187J. Br. Astron. Assoc. 118, 4, 2008

For many years, our traditional view of asteroids has beenlimited to those objects orbiting between the paths of Marsand Jupiter now known as the Main Belt. But within just thelast 15 years or so, our perspective has radically changedwith the discovery of many bodies occupying theEdgeworth−Kuiper belt and other so-called Scattered Diskobjects, the orbits of which extend well beyond the conven-tional confines of the solar system previously delineated bythe planets Neptune and Pluto. How did this come to pass?In this paper, I shall describe the evolution of our knowledgeof this system of minor planets, not only to provide you withsome answers to this question but also to illustrate how ourunderstanding of these mysterious objects has developedand how it might expand in the future. In so doing I shall alsotrace those activities which involved the BAA and moreparticularly the Asteroids and Remote Planets Section.

The early history of asteroiddiscovery

The tale of how asteroids were first discovered has beentold many times. It harks back to the latter years of the 18thcentury when scientific enquiry was developing apace in

many European countries. In particular, the last two decadesof the century witnessed an unprecedented increase in thepopularity of telescopic observation as a means of extend-ing our knowledge of celestial phenomena. William Herscheldiscovered the planet Uranus in 1781, and thanks to him andseveral other keen observers, many new comets were foundas were several new types of variable star, and companionsatellites were seen to circle both Saturn and Uranus.

As the 18th century drew to a close, astronomers includ-ing Wilhelm Olbers and Johannes Bode, each of whom hadalready discovered a comet, postulated the existence of nu-merous lesser bodies within the solar system. With this ideain mind, and at the behest of Baron von Zach, a group met on1800 September 20 in Lilienthal, Germany, and formed a soci-ety of 24 practical astronomers distributed throughout Eu-rope. The express purpose of this group, which came to beknown as the ‘Celestial Police’, was to investigate the regionof space between Mars and Jupiter close to the ecliptic,searching for new planets. Coincidentally and by chance,Giuseppe Piazzi, who was working on a star catalogue atPalermo Observatory, Italy, came across a moving star on1801 Jan 1, which he realised might be different from a comet,possibly another planet similar to Georgium Sidus (Uranus).Subsequent observations indeed showed it to be a smallplanet, and that it occupied the space between Mars andJupiter close to the distance where the Titius−Bode Law

predicted a planet to be. The objectwas named Ceres Ferdinandea, in hon-our of King Ferdinand IV of Sicily, andlater became known more simply asCeres. A learned account of the dis-covery of Ceres can be found in thebook Asteroids III written following theinternational conference held inPalermo in 2001 June.1

By 1807, thanks to the activities ofthe Celestial Police three further aster-oids were found, namely Pallas andVesta discovered by Wilhelm Olbers in1802 and 1807, and Juno by Karl Hardingin 1804. However, there the story re-mained and no others were found for afurther four decades. Books on as-tronomy during those times referred tothe solar system as comprising the con-ventional system of seven known plan-ets together with the first four asteroids,Ceres, Pallas, Juno and Vesta, thoughtby some to be the remains of a disrupted

AsterAsterAsterAsterAsteroids:oids:oids:oids:oids: past, past, past, past, past, pr pr pr pr present and futuresent and futuresent and futuresent and futuresent and futureeeeeRichard Miles

The 2007 BAA Presidential Address, delivered on 2007 Octo-ber 31 at the Geological Society Lecture Room, BurlingtonHouse, Piccadilly, London W1

Figure 1. Giuseppe Piazzi, discoverer of the first asteroid, with the title page of his Dellascoperta del nuovo pianeta Cerere Ferdinandea... (Dibner Library of the History of Science &Technology). Inset: HST image of 1 Ceres. (NASA/STScI)

188 J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

planet. More than two centuries on, we appear to have turnedfull circle as we now can anticipate the forthcoming visit totwo of these bodies by the space probe Dawn, launched on2007 September 27. Powered by a futuristic xenon ion drivethis probe is due to reach Vesta in 2011 August where it willremain in orbit until 2012 May before going on to visit Ceresin 2015 February.

By studying these two minor planets in unprecedenteddetail it is hoped that new light will be shed on the nature ofthe early solar nebula and the formation of the solar system.These advances will be a far cry from those early years of the19th century when, apart from their orbit (all four remainedbetween the orbits of Mars and Jupiter), little of fact wasknown about them. Herschel coined the term ‘asteroid’ inpart to distinguish them from the major planets. Attemptswere made to measure their angular diameter and size butthese remained highly speculative in that different observ-ers reported widely differing values. We now know that theirapparent size as seen from the Earth is smaller than the see-ing disk of stars as seen with those early telescopes. Schröterand others even claimed that they were surrounded by anatmosphere which varied in extent,2 which we now know isnot the case.

Asteroid discovery during the19th and 20th centuries

A long interval of almost 40 years accumulated without anyfurther finds until in 1845 Karl Hencke, a postmaster and ama-teur astronomer in Driesen, Prussia, discovered Astraea aftermore than a decade of intensive searching, followed in 1847

by Hebe.3 This advance reawak-ened interest in the subject. JohnHind, observing from GeorgeBishop’s observatory at Re-gent’s Park, London, discoveredIris and Flora also in 1847.4 Al-though a few keen amateurs weresuccessful in finding a handfulof new objects (the first ten as-teroids having been found by1849), the real advance in the rateof discovery was made possiblebeginning in the 1850s andthereafter when detailed starcharts became available, mostnotably the extensive BonnerDurchmusterung, comprisingthe positions of more than324,000 stars.5 Figure 3 illustratesthe accelerating rise in the dis-covery rate which took placethroughout the 19th century.

