J. Grigg, J. F. Skjellerup and their comet

Vistas in Astronomy, Vol. 34, pp. 1-10, 1991 Printed in Great Britain. All rights reserved.

0083--6656/91 $0.00 + .50 © 1991 Pergamon Press plc.

J. GRIGG, J. F. SKJELLERUP A N D THEIR COMET David W. Hughes D e p a r t m e n t of Physics, The University, Sheffield $3 7 R H , U . K .

Interest in P/Grigg-Skjellerup, a rather faint periodic comet, has been rekindled by the European Space Agency, who have recently redirected the GIOTTO spacecraft so that it will fly close by P/Grigg-Skjellerup in 1992. This possibility was widely communicated to the astronomical community in Mardl 1987 in the IAU Circular No. 4346. The comet has its ascending node very close to the Earth's orbit and its absolute magnitude of 12.45 makes its nucleus about 25 times smaller than that of P/Halley. G I O ' U r o was designed to visit P/Halley, and flew through the inner coma of that comet on 13/14 March 1986, on a Kamikaze mission of discovery. The discovery portion of the mission succeeded superbly but the Kamikaze did not quite occur and the spacecraft limped on, along its orbit of period ten months, to return past Earth in July 1990. It will now make another flyby in 1992. Both Grigg and Skjellerup were amateur southern hemisphere astronomers. Grigg was responsible for the discovery of three new comets, finding 1902 II, 1903 Ill and 1907 II. Skjellerup is credited with discovering five, 1920 I, 1920 IIl, 1922 I, 1923 [ and 1927 IX. Comet P/Grigg-Skjellerup is both 1902 lI and 1922 I. Abstract.

John Grigg (1838-1920) was born I on tile Isle of Thanet in Kent, and his father, James, (a prominent member of tile firm Wells & Co.) saw to it that he had a musical, scientific and commercial education. John started work in the music and furnishing trades, giving up much of his spare time to choral singing. In 1863 he emigrated to New Zealand, living in Auckland until 1868 and thea moving to Thanms, Auckland Province, North lsland (his observatory being at latitude 37 ° 8' 23.21" South, longitude 1lh 42m 10.57s East 2) where he remeined until his death. tie taught singing under the Education Board and composed the song "My own New ZeMand tlome" - a song that is said to rank almost as a nationM anthem. The 1874 transit of Venus revived his interest in astronomy and he theu erected an observatory at Thames which contained both a transit circle aud an equatorial telescope. lie joined the British Astronomical Association in 1897 and worked zealously for the cornet section. He also bccame it Fellow of the Royal Astrouomical Society on 1906 June 8. Grigg received two medals from the Astronomical Society of the Pacific for his cometary discoveries. He was also a successful cometary photographer, obtaining one of the few good plates of the bright comet 1901 I. Grigg was a frcquent lecturer on astronomical topics and coatributed a regulaa' astronomical column to New Zealand's Ola9o Wil~ess newspaper. Hc died ou 1920 June 20 at thc age of 82, leaving a son.

