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SkiesIdaho
Skies
July 2007
Vol. 4 No. 7
Idaho Skies is a column for beginning amateur astronomers and those interested in astronomy. Suggestions about the column are gladly accepted by the columnist, at paul.verhage@boiseschools.org
This month look for the star Arcturus, the lucida of the constellation Bootes the Herdsman. Arcturus is located 37 light years from earth. Therefore if you were born in 1970, Arcturus is your birthday star this year. The word Arcturus means “Bear Guardian” and refers to Arcturus’ closeness to the Great Bear, Ursa Major. As the constellation of the Big Bear wheels around the sky, the bright star Arcturus follows closely behind.
Arcturus is the brightest star in the northern hemisphere. There are three stars brighter than Arcturus, but they all lie south of the celestial equator. Did you know that the light of Arcturus was used to open the 1933 World’s Fair in Chicago? In 1933 the distance to Arcturus was determined to be 40 light years. Since the previous World’s Fair in Chicago occurred forty years earlier (that is in 1893), the light of Arcturus seen at the opening of the 1933 World’s Fair left the star the last time the World’s Fair was held in Chicago. To open the 1933 World’s Fair, a large telescope lens focused Arcturian starlight on a photocell (a light sensitive resistor) which triggered a relay and turned on the lights.
Arcturus is a K type star, which means its cooler than our sun (which is a G type). Instead of being yellow like our sun, Arcturus is cooler orange. However, because of its larger diameter, Arcturus shines over 100 times more brightly than our sun. If its infrared radiation is included, then Arcturus shines over 200 times brighter than our sun. Arcturus is a giant star 26 times larger than our sun. The motion of Arcturus through the galaxy indicates that the star did not form in the Milky Way Galaxy. Most likely Arcturus formed in a dwarf galaxy that got too close to our galaxy and was torn apart by gravity. The other stars from this long dead galaxy move in high speed, highly inclined orbits through the Milky Way like Arcturus.
To find Arcturus follow the bend of the handle of the Big Dipper. An arc drawn from the end of the Dipper’s handle (actually the Large Bear’s tail) leads you to Arcturus. In July Arcturus is the yellowish star slightly north-west of overhead.
July 1 – 7
July begins with the 45th birthday of the Kennedy Space Center. It was on July 1st, 1962 that America’s space center was opened for business. But rocket launches occurred on the cape before 1962; in fact there was one launch of a captured V-2 missile way back in the late 1940s. For more information on the cape check out its website, http://www.nasa.gov/centers/kennedy/home/index.html
Here’s a photographic opportunity. Saturn is rapidly approaching the sun and western horizon. But before it can, Saturn and Venus first cross paths. As soon as it gets dark on the 1st, look in the west for brilliant Venus. The larger but more distant Saturn will be the fainter star just to the upper right of Venus. The two are only about the width of a finger apart (when viewed from an outstretched arm). The close alignment of Venus and Saturn will make a great photograph if you have a camera, telephoto lens, and include an interesting landmark in the image.
The Mars Pathfinder (MPF) landed on Mars ten years ago on the 4th. Pathfinder was our first landing on Mars since 1976 (that’s 21 years between Mars landings). While you may not remember the Mars Pathfinder, you no doubt remember the little rover that it carried to Mars, Sojourner.
Mars Pathfinder was primarily a demonstration mission. JPL wanted to show that they could quickly design and land a small lander on Mars. To simplify Pathfinder’s landing, it was wrapped in air bags. Just before impact with the Martian surface the air bags inflated and the cocooned lander bounced to a stop at rocky Ares Vallis. After successfully landing, MPF was renamed the Carl Sagan Memorial Station in honor of Dr. Sagan who had passed away six months earlier.
An addition to the lander there was the rover. Sojourner weighed 23 pounds and was battery operated. A solar array on top of the rover kept the battery from discharging too rapidly, but it wasn’t enough to fully recharge the battery. JPL wanted to get a month out of the lander and a week out of the rover. But both functioned for more than 90 days. Sojourner and Mars Pathfinder returned over 16,000 images of the Martian surface along with measurements of the elements in 15 Martian rocks.
