367943 Duende

367943 Duende (provisional designation ) is a micro-asteroid and a near-Earth object of the Aten and Atira group, approximately 30 m in diameter. It was discovered by astronomers of the Astronomical Observatory of Mallorca at its robotic La Sagra Observatory in 2012, and named for the duende, a goblin-like creature from Iberian and Filipino mythology and folklore. Duende is likely an uncommon L-type asteroid and significantly elongated. For an asteroid of its size, it has a relatively long rotation period of 9.485 hours.

On 15 February 2013, Duende passed at a record distance of 27,700 km or 4.3 Earth radii from Earth's surface. Due to its close passage, its orbit was perturbed significantly enough that it changed from an Apollo asteroid to an Aten asteroid. Duende passage also coincided with the completely unrelated Chelyabinsk meteor, which entered Earth's atmosphere above Russia just 16 hours earlier.

Discovery and past risk assessments
Duende was discovered on 23 February 2012, seven days after passing 0.0174 AU from Earth, by the La Sagra Observatory in Granada Province, Spain, using a 0.45-m reflector which was remotely operated by amateur astronomers at the Astronomical Observatory of Mallorca.

The still relatively imprecise orbit deduced from the short arc of the 2012 observations already made clear that Duende would pass no closer to Earth's surface than 3.2 Earth radii during its 2013 passage. There was at the time, however, a cumulative risk of 0.033% (1 in 3,030) that Duende would impact Earth during one of its 2026 to 2069 approaches.

2013 passage
On 9 January 2013, Duende was observed again by Las Campanas Observatory and the observation arc immediately increased from 79 days to 321 days. On 15 February 2013 at 19:25 UT, Duende passed 0.0002276 AU from the center of Earth, with an uncertainty region of about 0.0000001 AU.

It passed 27743 km above Earth's surface, closer than satellites in geosynchronous orbit. It briefly peaked at an apparent magnitude of roughly 7.2, a factor of a few fainter than would have been visible to the naked eye. The best observation location for the closest approach was Indonesia. Eastern Europe, Asia, and Australia also were well situated to observe Duende during its closest approach.

Duende was not expected to pass any closer than 1950 km to any satellites. Goldstone Observatory observed Duende with radar from February 16 to February 20. Radar observations showed that it is an elongated asteroid with dimensions of 20 x. This gives Duende a geometric mean (spherical) diameter equivalent to 28 m.

During the close approach, an observational campaign involving 5 different telescopes in 4 different observatories was carried on in order to get information on the physical properties of this NEO.

Visible and near-Infrared photometry and visible spectroscopy were obtained at Gran Telescopio Canarias, Telescopio Nazionale Galileo and Calar Alto Observatory and put together. The classification using the M4AST online tool says this is an L-type asteroid. Those peculiar asteroids are characterized by a strongly reddish spectrum shortward of 0.8 μm, and a featureless flat spectrum longward of this, with little or no concave-up curvature related to a 1 μm silicon absorption band. Time-series photometry was also obtained in the Observatorio de La Hita (I95) and Observatorio de Sierra Nevada during two consecutive nights (15–16 February 2013). All of this data were co-phased to build a lightcurve of the object. This lightcurve is double-peak and presents large variations in magnitude, implying a very elongated object, which is compatible with radar observations. The amplitude of the lightcurve yields an axial ratio that, together with the long axis of 40 m inferred from the radar images by Goldstone, results in an equivalent diameter of 18 m, much smaller than the estimations before the close-approach.

The rotational period was precisely determined from the lightcurve obtaining a value of $9.526 AU$. This value is confirmed with an analysis of all the photometry of this objects reported to the Minor Planet Center. Using data pre and post close approach the authors find that the object suffered a spin-up during the event that decreased the rotational period from $20 m$ down to $18 m$ hours, which is compatible with the more accurate value estimated from the light-curve.



Orbital shift
The close approach to Earth reduced the orbital period of Duende from 368 days to 317 days, Its aphelion was reduced from 1.110 to 0.9917 AU, leaving it almost entirely inside Earth's orbit and perturbing it from the Apollo class to the Aten class of near-Earth asteroids.

Its next close approach to Earth will be on 15 February 2046, when it will pass about 0.0148 AU from Earth. Based on 7 radar observations, the next close approach to Earth similar to the 2013 passage will be on 16 February 2123, when Duende will pass no closer than 0.0002 AU from the center of Earth. For the 2123 passage, the nominal pass will be 0.003 AU from the center of the Moon and then 0.005 AU from the center of Earth.

Risks


Risk assessments calculated before the 2013 passage were based on a diameter of 45 meters and a mass of 130,000 tonnes. It was estimated that, if it were ever to impact Earth, it would enter the atmosphere at a speed of 12.7 km/s, would have a kinetic energy equivalent to 2.4 megatons of TNT, and would produce an air burst with the equivalent of 2.1 megatons of TNT at an altitude of roughly 10.1 km. The Tunguska event has been estimated at 3–20 megatons. Asteroids of approximately 50 meters in diameter are expected to impact Earth once every 1,200 years or so. Asteroids larger than 35 meters across can pose a threat to a town or city, and the Chelyabinsk meteor which serendipitously occurred on the day of the 2013 passage was due to a 17-meter asteroid. As a result of radar observations, it is now known that Duende is only about 30 meters in diameter. The table above uses Sentry's stony asteroid density of 2600 kg/m3, Sentry's atmospheric entry velocity (Vimpact) of 12.7 km/s, and an angle of 45 degrees.
 * The uncertainty region of Duende during planetary encounters is now well determined through 2123.
 * Duende was therefore removed from the Sentry Risk Table on 16 February 2013.
 * It is estimated that there are more than a million near-Earth asteroids smaller than 100 meters.

For kinetic energy at atmospheric entry, 3.3 Mt is equivalent to DF-4, 9 Mt is equivalent to Ivy Mike and 15.6 Mt is equivalent to Castle Bravo. For air burst energy, 530 kt is equivalent to W88 and 2.9 Mt is equivalent to R-12 Dvina.

Naming
This minor planet was named after the duende, fairy- or goblin-like mythological creatures from Iberian, Latin American and Filipino folklore. The approved naming citation was published by the Minor Planet Center 17 November 2013 (M.P.C. 85916).