2

D . W . Hughes

Grigg devised a semi-graphical method for producing ephemerides for periodic comets and used this with considerable success, being the first to recover comet P/Encke at its 1898 return. The only other observer of this comet (1898 III) was John Tebbutt (1834-1916), who saw it from his observatory at Windsor, New South Wales, Australia. Grigg's name is strongly associated with three comets. The first was 1902 II and this comet will be discussed further in this short paper. Grigg found it on the night of 1902 July 23 when sweeping in the Leo-Virgo region of the sky, using his 8.9 cm Wray telescope. His second comet, 1903 III, was a long period comet and is listed as having a parabolic orbital eccentricity of unity. 1903 III was discovered at magnitude 9.0 on 1903 April 17. Grigg was using a 7.6 cm refractor. The comet was in the constellation Eridanus and had passed perihelion about three weeks before, on March 26. Four years later, on 1907 April 8, came Grigg's third discovery, comet 1907 II. This was discovered at a magnitude of 6.5, after having passed perihelion on March 28. 1907 II has subsequently become known as Grigg-Mellish, John E. MeUish of Madison, Wisconsin discovering it independently on April 14. Until recently a this comet was thought to have a period of 165 y and to be a reappearance of the comet of 1742 but Brian Marsden 4 has concluded that the two orbits are unconnected and 1907 II is now listed as parabolic. Comet P/Grigg (1902 II) was discovered at magnitude 9.5, and only six very rough observations were made 5 between 1902 July 22 and 1902 August 3 as it moved south-east into Virgo. (Kronk 6 states, without reference, that it was observed on fourteen occasions). Bright moonlight then followed. Also there was an unfortunate miscarriage of a message to Pietro Baracchi (1857-1926), a Government Astronomer at the Melbourne Observatory, notifying him of the discovery. Quoting from Monthly Notices r, Mr Baracchi did not receive information until August 6, and made careful search over a wide area at the earliest possible opportunity, but without success. He therefore awaited confirmation or fresh information from other source; but unfortunately the comet was only seen by Mr Grigg. The observations are rough, but Mr Grigg has computed a set of elements from his data, which show that the comet when at its brightest must have passed through Taurus and Gemini in the preceding June. In these constellations (due to the proximity of the Milky Way) it was scarcely likely that the comet would have been seen in northern latitudes. It was decreasing in brilliancy when seen by Mr Grigg at the end of July, and under the adverse conditions of doubtful position and fading illumination it was scarcely likely that it would be recovered when intelligence reached this country. Knowledge of the discovery did not reach Europe until September. So Grigg was the sole observer and the sole orbit computer of this comet. The orbit was recorded as parabolic s. It now seems that the 1907, 1912 and 1917 returns of this comet were overlooked. Twenty years later, in the evening twilight of 1922 May 17, J. F. Skjellerup discovered ° a 12th magnitude comet moving north-eastward near the border between Gemini and Cancer. Mr William Reid, a friend and fellow amateur astronomer from Cape Town, South Africa, found it independently a few hours later but he asked I° that it be known only as Comet Skjellerup, since Skjellerup had priority. Both were awarded the Donohoe Comet Medal of the Astronomical Society of the Pacific for this discovery (Reid discovered six comets and was very successful 11 in encouraging other South African observers, such as Blathhurst, Ensor and Forbes, to become comet seekers). Skjellerup's final tally of comet medals was six, there being four Donohoe and two Donovan medals, the later being presented by the Donovan Trust, Sydney, for comets 1927 IX and 1941 IV. Unlike 1902 II, a comet that was only seen for twelve days, 1922 I was followed for four months as it moved rapidly through Cancer, Leo Minor (May), Ursa Major, Canes Venatici (June), Bootes