Earth reaches the aphelion of its orbit on the 6th at 6:00 PM (5:00 for Oregon and 7:00 for the Midwest). Being at aphelion means that earth’s at its farthest distance from the sun. Isn’t it funny how earth’s greatest distance from the sun corresponds to our hottest days? Actually that’s not surprising at all. The seasons are controlled by the planet’s tilt towards or away from the sun and not by its distance from the sun. When the northern hemisphere points towards the sun we have summer and when our hemisphere points away from the sun, we experience winter. By the way, earth’s distance from the sun today is 94 million 500 thousand miles.
The moon is last quarter on the 7th at 10:54 AM (9:54 for Oregon and 11:54 for the Midwest). Over the next two weeks the skies will remain dark later into the night making it easier to observe faint fuzzy astronomical objects like galaxies.
Would you like to know where Mars is located? On the morning of the 9th the moon will guide you to the red planet. Between 3:00 AM and dawn look for the crescent moon in the low east. Mars will be 5 degrees (or ten lunar diameters) to the lower right of the moon and will appear as a pale yellowish star.
While you’re looking at the moon on the 9th, it’s at perigee, or its closest distance to earth. This month the lunar perigee is only 229,000 miles away.
A show of hands please; who remembers Telstar 1? Does it make you feel old to know that Telstar 1 was launched 45 years ago on the 10th? Telstar 1 was an active communication satellite. Active communication satellites are ones that retransmit radio signals sent to them. This way the radio signal is amplified before a receiving station on the ground hears it. The stronger radio signal received on the ground means that a smaller antenna can be used to hear the signal. Some readers may also remember two earlier communication satellites named Echo. Echo 1 and 2 were reflective balloons that bounced radio signals back to earth. Since they didn’t amplify radio signals they are called passive communication satellites.
Telstar 1 was roughly spherical with a diameter close to one yard and a weight of 170 pounds. The power source for Telstar 1 was solar cells wrapped around the satellite. The output of the solar cells was rather low; they only produced about as much power as the light bulb in your refrigerator. Telstar was put into low earth orbit rather than geostationary orbit like most communication satellites today. Because of this low orbit, Telstar transmissions could only be received for 20 minutes at maximum. Telstar 1 remained active for about 18 months before failing permanently due in part to radiation damage caused by nuclear explosions in space.
The moon passes close to the Pleiades on the morning of the 10th. At around 4:00 AM you’ll find the Pleiades 5-1/2 degrees (eleven lunar diameters) to lower left of moon. Eleven diameters sounds like a lot, but the moon is not as large as it appears, especially when it’s close to the horizon. The angular distance between the moon and Pleiades this morning is about the same as the angular distance between the Pointer Stars in the Big Dipper.
Venus reaches greatest brilliance on the night of the 12th. As Venus passes between the sun and us, its gets larger and brighter. But at the same time it becomes a thinner crescent and therefore fainter. Greatest brilliance occurs when the thinnest and largeness of Venus reach a balance. Less than two degrees above Venus tonight is the star Regulus, the lucida of Leo the Lion. Saturn will also be six degrees to the right of Venus. Since it is summer and the sun sets late, Venus won’t be seen in very dark skies.
Is the end of your week free? The Table Mountain Star Party takes place from the 12th to the 14th. Table Mountain is located 20 miles northwest of Ellensburg (central Washington State) and is easily reached by traveling on I-90. You can get more information at their website, http://www.tmspa.com/
The moon is new on the 14th at 6:00 AM (5:00 for Oregon and 7:00 for the Midwest). In the next couple of days we’ll begin seeing the moon again in the west after sunset.
On the 16th the moon forms a small grouping with Venus, Saturn, and Regulus. The grouping is so small that all four objects will just fit within the field of view of binoculars. So take your binoculars and begin looking shortly before 10:15 PM. The grouping will be low in the west-northwest and set by 11:00 PM. Unfortunately this also means the skies will still be lit by twilight, cutting down on their contrast. In binoculars you’ll see a scene similar to this.
