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Meteors.

What is a meteor?

A meteor or "shooting star" is produced when an interplanetary dust particle (meteoroid) enters the Earth's atmosphere and deflagrates, leaving a train of excited and ionized particles along its path. If sufficient light is produced by the subsequent de-excitation, as well as various ionic processes, the meteor may be observed visually. Meteors can also be detected using techniques that rely on the scattering of radio signals by free electrons in their wakes.

The typical visible meteor is about the size of a grain of sand; a meteor the size of a grape produces a very memorable fireball. In general, meteors appear in the upper atmosphere, at an altitude of between 80 and 120 km and disappear between 60 and 80 km above the Earth.

Many meteors are sporadic, but many more are members of showers. The origin of sporadic meteoroids is still uncertain. They are probably very minor streams of low particle density, the remains of minor streams that have long dispersed, as well as solitary particles travelling in isolation. Under ideal conditions, about 10 sporadic meteors can be seen per hour.

Meteor showers

A meteor shower occurs when the Earth intersects a stream of meteoroids that has formed as a result of the disintegration of a larger body (usually a comet but occasionally an asteroid). Thus each shower recurs at the same time each year, and the meteors seem to radiate from a point (the radiant) corresponding to the direction from which the meteoroids approach Earth.

Table 9 lists meteor showers in need of observation during 2019. The date of maximum may vary by more than a day or so either side of that listed. Note that for the Alpha Monocerotids, which showed outbursts in the years 1925, 1935, 1985 and 1995, the possibility of enhanced activity exists again in 2019. Observers should be on the lookout for higher than normal rates, which if they materialise, will be of short duration. The 1995 outburst lasted just half an hour, when rates briefly reached ZHR=500.

Meteorites and impact craters

Most meteors vaporize during their descent, but a few survive and strike the Earth and are called meteorites. Once a meteorite lands on South African territory it belongs to the State (Act 25 of 1999) and is protected by the South African Heritage Resources Agency. Trading in South African meteorites is illegal.

A number of significant meteorite falls have been recorded in southern Africa. The Hoba Meteorite, located on a farm 18 km from Grootfontein in northern Namibia, is claimed to be the largest in the world. Believed to have fallen some 80 000 years ago, this 60-ton iron-nickel meteorite measures 3 m x 3 m x 1 m. lt was declared a National Monument in 1920. On 2002 July 21, at about 15:49, a meteor exploded over Lesotho, producing a strewn field on the Thuathe Plateau, about 12 km east of Maseru. The event was widely witnessed as a daytime fireball and at least 500 fragments were recovered with a total mass of about 30 kg. lt is estimated that on entry the meteoroid weighed about a ton. The most recent meteorite fall in southern Africa happened on 2018 June 02 at about 18:44, when asteroid 2018 LA collided with the Earth and exploded over Botswana. The asteroid was first detected eight hours before impact. The detonation was widely observed and captured on numerous security cameras. The potential strewn field location was triangulated from these video records, following which a search of the identified location led to the recovery of a single fragment (so far) weighing just under 18 grams.

The Vredefort structure (a UNESCO World Heritage Site since 2005) is currently regarded as the oldest and largest clearly visible impact structure on Earth, formed when a gigantic meteorite (larger than Table Mountain) hit the Earth some 2 billion years ago. The original crater is estimated to have been 250 - 300 km in diameter. The central uplift is near Parys in the Free State and the outer wrinkle is the Braamfontein Ridge in Johannesburg.

The Tswaing (Soutpan) Meteorite Crater is located about 40 km north of Pretoria, west of Hammanskraal. The impactor may have been a stony meteorite some 40 m in diameter. The Tswaing site is open to the public, and has an interpretive centre.

Caption: Figure 20. Three-image composite of the Vredefort Dome (30 km ground resolution) taken 2017 August 03 with the SCS Gecko lmager on the South African nSight-1 nanosatellite. lmage: SCS Aerospace Group.
Table 9. Meteor showers observable in 2019

Shower                 Date     Duration         Radiant
                       max.                      R.A.