J. Grigg, J. F. Skjellerup and their Comet

3

(June arid July), Corona Borealis, Hercules (July) and Ophiuchus (August). Throughout this time it was retreating from the Sun, the comet's heliocentricdistance increasing from 0.89 A U to 1.59 AU. It was lost when the magnitude dropped below 15. At the time it had the second shortest period of any comet whose orbit was well known, the record going to P/Encke (P = 3.31 y). John Francis Skjellerup12 was an Australian, born in Cobden, Victoria on 1875 May 16. Frank (a name he preferred to his two given names) was the tenth of thirteen children. His father died when Frank was stillvery young and his early lifewas hard. At fourteen he joined the Post Office and subsequently trained as a telegraph operator. In 1900 the South African Government appealed to the Australian Government for trained telegraphists,and Skjellerup applied and was accepted. He initiallysettled in Kimberly but soon moved to Cape Town. His interest in astronomy started in 1909 hut was considerably strengthened by the 1910 appearance of Halley's Comet. In 1912 he became the firstsecretary-treasurerof the newly founded Cape Astronomical Association, and he became a member of the British Astronomical Association in 1914. A note of Skjellerup's astronomical work can be found in the Journal of the British Astronomical Association (23, p. 210) where he describes some observations that he made on 1912 September 11 and 12, from Cape Town, of Comet Gale (1912 II). He also published (with A. W. Long) a series of observations of the magnitudes of variable stars. These can be found in Union Circulars between 1919 and 1921. One report came out in Nebst Beobachtungs-Zirkularen, 2, 77, (1921). In 1927 Skjellerup retired and returned to Australia, living in Oakleigh, a suburb of Melbourne. In 1933 he joined the Astronomical Society of Victoria and was the president of that society for three years, between 1942 and 1945. He died on January 6, 1952 at the age of seventy-five. Orchiston 12 points out that Frank Skjellerup was one of the independent discoverers of Comet Gale (1912 II). He found it on the evening of September 11, 1912. It was also "discovered" by a staff member of the Santiago Observatory, Chile, on the same night. Unfortunately Walter Gale of Sydney had found it a few days earlier. Skjellerup also recovered the periodic comet PonsWinnecke (1915 III), on October 31 of that year. By the end of the 1910s he was an experienced comet observer and hunter. J.F. Skjellerup discovered comet 1922 1 (P/Grigg-Skjellerup) from Rosebank, near Cape Town, South Africa. Another of his acknowledged cometary discoveries was 1920 I, which he found 33 ° from the Sun on 1919 December 19 at a magnitude of 8.5, while he was searching for the variable star RS Libra,. (This parabolic comet passed perihelion on 1920 Jan 3.17 and had a perihelion distance of 0.2977 AU). In the same year, on December 13, Skjellerup discovered comet 1920 III, when it appeared as a circular nebulosity of 10th magnitude near the star Alpha Hydrae. This comet passed perihelion (1.148 AU from the Sun) on December 11.5 and had an eccentricity of 0.99408. After leaving Leo it moved on to Leo Minor (1921 January) and Ursa Major (February and March). It was then lost as the magnitude dropped below 15. (This comet was discovered independently by another friend and amateur astronomer C. J. Taylor of Cape Town, South Africa, on December 8 i.e. five days before Skjellerup). In passing it is worth noting that Skjellerup and Reid also recovered Comet Encke on 1921 July 27. In October 1921 Skjellerup took a six month holiday and visited London. There he bought a 7.6 cm altazimuth Thomas Cooke f/15 refractor and this quickly replaced the Zeiss binoculars that he had previously used for the majority of his comet hunting. (This telescope is now at Victoria College, Melbournela.) Using this new telescope, Skjellerup discovered his fourth comet, 1923 I, on November 26, 1922 at a magnitude of 7.0 in the constellation of Crater. It passed perihelion on 1923 January 4.2232 at a perihelion distance of 0.92359 AU. The eccentricity was calculated to be 0.993735, giving the comet a period of about 1800 y. His fifth comet, 1927 IX, was discovered independently in the morning sky by at least 10 astronomers but the first to announce the discovery was the

4

D . W . Hughes

now-retired Skjellerup, observing from Oakleigh near Melbourne in Australia. On 3 December it had a magnitude of 3. Maristany in La Plata, Argentina independently announced its discovery on December 6 and obtained a precise measurement of its position. The comet has been named SkjeUerup-Maristany. Rather unfairly many other observations were belatedly reported, some actually going back to November 27. On December 18.1809 the comet passed its perihelion at a distance of 0.176157 AU from the Sun. The eccentricity was calculated to be 0.99984, giving it a rather imprecise period of 36,500 y. I find it hard to resist quoting Orchiston 12 at this stage. Apparently on the night of the comet discovery, Skjellerup had been awakened by his cat knocking something over in an adjoining room. "Noting that the sky was clear, he could not resist the temptation to take out the telescope and engage in a little comet searching before slipping back to bed. He met with almost instant success! Despite this, he was not particularly overwhelmed by the discovery; the local press reports that although he was "tremendously excited when he discovered his first comet .... repetition of the experience has blunted his zest." The comet created a great deal of local interest, as a conspicuous naked-eye object. It even inspired one writer to pen the following poem for the Melbourne newspapers: The comet that's known as the Skjellerup, At least cannot get one Fjellerup. In bed "Oriel" lies, And won't watch the skies, And says he don't care who the Hjellerup. On December 15, 1927, Skjellerup, lying on the ground and using the side of the house and a chimney as a screen from the direct rays of the Sun, was able to pick up the comet through binoculars when it was only about 1.5 ° from the Sun." Skjellerup's final comet discovery occurred much later. On 1941 January 21 he got up at 3 am to see Cunningham's Comet (1941 I). He found a fourth magnitude comet in the constellation of Norms, moving in the wrong direction for Comet Cunningham. The next day he reported this discovery to the Melbourne Observatory and to James Nangle, the Government Astronomer of New South Wales. It was subsequently learnt that De Kock had discovered the comet on December 15 and it was also independently discovered by Paraskevopoulos on January 23, both observing form South Africa. The comet is catalogued as De Kock-Paraskevopoulos but Orchiston 1~ makes a strong case for it to be renamed De Kock-Skjellerup 1941 IV. Let us return to the comet 1922 I (P/Grigg-Skjellerup). In July of that year R. T. Crawford and E. Meyer, at the University of California, computed TM an elliptical orbit and noted that this orbit had certain parameters that were similar to those of P/Grigg (1902 II). This supposition was supported by the work of Leuschner and Crommelin 15. Unfortunately this early orbit for 1922 I indicated erroneously that the period was 5.52 y. A later study by Gerald Merton TM found that the period was actually 5.0 years and the common identity of the two comets was firmly established. Merton went on to predict the parameters of the 1927 return, and noted that perihelion passage would occur on May 10. Using his own ephemerides Merton and F. J. Hargreaves recovered the comet on a photographic plate taken by Hargreaves on March 27 when it had a coma magnitude of 12. The predicted time of perihelion passage was less than five hours from the observed one. Comet P/Grigg-Skjellerup has been observed at every apparition since 1922. Even so, the 1947 apparition was dramatised by the prediction of perihelion passage being in error by five days and this led to the recovery being delayed by around four weeks. The comet normally is only

