Elsevier

Acta Astronautica

Volume 184, July 2021, Pages 251-258
Acta Astronautica

Research paper
Optical observations of the BepiColombo spacecraft as a proxy for a potential threatening asteroid

https://doi.org/10.1016/j.actaastro.2021.04.022Get rights and content

Highlights

  • We obtained ground-based optical astrometry of BepiColombo during its 2020 fly-by.

  • The astrometry allowed us to compute an accurate trajectory for the fly-by.

  • We compare the results with a ground-truth from radio tracking.

  • We used the campaign as a proxy for an incoming potentially threatening asteroid.

Abstract

We present the results of our ground-based astrometric observation campaign of BepiColombo during its Earth fly-by on 2020 April 10. The observational circumstances induced by the fly-by geometry made this event an excellent proxy for a close fly-by or an imminent impact of a natural body with a diameter of a few meters, with the added benefit of having a ground-truth orbit (from radio tracking) to compare with our purely optical orbit determination.

Introduction

BepiColombo is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) designed to study the planet Mercury [1]. Launched on 2018 October 20, the spacecraft trajectory is designed to take advantage of multiple fly-bys with the inner planets en route to Mercury. This included a single fly-by with Earth 1.5 years after launch, on 2020 April 10.

Since the fly-by date and circumstances were obviously well-known far in advance, we took advantage of the event to organize a coordinated observing campaign, in order to obtain the best possible optical astrometric coverage of the event. The results of such campaign are presented below. Additional observations designed for physical characterization were also obtained, but they will not be discussed in this work.

Section snippets

Observability and challenges

The incoming trajectory of BepiColombo resulted in an approach towards Earth at extremely low solar elongation, with the object remaining at an elongation of <30° for almost a month before the close encounter. Therefore, the ground-based observability of the fly-by event was limited to a few hours around the time of closest approach, plus the entire receding phase.

In order to obtain the best possible observational coverage, we therefore had to utilize telescopes located in the appropriate area

Datasets

Our analysis of the BepiColombo trajectory is mostly based on astrometric measurements extracted from image datasets collected during a dedicated observing campaign organized by ESA’s NEO Coordination Centre (NEOCC). Most images were obtained by telescopes having formal contracts with ESA in order to provide astrometric coverage of interesting targets selected by the Center. In some other cases, we obtained additional images via collaborations with other astronomers.

The network of telescopes

Results

The dataset outlined in Section 3 and presented in Table 3 forms the basis for our optical-only determination of BepiColombo’s trajectory during its Earth fly-by.

The main goal of this effort is to be able to compare the fly-by properties of this computed trajectory with the “ground truth” values available from the flight dynamics data. This will in turn give us information on the accuracy of the orbit determination process, the correctness of the astrometry we obtained, and the appropriateness

Reduced subsets

The numbers presented in Section 4.1 are all based on the entire available dataset. However, in order to understand the value of each observational circumstance, it is useful to analyze the results that we would have obtained if some crucial datasets had been missing.

Conclusions

As expected, the results collected during this observing campaign provide useful information that can be applied to natural objects, when observed in similar configurations. This is especially important for the case of imminent impactors, small objects detected during their incoming impact trajectory with our planet, often just a few hours before the time of impact.6

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank additional people who made this work possible, either by collaborating to the observing campaign, or by providing crucial date for the success of the campaign.

In particular, we would like to thank I. Reva and A. Serebryanskiy from the ISON team for the Tien-Shan observations.

We would also like to recognized the role of Alessandra Di Cecco (ASI) as responsible of the “Accordo ASI-INAF Detriti Spaziali 2019–2021” for making the observations from Castelgrande

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