On the 20th Mercury reaches greatest western elongation. Greatest elongations are generally the best times to see inferior planets because they’re at their greatest distance from the sun. But this month’s greatest elongation is not very good because Mercury still remains close to the horizon. So if you want to see Mercury, look for a star low in the east northeast at around 5:45 AM.
The moon passes close to Spica, the lucida of Virgo, on the evening of the 21st. Spica will be four degrees or eight lunar diameters from the moon’s right. Spica doesn’t look impressive, but it’s 260 light years away and over 2,000 times brighter than the sun. I guess if you’re a star 2,000 times brighter than the sun you can afford to look pretty average.
The moon is at first quarter on the 22nd at 12:30 AM (11:30 PM on the 21st for Oregon and 1:30 AM on the 22nd for the Midwest). Since the moon is half full, tonight would be a great time to look at the moon through your telescope or binoculars. You’ll see the greatest detail if you look along the straight edged boundary between day and night.
Less than three hours after the moon reaches the first quarter phase, it also reaches the apogee of its orbit around the earth. At 3:00 AM (2:00 for Oregon and 4:00 for the Midwest) on the 22nd the center of the moon will be 251,127 miles away from the center of the earth.
Thirty-five years ago on the 22nd the Soviet Venera 8 made the second successful spacecraft landing on Venus. Venera 8 was launched along with its twin, Venera 7, back in 1972. The previous Venera spacecraft were crushed by the atmosphere of Venus before they could land. So both Venera 7 and 8 were built stronger and included a refrigerator. Venera 8 survived for 50 minutes on the ground and reported that the lighting conditions on the surface were similar to an overcast day on earth. It also reported that the surface temperature was a toasty 900 degrees and that the air pressure 90 times greater than on earth. Venera 8 eventually failed due to the combined effects of the Venusian heat and pressure.
On the same day Venera 8 was landing on Venus the United States was launching Landsat 1 into earth orbit. Landsat was a weather satellite modified to take detailed and informative images of the earth’s surface. The images gave information on earthly resources like the oceans and rivers, forests, minerals, and weather. Landsat 1 did this with cameras that recorded images and light intensity in the visible and infrared spectrum. The satellite was placed into a nearly polar orbit so that nearly the entire earth’s surface was below its cameras at some point. Before being switched off in 1978, Landsat 1 discovered an island off the eastern coast of Canada which is now called Landsat Island.
The moon and Antares form a pair on the night of the 25th. Antares is the alpha star of Scorpius and is 600 light years away. Jupiter is the brighter object above Antares. Look in the low south once the skies get dark to see them.
Early on the morning of the 29th the South Delta-Aquarids meteor shower reaches it peak. Normally we would expect to see around 20 meteors per hour from this shower. But there are two strikes against it. First, it’s low with respect to the horizon. So the shower is best seen from the southern US or better still, south of the equator. The second strike is that this year the moon is nearly full at the shower’s peak. That means that by the time the moon sets dawn will be beginning. So the skies will never get dark for this shower. Oh well, there’s always next year.
The moon is full on the 29th at 6:48 PM (5:48 for Oregon and 7:48 for the Midwest). The full moon in July is often called the Thunder Moon.
This Month’s Topic
The most distant human-made object is over 100 astronomical units away from home. Over one hundred astronomical units away means Voyager 1 is more than 100 times more distant from the sun that the earth. Whereas it takes sunlight 8 minutes and 20 seconds to reach the earth, it takes almost 14 hours to reach Voyager 1. Voyager 1 and its twin Voyager 2 are traveling so fast that they are escaping our solar system and entering into interstellar space.
A space mission launched from earth that uses an energy efficient trajectory would take decades to reach the planets in the outer solar system. This most energy efficient trajectory is called a Hohmann Transfer Orbit. As described in 1925 by Walter Hohmann, the elliptical orbit neatly fits between the circular orbits of two planets involved. The period, or length of time required to transfer a spacecraft between two planets via a Hohmann transfer orbit is between the periods of the two planets involved. To get to distant planets like Uranus and Neptune requires decades via a Hohmann transfer orbit.