J. Grigg, J. F. Skjellerup and their Comet

5

observed for a short time near perihelion passage, this being due to the inner-solar system activity index1~ of the comet being larger than usual (the larger the activity index the greater the change in cometary coma brightness as a function of hdiocentric distance). The coma thus develops rapidly as the comet approaches the Sun and is depleted rapidly as it moves away. The orbital parameters of P/Grigg-Skjellerup are shown in Table 1 and a rough illustration of its present orbit is shown in Figure 1. It can be seen from this figure that the comet, with its perihelion close to the orbit of Earth and aphelion close to the orbit of Jupiter, will be prone to frequent orbital perturbations. These are manifest by changes in the perihelion distance, the period and to a lesser extent, the orbital inclination. For example, between the 1937 and the 1942 apparitions the period dropped from 5.02 to 4.90 y, the perihelion distance changing from 0.908 to 0.856 AU. Another perturbation took place between the appearances of 1961 and 1967. Sitarski is demonstrated that this was due to a close passage of Jupiter in March 1964, an encounter that resulted in a period change of 76.65 days. On March 17, 1964 the comet and the planet were only separated by 0.33 AU. The comet also passed within 0.18 AU of Jupiter in 1905 and this gave the comet a period of five years, which, due to Jupiter's 11.86 y period, prevented another close encounter for some time. Between 1902 and 1961 the orbits of Earth and P/Grigg-Skjellerup were always more than 0.08 AU apart 1°. Even so, the theoretical possibility of an associated meteor stream was mentioned in 1952 by Porter 2°. He calculated that the shower would maximise on April 26 and have a radiant at R.A. 109° and dec. - 3 7 °. Since 1964 the comet has moved onto an orbit which has a descending node that is less than 0.01 AU from the Earth's orbit and some of the meteoroids emitted during the comet's post-1967 perihelion passages have hit the Earth's atmosphere from time to time

First Point of Aries

Eorth

P/Grigg - S kjetterup" - P" - "

oscending node ond periheiion of comet, meteor shower occurs on April 23.

Figure 1. The present-day orbit of comet P/Grigg-Skjellerup. Note that the perihelion and aphelion are very close to the ascending and descending node respectively. The dashed line represents that part of the cometary orbit that is below the ecliptic.

