What Are Minor Planets Made of? Planetary Science with the SPHEREx 0.7 – 5.0 μm Solar System Object Catalog

The Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx), is a cosmological mission in NASA's Medium Explorer (MIDEX) astrophysics program that will launch in early 2025 and perform a 2-year all-sky near-infrared spectral survey [1-3]. (More details concerning SPHEREx are available at http://spherex.caltech.edu.)

The upcoming SPHEREx spectral survey provides a fantastic opportunity to detect, spectrally categorize, and catalog hundreds of thousands of solar system objects at WISE/NEOWISE sensitivity levels in 102 spectral bands between 0.75 - 5.0 µm with R = 41 to 135 [4] using a single stable, well-characterized and calibrated space-based remote sensing platform  [1-3]. Utilizing a sun-synchronous NEOWISE-like polar orbit, objects in the sky at ~90 deg elongation will be observed in each great circle. The Earth’s motion around the Sun advances the great circle’s longitude ~1 deg/day; taking data in both the leading/trailing directions so the entire sky’s range of longitudes is covered in 6 months.

In this work we give an overview of the SPHEREx mission, and describe the promise and challenges of taking the SPHEREx sky-survey LVF imaging data obtained by a mission designed to study billions of fixed galaxies and produce a moving object catalog with scientific content even greater than of the previous NEOWISE NIR photometric all-sky catalog, for ~15 times less cost. 

The resulting spectra could be used for everything from discerning new families of asteroids, comets, Trojans, Centaurs, and KBOs; to characterizing brand new interstellar objects; to mapping the zodiacal light with higher spatial resolution than has ever been done previously while searching for compositional structures; and to augmenting the science return from missions like DAWN, Psyche, Lucy, and NEO Surveyor [5]. In addition, there is great potential for overlapping synergies with results from missions running concurrently in the late 2020’s like JWST, WFIRST, Euclid, GAIA, TESS, eROSITA and LSST [3,7]. The SPHEREx Solar System pipeline will rely on a “forced photometry” approach, extracting signal for known bodies at their predicted locations on the sky, and thus benefit from the observations obtained from these and other platforms. With great care of converting the millions of individual SPHEREx LVF sky images into calibrated spectra with observational systematics, the resultant data will be capable of:

 

• Determination of the size, albedo, and composition of ~10⁵ asteroids [6, 7].
• Discovery of newly active asteroids and characterization of known episodically active asteroids and Potentially Hazardous Objects [6, 8].
• Characterization from 0.7-5.0 µm of Interstellar Objects passing through the SPHEREx sky survey from a stable, sensitive, above-the-atmosphere observatory.
• Spectral monitoring of the weather on Uranus, Nep- tune, and Pluto over weeks and years.
• Discovery, detection, and characterization of 1000’s of Centaurs and Comets, leading to better understanding of the origins and evolution of their primordial icy materials (especially CO₂ which is unavailable from the ground), as well as support of the ESA Comet Interceptor

Providing the planetary science community with an accurate object spectral data catalog containing ~10⁵ objects with IRTF/SpeX/Prism-like data will require the work of a dedicated team familiar with the SPHEREx mission, instrument calibration, and science data pipeline, time domain astronomical observations, and big dataset archiving at the PDS and IRSA.

  References  

1. Crill+ 2020, "SPHEREx: NASA's Near-IR Spectrophoto- metric All-Sky Survey", SPIE 11443, 114430I

2. Doré+ 2016, “Science Impacts of the SPHEREx All-Sky Optical to Near-Infrared Spectral Survey: Report of a Community Workshop Examining Extragalactic, Galactic, Stellar & Planetary Science”, eprint arXiv:1606.07039

3. Doré+ 2018, “Science Impacts of the SPHEREx All-Sky Optical to Near-IR Spectral Survey II: Report of a Community Workshop on the Scientific Synergies Between the SPHEREx Survey & Other Astronomy Observatories”, eprint arXiv:1805.05489

4. Mainzer+ 2015, "Space-Based Infrared Discovery and Characterization of Minor Planets with NEOWISE", in Hand- book of Cosmic Hazards and Planetary Defense, ISBN: 978-3- 319-03952-7. pp. 583-611

5. Mainzer+ 2023, PSJ  4, 224.

6. Lisse & Bauer, 2023, “Planetary Defense Use of the SPHEREx Solar System Object Catalog”, PDO White Paper submitted August 2023, arxiv

7. Ivezić+ 2019, "LSST: From Science Drivers to Reference Design and Anticipated Data Products", Astrophys J 873, 111

8. Ivezic, Z. et al. 2022, "Simulated SPHEREx Spectra of Asteroids and Their Implications for Asteroid Size and Reflectance Estimation", Icarus 371, 114696