Giotto and the Flyby of Comets Halley and Grigg-Skjellerup
Visiting a comet is spectacular, with all the logistics and calculations required to reach a very small object in space. What is even more amazing is when it is done twice. Giotto accomplished this in the late 80’s and early 90’s with much fanfare and success. How it accomplished this is just as amazing, and the science it gathered is still being explored to this day.
Goals, Development, and Launch
Giotto was ESA’s first deep space probe and initially a dual organization mission with NASA as the other partner. The mission was to be entitled the Tempel-2 Rendezvous and Halley Intercept Mission. However, budget hits forced the American space program to withdraw from the mission. ESA was able to get Japanese and Russian interests to join and keep the mission going (ESA “ESA”).
Giotto was launched with a few goals in mind. These included the return of color images of comet Halley, to determine what makes up the coma of the comet, find out the dynamics of the atmosphere and ionosphere, and determine what the dust particles are made up of. It was also tasked with finding out how the dust changed with time, how much gas was produced per unit of time, and explore the interactions the of the plasma formed from the solar wind hitting the particles around the comet (Williams).
With so much science to be done, one needs to make sure that you have all the instruments required. After all, once launched you have committed and there is no turning back. All of the following equipment was placed onto Giotto: a visual camera, neutral mass spectrometer, ion mass spectrometers, dust mass spectrometer, plasma analyzers, dust impact detector system, optical probe, magnetometer, energetic particle analyzer, radio science experiment. Of course, it needed power too so a 196 Watt solar cell array consisting of 5000 silicon cells was installed all around the surface of the probe. Four silver cadmium batteries were onboard as backup (Bond 45, Williams, ESA “Giotto”).
Moreover, how would this craft be protected? After all, it would be bombarded with particles as it flew close to the comet. A dust shield was created out of 1-millimeter thick aluminum with 12 millimeters of Kevlar beneath it. It was rated to withstand impacts of objects with mass 0.1 grams, based on the speed the particles would hit Giotto. With all of that in place, Giotto launched aboard an Ariane rocket on July 2nd 1985 (Williams, ESA “Giotto”).
To house all of this science, Giotto was based off a British Aerospace GEOS satellite, which is cylindrical in design with a height of one meter and a diameter of two meters. The top of the probe had a high-gain antenna while the bottom contained the rocket for maneuvering once in space (ESA “Giotto”).
March 1986 was the big event as half a dozen spacecraft approached comet Halley for a close-up look. Giotto got to within 596 kilometers of the nucleus (just 96 short of the target distance), encountering debris being ejected from the comet. Scientists were frankly surprised that Giotto emerged from its encounter functioning. However, a piece of dust 1 gram in size hit Giotto at 50 times the speed of sound, causing the probe to spin and temporarily lose contact with mission control. 30 minutes after the encounter, communication was reestablished and photographs were collected (Bond 44, Williams, ESA “ESA”).
Based on the collected data, the nucleus appeared to be 16 by 7.5 by 8 kilometers in size and was shedding up to 30 tons of material a second. About 80% of the gas the comet gave off was water-based with the remaining gas being made of carbon dioxide, carbon monoxide, methane, and ammonia. The dust that Giotto encountered was a mix of hydrogen, carbon, oxygen, nitrogen, iron, silicon, calcium, and sodium, and they hit in waves as gas layers separated from the comet. One of these was the isopause from 3,600 to 4,500 kilometers from the nucleus. This is where the pressure from the coma of a comet and solar wind balance each other out. Giotto hit one final layer at 1.15 million kilometers from the nucleus called the bow shock, or the place where the solar wind (which is pushing material off the comet) slows down to subsonic speeds. Surprisingly the surface was very dark and only reflected 4% of the light striking it. (Bond 44, ESA “Giotto”).