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D . W . Hughes

Table 1. The orbital parameters 2s of comet P/Grigg-Skjellerup year

T

q(AU)

1808 III

Mar 16

0.7310

0.7439

4.82

193.701

17.69

3.485

1902 II

July 3.503

0.75313

0.73269

4.83

350.732

221.265

8.299

1922 1927 1932 1937 1942 1947 1952 1957 1961 1967 1972 1977 1982 1987 1992 1997

May 15.694 May 10.249 May 12.609 May 23.061 May 23.380 Apr 18.137 Mar 11.158 Feb 2.634 Dec 31.357 Jan 16.475 Mar 2.652 Apr 11.000 May 14.996 Jun 20.1 Jul 24.9 Sep 3.8

0.88894 0.89270 0.90760 0.90788 0.85600 0.85313 0.85562 0.85529 0.85775 1.00285 1.00131 0.99339 0.98925 0.9933 0.9946 0.9968

0.695138 0.694235 0.690566 0.690526 0.703466 0.704273 0.703596 0.703639 0.702980 0.662435 0.662837 0.664705 0.665680 0.6648 0.6644 0.6639

4.98 4.99 5.02 5.02 4.90 4.90 4.90 4.90 4.91 5.12 5.12 5.10 5.09 5.10 5.10 5.11

355.004 355.030 355.234 355.317 356.363 356.396 356.361 356.328 356.384 359.170 359.282 359.322 359.328 359.31 359.27 359.34

215.8749 215.8586 215.7660 215.7288 215.4047 215.3779 215.3825 215.3880 215.3617 212.6855 212.6518 212.6449 212.6325 212.63 212.64 212.61

17.511 17.486 17.464 17.466 17.622 17.646 17.628 17.636 17.620 21.049 21.067 21.104 21.136 21.11 21.10 21.09

I V II III V II IV I IX I II VI IV

e

P(y)

w*

f~*

i*

and have produced a meteor shower. A brief, intense meteor shower, associated with P/GriggSkjellerup, was discovered 21 by southern hemisphere observers around 1977 April 23. It had a zenithal hour rate that peaked at around 40 h -1. The radiant was in the constellation of Puppis with approximate co-ordinates, right ascension 107 ° , declination - 4 5 ° , the geocentric velocity of the meteoroids being 15 km sec -l. As is to be expected, very few meteors were seen in the following year, the meteoroids still being orbitally clumped around the parent comet. Under these circumstances meteors are only seen when the comet is close by. In 1967, 1972 and 1977 the comet preceded the Earth to the minimum separation point between the two orbits by 97, 51 and 12 days. In 1982 the comet reached the closest separation point only three weeks after the Earth passed TM. The strength of the observed meteor showers on these occasions depends rather drastically on the shape of the meteoroid cloud. In the case of P/Grigg-Skjellerup, the recent perturbation will mean that the dust cloud on the orbit that comes close to Earth will have a very limited cross-section and could be either in front of, behind or around the comet dependent on the spin direction, the spin axis orientation and the distribution of active regions over the surface of the cometary nucleus. Considering the sparcity of land in the southern hemisphere and the limited number of meteor observers, the resultant meteor shower could be easily overlooked and the absence of observations of the shower is no indicator of the age or dustiness of the parent comet. An exciting recent advance in the study of P/Grigg-Skjellerup was made by Kres~k 2u, who has presented considerable evidence indicating that the comet 1808 III was also an apparition of P/Grigg-Skjellerup. 1808 III was discovered by Jean-Louis Pons from his Marseilles observatory