As more and more new as-teroids were identified, it becameclear that these were all located

between Mars and Jupiter, and this region became known asthe ‘asteroid belt’. An illustration of the distribution of whatwe now term the ‘main-belt’ asteroids is shown in Figure 4.Many hundreds of objects were found circulating close tothe plane of the ecliptic, confined there in part by the gravi-tationally dominant planet Jupiter. Beginning in 1891, pho-tography proved an effective means for identifying movingobjects such as asteroids and comets. Although asteroidsto all intent and purposes look entirely stellar when observedvisually, they can be easily differentiated by their apparentmotion relative to the fixed stars, in that they appear as ashort streak or trail on a photographic plate that has beenexposed for several hours whilst the telescope, on which thecamera is mounted, tracks the stars at sidereal rate.

As more and more were found it became clear that theirorbits were increasingly varied, such that in 1898 the veryfirst object was found which crossed the orbit of Mars andapproached the Earth quite closely. 433 Eros was discovered

Figure 2. William K. Hartmann’s artistic depiction of NASA’s Dawn space probe and the early solarsystem. (NASA)

Figure 3. Discovery statistics based on data available from theMinor Planet Center (see http://www.cfa.harvard.edu/iau/lists/NumberedPerYear.html)

189J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

during photographic searches by Gustav Witt in Berlin andindependently by Auguste Charlois in Nice. The unusualnature of Eros’ orbit was soon recognised at the time, sinceit is the first example of that special class of objects whichcan approach the Earth to within a distance of 0.2 AU or less,and which are now appropriately termed ‘near-Earth aster-oids’ or NEAs.

Between 1900 and 1959, the rate of discovery remainedroughly constant at about 32 objects per year, to reach atotal of 2,321. It was during this period when photographywas the prime astronomical tool that asteroids acquired thenickname ‘vermin of the skies’. The tell-tale sign of an as-teroid was the streak produced on a long exposure photo-graph, which was regarded by some as a distraction from

an otherwise perfect image: hence the nickname. Also atthis time, sufficient numbers had been discovered that theirorbital characteristics were able to be looked at from a sta-tistical standpoint. Groupings or families were recognised,for example sharing similar orbital inclinations and eccen-tricities, and these have therefore been interpreted as be-ing fragments from past asteroid collisions. Often there isone large body, such as in the case of Vesta, Pallas, Floraand Hygeia, accompanied by a swarm of much smaller as-teroids with similar orbits as shown in Figure 5, which isindicative of violent collisions occurring within the aster-oid belt in the distant past.

As a footnote to history, it should be mentioned that al-though Kiyotsugu Hirayama was the first to investigate theexistence of such asteroid families in a number of papersbeginning in 1918,6 William Monck, a founder member (No.12)of the BAA, had previously referred to the existence of smallgroups of asteroids for which he coined the term ‘family’back in 1888.7

Following the publication in 1960 of the Palomar−LeidenSurvey, which employed the 1.2m Schmidt to study a 12°×18°area of sky and discovered more than 2,400 small asteroidsdown to 20th magnitude, it was recognised that asteroidsgrow increasingly numerous at smaller and smaller sizes.However it was impractical to scan the entire sky in search ofasteroids down to sizes of a few kilometres using conven-tional plate photography. The revival of asteroid discoveryhad to wait for the application of CCD cameras and comput-ers to the task, for which they were eminently suited. TomGehrels established the first such concerted search pro-gramme using the 0.9m Spacewatch telescope on Kitt Peak,with an early CCD camera operating in drift-scan mode.8 From1992 to 1995, Spacewatch observed and automatically de-tected more than 60,000 asteroids down to a limiting magni-tude of about V=21 in an area of almost 4,000 sq.deg of skynear the ecliptic. In 1997, the Lincoln Near-Earth AsteroidResearch (LINEAR) project began and some ten years onhas discovered well in excess of 200,000 objects, of whichmore than 2,000 are NEAs and more than 200 are comets.

One consequence of the virtual explosion in the discov-ery rate during the last decade has been to make it extremelydifficult for amateur astronomers to discover asteroids them-selves. The automated surveys have swept the skies cleardown to 18th magnitude at least. To picture how the scenehas changed, consider Figure 6, which depicts the area ofsky in the vicinity of Messier 44, the Beehive Cluster, asseen on 2007 October 31 (the date of this address). Includedon the upper chart are all those asteroids which were knownat the time when the BAA Minor Planets Section was formedin 1984. In contrast the lower chart includes all asteroidsknown to date and clearly illustrates the extensive changethat has taken place during the intervening years. BeforeLINEAR began operations, BAA member Brian Manning,using photography, was able to discover or recapture some28 asteroids, breaking UK observer John Russell Hind’srecord of the 19th century.9 The most prolific UK amateurdiscoverer to date has been Peter Birtwhistle, observing fromGreat Shefford in Berkshire. Using 0.3m and more recently0.4m telescopes and a high-performance CCD camera, he

Figure 4. Illustration showing the orbits of main belt asteroids(shown in dark blue) and some comets. Adapted from an illustrationby Pedro Lacerda, Univ. of Hawaii and Univ. of Coimbra (see http://www.solstation.com/solsys/mb2comet.jpg)

Figure 5. Plot of proper inclination vs. eccentricity for currentnumbered asteroids (adapted from http://en.wikipedia.org/wiki/Image:Asteroid_proper_elements_i_vs_e.png)

190 J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

has found 82 new objects to date as serendipitous discover-ies, largely whilst engaged in imaging newly-discoveredNEAs. By 2007 October 31, a grand total of 387,205 newobjects had been designated of which just 14,148 have beennamed. One such name of relevance to the Association isthat of asteroid 4522 Britastra, named by the discoverer, pro-fessional astronomer and BAA member Ted Bowell, to com-memorate the BAA centenary in 1990.