Offline and Diagnosis
After successfully completing the Halley flyby, Giotto was put into a 6:5 orbital resonance with us, with us completing 5 orbits around the sun for every 6 Giotto does. Once this was done, Giotto was put into hibernation, waiting to wake up for another mission. Scientists began to take in inventory of what they had left and what was destroyed. Amongst the casualties were the camera, neutral mass spectrometer, 1 of the ion mass spectrometers, the dust mass spectrometer, and the plasma analyzer. However, the dust impact detector system, optical probe, magnetometer, energetic particle analyzer and radio science experiment survived and were ready for use. And with this in mind in June of 1991 the ESA approved of a mission for Giotto to do another flyby at a cost of $12 million (nearly $35 million today, a good deal). Preparation for this had already been done on July 2, 1990 when Giotto became the first space probe to use out gravity to alter its orbit. It travelled to within 23,000 kilometers of our surface, on course for Grigg-Skjellerup. It was then put back into hibernation as it travelled on (45).
After years of sleeping, Giotto was woken up on May 7, 1992 and on July 10, 1992 made a fly-by of Grigg-Skjellerup. This target was a choice of convenience, for it passes by every 5 years while Halley only makes an appearance every 78 years. But that does come at a price, for Grigg-Skjellerup has passed by the sun so many times now that much of the surface has sublimated leaving a very dull object, which doesn’t get very bright. That being said, Grigg-Skjellerup does not travel in a retrograde motion like Halley, so Giotto could approach the comet from a different trajectory and at a slower rate of 14 kilometers per second (42, 45).
Giotto was oriented at a 69-degree angle from the plane of orbit when it visited Grigg-Skjellerup, too steep for its shield to protect it from particulate. It had to be done, however, for there would have been no other way for the high-gain antenna to transmit data to Earth and because the batteries were dead and the only way the probe was getting power was from the solar panels facing the sun. Additionally, because the camera was not in commission after Halley, Giotto needed Earth to help keep the probe on track (46).
At a distance of 400,000 kilometers Giotto began to measure particulate from Grigg-Skjellerup, according to Andrew Coates of the Nullard Space Science Lab in Surrey, England. The manometer and energetic particle analyzer found that the turbulences were very different than those encountered with Halley. Unlike the high turbulence encountered at Halley Giotto found that smooth waves separated by about 1000 kilometers were the norm at Grigg-Skjellerup. As the probe approached the comet, the number of ions hitting it increased as the solar wind levels decreased. After passing the bow shock (which was less defined here than at Halley due to the distance away from the sun) at 7000 kilometers from the comet, the first carbon monoxide and water ions were detected. Even though the comet released 3 times as much gas as predicted, it was still 100 times less than the amount measured at Halley (46).
As Giotto neared the nucleus, the ion levels began to decrease as gas coming off the comet absorbed them and made them neutral. A magnetic field was also found and based off the levels found it seems as though Giotto went behind the comet and not in front. Eventually, Giotto got within 200 kilometers of the comet based off the Optical Probe Experiment equipment. Dust levels peaked shortly after this milestone. Giotto made it through its entire encounter without significant (and crippling) damage. Only 3 pieces of dust were detected on the Dust Impact Detector System. Of course it is likely that even more hits occurred but either they were of low mass or had less energy. Additionally, the dust shield was at that odd angle which did not favor good hits on the system. Something else did hit Giotto, however, because a change of velocity of 1 millimeter per second was detected along with a wobble (Bond 46-7, Williams, ESA “Giotto”).
Sadly Grigg-Skjellerup was the last comet Giotto was able to visit. After the encounter the probe had only 4 kilograms of fuel left, just enough to get it home. It did fly by us on July 1, 1999 with a closest approach of 219,000 kilometers and a speed of 3.5 kilometers per second for a final farewell to its homeport. Then, it sailed on for parts unknown (Bond 47, Williams).
Bond, Peter. “Close Encounter with a Comet.” Astronomy, Nov. 1993: 42, 44-7. Print.
ESA. “ESA Remembers the Night of the Comet.” ESA.in. ESA, 11 Mar. 2011. Web. 19 Sept. 2015.
---. “Giotto Overview.” ESA.in. ESA, 13 Aug. 2013. Web. 19 Sept. 2015.
Williams, Dr. David R. “Giotto.” Fnssdc.nasa.gov. NASA, 11 Apr. 2015. Web. 17 Sept. 2015.
© 2016 Leonard Kelley
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