Keeble Observatory
December 2003 Sky from the Keeble Observatory
You know those annoying commercials, with the pink bunny selling
a particular brand of batteries, that just keep on going, and going,
and going. A tiny space probe, launched in 1977 puts the bunny
to shame.
Voyager 1 and its near-twin Voyager 2 were launched to the outer
planets in 1977, a compromise version of what had been envisioned
as a grand tour. In the 1960s it was noted that the regular orbits
of the outer planets were aligning to make possible a single mission
to explore all the known planets beyond the asteroid belt. Taking
advantage of each successive planet's gravity, the Grand Tour probe
would sling-shot past Jupiter to gain the proper velocity to repeat
the maneuver at Saturn, Uranus, and Neptune, finally to reach distant
Pluto. However, the technical challenge and budget constraints led
to a less ambitious pair of probes designed for Jupiter and Saturn,
only. (At the time, NASA had a policy of launching deep space missions
in pairs, providing maximum redundancy in meeting the science objectives
for a relatively small increased cost . typically 20% more to build
and launch two compared to the cost of one.)
Voyagers were launched in the summer of 1977, each powered by a
small capsule of radioactive plutonium known as an RTG - (Radioactive
Thermal Generator) - and with an array of cameras and instruments
to explore the two largest planets in the solar system. The on-board
computers were a marvel of mid-1970s technology, with a whopping
64 kilobytes of on-board memory and CPU. During the cruise phase
to Jupiter, flight controllers entertained a change of mission.
If the Jupiter encounters by both probes and the Saturn encounter
by Voyager 1 were successful, they would reprogram the computer
on Voyager 2 and adjust its trajectory to attempt part of the grand
tour, even though the craft was not designed for the extended lifetime
required. We'll have more to say about Voyager 2 next month.
Voyager 1 successfully flew past Jupiter in March of 1979, nearly
two years after launch. The craft performed flawlessly, and made
its Saturn encounter in November of 1980. Its trajectory took it
over the south polar regions of Saturn, which flipped its orbit
up and out of the plane of the solar system. Its cameras were briefly
turned on in 1989 to capture a series of images used to make a family
portrait of the solar system. See Carl Sagan's book, The Pale
Blue Dot for the rest of that story.
Voyager 1 is now approximately 90 times further from the Sun that
you are as you read this page. To further grasp the scale of this,
light from the Sun takes about 500 seconds to reach Earth . nearly
12 � hours to reach Voyager 1. It is now the most distant human
artifact, coasting outward towards interstellar space at about 280
million miles per year - about 10 miles per second. Most of the
instruments have been turned off permanently, but the plasma experiment
measuring the solar wind flux remains operational. The RTG produces
sufficient electrical power to send telemetry at a level of a few
watts . requiring the resources of NASA's Deep Space Tracking Network
to detect the faint signals. Some measurements recently reported
suggest that Voyager is encountering the theoretical edge of the
solar system, where the tenuous solar wind piles into the interstellar
gas. Although a far better vacuum than we can produce in the laboratory,
the collision between these two produces a shock boundary and begins
to mix material from the solar system with the surrounding interstellar
medium.
Mission controllers estimate that fuel for thrusters and, more
importantly, the plutonium in the RTG will allow Voyager 1 to remain
in contact and continue reporting measurements until roughly 2020,
by which time we should know for certain whether the spacecraft
has really left the environs of the solar system and carried human
technology toward the stars. It just keeps on going, and going,
and going.
Lunar phases for December: Full Moon on the 8th, at 3:38 pm EST;
Last Quarter at 1:43 pm on the 16th; New Moon on the 23rd, at 4:44
am; First Quarter on the 30th, at 5:04 am.
In the early evening we'll see Mercury moving to its greatest eastern
elongation on the 9th - Just after sunset, you may see it low to
the southwest about 10 degrees off the horizon. Venus will be just
above and to the left of Mercury, and much brighter. To the southeast
you'll see red Mars, not as brilliant as during last summer's opposition
but still unmistakable. Saturn is coming to opposition on the 31st
so will be bright and accessible almost all night, nearly overhead
in Gemini at midnight. Jupiter rises at midnight.
Our overhead view a few hours after sunset has the Andromeda Galaxy
almost directly at zenith. Shifting your vision to the north you
will encounter the M or W shape of Cassiopeia (depending on which
way you're standing!). Ursa Major is low on the northern horizon.
From Cassiopeia, follow the Milky Way towards the eastern horizon,
passing through Perseus, Auriga, and Gemini (where Saturn lies this
month). The Twins (Castor and Pollux) are one above the other a
bit north of due east. Towards the southwest, you'll see Orion lying
on its side, with the familiar three stars of the belt almost
perpendicular to the horizon. Above Orion you'll note the Pleiades
cluster. From Cassiopeia westward, you'll find Cygnus, now standing
as the Northern Cross above the northwest horizon. From zenith south
you are looking out of the plane of the Galaxy. Mars spends the
month in Pisces, with relatively few bright stars competing.
For your own monthly star chart, you can direct your web browser
to http://www.skymaps.com.
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.
Copyright 2003
George Spagna