The expanding solar system

The exploration of the solar system by observation fromhere on Earth has been rather akin to the investigation of anear-impenetrable jungle in which more and more strangeexotic creatures are discovered. The longer the search con-tinues, the more aware and informed we become as to themultiplicity of animals which exist but which remain largelyhidden from sight. Although Percival Lowell began his searchin 1905 for the fabled entity Planet X, the landmark discoverytook until 1930 when after a long and protracted search ClydeTombaugh found what was believed to be the ‘ninth planet’,later named Pluto (Figure 7). The object appeared as a 15thmagnitude point of light, which to all intents and purposeswas the same as any other asteroid but its orbit was farremoved from the asteroid belt, so much so that Pluto wasconsidered to be a unique entity. We now know that Pluto isin fact one of the largest members of what is now termed theKuiper Belt, or more correctly the Edgeworth−Kuiper Belt inrecognition of the contribution made by BAA member

Kenneth Edgeworth in first postulating the existence of manyplanetesimals well beyond the major planets, which servesas a reservoir for comets which occasionally enter the innersolar system.10 Indeed, with some notoriety, the IAU hasrecently approved the reclassification of Pluto as a dwarfplanet which has been assigned the number 134340, one ofseveral almost equally large bodies beyond Neptune.11

Binary asteroids

We now know that many asteroids exist as binary objectsorbiting their common centre of mass. Indeed, one can arguethat the first such binary system was Pluto itself. In 1978James Christy at the US Naval Observatory noticed thatseveral images of Pluto which he had taken as part of a pro-gramme of astrometry appeared unusually elongated. Look-ing at other archive images it became obvious to Christy thatPluto had a close companion or satellite,12 one which helater named Charon and which is locked in synchronousrotation with Pluto circling every 6.39 days. The first con-ventional asteroid to be found associated with an encirclingnatural satellite was 243 Ida, when in 1994 images taken in1993 by the Galileo spacecraft were found to reveal thepresence of a small moon 1.4 km in size, now named Dactyl.

The existence of binary systems in which the two com-panions are similar in size has been suggested in the paston the basis of the existence of many paired impact craterson the Moon as well as on the Earth (e.g. Clearwater Lakesin Quebec, Canada). Apart from Pluto and Charon, the firstsuch system (1994 AW1) was discovered from 1994 obser-vations and reported in 1997 by Pravec & Hahn.13 Sincethat time the number of binary asteroids which have beenpositively identified from lightcurve data and direct detec-tion by radar from Arecibo and Goldstone has grown tonumber well over one hundred.

An interesting case is that of the long-lost asteroid 1937UB (Hermes), first sighted as a bright trail on a photographby Karl Reinmuth observing from Heidelberg on 1937 Octo-ber 28. Reinmuth was able to follow it for five days before itreceded and was lost. Believed to be almost 1 km in diameter,

Figure 6. Charts of M44 made using the planetarium programmeGUIDE 8.0, depicting the positions of all asteroids (red crosses)known by 1984 (upper chart) and 2007 (lower chart).

Figure 7. Clyde Tombaugh and his discovery photographs of Plutotaken on 1930 January 23 and January 29. (courtesy of Lowell Ob-servatory archives, available at http://en.wikipedia.org/wiki/Image:Pluto_discovery_plates.png and http://en.wikipedia.org/wiki/Image:ClydeTombaugh.jpg)

191J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

it is the largest object known to have passed within twolunar-distances of the Earth, which it did on 1937October 30.7, and most surprisingly it repeated this somefive years later coming even closer on 1942 April 26.8.14 Givenits size and speed (about 18 km/s relative) it packed the po-tential to cause massive devastation on the Earth. Then on2003 October 15, Brian Skiff of the Lowell Observatory Near-Earth-Object Search (LONEOS) found a 14th magnitude fast-moving object, which he posted to the Minor Planet Center’sNear-Earth Object Confirmation Page.15 BAA member NickJames imaged the then unknown object (see Figure 9) beforelater the same day from orbit calculations, the object waslinked to long-lost Hermes. Given the size of this potentialEarth-impactor, radar observations were scheduled the verysame day from Arecibo. These revealed it to be a binarysystem comprising two objects of similar size (about 400metres across) separated by just 1200 metres and spinningaround their common centre of gravity every 13.89 hours(see inset to Figure 9).

Centaurs and Trans-NeptunianObjects (TNOs)

Our knowledge of the outer solar system is really very re-cent. For fifty-seven years, 944 Hidalgo was the onlyasteroidal object known to travel beyond the orbit of Jupi-ter, but in 1977 Charles Kowal, who was engaged in a searchfor unusual solar system objects using the 1.2m Schmidttelescope at Palomar Observatory, discovered 2060Chiron.16 Chiron largely resides between the orbits of Sat-urn and Uranus and requires more than fifty years to com-plete a single, rather eccentric orbit around the Sun. For tenyears or so Chiron remained an enigma but then in 1988 itwas observed to undergo an outburst in brightness ofabout 0.7 magnitudes17 – Chiron was in fact a comet, and isnow known as periodic comet 95P/Chiron. In all about 50objects have been found which orbit further from the Sunthan Jupiter yet closer than Neptune and these are classedas Centaurs: a most appropriate name given what we nowunderstand of their nature, which for several is ‘half-comet,half-asteroid’.

Then during the 1990s several types of very distant aster-oid were discovered (see Table 1). Most notable was thevery first object, 1992 QB1, found by Dave Jewitt and JaneLuu,18 which was the first example of a TNO (sometimescalled cubewanos, a verbal play on the QB1 designation): aclass of object which on average orbits beyond Neptune.We also now know that Pluto has orbital characteristics whichit shares with a number of other objects called Plutinos, inthat they are all in 3:2 mean motion resonance with Neptune,orbiting the Sun twice for every three revolutions of Nep-tune. Plutinos are a special class of TNO as are ScatteredDisk Objects or SDOs. The latter comprise objects in eccen-tric orbits that take them well above and below the plane ofthe ecliptic, which are mooted to have once been ordinaryTNOs but which came too close to a major planet and werescattered by gravitational perturbation of their orbit.

