Planet Imager Research Articles

Imaging the 44 au Kuiper Belt Analog Debris Ring around HD 141569A with GPI Polarimetry

Abstract We present the first polarimetric detection of the inner disk component around the pre-main-sequence B9.5 star HD 141569A. Gemini Planet Imager H-band (1.65 μm) polarimetric differential imaging reveals the highest signal-to-noise ratio detection of this ring yet attained and traces structure inward to 0.″25 (28 au at a distance of 111 pc). The radial polarized intensity image shows the east side of the disk, peaking in intensity at 0.″40 (44 au) and extending out to 0.″9 (100 au). There is a spiral arm–like enhancement to the south, reminiscent of the known spiral structures on the outer rings of the disk. The location of the spiral arm is coincident with 12CO J = 3–2 emission detected by ALMA and hints at a dynamically active inner circumstellar region. Our observations also show a portion of the middle dusty ring at ∼220 au known from previous observations of this system. We fit the polarized H-band emission with a continuum radiative transfer Mie model. Our best-fit model favors an optically thin disk with a minimum dust grain size close to the blowout size for this system, evidence of ongoing dust production in the inner reaches of the disk. The thermal emission from this model accounts for virtually all of the far-infrared and millimeter flux from the entire HD 141569A disk, in agreement with the lack of ALMA continuum and CO emission beyond ∼100 au. A remaining 8–30 μm thermal excess a factor of ∼2 above our model argues for an as-yet-unresolved warm innermost 5–15 au component of the disk.

First Resolved Scattered-light Images of Four Debris Disks in Scorpius-Centaurus with the Gemini Planet Imager

Abstract We present the first spatially resolved scattered-light images of four debris disks around members of the Scorpius-Centaurus (Sco-Cen) OB association with high-contrast imaging and polarimetry using the Gemini Planet Imager (GPI). All four disks are resolved for the first time in polarized light, and one disk is also detected in total intensity. The three disks imaged around HD 111161, HD 143675, and HD 145560 are symmetric in both morphology and brightness distribution. The three systems span a range of inclinations and radial extents. The disk imaged around HD 98363 shows indications of asymmetries in morphology and brightness distribution, with some structural similarities to the HD 106906 planet–disk system. Uniquely, HD 98363 has a wide comoving stellar companion, Wray 15-788, with a recently resolved disk with very different morphological properties. HD 98363 A/B is the first binary debris disk system with two spatially resolved disks. All four targets have been observed with ALMA, and their continuum fluxes range from one nondetection to one of the brightest disks in the region. With the new results, a total of 15 A/F stars in Sco-Cen have resolved scattered-light debris disks, and approximately half of these systems exhibit some form of asymmetry. Combining the GPI disk structure results with information from the literature on millimeter fluxes and imaged planets reveals a diversity of disk properties in this young population. Overall, the four newly resolved disks contribute to the census of disk structures measured around A/F stars at this important stage in the development of planetary systems.

Irregular Dust Features around Intermediate-mass Young Stars with GPI: Signs of Youth or Misaligned Disks?

Abstract We are undertaking a large survey of over 30 disks using the Gemini Planet Imager (GPI) to see whether the observed dust structures match spectral energy distribution predictions and have any correlation with stellar properties. GPI can observe near-infrared light scattered from dust in circumstellar environments using high-resolution Polarimetric Differential Imaging with coronagraphy and adaptive optics. The data have been taken in the J and H bands over two years, with inner working angles of 0.″08 and 0.″11, respectively. Ahead of the release of the complete survey results, here we present five objects with extended and irregular dust structures within 2″ of the central star. These objects are FU Ori, MWC 789, HD 45677, Hen 3-365, and HD 139614. The observed structures are consistent with each object being a pre-main-sequence star with protoplanetary dust. The five objects’ circumstellar environments could result from extreme youth and complex initial conditions, from asymmetric scattering patterns due to shadows cast by misaligned disks, or in some cases from interactions with companions. We see complex U ϕ structures in most objects that could indicate multiple scattering or result from the illumination of companions. Specific key findings include the first high-contrast observation of MWC 789 revealing a newly discovered companion candidate and arc, and two faint companion candidates around Hen 3-365. These two objects should be observed further to confirm whether the companion candidates are comoving. Further observations and modeling are required to determine the causes of the structures.

