Analysis of Two-Band WISE Observations of Asteroids

Image Credit: NASA/JPL-Caltech

The Wide Field Infrared Survey Explorer (WISE) is a space-based telescope which conducted a full-sky survey in four infrared wavelength bands centered at 3.4, 4.6, 12, and 22 μm (called W1, W2, W3, and W4, respectively), yielding thermal observations of over 100,000 asteroids. In this project, we wished to elucidate unique aspects of asteroid thermal modeling with only two infrared bands instead of all four so that any potential biases or shortcomings could be anticipated and quantified for the upcoming two-band infrared surveys of asteroids, such as NEO Surveyor. Using diameter solutions based on a thermal model that was reparameterized by Myhrvold 2018, we compare diameter solutions for a set of over 4000 asteroids observed with WISE in two sets of two wavelength bands (W2-3 and W3-4) to previous results obtained by Myhrvold et al. 2022 with four wavelength bands (W1-4).

We found that diameters tend to be 10% smaller for W2-3 and 11% larger for W3-4 than for W1-4. We also found that diameter uncertainties tend to be 30% and 35% larger for W2-3 and W3-4 than for W1-4. While future surveys should take these biases into consideration, they should still be able to obtain valuable size information about these asteroids.

PSJ Publication

Detecting Rocky Surfaces with JWST

Image Credit: NASA/JPL-Caltech/R. Hurt, IPAC.

The James Webb Space Telescope (JWST) has allowed us to view the spectral signatures of terrestrial planets for the first time. The shapes of these spectral emission features can tell us not only about the atmospheres of these planets, but may be able to tell us about what the surfaces are made out of if the atmosphere is sufficiently thin and transparent across multiple wavelengths. In this study, we focused specifically on LHS 3844b, a hot (around 1000 K), rocky exoplanet that has been observed by the Spitzer space telescope to have a thin to non-existent atmosphere. We explored the feasibility of characterizing with JWST the atmosphere and surface of this exoplanet, before it is observed during Cycle 1 of JWST’s observing program. Considering the Spitzer observation of the planet at 4.5 microns, LHS 3844b was consistent with a relatively dark surface material such as a basaltic, metal-rich, or iron-oxidized surface, and atmospheres greater than 1 bar would be excluded assuming they contained greater than 100 parts-per-million of a near-infrared absorber, such as carbon dioxide. We find that about 3 secondary eclipse observations of the exoplanet should be enough for JWST to distinguish between plausible surface and atmospheric features.

We also performed a Bayesian retrieval analysis on simulated JWST data to see if we could correctly retrieve the atmosphere of the simulated planet that we put into the model. We found that since the standard techniques for retrieving the abundances of these atmospheric gasses don’t account for the features that could be added to a spectrum by the surface of an exoplanet, certain surface models can create a false atmospheric water detection. Therefore, any detections of water in a thin terrestrial exoplanet atmosphere should be interpreted with caution.

PSJ Publication

Finding Near-Earth Asteroid Sizes using Radar

Image Credit: Arecibo/S. J. Ostro et al.

Determining the sizes of near-Earth asteroids is very important for understanding the danger posed by a potential asteroid impact. Ground based planetary radar can provide precise and direct size measurements by sending a radar pulse to a specific near-Earth asteroid and measuring the reflected signal. To obtain a precise diameter, one needs to take radar measurements at many different orientations of the asteroid and use a complex shape modeling technique to construct a 3D model of the asteroid. While effective, this technique is quite time and work intensive, and requires many observations. The standard alternative is to estimate the diameter from a single radar image by visual inspection of the image. While fast and easy, this technique tends to underestimate the size of the observed asteroid. In this project, we tested a variety of techniques for asteroid diameter measurement with the goal of striking a balance between speed and accuracy.

After binning the image data into a plot of Doppler delay range vs. relative echo power, where range represents distance along the line of observation, we tested three models for obtaining a diameter from the data: a linear fit, a cosine scattering law, and a noise statistics method. After testing these models on radar data taken by the Arecibo Observatory of asteroid 66391 Moshup, we found that the noise statistics method was most robust for low signal-to-noise radar images, but that the linear fitting technique was most reliable, with average percent errors never exceeding 18%.

LPSC 2020 Abstract

Positional Accuracy of ZTF's Asteroid Streaks

Image Credit: ZTF/Caltech

The Zwicky Transient Factory (ZTF) is a camera made of a mosaic of 16 CCDs mounted onto the 48-inch Samuel Oschin Telescope at Palomar Observatory. ZTF is conducting a survey of the sky in search of near-Earth asteroids (NEAs), among other things. Particularly fast-moving NEAs will create a streak across a single image. An algorithm called ZStreak uses a stretched-PSF fitting model to detect these asteroid streaks and estimate their angular position on the sky. In this project, we compared ZStreak positional data on 125 well-known NEAs to predicted positions determined by NASA JPL’s Horizons System to determine the accuracy of ZStreaks positional fitting model and locate any systematic offsets.

We found that errors perpendicular to the direction of motion of the asteroid appeared to be random. Along the direction of motion, we found that ZStreak positions had a median error that was 0.35 arcsec ahead of the corresponding HORIZONS point, which could be further separated into a median error of 0.28 arcsec at the beginning of the streak and 0.48 arcsec at the end of the streak. Because most of this bias lies along the direction of the streak, it is likely that the source of this bias was an inaccuracy in the times listed as the shutter opening and shutter closing time. Therefore, improving the accuracy and precision of these listed times and ensuring these accurate times are received by the Minor Planet Center would allow researchers to better predict the object’s future orbital position.

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