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Nordic Optical Telescope helped in characterising Earth's temporary natural companion

Press release from Queen's University, Belfast.

The Moon is the brightest object in the night sky, and for that reason is often silently cursed at by astronomers. It is also the sole natural companion of the Earth in its celestial journey through the Universe - unless you take into account its smaller brothers and sisters - minimoons.

What are these minimoons? They are small asteroids that occasionally become companions to the Earth for a limited amount of time. On average, a minimoon is captured on an orbit around our home planet for a period of nine months and makes three orbits during that time. At any given time, the largest captured minimoon is just under a metre in diameter, and a three-metre object is captured on on average once a decade.

During spring of 2020, an international collaboration of 23 researchers from the UK, Italy, USA, Finland, Sweden, Spain and Canada performed a multi-instrumental observational campaign of asteroid 2020 CD3, Earth's second minimoon. The first one, 2006 RH120, rotated around the Earth 14 years ago, thus so far these objects have been a rare treat.

"The input of the Nordic Optical Telescope has been essential for our study", says Grigori Fedorets, an astronomer from Queen's University Belfast. "Through accurate measurements of its positions our group was able to prove its natural origin. Objects on geocentric orbits are often automatically considered to be of artificial origin, but not now!" Objects as small as minimoons, with a diameter of metres, experience a subtle but measurable non-gravitational force caused by solar radiation pressure. The high accuracy of the Gaia DR2 catalogue, the prime optics of the Nordic Optical Telescope (NOT) and excellent seeing conditions at the Roque de los Muchachos observatory resulted in high-quality astrometric measurements which allowed to measure the effect, but also to constrain the complicated orbit of 2020 CD3.

While one object is a discovery, two objects are already a population. Minimoons are expected to be discovered in high numbers in the following decade, with the commissioning of the Vera C. Rubin observatory and its Legacy Survey of Space and Time, expected to commence operations in 2023.

"Minimoons are exceptionally good targets for space missions", says Fedorets, "They come close to the Earth, and spend a significant amount of time in it's vicinity". A particularly interesting type of mission that could take advantage of the close and relatively long approaches of minimoons would involve bringing an entire small asteroid back to Earth for analysis. Before such a mission can take place, an orbit of a minimoon will need to be well constrained. We anticipate to make use of NOT for minimoon follow-up observations, especially once LSST is up. "NOT has proven to be a very capable instrument for this kind of work", says Fedorets.

The trajectory of 2020 CD3 in the Earth-Moon system. Video credit: NASA/JPL-Caltech.

Fedorets, Micheli, Jedicke et al. 2020, to appear in Astronomical Journal
ArXiv link

Here's what we know about Earth's new minimoon - Universe Today

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