J. Grigg, J. F. Skjellerup and their Comet

7

on 1808 February 6. Pons saw it again on February 9, but rather like a ghostly precursor of Grigg's sightings in 1902, no one else saw the comet. Pons was using his "Grand Chercheur" comet-seeking telescope, this having a field of view of around 4* 15'. The comet was seen moving very rapidly (2.30* d -1) past the globular clusters M10 and M12. The identification of 1808 III as a younger P/Grigg-Skjellerup requires a careful extrapolation back in time of the present orbit of the comet. Three groups have attempted this. Belyaev, KresLk, Pittich & Pushkarev 23 used the osculating elements of the 1952 IV return and included a weak non-gravitational effect. Carnsi, KresAk, Perozzi & Valsecchi 24 used the 1972 II return without including non-gravitational effects and Marsden 25 linked in all the 1902-1982 apparitions and also included a weak non-gravitational effect. The results are given in detail in KresLk's paper but the elements axe sufficiently similar for us to record a mean value in Table 1. In comparing 1808 III with the other apparitions one can see that there is considerable agreement (note that the individual values of the argument of perihelion and the longitude of the ascending node are of less importance than the combined value, the perihelion longitude w + f/). Further evidence for the identification comes from the ephemerides of 1808 III. The comet is found to be at its brightest on 1808 February 7, a date that is exactly between the two observations made by Pons. On the night before the discovery, the comet was at a geocentric distance of only 0.12 AU, a fact that explains the rather large estimate of 1" made by Pons for the diameter of the coma. The 1808 apparition of P/Grigg-Skjellerup was the most favourable one for observers in the northern hemisphere during the 19th and 20th centuries, even though the approximate 5:1 resonance between the revolution of the comet and the Earth makes the observing geometry of successive returns rather similar. So perihelion passage in 1813, 1818 and 1823 remained in March within one week of that of 1808, but unfortunately the close passage of Jupiter (0.57 AU) in 1809 increased the minimum Earthcomet distance. The brightest coma magnitude was between 9.5 and 10. In later apparitions the perihelion passage receded to February and the brightest coma magnitude dropped to between 11 and 12. Nineteen unobserved apparitions occurred between 1808 and 1902 and there were three close encounters 23 with Jupiter in that period. In 1845 the comet and the planet passed within 0.18 AU and this not only increased the inclination by 100 but also completely reversed the nodal line, making the comet a pre-perihelion southern sky object. So the brightest coma magnitude of 8 that occurred on 1883 February 9 was missed by the experienced northern comet hunters such as Swift, Barnard and Brooks. In 1881 the comet and Jupiter passed within 0.45 AU. Drastic changes in the orbit were avoided by the fact that the encounter velocity was relatively high (6 km s-l). Thirty-five orbital revolutions have occurred between the first and the last apparitions of P/Grigg-Skjellerup. The comet has been observed at 14 of these and over a whole range of observing geometries. It has never exhibited a distinct tail although some observers have reported on occasion, an elongation of the coma or a faint fan-shaped structure within it. The success of the orbital extrapolation over the 1902-1808 period indicates that the nongravitational effects were minimal. According to KresAk 22 this implies a low value for the ratio between the mass loss per orbit and the total cometary mass. It also might point to a low spin rate. The former of these statements indicates that the absolute magnitude of the comet (the apparent coma magnitude the comet would have if it was both 1 AU away from the Sun and observed from a distance of 1 AU) should have changed very little in the last two centuries. KresAk22 starts by trying to estimate how bright comet 1808 III was when Pons saw it. Unfortunately Pons, like many comet observers of his day, recorded no photometric information. But calculations can be made, using for example the dates of naked eye visibility, to reveal the apparent magnitudes of most of the comets that Pons discovered with his "Grand Chercheur'. KresAk concludes that 1808

8

D.W.

Hughes

III had a maximum brightness of 7.54-0.5 mag. Using the standard formula, such that

ml = H s o + 5 1 o g A + 1 0 1 o g r

,

(where A is the comet-Earth distance and r is the comet-Sun distance, both in AU), Kres~k found that/'/io (1808) = 12.2 -I"0.5. This is extremely close to the value of 12.5 that was obtained for the 1977 apparition (see Bortle26). Kres~,k22 has re-analysed the data of all the recorded apparitions, concentrating on only the magnitude estimates made visually using small telescopes of low magnification. These absolute magnitudes are shown in Figure 2 as are a set of Hi0 values given by Vsekhsvyatskii 2z'~s. The fairest conclusion that can be drawn from Figure 2 is that the absolute magnitude of the comet has not changed measurably in the last two centuries~. The median value of the absolute magnitude is 12.45. Hughes s° concluded that logR (km) = 1.114 - 0.51ogf - 0.2Hz0 , where R is the radius of the cometary nucleus and f is the fraction of the nucleus surface that is actively emitting gas and dust when the comet is close to perihelion. The constants have been obtained by using the mean observation of P/Halley, i.e.//10 = 5.5, f = 0.034 and R = 5.6 km. If it is assumed that the surface activity of P/Halley is typical and that P/Grigg-Skjellerup has a

I

~

I

I

I

I

I

l

l

I

I

4-

I0 "I-

11 12 E

o o o

o

o

o o

13

o8 8++o

g

o 4-

+

1/.