An Updated Visual Orbit of the Directly Imaged Exoplanet 51 Eridani b and Prospects for a Dynamical Mass Measurement with Gaia

Abstract We present a revision to the visual orbit of the young, directly imaged exoplanet 51 Eridani b using four years of observations with the Gemini Planet Imager. The relative astrometry is consistent with an eccentric ( ) orbit at an intermediate inclination ( °), although circular orbits cannot be excluded due to the complex shape of the multidimensional posterior distribution. We find a semimajor axis of au and a period of yr, assuming a mass of 1.75 for the host star. We find consistent values with a recent analysis of VLT/SPHERE data covering a similar baseline. We investigate the potential of using the absolute astrometry of the host star to obtain a dynamical mass constraint for the planet. The astrometric acceleration of 51 Eri derived from a comparison of the Hipparcos and Gaia catalogs was found to be inconsistent at the 2σ–3σ level with the predicted reflex motion induced by the orbiting planet. Potential sources of this inconsistency include a combination of random and systematic errors between the two astrometric catalogs and the signature of an additional companion within the system interior to current detection limits. We also explored the potential of using Gaia astrometry alone for a dynamical mass measurement of the planet by simulating Gaia measurements of the motion of the photocenter of the system over the course of the extended 8 yr mission. We find that such a measurement is only possible (>98% probability) given the most optimistic predictions for the Gaia scan astrometric uncertainties for bright stars and a high mass for the planet (≳3.6 M Jup).

She Space: A multi-disciplinary, project-based learning program for high school girls

Previous research investigating the under-representation of women in science and technology fields has identified various causes. These include, a lack of inspiring female role-models, limited exposure of girls to science, technology, engineering, and mathematics (STEM) subjects, gender-biased stereotypes, and preferences for different learning styles between genders. Dedicated to the ideas of women and space, the “She Space” research project for high school girls was a joint effort of the Earth and Planetary Image Facility (EPIF) at Ben-Gurion University in the Negev and Beit Yatziv. The participants included 20 high-school aged female students and a predominantly female staff of up to eight university researchers and advanced degree students. The goal of this project was to advance and support female students engaging with various scientific fields and, in particular, the field of space science. The project emphasized the quality and development of the research process and used project-based learning to help participants learn to use a research-based approach when encountering challenging scientific concepts or subjects. This, together with active learning and teamwork was set as the educational framework to meet the different learning styles and motivational needs of girls. Furthermore, special attention was given to the suppression of gender stereotypes by 1) introducing women role models 2) overcoming self-induced prejudices related to STEM abilities by introducing new and unknown subjects (remote sensing), about which participants lack preconceptions. Using participant surveys, we found that self-reported confidence with and interest in STEM subjects increased after the project. We also found that participants’ feelings about participating in an all-female program did not noticeably change after the program. However, the vast majority (87%) of participants did report that having a majority female staff was very important to them.

Detection of a Low-mass Stellar Companion to the Accelerating A2IV Star HR 1645

Abstract The ∼500 Myr A2IV star HR 1645 has one of the most significant low-amplitude accelerations of nearby early-type stars measured from a comparison of the Hipparcos and Gaia astrometric catalogs. This signal is consistent with either a stellar companion with a moderate mass ratio (q ∼ 0.5) on a short period (P < 1 yr), or a substellar companion at a separation wide enough to be resolved with ground-based high-contrast imaging instruments; long-period equal-mass ratio stellar companions that are also consistent with the measured acceleration are excluded with previous imaging observations. The small but significant amplitude of the acceleration made HR 1645 a promising candidate for targeted searches for brown dwarf and planetary-mass companions around nearby, young stars. In this paper we explore the origin of the astrometric acceleration by modeling the signal induced by a wide-orbit M8 companion discovered with the Gemini Planet Imager, as well as the effects of an inner short-period spectroscopic companion discovered a century ago but not since followed up. We present the first constraints on the orbit of the inner companion, and demonstrate that it is a plausible cause of the astrometric acceleration. This result demonstrates the importance of vetting of targets with measured astrometric acceleration for short-period stellar companions prior to conducting targeted direct imaging surveys for wide-orbit substellar companions.