I have already touched on Edgeworth’s idea that thereexists beyond Pluto a ‘reservoir’ of comets, which throughcollision or close approaches occasionally become short-period comets. In 1950, a few years after Edgeworth pro-posed his hypothesis, Jan Oort put forward a theory to ex-plain the existence of long-period comets by supposing thatthe Sun is associated with an enormous halo of cometarynuclei situated many thousands of AU beyond the outer

Figure 8. False-colour image of 243 Ida and companion Dactyl asimaged by the Galileo spacecraft. (courtesy NASA)

Figure 9. Four superimposed 30-sec exposures of 1937 UB (Hermes)taken on 2003 October 15 at 19:54−20:15 UT using a 0.30m Newto-nian and KAF-0401E CCD, showing the object’s motion (courtesyN. D. James). Inset: Doppler spectrum of the radar echo from 69230Hermes made using the Arecibo telescope on 2003 Oct 17, showingtwo clearly separated components. (courtesy J.−L. Margot). See alsohttp://www2.ess.ucla.edu/~jlmargot/NEAs/Hermes/

Table 1. Asteroids −−−−− some noteworthy milestones

1868 100th numbered asteroid (Hekate) discovered1891 First photographic discovery of an asteroid (323 Brucia) by

Max Wolf1898 First near-Earth asteroid (433 Eros) discovered by C. G. Witt/

A. Charlois1906 First Jupiter Trojan (588 Achilles) discovered by Max Wolf1918 Hirayama identifies the existence of asteroid families1921 1,000th numbered asteroid (Piazzia)1977 First Centaur (2060 Chiron) discovered by Charles Kowal1978 Pluto’s moon Charon, first binary asteroid, discovered by

James Christy1981 10,000th numbered asteroid (Myriostos, discovered in 1951)1992 First TNO discovered (1992 QB1)1993 First Plutino discovered (1993 RO)1996 First scattered disk object discovered (1996 TL66)2000 100,000th numbered asteroid (Astronautica)2003 First Inner Oort Cloud object discovered (90377 Sedna)

192 J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

regions of the then known solar system.19 This concept hasbeen given a name, the Oort Cloud, although it must be saidthat so far there is no direct evidence that it really exists. Thenearest we have come is with the discovery of 2003 VB12 byMike Brown. Now designated 90377 Sedna, it orbits close tothe plane of the ecliptic but its very eccentric orbit takes itout as far as 976 AU from the Sun! Some astronomers con-sider this is far enough that it should be assigned to a newclass of asteroid, an Inner Oort Cloud object.

In order to be detected at all, 90377 Sedna had to be of asignificant size. Figure 10 is an artist’s impression of thelargest TNOs known, drawn to scale. It seems that each ofthe objects depicted is unusual in some respect. 136199 Erisis an SDO, discovered by Mike Brown, Chad Trujillo & DaveRabinowitz, which has been found to be more massive thanPluto. We know this for sure since both objects have satel-lites, the orbital characteristics of which permit the mass ofthe primary to be determined with fair accuracy. Anotherunusual TNO depicted in Figure 10 is 2003 EL61, in that notonly does it possess at least two moons but it is also verysignificantly elongated, so much so that as it rotates it variesin brightness exhibiting an amplitude of 0.28 magnitudes anda period of 3.92 hours.20

The more TNOs that are found, themore opportunities amateurs will haveto observe these distant asteroids. Anexample of what can be done at presentis the lightcurve plot of 2003 EL61 inFigure 11, which was obtained byBAA member John Saxton using a0.30m Schmidt−Cassegrain telescopefrom Lymm, Cheshire. At the time ofthe observation, an 85% illuminatedgibbous Moon produced a bright skybackground, degrading the signal-to-noise of the 17th magnitude asteroid.In all 532 images of 30-sec durationwere used and by co-adding subsetsof these, a characteristic lightcurvewas produced having two maxima andtwo minima per revolution.

Asteroids and the BAA

In late 1980, at the request of the BAA Council, RichardBaum was asked if he would incorporate minor planet studyinto the programme of the Terrestrial Planets Section.21 TheMinor Planet Group was duly set up under the guidance ofAndrew Hollis as Coordinator, and the first observing projectinvolved visual observation of 18 Melpomene, which reachedmagnitude 7.9 in 1981 August, a particularly bright opposi-tion. Prior to this time there had been no attempt to organisegroups of observers in the study of minor planets apart frompossibly on the occasion of the two close approaches of 433Eros in 1930/’31 and 1974/’75, and the pioneering work ofGordon Taylor in the prediction of stellar occultations. Asummary of selected asteroid-related activities involving theBAA is given in Table 2. The Minor Planet Group became anindependent observing Section of the Association in 1984under the directorship of Andy Hollis, with myself as Assist-ant Director. Within 12 months it had grown into the Aster-

Figure 10. The largest known trans-Neptunian objects. (courtesy Wikipedia at http://en.wikipedia.org/wiki/Image: EightTNOs.png)

Figure 11. Lightcurve of trans-Neptunian object 2003 EL61 (cour-tesy J. M. Saxton)

Table 2. Asteroids −−−−− milestones of BAAinvolvement

1958 Gordon Taylor predicts first successfully-observed occulta-tion by an asteroid (SAO 112328 by 3 Juno)

1981 Minor Planets Group formed within the BAA TerrestrialPlanets Section

1984 Minor Planets Section formed (Andy Hollis as Director,Richard Miles as Assistant Director)

1984 First photoelectric photometry project (8 Flora)1985 Renamed Asteroids and Remote Planets Section to include

observations of Uranus, Neptune and Pluto1986 ARPS Observers’ Manual published1989 Occultation of star 28 Sagittarii by Titan widely observed in

the UK341990 Asteroid 4522 Britastra named by the discoverer Ted Bowell

to commemorate the BAA centenary1996 First asteroidal occultation observed from UK (892 Seeligeria

on Nov 9)2001 ARPS Section Meeting in Clanfield, Hampshire (Nov 3)2005 Roger Dymock appointed Section Director2006 ARPS Newsletter began in March

193J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

oids and Remote Planets Section (ARPS), taking on respon-sibility for not only asteroids but also planetary satellitesand the outer planets, Uranus, Neptune and Pluto.