[alpha] Crucids        Jan 19   Jan 06--Jan 28   12h 48m
[alpha] Centaurids     Feb 07   Jan 28--Feb 21   14 00
[gamma] Normids        Mar 13   Feb 25--Mar 22   16 36
[delta] Pavonids       Apr 06   Mar 11 -Apr 16   20 32
April Lyrids           Apr 22   Apr 16 -Apr 25   18 05
[pi] Puppids           Apr 23   Apr 15 -Apr 28   07 20
[eta] Aquariids        May 06   Apr 21--May 12   22 24
[theta] Ophiuchids     Jun 13   Jun 08--Jun 16   17 48
June Lyrids            Jun 16   Jun 11-Jun 21    18 32
July Phoenicids        Jul 13   Jul 10--Jul 16   02 08
Piscis Australids      Jul 28   Jul 19 -Aug 17   22 40
S. [delta] Aquariids   Jul 29   Jul 21-Aug 29    22 36
[alpha] Capricornids   Jul 30   Jul 15 -Aug 25   20 28
Orionids               Oct 21   Oct 02--Nov 07   06 20
Southern Taurids       Nov 05   Oct 01- Nov 25   03 20
Northern Taurids       Nov 12   Oct 01- Nov 25   04 00
Leonids                Nov 17   Nov 12--Nov 21   10 08
[alpha] Monocerotids   Nov 21   Nov 15--Nov 25   07 48
Dec. Phoenicids        Dec 06   Dec 03--Dec 09   01 12
Geminids               Dec 14   Dec 04--Dec 16   07 28
Puppid-Velids          Dec 29   Dec 05--Jan 07   09 56

Shower                                ZHR      Vel.   Watch
                       Dec                            Start

[alpha] Crucids        -63[degrees]   <5       50     00:00
[alpha] Centaurids     -59            5        60     22:00
[gamma] Normids        -51            8        56     00:00
[delta] Pavonids       -63            5        59     02:00
April Lyrids           +34            15       49     02:00
[pi] Puppids           -45            <5       18     19:00
[eta] Aquariids        -02            60       65     03:30
[theta] Ophiuchids     -20            5        27     20:00
June Lyrids            +35            5        31     23:30
July Phoenicids        -48            <5       47     23:00
Piscis Australids      -30            5        35     21:30
S. [delta] Aquariids   -16            25       42     22:00
[alpha] Capricornids   -10            5        25     20:00
Orionids               +16            30       68     00:00
Southern Taurids       +14            10       29     21:30
Northern Taurids       +23            5        31     21:30
Leonids                +22            5-10     70     03:00
[alpha] Monocerotids   +01            5-50     65     23:00
Dec. Phoenicids        -53            5        22     20:30
Geminids               +33            50       36     23:30
Puppid-Velids          -51            5        40     22:30

Shower                         Observing
                       End     prospect

[alpha] Crucids        03:30   Full Moon
[alpha] Centaurids     03:30   Favourable
[gamma] Normids        04:30   Good
[delta] Pavonids       04:30   Favourable
April Lyrids           05:00   Unfavourable
[pi] Puppids           23:00   Good
[eta] Aquariids        05:30   Good
[theta] Ophiuchids     05:30   Unfavourable
June Lyrids            02:00   Full Moon
July Phoenicids        05:00   Poor
Piscis Australids      05:00   Good
S. [delta] Aquariids   05:00   Good
[alpha] Capricornids   04:00   Favourable
Orionids               04:00   Poor
Southern Taurids       03:30   Good
Northern Taurids       03:30   Full Moon
Leonids                04:00   Poor
[alpha] Monocerotids   04:00   Good
Dec. Phoenicids        02:00   Poor
Geminids               03:00   Unfavourable
Puppid-Velids          03:30   Favourable

Key: Date max.: Meteor shower maxima may vary one or two days
either side of the predicted date. Prospective observers should
observe on a range of dates centred on the listed date. Radiant:
The radiant of most showers drifts slightly eastward each night.
The position given is for night of maximum. ZHR: Zenithal hourly
rate; expected maximum rate under observing conditions when stars
of 6.5-mag. can be seen and with the radiant at the zenith. Rates
under poorer conditions and when the radiant is low will
consequently be lower. Vel: Velocity (km x [s.sup.-1]).

Watch: Recommended watch times. Observing prospect: Showers listed
as "Favourable" have the best prospects for observation. Those
listed as "Good" may be observed with slight hindrance from the
Moon.

Impact craters in southern Africa

Name and location                       D      Age

Kalkkop (Eastern Cape, South Africa)    0.64   0.25
Kgagodi (Central Botswana)              3.50   < 180
Luizi (Shaba, DRC)                      17     < 575
Morokweng (North West, South Africa)    70     145.0 [+ or -] 0.8
Roter Kamm (Luderitz, Namibia)          2.5    3.7 [+ or -] 0.3
Tswaing (North West, South Africa)      1.13   0.220 [+ or -] 0.052
Vredefort (Free State, South Africa)    300    2023 [+ or -] 4

Key: D: Diameter (km). Age: millions of years
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Publication:Sky Guide Africa South
Date:Jan 1, 2019
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