When Halley's Comet rounds the sun a year and a half from now, Earth and comet will be on opposite sides of the star. The solar glare will obscure the comet for earthly observers. But the view from Venus should be spectacular.
That's why the US National Aeronautics and Space Administration (NASA) wants to use the Pioneer observatory, now orbiting Venus, to scan the comet.
This plan, announced earlier in the year, was successfully tested last April with Comet Encke. Now, according to a recent statement by the NASA-Ames Research Center, the results of that test have given Pioneer-Venus controllers confidence the plan will work.
This gives the United States a major opportunity to contribute to the massive international scientific effort being organized to study the famous comet.
Besides extensive ground-based efforts, that study includes close-in measurements and photographs to be made by five spacecraft - one from the European Space Agency, two from Japan, and two from the Soviet Union.
US scientists were disappointed when neither the Carter nor Reagan administration approved programs for a US Halley's mission. But NASA has now put together a program that gives the US a strong role in comet exploration even without a Halley's probe.
To begin with, NASA has diverted an old solar-wind monitoring satellite to cometary duty. Launched in August 1978 as an International Sun-Earth Explorer (ISEE-3), it is now called the International Comet Explorer (ICE).
With the agreement of European partners, NASA put the spacecraft through a series of complicated maneuvers last year. These ended with a close swing around the Moon on Dec. 22, 1983, which propelled ICE free of the Earth-Moon system.
Now ICE is headed for a rendezvous with Comet Giacobini-Zimmer Sept. 11, 1985 , when it is to probe the comet tail about 15,000 kilometers from the nucleus. Later, ICE is to measure the solar wind upstream of Halley's Comet.
ICE reportedly is in good shape for a spacecraft already more than three years past its designed lifetime, although its batteries are dead and it relies on solar-cell power.
It left the Earth-Moon system with 70 percent of its fuel reserves remaining, according to the NASA-Goddard Space Flight Center. Eventually, it might be possible to put the craft on a trajectory that would bring it close enough to be retrieved some time between the years 2012 and 2015.
Meanwhile, as part of the Halley's research effort, NASA plans to pick up the comet during the first mission of the shuttle-based Astro observatory.
This is a pallet, carried in the shuttle's cargo bay, with two wide-field cameras and three ultraviolet (UV) telescopes. The seven-day mission now is scheduled to overlap the arrival of the comet probes in March 1986. Astro data and photographs should be a valuable supplement to the probes' findings.
But the data now expected from the Pioneer-Venus craft will be a unique contribution to the Halley's studies. No other instruments will be as well placed to observe what happens when the comet swings around the sun at perihelion (closest approach) at about 180,000 kilometers an hour on Feb. 9, 1986.
These instruments also include a UV scanner to detect ultraviolet radiation from cometary material exposed to sunlight. Such measurements should help determine the comet's gas composition, gas-to-dust ratio, and the rate at which its water is vaporized. The three-month observing period is to begin in December 1985.
Here again, NASA is making new use of an old spacecraft, which was also launched in 1978.
In this case, the project involves substantial redirection of the spacecraft from its Venus-oriented attitude. This is the maneuvering that NASA tested with Comet Encke.
A short-period comet, Encke returns every 3.3 years. Its nucleus is believed to be about 2 kilometers in diameter. Controllers at the NASA-Ames center reoriented Pioneer-Venus April 13 to observe the comet and a calibration star Spica for about 11/2 days. The comet was 85 million kilometers from the sun and almost twice as far from the spacecraft.
Although this was only a test, Pioneer-Venus did return new information. Its UV data on hydrogen indicate the comet was losing water at about three times the expected rate, according to Ian Stewart of the University of Colorado, head of the research team.
This might be due to the way dust and ice are mixed together in the comet nucleus or to the crumbling of surface features not included in the scientists' model of Encke.