The advances in technology starting in the late 1970shave made an enormous difference to amateur astronomyand the potential opportunities for observing asteroids. Thefirst phase of this rise involved the development of solid-state electronics, which enabled keen amateurs like myselfand Andy Hollis to construct photometers built aroundphotomultipliers as the light-sensitive detector.22 These pho-tometers were single-channel devices operated manually andrequired a good deal of manual dexterity and perseverance,in that the telescope had to be moved three times to afford asingle magnitude determination: first to the target asteroid,then to an empty region of background sky and thence to asuitable comparison star. Despite this they were extremelyaccurate and had an enormous dynamic range, enabling oneto use comparison stars of any brightness. Keen amateursalso ventured into the realm of astrometry by either buildingtheir own plate-measuring machines orgetting access to the large and expen-sive Zeiss machines found at profes-sionally-run establishments.

Computers were also evolving andduring the mid-1980s it was possible touse hand-held programmable calcula-tors to record and print out the photo-metric measurements directly. By 1988,portable computers had arrived and thedata could be recorded to disk and dis-played on a flat-screen monitor in realtime. However, the one technologicalrevolution which really made the differ-ence was the appearance of commer-cially-available CCD cameras around1990. The rest is history as they say, inthat these devices could readily be em-ployed for both brightness and posi-

tion measurement with high precision − the perfect answerto an asteroid observer’s dreams.

Meetings and publications

We have seen a gradual expansion of asteroid-related activi-ties over the years thanks to technology and computers, inparticular the introduction of e-mail and most strikingly inrecent years the Web (the Minor Planet Center website toname but one), and search engines such as Google, all ofwhich have enabled ready access to vast amounts of infor-mation. However, despite this high level of electronic com-munication, observers like to meet together from time to timeto discuss topics of common interest face to face. In thisrespect, nothing has changed throughout the long historyof the BAA, with meetings being as popular as ever. Thefirst ARPS Section meeting was a session at the joint BAA/IAPPP symposium at Herstmonceux in 1984 September.23

Other notable meetings of late include the workshop held atClanfield, Hampshire on 2001 Nov 3 (see Figure 13) and theSection meeting held as part of the Winchester Weekend in2005 with Roger Dymock as Section Director, this being ineffect the 21st anniversary of the Section’s formation. Linkswith professional astronomers are important and at the lat-est ARPS meeting at Newbury in 2007 June, Alan W. Harrisand Mark Kidger contributed to the proceedings.24

Astronomy is a pursuit of people worldwide, the nightsky being accessible to all, and so international cooperationis an essential feature of the subject. Section members haveattended international gatherings such as those involvingthe European Asteroid Occultation Network (EAON), andthe Meeting on Asteroids and Comets in Europe (MACE) toname but a few.

Prior to the formation of the ARPS, little by way of litera-ture was available to guide the would-be observer of aster-oids. An observers’ manual covering all aspects of the sub-ject was produced to fill this gap in 1986 but it has not been

Figure 12. Andy Hollis (right) and Richard Miles (left) with some oftheir children in 1982. The 0.35m Cassegrain telescope at Andy’sOrmada Observatory can be seen in the background.

Figure 13. Participants at the ARPS Section Meeting and workshop held at the HampshireAstronomy Group’s observatory near Clanfield on 2001 November 3. (courtesy H. McGee)

194 J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

updated since that time. In the meantime much has changedand so a complete revision of this manual is needed. Thework of Section members is reported in a number of ways;through papers published in the Journal, formal submis-sion of observations to the Minor Planet Center and otherorganisations, publishing further afield, and last but not leastin the Section Newsletter Impact, which Roger Dymock hascompiled and published electronically since 2006 March.

Occultations

During the early 1950s, Gordon Taylor, an astronomer work-ing at the Royal Greenwich Observatory and BAA mem-ber contributing to the work of the Computing Section,began to provide predictions of possible stellar occulta-tions by the first four asteroids, 1 Ceres to 4 Vesta. Inthose early days, few asteroids had accurately knownorbits and the available star catalogues were of limitedvalue. Despite these shortcomings, the first claimed suc-cess was on 1958 February 19 when 3 Juno was seen tooccult BD +6° 808 from Malmo, Sweden, although there issome doubt about this since the brightness dropwas only about 0.6 magnitudes for this event.25,

26 The first undisputed success was a photo-electric recording of the occultation of BD −5°5863 by 2 Pallas from Naini Tal, India on 1961October 2.25

Although many stellar occultations have beenwitnessed worldwide since this time, the firstpositive event seen from the UK was by thepresent author and involved 892 Seeligeriaocculting GSC 4695 0543 on 1996 November 9,thanks to predictions generated by myself us-ing Brian D. Warner’s Asteroid Pro Version 2.0

software. Predictions using this software continued to beissued and this effort was rewarded on 1998 September 10,when Hazel McGee timed an occultation by 1574 Meyer,and on 1998 December 18 when Fiona Vincent observedan occultation by 70 Panopaea.26 Occultations continueto be an important aspiration for UK-based observers, forwhich predictions are now distributed by Andrew Elliott.In recent years we seem to have suffered a run of bad luckwith few favourable events and bouts of poor weather atcritical times. If luck averages out in the long run then wemay be destined for some spectacular events within thenext few years.

