Astronomy Now - January 2021

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Earth’s second temporary minimoon gives up

its secrets

ew details regarding a ‘minimoon’, temporarily in orbit around Earth, are providing clues about

about a population of small asteroidal bodies that have hitherto been unknown.

Despite their name, minimoons are not proper moons. ey are small, captured asteroids that are
eventually ung back out into interplanetary space. e rst minimoon to be discovered, catalogued
2006 RH120, is at most three metres across and makes close encounters of Earth every 20 years,
when Earth’s gravity can capture it for a time. e last occasion this happened was in 2006–2007.

In February 2020, a second minimoon, 2020 CD3, was discovered by the Catalina Sky Survey.
Given its small size, it was initially thought that it could be a discarded rocket-stage from NASA’s
lunar Surveyor 2 mission, dating back to 1966. However, observations by the 4.3-metre Lowell
Discovery Telescope in Arizona have proven that 2020 CD3 is a captured asteroid. Based on its
colour, it is a silicate asteroid no larger than 1.5 metres across, and its light curve – which describes
how its brightness changes as it spins – indicates that it is rotating once every three minutes. Its
orbit brings it as close as 13, 000 kilometres at perigee, and it will remain in captured orbit for 2.
years. “e rotation rate was probably the largest unanswered question of this research,” says Grigori
Fedorets of Queen’s University Belfast, who led an international team studying the minimoon. “e
Lowell team showed that it rotates slower than anticipated for objects of this size range.” is
suggests that we don’t know as much about this scale of asteroid as we thought, but that could be set
to change, according to Fedorets. “Minimoons are expected to be discovered in high numbers in the
following decades, with the opening of the Vera C. Rubin Observatory [an 8.4-metre survey
telescope in Chile] expected in 2023,” he says.

Earth’s second-known ‘minimoon’ – 2020 CD3 – imaged by the Gemini North Telescope.

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Earth’s second temporary minim...

January 2021

Astronomy Now