In 1961 a graduate student at JPL discovered that a planet’s gravity can be used to accelerate a spacecraft. Relative to the moving planet, the spacecraft approaches and leaves the planet at the same speed. But from the sun’s perspective, the spacecraft gains the speed of the planet. To balance the change in the spacecraft’s kinetic energy, the planet loses the same amount of energy that the spacecraft gained, which slows down the planet. But since planets are so much more massive than spacecraft, the change in the planet’s speed is insignificant (on the order of one foot per trillion years).
The outer solar system is opened to exploration once the spacecraft reaches Jupiter. Jupiter’s gravity does the rest, if a second planet is properly aligned with the spacecraft’s new trajectory. In 1965 Gary Flandro, also working at JPL, discovered that every 175 years the outer gas giants align in such a way that a spacecraft can visit each one by utilizing the gravity of the previous planet. Fortunately, the next window for such a mission began in 1976 and would last for two years. The mission to explore the outer solar system with gravity assists was given the name, the Grand Tour.
Budget cuts at NASA forced the cancellation of the Grand Tour mission a few years later. In part this was due to the cost of developing spacecraft electronics sufficiently reliable for a 15 year space flight. At the time the cost of two of these missions was estimated to be $750 million. After the cancellation a variation of the Mariner spacecraft was developed to explore just Jupiter and Saturn by using a gravity assist from Jupiter. The spacecraft was named Mariner Jupiter Saturn (MJS) and approved in May 1972.
One nearly unique piece of electronics inside MJS was its flight computer. The flight computer gave MJS the ability to control its mission when communications between the MJS and earth was too great to allow real time control. The flight computer was also reprogrammable during the mission. This gave it the ability to improve with age, something that a lot of people aren’t particularly good at. Flight computers have been used in manned spacecraft since 1965, but unmanned spacecraft didn’t start using them until 1972. This made the MJS flight computer one of the earliest ones.
For electrical power in deep space where there is very little sunlight, MJS carried two Radioisotope Thermoelectric Generators (RTG) on booms mounted a short distance way from the spacecraft. Using plutonium as a heat source, the RTGs generated 470 watts of power at the time of launch (or about 1/4th the average power used to run a house). The decay of plutonium was slow enough that the RTGs would be able to power the spacecraft for nearly fifty years.
Six months before their launch, the MJS spacecraft were renamed Voyager 1 and 2. On August 20, 1977, Voyager 2 was launched by a powerful Titan-Centaur rocket. A month later Voyager 1 was launched. Launched with a greater speed, Voyager 1 would arrive at Jupiter before Voyager 2. Both Voyager 1 and 2 were launched from earth with enough speed to reach Jupiter, but not enough to reach Saturn, left alone escape the solar system. But their gravity assists by Jupiter would be sufficient to boost both to solar escape velocity.
Voyager 1 arrived at Jupiter in less than two years on March 5, 1979. It returned 18,000 images of Jupiter and its four Galilean satellites. In addition it made measurements of the Jovian magnetic field and radiation belt. Radiation near Jupiter was discovered to be 1,000 times greater than the lethal dose for human beings. Jupiter’s powerful magnetic field was responsible for trapping this immense amount of radiation.
Images taken by the Voyagers showed that Jupiter’s innermost moon, Io, was the most volcanic body in the solar system. Eight volcanic eruptions were visible at the same time, and one volcano was observed ejecting a lava fountain to a height of 165 miles high (or 30 times higher than Mt. Everest). The second innermost satellite, Europa, was discovered to be the smoothest body in the solar system. It was covered with an icy shell that may hide a deep ocean. Voyager 1 and 2 increased their speed by 30,000 miles per hour from Jupiter’s gravity assist.
Two years after their Jupiter flyby the Voyagers reached Saturn. Saturn’s rings were found to consist of ten thousand narrow rings. Titan, viewed close up by Voyager 1, was discovered to be enshrouded in a thick blanket of orange smog. Analysis of the data collected by Voyager 1 indicated that the atmospheric pressure on the surface of Titan was 50% greater than the pressure on earth. Another interesting satellite of Saturn that was photographed by the Voyagers was Mimas. At first glance one would think the 250 mile diameter satellite was the Death Star from Star Wars. A 80 mile diameter crater named Herschel gave the moon an appearance similar to the Death Star. An equivalent crater on earth would be nearly as wide as the United States.