Photometry and rotationallightcurves

During the early years of the Section, photoelectric pho-tometers were used to measure the brighter asteroids, oneexample being the 1984 campaign to determine the rota-tion period of 8 Flora. It was a difficult objective sinceFlora is almost spherical in shape and exhibits a lightcurveof very low amplitude. The outcome of this joint effort byobservers in the UK, France and Australia was the identi-fication of a possible rotation period of 12.79 hr as well asan estimate of the pole position.27 The value currentlyaccepted for the period of 8 Flora is 12.799 hr.20

A fundamental restriction of the photoelectric photometerfor asteroid work was that the image of the target object hadto be centered in a small aperture hole typically 1−2mm indiameter, so as to exclude extraneous stars and restrict thesky background contribution. As such, the object had to bereadily visible, which largely limited photometry to aster-oids of 12th magnitude or brighter. When CCD cameras foundtheir way into the hands of observers, a revolution ensued.Not only could objects down to 18th magnitude be meas-ured but there was no need to move the telescope to find asuitable comparison star or to measure the sky backgroundsince everything the photometricist needed was usually onthe one image frame. The potential for CCD photometry isvirtually unlimited except that handling the large datafilesand analysing the images to extract magnitude informationis a non-trivial task. Software such as AIP4WIN and Canopus

Figure 15. Composite lightcurve of 8 Flora in 1984.27

Figure 14. The cover of the ARPS observers’ handbookpublished in 1986.

195J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

has made differential photometry a good deal easier, but weare still some way from attaining the holy grail whereby themagnitude information is available in near real-time thus pro-viding direct feedback to the observer on progress duringan actual observing run.

Surprisingly, after all these years of space-borne observa-tories, we still lack an accurate catalogue of broadband mag-nitudes (viz. B,V, R and I) for stars suitable for use as com-parisons down to 15th magnitude. This limitation has been afundamental restriction preventing the determination of truemagnitudes as a matter of routine. Instead, most observershave reported differential magnitudes and so objects withparticularly slow rotations have tended to be overlooked.The most accurate source of all-sky V magnitude data is thatprovided by the Hipparcos mission, but on average, thereare only about three stars available per square degree of sky.

To overcome this shortcoming, the author mounted short-focus telescopes with a wide field of view alongside his maininstrument so that Hipparcos stars could be imaged throughfilters at the same time as a target asteroid and other nearbystars using the main scope. The advantage of this approachis that it yields an accurate measurement of the V magnitudeof the asteroid and so, unlike differential methods, combin-

ing observations from different nights made overperiods of many weeks becomes a straightforwardexercise. One merely has to apply a correctionwhich takes into account the varying heliocentricdistance, Earth-asteroid distance, and asteroidphase angle so that every night’s set of measure-ments can be combined. Figure 16 is an example,where 17 observing runs on 130 Elektra, some last-ing just a few minutes, have been plotted colour-coded on one composite rotational lightcurve,yielding in the process an accurate and unambigu-ous rotation period for this object. The acceptedvalue for the period of Elektra is 5.225 hr.20 Clearly,CCD photometry can reach levels of precisionwhich were unthinkable just 20 years ago, and itwill continue to prove a valuable tool in the handsof the amateur for many years to come.

The CCD’s other great strength is its capacity to record andmeasure faint objects. Figure 17 depicts the results of one ofthe author’s several observing runs on the unusual object137924 (2000 BD19), the orbit of which is so extreme that it isnot only a Mars-crossing asteroid but is also an Earth-crosser,Venus-crosser and Mercury-crosser. Indeed, it approachescloser to the Sun than any other body in the solar system forwhich an accurate orbit is known. No other lightcurve of thisobject is known to the author. It appears to rotate every 10.6hours or so, but by combining the results of several observ-ing runs when the object reached almost 18th magnitude, itshould be possible to derive a much more accurate rotationperiod and an estimate of the object’s shape.

Given our normally unreliable weather, it is not alwayspossible to obtain sufficient photometric coverage of favour-ably-placed asteroids and so it is often best to organise aninternational observing campaign so that intensive cover-age of any one object can be achieved. In future, it is hopedthat more observers will undertake absolute photometry ofthis kind. One new opportunity, where amateurs will be ableto cooperate, will be in quantifying the ‘opposition effect’ orphase angle brightening of various different classes of mi-nor planet. The task here will be to try to measure the bright-ness over a range of phase angles, and especially at verylow phase angle on the night of opposition.

Astrometry

In some ways we have now reached a ‘golden age’ for ama-teur astronomers, who by taking advantage of modern tech-nology can contribute equally to the work of the profes-sional community. Several amateurs have come from a back-ground in computing and have been able to contribute byproviding software tools which facilitate analysis, for exam-ple of CCD images. One notable example is the Austrianamateur Herbert Raab. By writing the Astrometrica software,and by providing support via his user group, he has pro-vided the amateur with a powerful tool for measuring theastrometric positions of celestial objects, more especially

Figure 16. Composite lightcurve of 130 Elektra based on 17 nights of observationby the author during 2005 February 14−April 10.

Figure 17. Lightcurve of 2000 BD19 obtained during a single 12-hourphotometric observing run by the author on 2006 February 9/10.

196 J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

moving ones such as comets and asteroids.28 Astrometricais worlds apart from the technology of plate-measuring ma-chines and is a joy to use. It is particularly useful for measur-ing the positions of fast-moving objects such as NEAs. Onewell-known exponent of this pursuit is Peter Birtwhistle, men-tioned already with regard to his asteroid discoveries. Pe-ter’s particular interest is in following up newly-discoveredobjects which appear on the NEO Confirmation Page. At thetime of writing he has reported no less than 9,248 accurateastrometric positions of NEAs, all since 2002 June 10.

Near-Earth asteroids

There is no doubt in my mind that near-Earth asteroids arethe most exciting objects for observers at present and willremain so for years to come. We still know the orbits of onlya small fraction of objects which are less than 300 metres insize. So any object which is detected approaching to withinsay 10 lunar-distances of our planet is invariably a newly-

discovered one and may have originated from al-most any region of our solar system, having beenperturbed into its present orbit by gravitationalforces following a close approach to a planet orother object, or indeed it may be a fragment leftover from a collision many millions of years ago.So here is our chance to pay close attention tothese strange and varied bodies: some may bederived from the interior of much larger asteroids,some may be desiccated remnants of comets, oreven material ejected from a planet’s surface by aparticularly energetic impact. With the recent dra-matic success of automated searches, more andmore NEAs are being found, many before theymake very close approaches to the Earth with somepassing between us and the Moon.

