Temporarily Captured Orbiters (a.k.a. Mini-moons)
My interest in searching for serendipitous images of meteoroids branched unexpectedly when it became obvious that some meteoroids impact our atmosphere at very low speeds. The minimum speed that a solar system object will hit our atmosphere is approximately 11.2 km/s. This corresponds to the speed an object would have if you held it stationary with respect to the Earth at a very large distance (say the distance to the Moon or more), and let it "drop" to the Earth. Anything less than 11.2 km/s means the object came from the Earth. Objects that have a verly low relative velocity to the Earth, i.e. objects orbitting the Sun in orbits very similar to ours, may enter into "orbits" around the Earth. These orbits are less stable than the traditional orbits we think of, and can be very chaotic. Most such objects stay close to Earth for a short peiod of time and then return to a heliocentric (sun-centred) orbit. A few hit the Earth. On January 13, 2014, cameras in the Czech Republic observed such an object as a fireball. After some modelling of the atmospheric behaviour of the object by Pavel Spurný and Jiří Borovička, I calculated the possible trajectories of the object; what was found was very interesting....
Paper: Clark, David L.; Spurný, Pavel; Wiegert, Paul; Brown, Peter; Borovička, Jiří; Tagliaferri, Ed; Shrbený, Lukáš (2016) Impact detections of temporarily captured natural satellites, The Astronomical Journal, Volume 151, Issue 6, article id. 135,
Abstract: Temporarily Captured Orbiters (TCOs) are Near-Earth Objects (NEOs) which make a few orbits of Earth before returning to heliocentric orbits. Only one TCO has been observed to date, 2006 RH120, captured by Earth for one year before escaping. Detailed modeling predicts capture should occur from the NEO population predominantly through the Sun-Earth L1 and L2 points, with 1% of TCOs impacting Earth and approximately 0.1% of meteoroids being TCOs. Although thousands of meteoroid orbits have been measured, none until now have conclusively exhibited TCO behaviour, largely due to difficulties in measuring initial meteoroid speed with sufficient precision. We report on a precise meteor observation of January 13, 2014 by a new generation of all-sky fireball digital camera systems operated in the Czech Republic as part of the European Fireball Network, providing the lowest natural object entry speed observed in decades long monitoring by networks world-wide. Modeling atmospheric deceleration and fragmentation yields an initial mass of ~5 kg and diameter of 15 cm, with a maximum Earth-relative velocity just over 11.0 km/s. Spectral observations prove its natural origin. Back-integration across observational uncertainties yields a 92 - 98% probability of TCO behaviour, with close lunar dynamical interaction. The capture duration varies across observational uncertainties from 48 days to 5+ years. We also report on two low-speed impacts recorded by US Government sensors, and we examine Prairie Network event PN39078 from 1965 having an extremely low entry speed of 10.9 km/s. In these cases uncertainties in measurement and origin make TCO designation uncertain.
Highlighted on The American Astronomical Society's NOVA page:
A capture image of EN130114 (Digital autonomous observatory at Kunžak) with spectrum (L. Shrbený)
The Eureka animation:
When animating the integrating the possible trajectories of EN130114, the interaction with the Moon immediately jumped out.