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Making and Reporting Fireball Observations

 

gharw001r.jpg
The brilliant fireball of November 1, 2006, as seen from Hyde Park, London, in an oil-painting by Garry Harwood. © Garry Harwood, 2006. Click on the image to see a report on this fireball, and for links to more of Garry's work.

 

Contents

 

Introduction

A fireball is an especially bright meteor. By internationally-agreed definition, any meteor that attains magnitude -3 or more is classed as a fireball. An object this bright is more likely to be noticed by even casual witnesses outdoors at night - or rarely even in daytime. Sometimes, the meteoroid that produced a fireball may be large and robust enough to reach the Earth's surface still partially intact as one or more meteorites. If enough good-quality observations of such a fireball's path through the sky are made to allow an accurate estimate of its atmospheric flight, it may be possible to recover these surviving pieces.

Unfortunately, most fireballs are reported by single witnesses, and many are spotted by people with little experience of recording meteors, who may easily forget to note some items of possibly critical importance in the excitement of seeing a spectacular fireball. It is very easy for even highly experienced meteor watchers to do this too, so there is no shame attached! The following notes should help you make better records of any fireballs you see.

The SPA Meteor Section collects and analyzes data on fireballs seen primarily from the British Isles and places nearby. Send your sightings to meteor@popastro.com. If you observed a fireball from North America, please report it to the American Meteor Society instead. If your sighting was made from elsewhere in the world, or in all cases where you are not sure where to report it to (or even if what you saw was a fireball), please contact the Director for advice. A copy of your sighting should be sent to the International Meteor Organization's Fireball Data Center, FiDAC, too, at fidac@imo.net.

Essential items to record

A fireball sighting should be recorded immediately after the event. This is very important, as you may easily forget or accidentally exaggerate some details if you delay. The three essential points to give in your report are:

  1. Exactly where you were (give the name of the nearest town or large village and county if in Britain, or your geographic latitude and longitude if elsewhere in the world, as well as the place and country name);
  2. The date and timing of the event (please be sure to state clearly whether this was in Universal Time, UT, which is the same as Greenwich Mean Time, or in local clock time); and
  3. Where the fireball started and ended in the sky, as accurately as possible, or where the first and last points you could see of the trail were if you did not see the whole flight.

Estimating sky-positions

In order to try to reconstruct accurately what you saw, so it can be compared with what other witnesses elsewhere spotted, and thus attempt to triangulate to its atmospheric trajectory, it is absolutely vital you try to describe where the meteor was in the sky as clearly as you can. The key elements are to note the positions for the start and end of its visible trail, or the first and last points of that trail if you did not see the whole thing (which is quite common).

If the fireball was seen at night with plenty of stars visible, and you are familiar with the astronomical convention of using Right Ascension, RA, and Declination, Dec, to describe sky-positions, you can give these points in RA and Dec.

If the fireball was seen in daylight, or twilight too strong to see sufficient stars, or if you are not familiar with estimating RA and Dec positions, please use altazimuth positions instead.

  • Altazimuth system: The altazimuth system is a means of giving angular positions in the sky based on the true horizontal for where you were at the time. Any point in the sky can be determined by two quantities, the altitude or elevation in degrees measured vertically up from the horizontal, and the azimuth, literally "directions", in degrees measured around the sky parallel to the horizontal. Altitude starts at 0° at the horizontal and runs up to the zenith, or overhead point, at 90° elevation. Azimuth is measured from true north eastwards (that is, clockwise), thus 000° is due north, 090° is due east, 180° is due south, 270° is due west, and so forth. It is useful to give altitude values as two-digit numbers, and azimuth ones as three-digits to help you avoid confusion.

To help you in making these angular measures, the distance across the clenched knuckles of your hand held at arm's length from your eye gives an angle of about 10°. The distance between the outer edges of your thumb and fourth finger of your outspread hand, again held at arm's length, forms an angle of about 22°. A single fingertip at arm's length makes an angle of about 1°. You can test this smallest measure quite easily against the diameter of the full Moon, which is about half a degree across. If you measure the azimuth positions using a magnetic compass, please be sure to state this, as there is a discrepancy between true and magnetic north which sometimes needs to be allowed for. If you are concerned you may have given the positions incorrectly, you can easily allow a rough confirmation by also stating in your description the approximate compass directions, such as 'south-southeast', 'northwest', etc., as well as the azimuth.

Other items to note

The following elements are listed in a crude descending order of importance and/or their relatively lower chances of occurring with any given fireball.