Model of the Search for Extraterrestrial Intelligence with Coronagraphic Imaging

Abstract We present modeled detection limits of the Gemini Planet Imager (GPI) and the Wide-Field Infrared Space Telescope (WFIRST) to an optical and infrared laser which could be used by an extraterrestrial civilization to signal their presence. GPI and WFIRST could utilize a coronagraph to search for extraterrestrial intelligence in the present and future. We use archival data for GPI stars and simulated WFIRST observations to find the detectable flux ratio of a laser signal to residual scattered starlight around the target star. This flux ratio is then converted to detectable power as a function of distance from the parent star. For GPI, we assume a monochromatic laser wavelength of 1.55 μm and a wavelength of 575 nm for WFIRST. We assume that the lasers are projected through a 10 m aperture, and that the intensity of the laser beam follows a Gaussian profile. Our analysis is performed on six stars with spectral types later than F within 20 pc (with an emphasis on solar analogs at different distances). The most notable result is the detection limit for τ Ceti, a G5V star with four known exoplanets, two of those within the habitable zone (HZ). The result shows that a 24 kW laser is detectable from τ Ceti from outside of the HZ with GPI and a 7.3 W laser is detectable from within τ Ceti’s HZ by WFIRST.

Asymmetries in adaptive optics point spread functions

An explanation for the origin of asymmetry along the preferential axis of the point spread function (PSF) of an AO system is developed. When phase errors from high-altitude turbulence scintillate due to Fresnel propagation, wavefront amplitude errors may be spatially offset from residual phase errors. These correlated errors appear as asymmetry in the image plane under the Fraunhofer condition. In an analytic model with an open-loop AO system, the strength of the asymmetry is calculated for a single mode of phase aberration, which generalizes to two dimensions under a Fourier decomposition of the complex illumination. Other parameters included are the spatial offset of the AO correction, which is the wind velocity in the frozen flow regime multiplied by the effective AO time delay and propagation distance or altitude of the turbulent layer. In this model, the asymmetry is strongest when the wind is slow and nearest to the coronagraphic mask when the turbulent layer is far away, such as when the telescope is pointing low toward the horizon. A great emphasis is made about the fact that the brighter asymmetric lobe of the PSF points in the opposite direction as the wind, which is consistent analytically with the clarification that the image plane electric field distribution is actually the inverse Fourier transform of the aperture plane. Validation of this understanding is made with observations taken from the Gemini Planet Imager, as well as being reproducible in end-to-end AO simulations.

A Robust Ground‐to‐Image Transformation Algorithm and Its Applications in the Geometric Processing of Linear Pushbroom Images

AbstractLinear array cameras are widely used in the fields of aerial and space remote sensing. The ground‐to‐image transformation plays a significant role in the geometric processing procedures of linear pushbroom images, which is time‐consuming due to multiple iterative calculations. Moreover, the detectors of the linear array may exhibit a curved shape due to distortions, resulting in a more complex ground‐to‐image transformation. This paper presents a novel ground‐to‐image transformation algorithm for linear pushbroom images based on a rigorous sensor model. We regard the distance between the back‐projected image point and the curved shape linear array as the “generalized distance.” The idea of generalized distance prediction was developed to efficiently determine the best scan line corresponding to the ground point. The plane analytic geometry is employed to solve the practical problems of the ground‐to‐image transformation, including the iterative direction, convergence condition, and subpixel interpolation. The applications of the proposed method in orthorectification and image matching were investigated. The proposed method was comprehensively tested using airborne images, Earth observation satellite images, and planetary images. The experimental results demonstrate that the proposed method (1) delivers high accuracy (better than1 × 10−4pixels) and efficiency, (2) is better than the cutting‐edge method based on the geometric constraints of central perspective plane for the linear pushbroom images with distortions, and (3) provides promising applications for generating orthophotos and image matching. The proposed method is easy to implement and efficient, which can greatly enhance the geometric processing abilities for various types of linear pushbroom images.

TESS Hunt for Young and Maturing Exoplanets (THYME): A Planet in the 45 Myr Tucana–Horologium Association

Abstract Young exoplanets are snapshots of the planetary evolution process. Planets that orbit stars in young associations are particularly important because the age of the planetary system is well constrained. We present the discovery of a transiting planet larger than Neptune but smaller than Saturn in the 45 Myr Tucana–Horologium young moving group. The host star is a visual binary, and our follow-up observations demonstrate that the planet orbits the G6V primary component, DS Tuc A (HD 222259A, TIC 410214986). We first identified transits using photometry from the Transiting Exoplanet Survey Satellite (TESS; alerted as TOI 200.01). We validated the planet and improved the stellar parameters using a suite of new and archival data, including spectra from Southern Astrophysical Research/Goodman, South African Extremely Large Telescope/High Resolution Spectrograph and Las Cumbres Observatories/Network of Robotic Echelle Spectrographs; transit photometry from Spitzer; and deep adaptive optics imaging from Gemini/Gemini Planet Imager. No additional stellar or planetary signals are seen in the data. We measured the planetary parameters by simultaneously modeling the photometry with a transit model and a Gaussian process to account for stellar variability. We determined that the planetary radius is 5.70 ± 0.17 R ⊕ and that the orbital period is 8.1 days. The inclination angles of the host star’s spin axis, the planet’s orbital axis, and the visual binary’s orbital axis are aligned within 15° to within the uncertainties of the relevant data. DS Tuc Ab is bright enough (V = 8.5) for detailed characterization using radial velocities and transmission spectroscopy.

