|Number 20: 24/05/2003
A scientific publication by SGF and NEODyS
|NEOs' Spacemissions Special: mission GIOTTO
"The night of the comet " by Ettore Perozzi - Telespazio
Figure 5. A composite image of the nucleus of comet Halley composed of 68 images at a resolution varying from 500 m px-1 to 50 m px-1 near the active region.
Figure 6. A tableau of features. Sections of the composite image (centre bottom) have been extracted and expanded by a factor of 3 to show, in detail, notable features on the nucleus. The position of each expanded section is marked as a box on the composite and a corner of each section is linked to its counterpart by a line. Non-linear enhancement has been applied to provide improved contrast.
Figure 8. This Vega image led someone to believe that Halley were a "double object"
Figure 9. Giotto flyby sequence
March 13, 1986
2 :00 pm
At the European Space Operations Centre (ESOC, Darmstadt, Germany) the last pre-encounter press conference is particularly interesting because of the participation of the Russian and Japanese delegates. Preliminary results from their missions toward comet Halley are expected. Both Vega 1 & 2 (Russian) and Sakigake & Suisei (Japan) have already encountered Halley, even if from a farther distance than what it is planned for Giotto, and with less powerful instruments. False colour images (i.e. processed using an artificial scale of relative luminosity) of Halley's nucleus are shown but they allow different interpretations. In particular it is not clear if the two bright features detected imply a somewhat "double" object or they are just an artifact due to the presence of large dust clouds obscuring part of the nucleus. Hopefully, Giotto will answer the question. The Japanese spacecrafts have been severely damaged by impacts during flyby and since this is a major concern also for Giotto (at a relative velocity of 70 km per second even the tiniest dust particle becomes a dangerous projectile), discussion rises high on this point. What about the more massive Vega spacecrafts? They survived, but 80% of the solar panels is destroyed. What was the particle flux encountered at their minimum approach distance (8000 km)? Good news: less large particles than expected.Among the participants there is Fred Whipple, who first proposed, back in the '60s, the now widely accepted dirty snowball model for the nucleus of comets. A 1:1 model of the Giotto spacecraft is also displayed and at the end of the press conference it is selected as a proper background for an historical group photograph.
8 :00 pm
ESOC enters into safety mode: access is restricted and entering into different areas is ruled by coloured badges. Green is for the press, Blue are scientists involved in the mission, White if you are a VIP. Yellow means following the event from the wide screens installed at the canteen (where food and beer are also served through the night), while wearing a Red Badge lets you in everywhere. Mixed colours are also found: Yellow badges with a Green stripe, Blue and White, and so on: a multicolour tribute to German efficiency.Let us take a blue badge and climb to the top of the ESOC building, where science is. A long corridor leads to ten rooms, one for each instrument on-board Giotto. Inside, the usual parade of computer screens and printers for performing quick look analysis of the incoming data. The most visited room is that labelled HMC: Hally Multicolour Camera. Here, starting at 9 pm and at a pace of one every 4 seconds, the incoming images of the comet will be processed and displayed.
9 :00 pm
Waiting for the first Giotto image. As time goes by, tension rises: past communication problems are back in everyone's mind - like those which occurred not long ago, when a bulldozer mistakenly cut the wire connection to the DSN antenna, down yonder in Australia. At 9.30 pm video screens are still blank.
10 :10 pm
Clappings coming out of the HMC room indicate that they finally made it: a few minutes later Comet Halley as seen from Giotto is on each and every screen. The "Night of the Comet" has just begun - a long night since the spacecraft is presently more than 700.000 km away from its target and the success of the mission will be decided only in the last few minutes, when it will enter in the so-called nucleus mode. In fact, data coming from the Vega missions have allowed to reduce the uncertainty of the position of Halley's nucleus and Giotto should then be able to flyby it at less than 500 km.At the moment all scientific instruments are alive and well. Those measuring the interaction of the comet with the solar wind have already detected some turbulence, while the dust particle counter is, fortunately, quiet. Nothing relevant is going to happen in the next two hours, mostly devoted to routine operations.
