4th Annual Andrew Trimble Memorial Lecture 6th February 2006
“Deep Impact: The story so far”

Professor Alan Fitszimmons, QUBThe 4th annual Andrew Trimble Memorial Lecture took place at the Thompson Primary School, Ballyrobert at on Monday 6th February.

Professor Alan Fitszimmons from the Astrophysics department of Queen's University, Belfast gave us a fantastic lecture on "Deep Impact the Story so Far". The lecture covered the Deep Impact mission in detail as Alan showed us the many images captured before during and after impacting a copper projectile into Comet Tempel 1 on 4th July 2005.

Our EAAS Chairman, Mark Stronge started the meeting by highlighting the new features of the EAAS newsletter. There is a detailed guide to the moon and its phases which also details the times of rising and setting for both the moon and sun and any interesting planetary conjunctions. He highlighted an interesting Mercury - Uranus conjunction between the 11th and the 16th of February low in the SW in the early evening . Mark also talked about a new comet - Comet McNaught C/2005 E2 at Mag 9.1, which is visible low in the WSW in Pisces as darkness falls, but which has not been observed from the UK yet. Mark then discussed the possibility of a 2006 Messier Marathon and suggested dates when it could be arranged for.

Mark then introduced Robert Dick to say a few words about Andrew Trimble. Robert is the son-in-law of Andrew Trimble, who was a founding member of the East Antrim Astronomical Society and for whom this lecture is held in his memory. Andrew Trimble was passionate about his astronomy and Robert had great pleasure in welcoming Prof. Alan Fitzsimmons to deliver the Andrew Trimble Memorial Lecture.

Robert Dick tells us about Andrew Trimble

Prof. Fitzsimmons thanked the society for inviting him to deliver the lecture. The subject matter was “Deep Impact: The story so far”. This lecture dealt with the NASA mission Deep Impact which successfully launched in January 2005 and fired an impactor into Comet 9P/Tempel 1 in July 2005, the first mission of its kind. He first outlined an overview of the lecture which broke down into the following five areas:

  1. What is a Comet?
  2. Comet 9P/Tempel 1
  3. The Nucleus
  4. The Actual Impact
  5. The Future

Prof. Fitzsimmons first outlined what a comet is. There is the nucleus which is made mostly of water ice and can be between 1 and 10 km in size, then there is the coma which surrounds the nucleus. The coma can be up to 100,000 km in size. The two tails were then outlined. There is the dust tail and the ion tail. Both form when a comet comes into the inner solar system and is heated up by the Sun. The dust tail can be up to 1 million km long and the ion tail up to 10 million km long. The ion tail is formed when the ice in the comet turns into gas and becomes ionised and is caught by the solar wind. It was pointed out that comets are made of material which is thought to be the building blocks of the solar system, the stuff that the planets are made of.

Alan in full swing

Prof. Fitzsimmons then outlined the history of missions to comets. The first was Giotto - an ESA mission to Comet Halley in 1986. Halley was the first nucleus to be studied and it was calculated that its nucleus was 15 km by 7 km in size. The second visit to a comet’s nucleus was Deep Space 1 in 2001 which visited Comet Borrelly, its nucleus was found to be 3 km by 1.5 km in size. Then there was the visit to Comet Wild 2 in 2003 and its nucleus was found to be 5 km wide.

Prof. Fitzsimmons then gave a brief history of Comet 9P/Tempel 1. It was discovered in 1867 and found to have a highly elongated orbit which brought it close to Jupiter. Jupiter’s gravitational influence affects the comet’s orbit every time it comes into the orbital plane of the solar system and this was demonstrated as Prof. Fitzsimmons showed the comet’s calculated orbital path in 1881, 1887 and 1953. It is thought that when it was discovered in 1867, it was the comet’s first visit into the solar system from its starting point in the Edgeworth-Kuiper Belt. The comet has a highly tilted orbit compared to the Earth and it was pointed out that NASA had calculated that July 2005 was the perfect time to send a spacecraft to the comet when the orbital planes of the comet and Earth were aligned.

The Deep Impact mission was delayed 1 year and was originally due for launch in January 2004. The delay meant that the spacecraft was sent directly to the comet, instead of completing 1 full orbit of the Earth before heading for the comet, as was the original plan. We were then shown a video of the launch on 12th January 2005 with the spacecraft onboard a Delta 2 rocket. The mission was then outlined, showing how the impactor was released from the main craft 24 hrs before the encounter with the comet and how the main craft manoeuvred itself away from the comet’s path. Prof. Fitzsimmons pointed out that if things had gone wrong and the impactor had not released properly, then the entire craft was to impact on the comet!

A series of images from the Faulkes Telescope from November 2004 onwards showed how the comet gradually brightened as it moved into the inner solar system. Through the use of light curves, Prof. Fitzsimmons showed us that the rotation of the nucleus of the comet was 40 hours. Pictures from the spacecraft itself and an animated movie showed how the comet went through a huge outburst of material from the 22nd to the 24th June, a couple of weeks before impact. It was only 2 days before impact that the first pictures of the nucleus itself were taken by the spacecraft.

Prof. Fitzsimmons observed the impact from Hawaii, using the Faulkes Telescope North. In terms of the impact itself, the impactor didn’t actually hit the comet, the comet ran into the impactor. The comet was moving at 30 km/s, with the impactor only moving at 20 km/s! Impact occurred at 05:45 UT on July 4th which ironically was still July 3rd on the West coast of the USA and Hawaii, the day before American Independence Day! The timing of the impact was significant as the comet was quite low in the sky and set early in the evening so telescopes in California and Hawaii were still in a position to view the comet, before it was lost from view. We were shown the movie from the impactor camera as it got closer and closer to the comet with the last image being only 4 seconds before impact.

A good tun out

The analysis of the results from the mission is still ongoing but many things have been learnt already. The nucleus has been determined to be 8 km long and 4.5 km wide and that there is evidence of impact craters on the comet. The comet is a dark red colour and is a very dark object, only reflecting 4% of light it receives from the sun. This was the first detection of water ice on the comet in solid form. Another interesting result was that the distribution of material in the nucleus was not constant, which had been previously assumed. In the North of the nucleus, Carbon Dioxide only accounted for 3% of the material, whereas it accounted for 14% in the South of the nucleus. In terms of the comet’s brightness, 30 minutes after impact the magnitude of the comet was 13.1 compared to 14.6 pre impact.

The crater which was formed due to the impact was not visible from the main craft due to the amount of material which was expelled from the nucleus. The impactor weighed one third of a ton and managed to expel one thousand tonnes of material! The density of the comet is only 40% than that of ice and the comet itself is highly porous. There is more dust than ice on the comet and the make up of the comet’s surface is different from that of the nucleus. In terms of the future, there is a possibility of the main Deep Impact craft visiting Comet 85P/Boethin. This is because the craft will come past the Earth in 2007 with a possible comet rendezvous in 2008. Professor Alan Fitzsimmons gave an excellent lecture and he had many questions to answer.

At the end of the meeting, the Mayor of Newtownabbey, Alderman Billy DeCourcy presented Alan Fitszimmons with a piece of Trone Crystal engraved with the official EAAS logo, on behalf of the Trimble family and society members and in appreciation of the excellent lecture.

Mayor, alderman Billy DeCourcyMayor and Alan Fitszimmons

After the meeting there was a cup of tea provided and the members and public had some time to chat with Alan and discuss the latest news in the astro community.

Mayor and Mark StrongeMayor Billy DeCourcy, Speaker Prof. Alan Fitszimmons and EAAS Chairman Mark Stronge



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