Giant Planets and Brown Dwarfs Form in Different Ways

Once thought to be part of the same population, planets larger than Jupiter and “failed stars” likely grow via different mechanisms, the Gemini Planet Imager Exoplanet Survey has shown.

Kinematic Approximation of Position Accuracy Achieved Using Optical Observations of Distant Asteroids

NASA’s Deep Space 1 mission demonstrated that a spacecraft can be navigated autonomously during deep-space cruise operations using only optical navigation measurements. A methodology is developed to evaluate the feasibility and accuracy of the Deep Space 1 orbit determination approach throughout the solar system as a function of a spacecraft’s imaging capabilities. Feasibility can be addressed by comparing the apparent magnitudes of the known population of asteroids against the imaging system capabilities. An upper limit on the accuracy of the spacecraft position estimate at a given location can be formulated by assuming observations of at least two asteroids simultaneously. Example results are presented for three camera implementations that span the range of capabilities flown in deep space to date using orbit and absolute magnitude data for 50,129 of the brightest known asteroids. Broadly speaking, achievable accuracies range from approximately 200 to 12,000 km () interior to the main asteroid belt and from 50 to 2000 km within the main belt, depending strongly on the chosen camera implementation. Between the main belt and Jupiter, only a current state-of-the-art imaging system is consistently capable of kinematic positioning using only asteroids. Beyond Jupiter, there are insufficient known asteroids to support this approach without including images of planets and moons. Although these levels of accuracy are far inferior to those achievable with radiometric navigation, they may yet be sufficient to satisfy cruise-phase requirements for many deep-space missions.

Observable scattered light features from inclined and non-inclined planets embedded in protoplanetary discs

ABSTRACT Over the last few years instruments such as VLT/SPHERE and Subaru/HiCIAO have been able to take detailed scattered light images of protoplanetary discs. Many of the features observed in these discs are generally suspected to be caused by an embedded planet, and understanding the cause of these features requires detailed theoretical models. In this work we investigate disc–planet interactions using the pluto code to run 2D and 3D hydrodynamic simulations of protoplanetary discs with embedded 30 and 300 M⊕ planets on both an inclined (i = 2.86°) and non-inclined orbit, using an α-viscosity of 4 × 10−3. We produce synthetic scattered light images of these discs at H-band wavelengths using the radiative transfer code radmc3d. We find that while the surface density evolution in 2D and 3D simulations of inclined and non-inclined planets remain fairly similar, their observational appearance is remarkably different. Most of the features seen in the synthetic H-band images are connected to density variations of the disc at around 3.3 scale heights above and below the mid-plane, which emphasizes the need for 3D simulations. Planets on sustained orbital inclinations disrupt the disc’s upper atmosphere and produce radically different observable features and intensity profiles, including shadowing effects and intensity variation of the order of 10–20 times the surrounding background. The vertical optical depth to the disc mid-plane for H-band wavelengths is τ ≈ 20 in the disc gap created by the high-mass planet. We conclude that direct imaging of planets embedded in the disc remains difficult to observe, even for massive planets in the gap.

The Gemini Planet Imager Exoplanet Survey: Giant Planet and Brown Dwarf Demographics from 10 to 100 au