+

÷

15 t

1808

~

q- O - k

t

x

t

J

t

I

t

9

t

1902

12

1922

32

z.2

52

62

72

1982

Date Figure 2. P/Grigg-Skjellerup's absolute magnitude (i.e. the apparent magnitude at a distance of 1 AU from both the Sun and Earth) is plotted as a function of the date. The open circles have come from Kres~l¢ ~2 and the crosses from Vsekhsvyatskii aT,2s. Using only the Kres~J¢ results, the median absolute magnitude for the period 1808 to 1982 can be seen to be 12.45.

J. Grigg, J. F. SkjeUerup and their Comet

9

similar value, the formula quoted above becomes log R (km) = 1.848 - 0.2H10 • So the nucleus of P/Grigg-Skjellerup has a mean radius of 0.23 kin, some 25 times smaller than P/Halley. P/Grigg-Skjellerup is nothing if not a very faint comet. As an example of this, it was recovered as 1986m by K. Birkle of the Max-Planck Institut ffir Astronomie, Heidelberg, using the 3.5 m telescope at Calar Alto, with a CCD camera at the prime focus. The first detection was made on August 12 when the comet was stellar in appearance and had a red (R) magnitude of 22 (see IAU Circular No. 4255). The European Space Agency is now actively involved in the Giotto Extended Mission (GEM). The spacecraft was successfully reactivated on 1990 February 19 and was found to have "seven fully operational or only marginally degraded instruments sl". Unfortunately the camera is unable to detect anything. In March the hydrazine thrusters on Giotto were fired in order to divert the spacecraft onto the orbit required for the Earth flyby. Giotto flew past the Earth on 1990 July 2 at a minimum distance of 22730 kin. This was the first Earth gravity assisted manoeuvre ever performed on a spacecraft. Giotto is now on its second hibernation. It is planned to reactivate it in mid-May 1992 and it will encounter P/Grigg-Skjellerup in July 1992. The flyby speed will be 13,9 km s-l,a mere 20% of its encounter speed with P/Halley. This, coupled with the fact that the dust and gas production by P/Grigg-Skjellerup is about 0.75% of that of P/Halley, makes the dust hazard insignificant. There is thus a negligible risk of telemetry loss. As the camera is not working there is no requirement that the spacecraft encounters the comet on the sunward side. The present feeling among the principal investigators is that Giotto should be aimed directly at the nucleus of P/Grigg-Skjellerup. The chance of a direct hit is minute. At the present time the comet's orbital uncertainty results in a 2000 km error ellipse. Ground-based astrometry at a time nearer to flyby could improve this to 800 km. The Hubble Space Telescope Fine Guidance Sensors would drop this error to 100 km. What better to ask Giotto to do than to follow its visit to a big retrograde periodic comet, by a visit to a small prograde one.

Acknowledgements.

I would like to thank Wayne Orchiston of the Gisborne Museum, New Zealand and Peter Hingley and Mary Chibnall of the Royal Astronomical Society Library for their help. 1 Report of the Council of the RAS, Mon. Not. R. astr. Soc.,81,258-258 (1921). 2 Stroobant, P. et al., Lea Observatoires astronomiques et les astronomes, Bruxelles, Hayes, imprimateur de L'Observatoire Royal de Belgique, 1907, p. 235. See also Pubs. Astron Soc Pacific, 16, 28-29 (1904). 3 Weiss, E., Denkschr. Akad. Wiss. Wien, 84, 9 (1909). 4 Marsden, Brian G., Q. J. Roy. astr. Soc. , 26, 79, (1985). 5 Mon. Not. R. astr. Soc. , 83, 244 (1923). 6 Kronk, Gary K., Comets, a descriptive Catalog, Enslow, Hillside, New Jersey, (1984), p. 255. 7 Mon. Not. R. astr. Soc., 63, 242 (1903). 8 The Story of Comets, by George F. Chambers, Oxford University Press. 1910. p. 256. 9 Mon. Not. R. astr. Soc. , 83, 288 (1923). 10 Pubs. Astron. Soc. Pacific, 35, 80-81 (1923). 11 Brown, P.L., Comets, Meteorites and Men, Robert Hale & Co. London (1973), p. 131.

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