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

After my PhD in 2011, I was 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, and trajectory analysis tools to determine the fuel cost to reach any candidate objects.

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.

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.

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