Abstract We present a statistical analysis of the first 300 stars observed by the Gemini Planet Imager Exoplanet Survey. This subsample includes six detected planets and three brown dwarfs; from these detections and our contrast curves we infer the underlying distributions of substellar companions with respect to their mass, semimajor axis, and host stellar mass. We uncover a strong correlation between planet occurrence rate and host star mass, with stars M * > 1.5 M ⊙ more likely to host planets with masses between 2 and 13M Jup and semimajor axes of 3–100 au at 99.92% confidence. We fit a double power-law model in planet mass (m) and semimajor axis (a) for planet populations around high-mass stars (M * > 1.5 M ⊙) of the form , finding α = −2.4 ± 0.8 and β = −2.0 ± 0.5, and an integrated occurrence rate of % between 5–13M Jup and 10–100 au. A significantly lower occurrence rate is obtained for brown dwarfs around all stars, with % of stars hosting a brown dwarf companion between 13–80M Jup and 10–100 au. Brown dwarfs also appear to be distributed differently in mass and semimajor axis compared to giant planets; whereas giant planets follow a bottom-heavy mass distribution and favor smaller semimajor axes, brown dwarfs exhibit just the opposite behaviors. Comparing to studies of short-period giant planets from the radial velocity method, our results are consistent with a peak in occurrence of giant planets between ∼1 and 10 au. We discuss how these trends, including the preference of giant planets for high-mass host stars, point to formation of giant planets by core/pebble accretion, and formation of brown dwarfs by gravitational instability.

A GENERIC RIGOROUS SENSOR MODEL FOR PHOTOGRAMMETRIC PROCESSING OF PUSHBROOM PLANETARY IMAGES

Abstract. Currently, each planetary exploration mission team always develops its own software modules to support the photogrammetric processing of planetary images, and as a result of that the main drawbacks are lacking software reusability and the high cost of software development and maintenance. This is mainly due to that there is lack of a highly universal sensor model in the planetary mapping community. This paper presents a generic rigorous sensor model (RSM) for the photogrammetric processing of pushbroom planetary images. The main contributions of this paper include: (1) the implementation details of the generic RSM; (2) the optimized coordinates transformation methods between 3D ground points and 2D image points for linear pushbroom images; (3) a pipeline to acquire exterior orientation (EO) parameters for each planetary image. The generic RSM is developed based on the methodology used in airborne linear scanners ADS40. Specifically, the generic RSM comprises of a camera file and an orientation data file for each image. The camera file stores each detector’s calibrated image coordinates and the orientation data file contains each scan line’s EO parameters, such that the RSM can perform coordinates transformation among pixel coordinates, focal plane coordinates and ground coordinates. Furthermore, the generic RSM supports varying exposure time, summing mode and image distortions, which are typical problems that need to be solved in planetary mapping. We tested the generic RSM with Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC), Chandrayaan-1 Moon Mineralogy Mapper (M3) and Mars Express (MEX) High Resolution Stereo Camera (HRSC) images. The geometric accuracy and computational efficiency of the developed generic RSM were compared with the famous planetary mapping software, namely Integrated System for Imagers and Spectrometers (ISIS). The experimental results demonstrate that the generic RSM has the merits of processing various types of pushbroom planetary images with a unified way and decreasing the software development and maintenance burden. Moreover, the developed generic RSM significantly improves the computational efficiency of orthophoto generation and tie points extraction for pushbroom planetary images.

ASSESSMENT OF FEATURE DETECTORS AND DESCRIPTORS IN REMOTE IMAGES OF PLANETARY BODIES

Abstract. Algorithms to detect, describe, and match common features in image sets are expanding to new worlds with the integration of the OpenCV feature matching framework in a popular mapping tool for planetary images (ISIS3). These algorithms provide a new approach to register images and build image-based control networks. However, the natural landscape of the Moon and other planetary bodies pose new challenges such as numerous features that appear similar (i.e., impact craters and boulders). In addition, planetary image sets vary in scale, orientation, and noise properties, especially when conducting cross instrument and cross mission comparisons. This study assesses a collection of common feature detector and descriptor algorithms to examine how they adapt to these challenges. With our analysis, we did not identify an ideal detector and descriptor combination that exists for our diverse lunar dataset. However, we did identify where particular algorithms succeed and identify their shortcomings. By knowing these capabilities, users can identify the proper set of algorithms to apply to an image set given the presence of noise and variations to scale and orientation.