  • Apparent speed: If you are reasonably familiar with meteor apparent velocities, you can use the usual 0 to 5 meteor-speed scale from "Observing Meteors", where 0 = stationary (the fireball appeared as a point of light, as it was heading directly towards you through the atmosphere; also called a point-source meteor), 1 = very slow, 2 = slow, 3 = medium speed, 4 = fast and 5 = very fast. The scale gets easier to use the more meteors you have seen. As compared to objects like distant aircraft or artificial satellites, even a very slow meteor can seem remarkably quick! If you have little or no experience of seeing meteors, you may find it easier to estimate how long the fireball was visible for in partial or complete seconds. Very few fireballs last more than 5 or 10 seconds, and only man-made re-entry fireballs are likely to last for several tens of seconds up to a couple of minutes. If the object was visible for longer than these times, it may not have been a fireball. Some objects which may be mistaken for fireballs are discussed below under the "Fireball or not?" section.
  • Trains: Some meteors, especially the faster and brighter ones, may leave a glowing ionization train along their trajectory after they have disappeared. If so, estimate how long this took to fade from view in seconds. Rarely, the train may last several minutes, when sketches could be made to show how it changed over time, or images taken. Very rarely, typically seen only with deeply-penetrating meteors, a dust or smoke train may be left along the object's flight path. The duration, appearance and colour should be noted if so.
  • Sounds: Noises may occur associated with some brighter fireballs. Sometimes these occur simultaneously with the meteor's flight, and are typically described as hissing, whooshing or crackling, though others have been reported. Although long dismissed as illusory, a tiny number of these electrophonic sounds have been recorded, so some are genuine. They may be due to very low frequency radio waves being detected audibly by some unknown mechanism, and are extremely rare. It is important to rule out as far as possible any terrestrial noises that happened by-chance at the same time as the meteor appeared, such as the wind in leaves or overhead wires, or even the sound of your hair brushing against your collar, but if you are convinced the sound probably did not have such a mundane cause, you should also send details from your report to the Global Electrophonic Fireball Survey. More common, but still very rare, are noises heard tens of seconds to a few minutes after the fireball has ended, usually rumblings or bangs. These are due to acoustic shock waves passing through the atmosphere, "sonic booms", often caused by severe detonation events in the fireball's flight. Again, terrestrial causes need to be ruled out where possible. In all cases, note down what sounds you heard, and what the approximate delay was between your seeing the meteor and hearing the sounds.
  • Fragmentation: Give details of any fragmentation the fireball showed, which could include a breakup into several smaller meteors (try to state their number if possible), or sparkling material seen along the track while the object was in flight, possibly due to molten material spraying off a rotating main body.
  • Colours: Describe any colours seen using only pure hues, that is: red, orange, yellow, green, blue, violet, white. So state "blue-green" not "turquoise", for example. The human eye is not sensitive enough to usefully tell what chemical components may have produced which colours, and of the spectroscopically-identified meteoric elements, unfortunately far too many are capable of generating common fireball colours, such as blue, green and yellow. Some of the colours result from atmospheric components anyway.
  • Magnitude: Try to give an estimate of how bright the event was. This is probably the most difficult thing to say with precision. In Observing Meteors there are some notes on estimating fireball brightnesses and a list of comparison objects which may be of help. Details from the comparison list are as follows. The planet Jupiter at its brightest reaches magnitude -2.9 (its opposition magnitude range is -2.3 to -2.9), thus any meteor as bright as Jupiter or brighter will be a fireball. The planet Venus at its most brilliant reaches about magnitude -4.8, but after that we have only the Moon and Sun as guides. The 4 or 24 day old crescent Moon is about magnitude -8; first or last quarter Moon is around magnitude -10; the 11 or 18 day gibbous Moon is roughly magnitude -11/-12; and full Moon is magnitude -13. The Sun is magnitude -26. Over-estimating a fireball's brightness is very common, so if in doubt, use a range of possible value, such as -7 to -9, without guessing at a single number.

Bolides and superbolides

In the meteor literature, you may come across these two terms. They both refer to very brilliant fireballs.

Bolide, from the Greek bolis, "missile" or "thrown spear", is a term not properly defined anywhere. Some authorities say it can be used for any bright meteor interchangeably with "fireball", others that the defining brightness is magnitude -9, still others that the meteor must be very bright and produce sounds (either acoustic or electrophonic). As a term in meteor astronomy, it is usually applied to bright, spectacular fireballs, with little regard for the specifics of the event.

A superbolide is a fireball-class meteor bright enough to be detectable from space by US Department of Defense, or similar, satellite sensors. Its defining minimum brightness is about magnitude -17. These are incredibly rare, but are visible even in broad daylight when they do appear. There is also a very good chance meteorites will result from such an event.

Fireball or not?

Not all very bright objects seen moving across the sky are fireballs. Three of the commoner objects mistaken for fireballs are aircraft lights, satellite flares and "sky-lanterns". Notes on how to help recognise them are given below.