When a new object is announced on the NEOCP,the priority is to obtain astrometry of it so that itsorbit can be defined more accurately. If only a few

positions are obtained over a short time interval, say lessthan 24 hours, then the object can easily be lost. A propor-tion of these objects are classed as ‘potentially hazardousasteroids’ or PHAs, i.e. asteroids whose orbit approacheswithin 0.05 AU of the Earth’s orbit, and which are brighterthan an absolute visual magnitude H = 22.0, correspondingto a diameter of at least 110−240 metres. At the time of writing(2008 June 11), there are 957 known PHAs.29 Objects whichapproach very close to the Earth astronomically speaking,say closer than our Moon, are typically much less than 50min size and would therefore not fall into the PHA category ascurrently defined. So far some 68 objects are known to havepassed by closer than the Moon; the first was picked up in1991 and the largest was 2002 MN at H=23.4 or around 60−120m diameter. One example is 2007 EH, shown imaged byPeter Birtwhistle in Figure 20 when it was moving at an angu-lar speed of 1249 arcseconds per minute.

In addition to astrometry, NEAs passing close-by are in-teresting photometric targets since it is the only opportunity

Figure 18. An example of a ‘plate solution’ obtained using the software Astromet-rica. In this case, 128 star positions (circled in green) were used to achieve apositional accuracy of about ±0.3 arcseconds. (courtesy of H. Raab)

Figure 19. Photograph of Peter Birtwhistle surrounded by meteor-ites at the Naturhistorisches Museum, Vienna, taken by the authoron the occasion of the Meeting on Asteroids & Comets in Europe[MACE] on 2006 May 12–14.

Figure 20. Stacked image showing the 12m object 2007 EH passingless than half the Moon’s distance from the Earth. (P. Birtwhistle)

197J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

we have to study such small objects close up. However,measuring the rotational lightcurve of a very fast movingobject or VFMO can be a considerable challenge, not leastbecause of the rapid motion across the field of view causingextreme trailing and the need to continually chase the objectalong its trajectory. In the case of Figure 21, the image ismade up of three 0.3-sec exposures tracked to follow theasteroid and stacked using Astrometrica. Stacking in thisway increases signal to noise, permitting higher precisionwhen the resultant stack is measured photometrically. Fig-ure 22 illustrates the brightness changes of the PHA 2005WC1 turning end-over-end as it traversed the sky. In thiscase a period analysis performed using the Peranso soft-ware, written by amateur astronomer Tonny Vanmunster, un-ambiguously shows that it rotates every 2.57 hr.30

When contemplating photometry of newly-discoveredNEAs, it is usually imperative to start observing as soon aspossible after the discovery announcement, preferably withina few hours, since the total apparition when the object isvisible may only be a day or two. Consequently, the morepeople who participate in a photometry campaign the better.Figure 23 shows the composite rotational lightcurve of the250m PHA 2006 XD2, obtained from images by Nick Jamesand Fiona Vincent, folded on a period of 3.7 hr. The smallestNEAs, those less than about 150 metres in size, tend to ro-

Figure 22. Asteroid 2005 WC1 makes more than three complete revolutionsduring the 9-hr observing run by the author on 2005 December 6/7.

Figure 23. Composite lightcurve of 2006 XD2 (period = 3.70 hr).From an ARPS observing campaign launched the day after the dis-covery announcement on MPEC 2006-X43 (2006 December 13).

Figure 21. Part of the NEO Confirmation Page screen downloaded in 2007 (courtesy Minor Planet Center).An image of one of the two LINEAR robotic survey telescopes has been added as the inset. (courtesy LincolnLaboratory)

tate very quickly as theyoften comprise mono-liths of rocky materialformed by fragmentionduring a collision be-tween two larger bodies.To improve our under-standing of the dynam-ics and evolution of oursolar system, it is impor-tant to survey small as-teroids photometricallyso as to determine howtheir spin rate varies withsize and to see whethersome of these objects aretumbling throughspace, i.e. simultane-ously rotating about twoaxes. Several such aster-

oids are known to be ‘superfast rotators’, turning a full 360°in just a few minutes.20

The unique case of AL00667

Of the 68 objects known to have passed within theMoon’s orbit, many were detected after reachingperigee, i.e. whilst on the outward departing stageof their journey. It is not so surprising therefore thatno object detected whilst still incoming has beenfound to be on a collision course with the Earth.One day in the near future this will undoubtedlyhappen. However, we have already had a practiceexercise ahead of the real thing in the case of NEOCPobject AL00667 on 2004 January 13. This object,now designated 2004 AS1, was detected by LIN-EAR, and the initial orbit posted on the Minor PlanetCenter (MPC) website indicated that it was very

198 J. Br. Astron. Assoc. 118, 4, 2008

Miles: Asteroids – past, present and future

close and moving towards the Earth. The author was one ofthe first people to notice its unusual nature and that the appar-ent trajectory placed it on a direct course towards the Earth. Itwas evident that the collision if it were to happen would haveoccurred within about 24−36 hours.

The weather across most of Europe was poor that nightapart from southern England. Peter Birtwhistle was unableto observe the object as his telescope could not reach theregion of sky in question, but the author spent about onehour imaging the area from Golden Hill Observatory withoutfinding the moving object, concluding therefore that it wastravelling on some other trajectory and was not in fact athreat to the Earth. By the time a message was posted to thiseffect on the Minor Planet Mailing List,31 the skies hadclouded over in Dorset and it had begun to rain. Some fivehours passed before two US amateurs joined the search forthe object, which had been reckoned to have a 1 in 4 chanceof being on an Earth-impact trajectory. It was not detectedby anyone that night because, instead of approaching us, itwas in fact receding and therefore much further away thanfirst thought and growing fainter. It was finally recovered byLINEAR and shown to be an ordinary Apollo-type asteroidorbiting the Sun every 1.11 years and reaching a periheliondistance of 0.88 AU.

