Alex Harrison Parker | Planetary Astronomy

Planetary Astronomer, Southwest Research Institute

Research scientist in planetary astronomy at the Southwest Research Institute, supporting NASA's New Horizons mission to Pluto, and developing the post-Pluto mission into the Kuiper Belt. Expert in the dynamics of binary minor planets, detection and characterization of trans-Neptunian objects, and the origin of the architecture of our Solar System.


Exploring the Kuiper Belt with New Horizons

Since my PhD in 2011, I have been engaged in the search for a Kuiper Belt object to send NASA's New Horizons spacecraft to after its Pluto encounter in 2015. This post-Pluto mission through the Kuiper Belt will be the only in-situ exploration of this region of the outer Solar System in the foreseeable future. 

Our search targeted the area of sky where Kuiper Belt objects that will eventually pass New Horizons currently reside. Unfortunately, this area of sky is deep in the galactic plane, and the background is packed with stars. I developed difference-imaging and digital tracking techniques to suppress the starlight while leaving behind the slowly-moving Kuiper Belt objects of interest. 

This search culminated in 2014 with the discovery of a targetable Kuiper Belt Object in Hubble Space Telescope data. New Horizons completed a targeting maneuver in November 2015 and is on-course for a New Years Day 2019 flyby of 2014 MU69.

Long-range science: Search for close binaries

During flybys at moderate range (0.1-0.5 AU), New Horizons' LORRI camera will have nearly an order of magnitude the resolving power of the Hubble Space Telescope when searching for close companions around Kuiper Belt objects. Because of this, New Horizons will be able to detect closer binaries than have ever been seen before. The presence or absence of these binary systems will put powerful new constraints on the formation mechanisms of binary Kuiper Belt objects in general. As the video above and the figure below illustrate, the majority of these flybys occur in the 2018-2020 timeframe.

This interactive figure illustrates the flyby distances of Kuiper Belt Objects my team has discovered (gold), compared to all historic flybys of outer solar system minor planets by the Pioneer and Voyager missions. Mouse-over each point for spacecraft, object, and flyby details. These flybys were not know about ahead of time; instead, I identified them by backwards-integrating all currently known Kuiper Belt Objects, Centaurs, and Jupiter Family Comets in the presence of the giant planets and searching for close encounters. Because of our survey, we have already discovered a handful of Kuiper Belt objects that pass closer to New Horizons than any other outer solar system body has passed any other spacecraft in all of history.

Dynamics of Minor Planet Populations

Neptune Trojans are valuable tracers of the history of Neptune's orbital migration. In 2011 I serendipitously discovered and characterized the L5 Neptune Trojan 2011 HM102, which is the highest-inclination stable Neptune Trojan known as well as the largest L5 Trojan object known in the entire solar system. The extreme orbit of 2011 HM102 prompted me to characterize the overall distribution of Neptune Trojan orbits, which required developing new survey-agnostic statistical characterization techniques to remove discovery biases present in the sample of known Neptune Trojans.

Using these new statistical techniques, I produced the first quantitative measure of the intrinsic orbit distribution of the Neptune Trojans, confirming that they are a very dynamically-excited population. Using a large series of numerical n-body simulations, I demonstrated that if Neptune migrated as quickly as some models suggest, the bodies that became Neptune Trojans must have been pre-excited before Neptune's arrival.

Binaries in the Kuiper Belt are extremely sensitive probes of the environment they are embedded in, and are the only observable population of unambiguously-primordial binary minor planets in the entire Universe. I study their orbital and physical properties in order to explore the history of the outer Solar System and the mechanisms of planet formation.

The Widest Seven Binaries

As part of the research I performed for my PhD, I measured the properties of the seven widest-known Kuiper Belt binary systems. I proposed for, acquired, and processed optical imagery from the 8-meter Gemini North observatory of each of these systems, and compiled and processed archival data from over the last decade. I used this time-series imagery to measure the sky-plane motion of the binary components about their mutual barycenters, and developed modeling software to determine their orbital properties from this apparent motion. I found that these binaries have orbits inconsistent with the predictions of classical theories of binary formation. A new mechanism for binary production is needed; recently it has been shown that binary objects may be a side-effect of gravitational collapse-driven planetesimal formation, and binaries formed through this process may have properties more in-line with those observed. 

Delicate Duos

Wide binaries are easily unbound by collisions or gravitational perturbations. Impacts with objects as small as one kilometer across are sufficient to unbind some systems, making the continued existence of these binaries a powerful constraint on the total number of one-kilometer-sized bodies in the outer Solar System. In addition, the ease of disrupting these systems suggests that the populations that host them were not subjected to violent close encounters with Neptune at any point in their history. An epoch of intense collisional processing or violent encounters with Neptune are required by some models of Kuiper Belt formation; my work strongly suggests that the continued existence of wide binaries in the Kuiper Belt rules such models out.

   Logo by Samantha Fraser

 Logo by Samantha Fraser

Outer Solar System Origins Survey

The Outer Solar System Origins Survey (OSSOS) is a Canada France Hawaii Telescope Large Program designed to push the state of the art in outer Solar System minor planet observational science by doubling the currently known sample of Kuiper Belt objects with well-measured orbits. I lead the effort in this survey to characterize the binary properties of the discovered objects, and use these binaries to illuminate the past conditions in the outer Solar System.

Preparing for the Future of Ground-Based Planetary Astronomy

Next-generation ground-based surveys like LSST will produce an unprecedented deluge of data relevant to understanding minor planet populations and their implications for our Solar System's history. Careful modeling of the yields of these surveys is crucial to motivate algorithm development prior to survey deployment. I am extremely interested in seeing the planetary astronomy community fully leveraging the data produced by these surveys from day zero, and have made an effort to estimate the number of binary Kuiper Belt Objects likely to be characterized by these surveys, the frequency that transient dust clouds produced by collisions in the Kuiper Belt will be detected by these surveys, and have developed a small suite of optimization algorithms to improve the computational tractability of processing the LSST "Deep Drilling" fields. 

Asteroid Families in Color

Some of my earliest research work involved linking asteroid dynamics to their physical properties. As an undergraduate at the University of Washington, I mined the latest release of the Sloan Digital Sky Survey Moving Object Catalog to explore how the photometric colors of asteroids could be used to refine asteroid family definitions and improve dynamical interloper rejection.

  Figure from  Parker et al. (2008).  Selected as the cover illustration for the journal Icarus' November 2008 edition. 

Figure from Parker et al. (2008). Selected as the cover illustration for the journal Icarus' November 2008 edition. 

This figure illustrates the orbital properties of the asteroids seen by the Sloan Digital Sky Survey: specifically, their inclination vs. their eccentricity. Vertically, the three sets of plots show different subsets in semi-major axis (or mean distance from the Sun).

The colors of the points are a representation of the photometric colors measured by the Sloan camera, and are indicative of the surface properties and compositions of each individual asteroid.

The left panels show the entire population, while the right panels illustrate the collisional families identified during my work. These families were created by the destruction of a large parent object through a catastrophic collision. All of the fragments share similar chemical composition and similar orbits, and so appear as clusters of objects in their orbital parameters, and these clusters share common colors. The dominance of blue (C-type) families in the outer belt is clear, as is the dominance of red families (S-type) in the inner belt.

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