To see Titan close up, Voyager 1 ended up on a trajectory that took it out of the plane of the solar system. Hence Voyager 1 was unable to visit any other planets after Saturn. But with the success of Voyager 1, Voyager 2 was sent through the Saturnian system on a trajectory that would to take it on to Uranus. After successfully exploring Saturn, money for the Uranus flyby was granted. Later a follow on flight to Neptune was also approved by NASA. Voyager 2’s five year mission was now extended to 12 years.
On January 24, 1986 Voyager 2 encountered Uranus. Voyager 2 photographed its dark rings and moons. One satellite, Miranda, was discovered to have experienced intense geologic activity in the past. Ancient tidal heating created up-wellings of material called corona and a splitting of the surface that resulted in dropped valleys called grabens. The result is that the satellite looks a lot like a cosmic quilt. The other major satellites of Uranus were founded to be frozen ice balls. Images show that as these satellites solidified their surfaces cracked. Since Uranus is turned on its side, Voyager 2’s passage through the Uranian system was over very quickly. Voyager shot through the plane of the rings and satellites like an arrow through a bull’s eye.
Voyager 2’s closest approach to Neptune occurred on August 25, 1989, 12 years after its launch. One of the interesting features found on Neptune was its great dark spot. The dark spot is as large as the earth and appears to be a hole in Neptune’s atmosphere (think ozone hole on earth). Our first view of a Kuiper belt object may have been Voyager 2’s image of Triton, the largest satellite of Neptune. Triton is the coldest body of the solar system yet visited. Voyager 2 showed that there’s an exceedingly thin atmosphere of nitrogen was on Triton. Even through the atmosphere has the tinniest fraction of our earth’s atmospheric pressure, it’s still capable of supporting icy nitrogen clouds. A slightly pinkish polar cap was discovered on Triton. On Triton’s surface, dark patches were visible. These patches appear to be material blown out of ice volcanoes or geysers.
The Voyagers are now 30 years into their missions. Their radioisotope electric generators are currently producing on the order of 300 watts of power and will continue to decrease over time. Some science instruments have been shut off to save Voyagers’ dwindling power. But with a bit of good fortune, both spacecraft will continue to function until the year 2020.
The Voyagers have explored four gas giants and 48 satellites. These worlds changed from spots in a telescope to worlds. The Voyagers are now traveling through the outer solar system and soon should begin returning information on interstellar space. In another 6,500 years Voyager 2 will fly close to Bernard’s star and in 38,000 years Voyager 1 will pass close to a star named AC+793888. In case either Voyager is found some where deep in time and space, each has a gold record mounted to their side. The record contains the sights and sounds of our blue home planet. The sounds, music, and images contained on the record will out-survive anything so far created by humans. I hope some day that we can travel fast enough to catch up to the Voyagers. If so, I want them left to their course through the galaxy. But if humanity doesn’t survive for the long run, then a record of our existence will remain in a readable state for at least another one billion years.
The cost for both Voyager spacecraft is 20 cents per American per year. That’s not a bad return on an investment that’s less than the cost of a candy bar per year.
Observer’s Handbook 2007, The Royal Astronomical Society of Canada
Space Calendar, <http://www.jpl.nasa.gov/calendar/>
Night Sky Explorer (software)
Stars, <http://www.astro.uiuc.edu/~kaler/sow/>
Mars Pathfinder, <http://mars.jpl.nasa.gov/MPF/>
Telstar, <http://en.wikipedia.org/wiki/Telstar_1>
Venera 8, <http://nssdc.gsfc.nasa.gov/nmc/tmp/1972-021A.html>
Landsat 1, <http://en.wikipedia.org/wiki/Landsat_1>
South Delta Aquarids, <http://en.wikipedia.org/wiki/South_Delta_Aquarids>
Voyagers (1977 – present), <http://filer.case.edu/sjr16/advanced/20th_far_voyagers.html>
University of California, Riverside, 25 Years of Voyager<http://spacephysics.ucr.edu/>
Dark Skies and Bright Stars,
Your Interstellar Guide