  • Aircraft lights: Ordinarily, aircraft headlamps, searchlights, landing or cabin lights are not likely to be mistaken for anything else, but sometimes these can be less obvious. Repeat flashes from landing lights in a similar area of sky soon afterwards mean the object was not meteoric. Afterburners on a distant jet aircraft, where no sound from the vehicle was heard, can look like a point-source, or short-pathed, meteor or fireball, if in just the right orientation, and can sometimes seem to move across the sky leaving a trail or streak from the exhaust gases. In all cases, the movement is liable to be slower than most natural meteors, but the best way to tell what was seen is to check the object with good, firmly-held binoculars, as these should enable other tell-tale details to be spotted. If the object streaks across the sky and is gone before you can swing your binoculars into action, it may well have been a meteor after all!
  • Satellite flares: While the Sun is not far below the horizon, artificial satellites orbiting high above the Earth may be seen due to reflected sunlight. In winter, this situation occurs only for a few hours after sunset or before sunrise, but during the British summer when twilight never ceases, satellites can be seen throughout the night. Many of these are quite faint and continue to track over the sky for several minutes, so are quite unlike natural meteors, and could not be mistaken for them. However, there are also many Iridium communication satellites orbiting the Earth, and some others, which have solar panels that can reflect the Sun brilliantly, creating the impression of a dramatic, almost meteoric, flare. These have been recorded as reaching quarter-Moon brilliance, before fading to about magnitude +2 to +3, or disappearing entirely. Although as with all man-made objects orbiting the Earth, their apparent speeds are far below those of even the slowest natural meteors, the short-lived nature of their flares can sometimes make them much more difficult to tell apart from a genuine fireball's flare. If you are not sure whether what you saw was a satellite flare or a fireball, make an immediate record of what you saw as usual, taking particular care to note the exact time, and the flare's altitude and azimuth. Then as soon as possible, check the Heavens Above website to see whether any of the events listed there coincided with the event you observed. The Heavens Above site can also provide predictions for Iridium flares for any given location, which information can be used to make a few observations of these flare events to familiarise yourself with their appearance. This experience should make it easier to separate them from possible meteor flares in future.
  • Sky-lanterns: Sky-lanterns: These are small, roughly metre-high, paper hot air balloons, each powered by a burning wick suspended below it. Released singly or sometimes in groups to "celebrate" some event, and moving with the wind, they first started to appear as objects mistaken for meteoric fireballs in the summer of 2007 over Britain, and have recurred with increasing frequency ever since. They might occur at any time, but there has been a clear tendency for sighting numbers to rise significantly during holiday periods (notably in July to September, around October 31 to November 5, and from mid December to early January). They can look like literal "balls of fire", but typically are much slower-moving than meteors (natural meteors rarely last more than a few seconds, but lanterns can remain in-sight for several minutes). Colours commonly reported are yellow, orange or red. However, coloured or patterned balloon-bags may lead to other shades being seen too, including black, the latter either because of shadow effects, or printing on the balloon. Genuine fireballs never show black, only brilliant, radiant light. Sometimes the objects are clearly burning or flickering, occasionally dropping flaming or glowing material vertically below them, and they can also seem surprisingly bright for their size, because effectively, what is seen is something approximating to a rounded, metre-high candle flame, due to the illuminated balloon-bag. There have been sightings of a short, dark, smoky "tail" from some lanterns, not always along the object's line of motion (a meteor may also show a tail, but at night this will be generally bright, and have a streak-like appearance along the meteor's path). The fact lanterns can appear in groups has also led to numerous "UFO" or "mystery lights in the sky" press stories. In theory, lantern mass-releases are supposed to be notified to the UK aviation authorities in advance, but it is clear this does not always happen, nor is it obvious just how many sky-lanterns constitute a "mass-release". Worryingly, there have been growing numbers of emergency service alerts, especially to lifeboats, because of red or orange lanterns being mistaken for distress flares. To tell lanterns apart from meteoric fireballs or other astronomical targets like artificial satellites, again the best way is to examine them with good binoculars. This should enable an easy confirmation of what they were. Very few genuine meteors will remain visible for long enough to let you get binoculars into action and properly focused! More information about sky-lanterns and the confusion their appearance may cause has featured in the SPA's Electronic News Bulletins, ENBS, and on the Forums, several times. See for instance the notes and links in ENB 270.

In all cases, if you are not sure whether you saw a genuinely meteoric fireball or not, please send the Meteor Director as full a description as possible of what you saw, from where and when.

Reporting your sighting

There are two ways of reporting your fireball observation. You can send a written description using the notes above for guidance in an e-mail or by ordinary mail (please enclose an e-mail address valid for at least the next few weeks, or an SAE by post, if you would like a reply), or you can fill out and send in a copy of the Fireball Report Form. This provides a more formal framework for reporting exactly the same information for those who may find it helpful.

Once your report is ready, either send it to the Meteor Director by ordinary post (whose address is in the SPA's magazine Popular Astronomy), or by e-mail to meteor@popastro.com.

Notes on the more interesting or better-seen fireballs feature regularly in the ENBs and sometimes on the SPA Forums, while the Recent Fireball Sightings webpage has a regularly-updated list of fireball reports seen from the British Isles and nearby.

Alastair McBeath