Asteroid Science with ESA Euclid: Results from the Ecliptic PDC Campaign

Introduction

The European Space Agency's Euclid space telescope provides a unique opportunity for Solar System science due to its capabilities in both visible and near-infrared wavelengths, thanks to the visible imager (VIS) and the near-infrared spectrophotometer (NISP). During a week-long Phase Diversity Calibration (PDC) campaign in December 2023, Euclid observed regions along the ecliptic plane, offering a special dataset for asteroid detection and characterization. The campaign's observation strategy was designed to follow the apparent motion of most asteroids to increase the probability of obtaining multiple observations of the same objects. These observations represent Euclid's first deliberate sampling of Solar System objects and serve as a testbed for the Wide Survey. We present results from our analysis of these observations, demonstrating Euclid's potential for asteroid science.

 

Background

Studying asteroids provides insights into the formation and evolution of our Solar System. Asteroids' compositional and dynamical distribution helps us understand planetary migration and other processes that have shaped the Solar System over 4.5 billion years.

While many asteroids show relatively similar reflectance spectra in visual wavelengths, they differ significantly in NIR (DeMeo et al. 2009). NIR data for asteroids remains relatively scarce, with the largest catalog containing only about 35,000 objects (Popescu et al. 2016). Unlike surveys such as Gaia and LSST (visual only) or VISTA and UKIDSS (NIR only), Euclid covers both spectral regions with a single telescope. During its 6.5-year Wide Survey, Euclid is expected to observe up to 150,000 Solar System objects (Carry 2018), even though it primarily targets regions away from the ecliptic plane where asteroid densities are lower.

 

Methods

The ecliptic campaign consisted of approximately 150 observations covering an area of ~18 square degrees between one degree on either side of the ecliptic plane. We developed a pipeline capable of processing Level 1 (raw) VIS data, which will be essential for timely analysis and potential follow-up observations during the Wide Survey.

Our data processing pipeline begins with preprocessing, adapted from Nucita et al. (2025), including standard reduction steps (such as bias and flat corrections), cosmic ray removal, and astrometric and photometric calibration. For streak detection, we utilize the StreakDet software (Virtanen et al. 2016) to identify asteroid streaks in single exposures, using parameters optimized for Euclid VIS images (Pöntinen et al. 2020). We then employ an algorithm to link streaks across Euclid's four dithered exposures per field, significantly reducing false positives. We carry out PSF photometry via StreakDet, aperture photometry via TRIPPy (Fraser et al. 2016), and trailed photometry (Devogèle et al. 2024). Finally, we use the OpenOrb software (Granvik et al. 2009) to link objects between fields and generate probability density functions for orbital elements.

 

Results

Our pipeline detected over 43,000 individual asteroid streaks across the observed fields. Cross-matching with the SkyBot database (Berthier et al. 2006) revealed that approximately 53% of these streaks correspond to 2,334 known asteroids, with an average of 10 streaks per object. The remaining 47% of streaks originate from previously unknown objects, suggesting the discovery of roughly 2,000 new asteroids in this dataset. The false positive rate of our pipeline is estimated to be less than 1%.

Most detected objects belong to the Main Asteroid Belt, with additional detections including near-Earth asteroids, Mars-crossers, and objects in the Cybele and Hilda populations. Most detected asteroids have apparent magnitudes between 22 and 25. We also detected an unknown comet or active asteroid in seven fields. This object, with an apparent magnitude of 20-21, displays a clearly visible tail in all observations. Orbital fitting suggests the object is moving away from the Sun, with a distance of approximately 2-4 AU from Euclid at the time of observation.

While the short observational arcs (typically 1-7 hours) limit the precision of orbital solutions from Euclid data alone, we estimated orbital distributions for the objects. Asteroids observed across multiple fields over periods up to 27 hours yielded significantly improved orbital constraints.

Additionally, this extensive collection of real asteroid streaks is valuable for retraining the deep learning-based asteroid detection pipeline developed by Pöntinen et al. (2023), which, when tested with simulated data, could reach shorter and fainter streaks than StreakDet, potentially increasing the number of detected objects by up to 50%.

 

Conclusions

The Euclid ecliptic plane observations have demonstrated the telescope's capability for asteroid science, and it has served as a proving ground for the Wide Survey. Furthermore, it highlights the usefulness of Euclid’s ecliptic campaigns: by pointing the telescope toward the ecliptic plane, we observed approximately ten times more asteroids during one week than would be expected from a comparable period of the Wide Survey, which focuses away from the ecliptic plane.

While our analysis of VIS data is well-advanced, work on NISP data calibration and analysis continues. The combination of VIS and NISP data provides valuable insights into asteroid compositions, helping address key questions about Solar System formation and evolution. The PDC campaign has confirmed Euclid's potential as a significant contributor to small-body science, complementing other current and upcoming surveys through its fairly unique observational capabilities.

 

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