23 :00 am
We take a break then and get out of the ESOC main building. Right outside the entrance gates a small post office has been built for the occasion by the Deutsche Bundespost. There you can mail letters or postcards which will be marked with a special logo commemorating the Giotto-Halley encounter. Tomorrow is too late, since both office and logo-marker will be dismantled. Thus it appears worthwhile joining the long cue for profiting of this rather unusual aspect of the space business.
March 14, 1986
00 :02 am
There is turmoil in the corridor of the scientific intruments: the dust particle counter has just detected some impacts. The news is acknowledged with mixed emotions: it is the proof that the instrument is working properly, but at the same time it rings an alarm on impacts. One hour has still to go before the closest approach and the spacecraft is 250.000 km away.The HMC group is particularly concerned because while all the other instruments are well protected by the spacecraft double-shield, the camera needs a small mirror to protrude from the spacecraft main body and watch directly the comet thus sending images to the HMC optics and electronics. Every impact, even the tiniest, produces a small crater on the mirror, contributing in the long run to a progressive decay of the quality of images. A problem well known: what it is not known is the timescale of the process: will the mirror succeed to remain sufficiently clear until closest approach?
00 :20 am
Halley now fills almost entirely Giotto's camera field of view. The HMC room is overcrowded, as everyone wishes to give a contribution to analysing the images. Suddenly, the lights go off: a shot of panic runs through the bones before realizing that nothing really serious has happened. It was just an unlucky guy who laid on the wall right over the light switch, turning it off. But it's enough: all people not directly involved in HMC operations is kindly invited to leave the room. No additional risk is allowed, beyond the many that Giotto is already going to face in a short while.
00 :40 am
Coming down from the science corridor we use our green badge to enter the press room. There one can follow an interesting interview with Jan Oort, whose hypothesis on the existence of a "cloud of comets" surrounding our Solar System has revolutioned cometary research. The lucky old man has witnessed the spectacular 1910 Halley's return, which he observed as a naked eye object, and hopes on this very night to have a second, much, much closer look at the comet. To be honest, we need to say that the images which appear on the screens are not "user friendly". False colours are certainly meaningful for science, but they often tend to hide some basic visual information one is used to have from a photograph that is in our case: where is Halley's nucleus? Does it correspond to the green or to the yellow profile? How close Giotto is to the comet? Am I looking to the whole nucleus or just to one region of its surface?We then decide to choose a different strategy to follow properly the crucial phases of the encounter by using a chronometer and some quick calculations. At the very high speed with which Giotto is flying toward Halley, we know that 4 minutes before closest approach (occurring at time T) it will be 16000 km away from the nucleus; at time T-2 minutes the distance is halved: 8000 km (corresponding to the minimum approach distance of the Vega spacecrafts). If one continues halving time and space in this way while checking if images are still arriving, it is possible to estimate whether the mission is achieving its goal.
00 :50 pm
T-1 minute: the distance is 4000 km and images are still clear. There is silence now in the room, filled with anticipation. A few seconds later the feeling of success is in the air: no-one is anymore sceptical on the fact that Giotto has successfully achieved its very close encounter with Halley's comet. As soon as one starts to believe that nothing bad is going to happen anymore, the screen suddenly fills up with multicolour dots and a voice from the control room tells us that contact with the spacecraft is lost. But it happened late enough, success is granted anyway! Post processing of data will in fact reveal that the HMC mirror has degraded completely when the spacecraft was less than 1000 km from the nucleus and that Giotto lost contact a few seconds later, just before achieving a minimum flyby distance as low as 250 km!
03 :30 pm
The attitude recovery system of the spacecraft has worked out successfully to align again the Giotto antenna toward the Earth and the spacecraft is again under control. It seems the last good news of this historical night, when, an clear good old black-&-white image is shown on a secondary screen. Right in the centre of the frame a strange object appear: one can clearly see the bordering line separating the night side from that directly lightened by the Sun (it is the terminator: only the more optimistic hoped to see it!) and two regions from where powerful jet streams are ejecting ice and dust into space. A small crowd gathers around the screen, watching in silence: here it is, that potato-shaped object floating inside a dense dust cloud, that is the true face of Halley's comet.