September 2000 Sky from the Keeble Observatory
Aficionados of Star Trek will recognize the name "Deep Space 1" as a large base of operations for the fictional Star Fleet, far from our own solar system. NASA has its own Deep Space 1, and itís neither large nor outside our solar system. Rather, it is an experimental, rather small, robotic spacecraft; itís part of NASAís "New Millennium Project." Launched in October 1998, its objectives were to test a number of new technologies with a relatively inexpensive platform. Novel notions, such as autonomous navigation without having to send instructions from ground stations, were part of the mission. But, the centerpiece of the craft is its unique ion propulsion system.
Rockets rely on the "conservation of momentum" to propel a payload forward by throwing material backward at high speeds. Itís the same phenomenon which causes your shotgun to kick back into your shoulder when you pull the trigger. If you throw a snowball while standing on your skates in the middle of the rink, you will recoil in the opposite direction. The faster you throw it, or the heavier the snowball thrown at the same speed, the faster you will rebound. Imagine throwing lots of snowballs in sequence Ė your speed will increase with each throw.
Chemical rockets, like the boosters on the Space Shuttle, or the expendable booster used to launch Deep Space 1, make their "snowballs" travel very fast by burning their fuel and making the exhaust gases very hot. The speed of the exhausted gas is on the order of the speed of sound, about 300 meters per second (or, roughly, 700 miles per hour). But, fuel is massive, and to get it hot you have to burn it quickly. From launch to orbit is a matter of minutes, then the fuel is gone. If you want to go to deep space, you need to carry more fuel and fire your rockets for longer, but itís still only minutes. DS1 relies on less exhaust mass moving much faster. Xenon gas (the same gas used in flash tubes and some headlights) is ionized in a small electric furnace and expelled at nearly 30,000 meters per second! Far less mass is expelled (about 3.5 ounces per day), so the thrust is very small, roughly the same force exerted on your hand in hold a piece of paper. But, the ion drive can run for a very long time, efficiently boosting the speed of the space craft while expending very little fuel.
The ion drive on DS1 has been thrusting continuously for over 200 days, and could potentially operate for more than 583 days by the end of its extended mission. The technology trials have been completed. DS1 flew by an asteroid as part of that demonstration, returning low-resolution images before the Shoemaker NEAR spacecraft surpassed them in quantity and quality by orbiting another asteroid. The extended mission is now to rendezvous with Comet Borrelly in September 2001. The star-tracker navigation system failed shortly after the official completion of the primary mission. But, ground controllers were able to reprogram the on-board computer to use one of its on-board cameras as a navigation sensor, all while the spacecraft was some 200 million miles from Earth. So, DS1 just keeps going and going. Maybe we should rename it Energizer Bunny 1!
Lunar phases for September: First Quarter on the 5th, at 12:27 pm; Full Moon on the 13th, at 3:37 pm; Last Quarter on the 20th, at 9:28 pm; New Moon on the 27th, at 3:53 pm.
Venus and Mercury are still visible in the early evening twilight. Venus sets about an hour after sunset, Mercury is so low that itís unlikely to be seen through the near-horizon haze. Look to the west to west-southwest, low on the horizon. Jupiter rises about 4 hours after sunset, Saturn about a half-hour earlier. Jupiter will be quite bright, Saturn less so. Early in the month is a good time to look at Saturnís rings, which are inclined almost 25 degrees from being edge on. Jupiter and Saturn will be high and to the south at sunrise, with Saturn about 10 degrees to the right. Donít confuse Saturn with Aldebaran, which will appear somewhat closer to Jupiter, about 5 degrees below. Mars is low in the east at dawn.
At mid-month, our overhead view about 3 hours after sunset is dominated by the bright triangle of Deneb, Vega, and Altair. Deneb is at the northernmost end of Cygnus, which forms an obvious cross, aligned roughly northeast to southwest. The long axis of this "Northern Cross" is roughly parallel to the plane of the Milky Way, our home Galaxy. . Vega lies to the west, and Altair to the south. A larger triangle is formed by Vega, Arcturus, and Antares. Vega is high overhead, with Arcturus the bright star near the western horizon, just a bit north of the point where the sun has set. Antares is the bright red star near the southwest horizon.
Binoculars and a clear, dark, night will reveal the faint glow of the Milky Way, dividing the sky roughly in half, to be made of myriad faint stars. Their collective glow is visible only when you get away from city lights and dust and haze. Galileo was the first to note these otherwise invisible stars when he turned his newly-built telescope to the skies almost 350 years ago. Prior to that, the Milky Way was thought to be a luminous ribbon of fluid. Mythology told of its being a road to the heavens. Towards the northeast, just at the edge of the Milky Wayís glow, you may discern a faint patch of light on a truly dark night. Turn your binoculars to that patch and youíll see the Andromeda Galaxy, roughly what our own Galaxy would look like at a distance of 2 million light years.
The same binoculars will allow you to discriminate the fainter "star" near Vega as a binary. A modest telescope will further resolve this binary into a pair of binaries, the so-called "Double Double."
Dr. George Spagna