The MPC has now changed its procedures to highlightpotential Earth-impacting trajectories automatically beforeposting predictions on the NEOCP webpage. In the case ofAL00667 the general media were not informed of the hap-penings at the time, the first reports only appearing somefive weeks later.32

The future beckons...

As we have seen, our understanding of asteroids and othersmall solar system bodies is expanding rapidly and will con-tinue to do so in the coming years. The boundaries separat-ing comets from asteroids will become increasingly blurred.

Several large new survey telescopes are being constructedand are due to come on line within the next decade.33 Theseinclude

– the 4.3m DCT or Discovery Channel Telescope, whichwill be sited near Lowell Observatory, Arizona and have a2 sq.deg. field of view. It is due for completion in 2010.

– the 4×1.8m Pan-STARRS (an acronym for Panoramic Sur-vey Telescope And Rapid Response System), equivalentto a single 3.6m mirror with a 3° field of view, which willprobably be located on Mauna Kea, Hawaii.

– the 8.4m LSST or Large Synoptic Survey Telescope,which is planned to have a field of view of more than 3°and should see first light in 2015 at Cerro Pachón, north-ern Chile.

We have much to look forward to, as and when these futur-istic instruments embark on their search and mapping of thesolar system. No doubt these advances will provide new

opportunities for the keen amateur to play his or her part inthe exploration of the solar system ‘jungle’, with many strangenew ‘creatures’ yet to be found. Let’s go hunting!

Address: Grange Cottage, Golden Hill, Stourton Caundle, Dorset DT102JP. [rmiles@baa.u-net.com]

References and notes1 Foderà Serio G., Manara A. & Sicoli P., pp. 17−24 in Asteroids III,

(eds. Bottke W. F., Cellino A., Paolicchi P. & Binzel R. P.), Univ.of Arizona Press, Tucson, 2002

2 Linnington R. T., A Compendium of Astronomy, Whittaker,Treacher & Co., 1830

3 Schmadel L. D., Dictionary of Minor Planet Names, 4th edn.,Springer−Verlag, 1999. See entries for specific objects.

4 Obituary notice for J. R. Hind, MNRAS, 56, 200−205 (1896)5 Argelander F. A., Astron. Beobacht. Sternwarte zu Bonn, 3 (1859);

ibid., 4 (1861); ibid., 5 (1862)6 Hirayama K., Astron. J., 31, 185−188 (1918)7 Monck W. H. S., Sidereal Messenger, 334 (1888)8 Gehrels T., Space Sci. Rev., 58, 347–375 (1991)9 Mobberley M., Astronomy Now, 18(3), 75−76 (2004)

10 Hollis A. J., J. Brit. Astron. Assoc., 106(6), 354 (1996)1 1 Green D. W. E., IAU Circular no.8747, 2006 September 131 2 Marsden B. G., IAU Circular no.3241, 1978 July 71 3 Pravec P. & Hahn G., Icarus 127, 431−440 (1997). The compan-

ion to 1994 AW1 was discovered from January 1994 observationsby S. Mottola et al. from ESO and Ondrejov observatory. A goodresource listing details of binary and multiple systems can befound at: http://www.johnstonsarchive.net/astro/asteroidmoonsall.html

1 4 Orbital calculations for Hermes by Jon Giogini based on radarastrometry from Arecibo (see http://www2.ess.ucla.edu/~jlmargot/NEAs/Hermes/ca)

1 5 See the Minor Planet Center’s NEO Confirmation Page at http://www.cfa.harvard.edu/iau/NEO/ToConfirm.html

1 6 Kowal C. T., in Asteroids: Their Nature and Utilization, 2nd edn.,John Wiley/Praxis Publishing, 74−77 (1996)

1 7 Tholen D. J., Hartmann W. K. & Cruikshank, D. P., IAU Circularno. 4554, 1988 February 24

1 8 Jewitt D. C. & Luu J. X., IAU Circular no. 5611, 1992 Sept 141 9 Oort J. H., Bull. Astron. Inst. Netherlands, XI (408), 91−110

(1950) available at http://articles.adsabs.harvard.edu/20 Harris A. W. & Warner B. D., List of known lightcurve param-

eters updated on 2008 March 17 at: http://www.minorplanetobserver.com/astlc/LightcurveParameters.htm

2 1 Baum R., J. Brit. Astron. Assoc., 93(6), 260−263 (1983)2 2 Miles R., J. Brit. Astron. Assoc., 117(4), 181 (2007)2 3 Hollis A. J., J. Brit. Astron. Assoc., 94(6), 272 (1984)2 4 Dymock R., J. Brit. Astron. Assoc., 117(5), 273 (2007)2 5 Taylor G. E., The Observatory, 82, 17−20 (1962); Sinvhal S. D.,

Sanwal N. B. & Pande, M. C., The Observatory, 82, 16−17 (1962)2 6 Worldwide occultations database. See entries (incl. Note A re. the

occultation of 3 Juno in 1958) at http://mpocc.astro.cz/world/mpocc1.txt

2 7 Hollis A. J., Bembrick C. S., Dumont M. & Miles R., J. Brit.Astron. Assoc., 97(4), 220−223 (1987)

2 8 Raab H., Astrometrica, v.4.4.2.366 (2008 Mar 2) available athttp://www.astrometrica.at/

2 9 PHA website: http://neo.jpl.nasa.gov/neo/pha.html30 Vanmunster T., Peranso, v.2.31 (2008 June) available at http://

www.peranso.com/3 1 Minor Planet Mailing List available at http://groups.yahoo.com/

group/mpml/3 2 Whitehouse D., BBC News website, 2004 February 24 at http://

news.bbc.co.uk/1/hi/sci/tech/3517319.stm3 3 Trujillo C. A., pp. 573−585 in The Solar System beyond Neptune,

eds. Barucci M. A. et al., Univ. of Arizona Press, 200834 Miles R. & Hollis A. J., J. Brit. Astron. Assoc., 104(2), 61–76

(1994)

Address given 2007 October 31; revised text received 2008 June 13