New Horizons crosses the solar system will help to understand the generation, the transport, and the loss processes of IDPs. The measurements of the density variations outside the orbit of Jupiter will allow for estimating the rate of dust production in the Kuiper Belt, the collisional history of the region, and the mass distribution of the primordial KBO population (Stern, 1996).
3. Instrument Description
The measurement requirements of the SDC instrument were established by estimating the particle mass detection limit needed to resolve the expected features of the dust distribution. The characteristic width of the predicted resonance structures is on the order of 1 AU, hence a spatial resolution of ≈0.1 AU assures adequate sampling. With the characteristic spacecraft speed of ≈13 km/s during cruise, this is equivalent to an integration period of one week. Based on the current models (Divine, 1993), a detector with a lower mass detection limit of about 10−12 g and 0.1 m−2 surface area is predicted to have on the order of 1IDP hit per week outside of dust structures. SDC was designed to resolve the mass of IDPs in the range of 10−12 < m < 10−9 g within factors of <2, covering an approximate size range of 0.5−5 μm in particle radius. Bigger grains are also recorded without the ability of resolving their mass.
To process SDC data, all grains are assumed to be on circular, Keplerian orbits. For each detected impact, an impact angle is calculated between the grain and SDC area vector. Impacts with angles greater than 45° are rejected as noise since the charge generated by such an impact would be below our detection limit. From the set of valid impacts, measured dust grain fluxes are calculated. Densities are calculated from these fluxes using the circular orbit assumption and local spacecraft velocity for each impact.
4. SDC Measurements Through 2012
When SDC data are received on Earth, they are sent to the New Horizons mission operations center at the Johns Hopkins University Applied Physics Laboratory. The SDC data, as well as several ancillary spacecraft data packets, are also sent to SDCs home institution, the Laboratory for Atmospheric and Space Physics (LASP). Simultaneously, it is also received at the Tombaugh Science Operations Center (TSOC) of the Southwest Research Institute (SwRI) in Boulder. The data are sent through “levels” of processing software that progressively decode more of the packets and send the decoded data to a database. The SDC final science product includes noise corrected impact rates as function of mass (Horányi et al., 2008).
5. Comparison to Model Results
SDC data provide a unique opportunity to validate model results of the IDP distribution past 18 AU with direct measurements. The model we use to compare SDC data to is described in Han et al. (2011a). Briefly, we traced dust grains over a range of grain radii from the EKB throughout the solar system under the influence of gravity, solar radiation pressure and solar wind drag in order to establish a statistical equilibrium density distribution for each grain size. All grains in this model were started in the region 35 < r s < 50 AU, with the orbital elements randomly selected from those of three categories of KBOs in the ratio of Classical : Scattered : Plutinos = 0.67 : 1.00 : 0.25 (Kavelaars et al., 2009). Integration of the grain trajectory was stopped when a grain arrived near the Sun (r s < 0.1 AU) or left the solar system (r s > 200 AU) with net positive orbital energy.
To date, the two dedicated measurements of interplanetary dust grain density in the outer solar system come from SDC (Horányi et al., 2008; Stern, 2008; Poppe et al., 2010a) and the meteoroid detector on the Pioneer 10 spacecraft (Humes, 1980). Both SDC and Pioneer 10 flux measurements are converted to densities by dividing out the theoretical dust grain impact velocity as a function of heliocentric distance, as described in the Instrument Description.
In order to compare the model-predicted densities with the experimental measurements, the model densities at each grain radius were added together after appropriately weighting by the assumed power law distribution (Stern, 1996; Yamamoto and Mukai, 1998), , where is the differential mass production rate of dust grains between m and dm in the Edgeworth-Kuiper belt, is a normalization constant, and m o = 10−11 g. The net mass production rate in the Edgeworth-Kuiper belt, , is found by integrating the distribution from mmin = 10−14 g (a d = 0.1 μm) to mmax = 10−8 g (a d = 10 μm) (Stern, 1996; Yamamoto and Mukai, 1998). The data was compared to the model by using a two-parameter χ2 goodness-of-fit function. Varying both and α, the minimumχ2 value was found to occur at and α = 3.02 ± 0.04 (Han et al., 2011a), with the mass production rate well within previous theoretical estimates (Stern, 1996; Yamamoto and Mukai, 1998). Using the best-fit values for the mass production rate and the power law index, the model predicted densities can be compared with the observations, as shown by the blue line in Fig. 2. Naturally, these estimates could be greatly improved if there was another set of measurements along a different path than New Horizons, cutting through our solar system in a different direction. For completeness we note, that beyond Saturn a significant part of the dust population is expected to be from interstellar dust particles. The characteristic radius of these grains is on the order of 0.3 μm, and they remain undetected by SDC (Landgraf, 2000; Sterken et al., 2012). In addition to the previously mentioned dust grain populations, SDC will also be flying though the Pluto-Charon dust cloud as it approaches the Pluto-Charon system. Recent simulations show this impact-generated dust cloud contains grains ejected from Pluto and its moons and predicted fluxes for SDC are ~10−4 m −2 s−1 (Poppe and Horányi, 2011).
Based on in-flight test and calibrations, and the science data as of to date, we expect SDC to continue to make dust measurements beyond Pluto. The Pluto encounter will take place in July 2015, at a distance of 32 AU from the Sun. New Horizons is expected to remain fully functional and operate for decades to come, reaching deep into the Edgeworth-Kuiper belt. Combining interplanetary dust grain measurements by SDC with a dynamical dust grain tracing code, enabled us to estimate the mass production rate g/s and power law index of the initial size distribution α = 3.02 ± 0.04 (Han et al., 2011a). SDC will continue to provide insight about the dust production at the outskirts of our Solar system and enable comparisons with dust disks around other stars.
This work was supported by the New Horizons Mission. Dr. A. Poppe, now at UC Berkeley, was the previous lead graduate student to oversee the operations of SDC.
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