April 7, 2020

Astro-Event: An Occultation of Venus.

Occultations are one of the few split-second events in the field of observational astronomy. Unlike most spectacles which may take longer than the average human life span, (the orbits of many double stars comes to mind!) An occultation of a star or planet can occur with abrupt swiftness. Such an event can serve as a calibration, a precise measurement in time and space of size and position.

Such an event occurs on September 11th, 2010 as the 3 day old crescent Moon will cover the planet Venus. This event will be visible in afternoon skies for observers from the southern Indian Ocean, across South Africa and into the Atlantic. The rest of the world will see a near miss. Both Venus and the Moon will be visible to the naked eye in broad daylight, and now would be a good time to attempt the interesting feat of spotting Venus in the daytime, using the Moon as a guide. The “shadow” footprint of Venus graces the Earth from 10:47 to 15:04 UT. Venus itself is at crescent phase, about 33” in size and 35% illuminated. Good luck and clear skies!

The astro-term for this week is Solar Sail. Long a dream of science fiction writers, a solar sail would provide propulsion via the action of solar radiation pressure against a large ultra-thin reflective sail. The obvious advantage for such a propulsion system is that the craft would not have to carry its own fuel and would be capable of course changes over time.  Disadvantages would include a low thrust ratio aspect and the fact that it would only work well in the inner solar system, as the solar radiation pressure drops off as you recede from the Sun via the inverse square law. Most spacecraft would probably be hybrids, deploying solar sails in their cruise phase and then using chemical thrusters for landing or rendezvous (remember Count Dooku’s ship in Star Wars: Attack of the Clones?) The first successful test of solar sail technology has been demonstrated in the form of the Interplanetary Kite-craft Accelerated by Radiation Of the Sun (IKAROS) launched by the Japanese Aerospace Exploration Agency (JAXA) enroute to Venus. Launched on May 21st, 2010 and deployed on June 10th, IKAROS sports a polyimide sail 200 m2 in size. The Planetary Society had sought to test this technology in 2005 aboard the ill-fated Cosmos-1. Another interesting proposal is a solar sail that utilizes the pressure created by a high energy laser or maser that would say, be based on an asteroid or the farside of the Moon, providing a steady push on outbound spacecraft.

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