APAS 1110 Summer 1996
Cool Image of the Day -- June 19
The Halley Encounter
The spacecraft encountered Halley on March 13, 1986, at a distance of 0.89 AU from the Sun and 0.98 AU from the Earth and an angle of 107¡ from the comet-sun line. The goal was to come within 500 kilometers (310 miles) of Halley's comet at closest encounter. The actual closest approach was measured at 596 kilometers (370 miles). The spacecraft had a dust shield consisting of a front sheet of aluminum (1-millimeter thick) and a 12-millimeter (.4-inch) kelvar near sheet separated by 25 centimeters (10 inches), which could withstand impacts of particles up to 0.1 g.
All experiments performed well and returned a wealth of new scientific results, of which perhaps the most important was the clear identification of the cometary nucleus. Fourteen seconds before closest approach, Giotto was hit by a "large" dust particle. The impact caused the spacecraft angular momentum vector to shift 0.9 degrees. Scientific data were received intermittently for the next 32 minutes. Some experiment sensors suffered damage during this 32-minute interval. Other experiments (the camera baffle and deflecting mirror, the dust detector sensors on the front sheet of the bumper shield, and most experiment apertures) were exposed to dust particles regardless of the accident and also suffered damage. Many of the sensors survived the encounter with little or no damage. Questionable or partially damaged sensors included the camera (later proved to not be functional) and one of the plasma analyzers (RPA). Inoperable experiments included the neutral and ion mass spectrometers and one sensor each on the dust detector and the other plasma analyzer (JPA).
Nucleus
For the first time images of a comet nucleus could be taken during the encounters with
comet Halley. The nucleus was larger then expected at 16 by 7.5 by 8 kilometers (10 by 4.7
by 5 miles), implying a lower albedo (~0.04) than predicted. It was confirmed that the
nucleus can be described as a dirty snowball or even as an icy dirt ball. The nucleus is
probably very fluffy and might have a bulk density as low as 0.3 g cm-3. The discovery of
gas and dust jets being emitted from the nucleus suggested that only about 10% of the
surface was active. The structures clearly identified in the images, such as hills and
depressions, were obviously related to a continuous variation in the surface morphology.
Neutral Gas and Ion Composition
The chemical composition of the gas changes with distance from the nucleus in a complex chain of chemical reactions. The 'parent molecules' at the beginning of this chain reflect the composition of the nucleus. As expected, water (H20) was found to be the dominant (80%) parent molecule. Other parent molecules are carbon monoxide (CO) at 10%, carbon dioxide (CO2) at 2.5%, CH4 (7%), NH3 and at the 0.1% level HCN and various hydrocarbon (C2Hn, C3Hm). Iron (Fe) and Sodium (Na) were also found, and S2 and H2S are strongly suspected to be other parent molecules.
The cometary gas is ionized by solar ultraviolet radiation, by electrons and by charge exchange with the solar-wind plasma. A large number of ionic species were identified, among them H30+ (the most dominant ion near the nucleus), H20+, OH+, C+. CH+, 0+, Na+, C2+, S+ and Fe+.
At the time of the Giotto encounter on 14 March 1986 at about 0.9AU heliocentric distance, comet P/Halley emitted about 18 metric tons (19.8 tons) of gas every second, together with the emission of dust, which was of nearly the same order, an estimated 30 metric tons (33 tons) of material was released every second by the nucleus.
Previous Cool Images of the Day