Performance of the Gemini Planet Imager Non-redundant Mask and Spectroscopy of Two Close-separation Binaries: HR 2690 and HD 142527

Abstract The Gemini Planet Imager (GPI) contains a 10-hole non-redundant mask (NRM), enabling interferometric resolution in complement to its coronagraphic capabilities. The NRM operates both in spectroscopic (integral field spectrograph, henceforth IFS) and polarimetric configurations. NRM observations were taken between 2013 and 2016 to characterize its performance. Most observations were taken in spectroscopic mode, with the goal of obtaining precise astrometry and spectroscopy of faint companions to bright stars. We find a clear correlation between residual wavefront error measured by the adaptive optic system and the contrast sensitivity by comparing phase errors in observations of the same source, taken on different dates. We find a typical 5σ contrast sensitivity of (2–3) × 10−3 at ∼λ/D. We explore the accuracy of spectral extraction of secondary components of binary systems by recovering the signal from a simulated source injected into several data sets. We outline data reduction procedures unique to GPI’s IFS and describe a newly public data pipeline used for the presented analyses. We demonstrate recovery of astrometry and spectroscopy of two known companions to HR 2690 and HD 142527. NRM+polarimetry observations achieve differential visibility precision of σ ∼ 0.4% in the best case. We discuss its limitations on Gemini-S/GPI for resolving inner regions of protoplanetary disks and prospects for future upgrades. We summarize lessons learned in observing with NRM in spectroscopic and polarimetric modes.

MEMS Deformable Mirrors for Space-Based High-Contrast Imaging

Micro-Electro-Mechanical Systems (MEMS) Deformable Mirrors (DMs) enable precise wavefront control for optical systems. This technology can be used to meet the extreme wavefront control requirements for high contrast imaging of exoplanets with coronagraph instruments. MEMS DM technology is being demonstrated and developed in preparation for future exoplanet high contrast imaging space telescopes, including the Wide Field Infrared Survey Telescope (WFIRST) mission which supported the development of a 2040 actuator MEMS DM. In this paper, we discuss ground testing results and several projects which demonstrate the operation of MEMS DMs in the space environment. The missions include the Planet Imaging Concept Testbed Using a Recoverable Experiment (PICTURE) sounding rocket (launched 2011), the Planet Imaging Coronagraphic Technology Using a Reconfigurable Experimental Base (PICTURE-B) sounding rocket (launched 2015), the Planetary Imaging Concept Testbed Using a Recoverable Experiment - Coronagraph (PICTURE-C) high altitude balloon (expected launch 2019), the High Contrast Imaging Balloon System (HiCIBaS) high altitude balloon (launched 2018), and the Deformable Mirror Demonstration Mission (DeMi) CubeSat mission (expected launch late 2019). We summarize results from the previously flown missions and objectives for the missions that are next on the pad. PICTURE had technical difficulties with the sounding rocket telemetry system. PICTURE-B demonstrated functionality at >100 km altitude after the payload experienced 12-g RMS (Vehicle Level 2) test and sounding rocket launch loads. The PICTURE-C balloon aims to demonstrate contrast using a vector vortex coronagraph, image plane wavefront sensor, and a 952 actuator MEMS DM. The HiClBaS flight experienced a DM cabling issue, but the 37-segment hexagonal piston-tip-tilt DM is operational post-flight. The DeMi mission aims to demonstrate wavefront control to a precision of less than 100 nm RMS in space with a 140 actuator MEMS DM.

Polarization aberration analysis for the PICTURE-C exoplanetary coronagraph

The Planetary Imaging Concept Testbed Using a Recoverable Experiment - Coronagraph (PICTURE-C) mission will directly image debris disks and exozodiacal dust around nearby stars from a high-altitude balloon using a vector vortex coronagraph. One of the many starlight leakage sources that can degrade the performance of the coronagraph is polarization aberration induced by the reflective optical coatings. A polarization ray trace of the PICTURE-C telescope and coronagraph is combined with a physical optics wavefront propagation simulation to quantify the expected amount of coronagraph leakage due to polarization aberration. The simulations show the leakage is below the budgeted contrast of 1 × 10 − 8.

High-accuracy location algorithm of planetary centers for spacecraft autonomous optical navigation

This study provides an on-board image processing algorithm for a spacecraft optical navigation sensor system. We propose a high-precision planetary center localization algorithm that provides effective information for subsequent spacecraft navigation measurement considering the geometric properties and special texture features of the planetary image. First, we combine a morphological opening with a minimum spanning circle method to preprocess an image. Second, a gradient direction partial area effect method in the edge region of interest is proposed for high-precision subpixel edge extraction. This study combines the law of edge energy distribution with a gradient direction partial area effect on planetary edge detection. A nonlocal filtering method is proposed to improve the results under complex conditions. Finally, the center coordinates of the planet are obtained by fitting the circular arc edges using least squares. The simulated and real images show that our algorithm can obtain high-precision subpixel edges and center coordinates for regular planetary images, with strong robustness to planetary images with noise, complex background, and rich texture.