Scientists have sent probes to investigate many kinds of celestial objects. In 1995, the Ulysses probe was launched towards the Sun and took readings of the solar wind and the star’s magnetism. The Giotto probe, launched in 1986, battled its way past flying debris and gas into the heart of Halley’s Comet, taking incredible pictures of its nucleus. Asteroids have also been visited by space probes. The Near Earth Asteroid Rendezvous probe landed on the asteroid Eros in 2001.
The 1985-1986 emergence of Halley’s Comet, the first since the advent of the space age, was explored by a variety of spacecraft. The Vega 1, launched by the USSR together with the Eastern-block alliance, passed 5523 miles from the Comet’s nucleus at 7:20:06 Universal time. It indicated that the Comet was about 300 miles closer to the sun than had been predicted. The Japanese spacecraft, Suisei, was created to map the distribution of neutral hydrogen atoms outside Halley’s visible coma. Its pictures indicated that the Comet’s output of water varied between 25 and 60 tons per second. Five days after the Vega 2’s passage through the Comet, the Giotto (sponsored by the European Space Agency) probe appeared. Giotto’s close approach took place 3.1 minutes after midnight UT on March 14th; the craft had passed 376 miles from its target. Giotto’s data indicated that the nucleus was bigger than expected, and that the Comet was composed primarily of water, CO2 and N2. The Vegas and Giotto found that as the solar wind approaches Halley, it slows gradually and the solar magnetic lines embedded in the wind begin to pile up. Pick-up ions, from the Comet’s halo of neutral hydrogen, were found in this solar wind. Sensors on the Vega spacecraft found a variety of plasma waves propagating inside the bow wave. In order to synthesize all the results, a conference on the exploration of Halley’s Comet will be held this October
Data on the nitrogen-containing compounds that observed spectroscopically in the coma of Comet Halley are summarized, and the elemental abundance of nitrogen in the Comet Halley nucleus is derived. It is found that 90 percent of elemental nitrogen is in the dust fraction of the coma, while in the gas fraction, most of the nitrogen is contained in NH3 and CN. The elemental nitrogen abundance in the ice component of the nucleus was found to be deficient by a factor of about 75, relative to the solar photosphere, indicating that the chemical partitioning of N2 into NH3 and other nitrogen compounds during the evolution of the solar nebula cannot account completely for the low abundance ratio N2/NH3 = 0.1, observed in the Comet. It is suggested that the low N2/NH3 ratio in Comet Halley may be explained simply by physical fractionation and/or thermal diffusion.