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Asteroids.

Building rubble of the solar system

Asteroids (minor planets in the inner solar system) are the bits and pieces left over after the formation of the inner planets, including the Earth.

There are millions of these objects, and most are located in the main belt, between the orbits of Mars and Jupiter. ln the past, asteroids have collided with the Earth, significantly modifying our planet's biosphere, and will continue to do so in the future. They are also the source of most meteorites that have struck our planet's surface.

There are 187 main-belt asteroids larger than 100 km in diameter, and 474 that are larger than 50 km. The total mass of all the asteroids is about 4% of the mass of the Moon. The largest object in the asteroid belt is the dwarf planet Ceres, measuring 975 x 910 km. The next largest are the asteroids (2) Pallas and (4) Vesta, both about 530 km in diameter.

An asteroid is formally referred to as a number in brackets followed by a name. lf newly discovered, its temporary designation is made up of the year of discovery, two letters and, if need be, further digits, until a formal name is agreed upon. As of this writing (2018 September) there are 783 768 known asteroids, of which 523 584 are numbered.

Orbits of the asteroids

An intriguing group of asteroids, known as the Centaurs, is found orbiting between the main giant planets. These include (2060) Chiron (also known as comet 95P Chiron) orbiting between Saturn and Uranus; (5335) Damocles with an orbit ranging from near Mars to beyond Uranus; and (5145) Pholus, with perihelion less than Saturn's and aphelion greater than Neptune's. They are thought to be objects in transition between the outer and inner solar system bodies and are all potential comets (see Figure 18 on p 80).

Some asteroids move in orbits that bring them close to Earth, creating potentially hazardous encounters. June 30 is recognized annually as International Asteroid Day, a global event to raise awareness about asteroids and what can be done to protect the Earth.

The so-called near-Earth objects (NEOs) approach the Earth within 1.3 Astronomical Units (AU). The largest known NEO is (1036) Ganymed, a 31.7-km-diameter asteroid with an orbital period of 4.34 years. About 2 000 known NEOs approach within 0.04 AU of the Earth, the largest of which is (4179) Toutatis, measuring 4.6 x 2.4 x 1.9 km. Close approaches predicted in 2019 are listed in Table 6. More than 200 asteroids are known to cross the Earth's orbit. (3200) Phaethon passes closer to the Sun (within 0.14 AU) than any other known object except for the occasional comet. As of this writing (2018 September) there are 18 562 known near-Earth asteroids (of which 895 are one kilometre or larger) and 107 near-Earth comets.

Observing asteroids

Though they are rarely visible with the unaided eye, many asteroids are visible through binoculars or small telescopes. As the name "asteroid" suggests, they appear starlike in a telescope. One way to distinguish an asteroid from a star is by its motion, typically 30 arcseconds per hour relative to the background stars.

Asteroids are usually much closer to us at opposition than at other times, and are therefore brighter and easier to see. Main-belt asteroid (4) Vesta (525 km diameter, 3.63 year orbital period) reaches opposition on November 12 (magnitude +6.5) at a distance of 1.56 AU. lt is at solar conjuction on March 07 (distance 3.34AU) and is stationary on September 25. During the year it lies in Aquarius, Aries, Capricornus, Cetus, Pisces and Taurus.

The irregular shape and rough surfaces of asteroids are evident in the detectable variation of reflected sunlight as seen from Earth. This variation, caused mainly by rotation, is usually repeated after several hours. Rotational periods are found to be between five and eight hours, although a few have rotational periods in excess of 24 hours.

Minor planet occultations

Occasionally, an asteroid may move in front of a star as seen from Earth, an event known as an occultation. Occultations of selected bright stars during 2019 are shown in Table 7. Such an event is of great interest, since the size and shape of the asteroid can then be measured. The duration of these occultations rarely exceeds 30 seconds. lf the minor planet has a moon, it may be seen (or even discovered) during such an occultation.

Asteroid (4) Vesta occults UCAC5 491-004294, a magnitude +12.1 star, on November 27 at 01:38. The duration of the occultation is 47.2 seconds.

Asteroid (3) Juno occults two stars during the year: UCAC5 458-004988 on January 24 at 19:51 and UCAC5 506-009319 on March 22 at 20:31.

Caption: Moon

Caption: Ceres

Caption: Pallas

Caption: Juno

Caption: Vesta
Table 6. NEO Earth close approaches during 2019

NEO designation   Date of closest   Close approach   Close approach
                                                     velocity (km/s)
                  Earth approach    LD     AU

(2012 KT12)       May 18            1.00   0.00257   3.95
(2016 NO56)       Jul 07            1.00   0.00258   12.2
(2016 GE1)        Apr 04            1.11   0.00285   10.13
(2015 EG)         Mar 04            1.15   0.00295   9.64
(2017 KP27)       Sep 26            2.03   0.00522   4.8
(2013 YM2)        Jan 09            6.21   0.01596   4.28
(2017 PV25)       Feb 12            7.09   0.01821   6.09
(2017 SL16)       Sep 21            7.49   0.01925   6.47

NEO designation   Estimated
                  diameter

(2012 KT12)       15-33 m
(2016 NO56)       20-44 m
(2016 GE1)        13-28 m
(2015 EG)         19-43 m
(2017 KP27)       18-41 m
(2013 YM2)        15-34 m
(2017 PV25)       32-71 m
(2017 SL16)       18-41 m

Key: LD: Lunar Distance, average distance between the Earth
and Moon, about 0.00257 AU.

Table 7. Asteroid occultations of bright stars

Date     Time    Asteroid        Star               Mag     Duration

Jan 10   23:04   1998WY31        UCAC5 572-029554   10.69   6.9
Jan 27   02:57   469 Argentina   UCAC5 604-040205   10.82   10.2
Feb 02   02:23   980 Anacostia   TYC 5503-00114-1   9.84    7.4
Feb 19   21:14   276 Adelheid    TYC 5414-01843-1   10.37   11.6
May 12   18:09   958 Asplinda    TYC 5527-00524-1   11.05   5.9
May 22   02:31   914 Palisana    TYC 6282-00100-1   9.48    7.1
Jun 06   23:46   514 Armida      UCAC5 330-088505   10.98   8.7
Jun 19   04:38   72 Feronia      TYC 0009-00704-1   10.84   3.6
Jun 21   03:27   605 Juvisia     UCAC5 200-171076   8.95    6.5
Jun 23   03:26   79 Eurynome     TYC 6265-02492-1   9.81    5.5
Jul 07   21:28   410 Chloris     TYC 6245-00158-1   10.49   20.7
Jul 10   05:20   379 Huenna      TYC 6307-01055-1   8.84    9.1
Jul 15   21:40   373 Melusina    TYC 7964-00671-1   10.76   9.8
Jul 24   21:04   965 Angelica    TYC 5549-00888-1   10.90   3.3
Aug 02   20:49   1877 Marsden    TYC 7840-01158-1   10.48   5.3
Sep 21   04:39   423 Diotima     TYC 5271-00018-1   10.41   17.2
Oct 18   21:47   410 Chloris     UCAC5 307-228929   10.04   4.5
Oct 21   19:49   1114 Lorraine   UCAC5 391-110331   10.92   3.1
Dec 29   21:51   10 Hygiea       UCAC5 563-007362   10.81   48.9

Key: Mag: V magnitude of the occulted star. Duration: Duration of
the event (seconds).
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Publication:Sky Guide Africa South
Date:Jan 1, 2019
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