IAU Homepage on Solar Eclipses
Fred Espenak's Eclipse Home Page
Nine Planets Page
Solar System Simulator from JPL
Planets Page (SEDS)
Space Library: Solar System Simulations
Solar System Simulator from JPL
List of Solar System Info
Lunar and Planetary Institute's 3D views
Great Images in NASA (GRIN)
Solar System on the Web
Views of the Solar System by Calvin J. Hamilton
The Nine Planets
Orrery: The current planet positions in their orbits
MURRAY HILL, N. J. - Hurtling through the frigid expanse beyond the planets, NASA's aging spacecraft Voyager 1 has reached the edge of the Solar System, where it has encountered a massive shock wave, according to a paper that will be published in Nature on Nov 6, 2003, by a team of scientists that includes Louis Lanzerotti of Bell Labs, the research and development arm of Lucent Technologies.
More than eight billion miles from Earth, the farthest an operating spacecraft has ever journeyed, Voyager 1 is providing scientists with data from a fascinating yet little understood region of space - the frontier where the Sun's influence begins to wane, and the tenuous vastness of interstellar space takes over.
It is here that charged particles streaming out from the Sun - called the solar wind - bump into the ionized gas and dust that is spread thinly between stars, causing a shock wave in the process.
Data of how the solar wind behaves will provide scientists with knowledge useful not only to astronomy but to terrestrial concerns such as how solar emissions affect wireless telephone calls, satellite communications and electric power grids.
"When the Voyager missions were launched in 1977, we never thought the instruments we developed more than three decades ago would one day probe the edge of the Solar System," said Lanzerotti, an expert on how the solar wind affects terrestrial communications. Lanzerotti joined Bell Labs in 1965 and has been involved with NASA's Voyager missions since their inception in 1972. In honor of his contributions to space physics, the International Astronomical Union has named an asteroid after him, Minor Planet 5504 Lanzerotti. He now divides his time between Bell Labs, where he is a consultant, and the New Jersey Institute of Technology, where he is a distinguished professor at the Center for Solar Terrestrial Research.
"Voyager 1 and 2 provided us with unparalleled views of the planets. I find it very exciting that Voyager 1 will soon begin to explore the vastness of the interstellar medium," he said.
Astronomers now know that the space between stars, once thought to be completely empty, is filled with a dilute plasma of gas and dust termed the interstellar medium. The solar wind blows out a giant bubble called the heliosphere within the interstellar medium, and the boundary between the heliosphere and the interstellar medium is a place where a lot of interesting physical phenomena take place.
Much as a shock wave precedes a supersonic airplane, astronomers think that a "termination shock" occurs near the edge of the solar system. This is a region where the speed of the solar wind drops dramatically as the wind brakes as it mixes with the interstellar medium, and where the density of ionized particles increases many times. It is also a region where some ions from the interstellar medium, relics of previous generations of stars, manage to diffuse through the edge of the Solar System and are accelerated tremendously.
Using an instrument on Voyager 1 called the low energy charged particle detector, Lanzerotti and his colleagues found evidence of all three effects. They saw a hundred-fold increase in the number of charged particles detected during a six-month period starting in August 2002; they deduced that the speed of the solar wind had dropped by a factor of seven; and they detected ions that came from beyond the Solar System.
"When we saw all that, we were pretty sure that we had reached the termination shock," Lanzerotti said.
Other members of the scientific team were Stamatios Krimigis (team leader), Robert Decker and Edmond Roelof of Johns Hopkins University; George Gloeckler, Douglas Hamilton and Matthew Hill of the University of Maryland; and Thomas Armstrong of the University of Kansas.
As the team analyzed its data from Voyager 1, it noticed something strange. It seemed that after six months, in early 2003, the termination shock region moved outwards, possibly as a result of increased solar activity. Scientists postulate that the heliosphere is a dynamic entity, which expands and contracts with the Sun's 11-year activity cycle.
"Solar eruptions must have caused the solar wind to pick up speed," said Lanzerotti. "That forced the heliosphere to expand outwards, and the termination shock must have moved outwards as well. Voyager 1 will probably encounter it again."
The team expects to get confirming data from a similar detector on Voyager 2, which is expected to follow Voyager 1 to the edge of the Solar System. The Voyager probes, among NASA's most successful spacecraft, are expected to provide data until approximately 2020, when they will exhaust their power supply from nuclear isotopes and drift off into space."Meanwhile, they will continue to provide a treasure trove of wonderful, sometimes unanticipated data about the far reaches of the Solar System and push the frontiers of our knowledge," Lanzerotti said.
The DPS fully supports the plan for Solar System exploration just released by the National Research Council "New Frontiers in the Solar System: An Integrated Exploration Strategy". The DPS was actively involved in the Survey, providing ad-hoc reports written by its members as input to the NRC Survey Panels. The Survey provides a science community consensus on priorities for planetary missions and ground-based research for the next decade.
The key overall recommendations for non-Mars planetary missions are
1) maintenance of the Discovery program of low-cost (total mission cost less than $325M) missions at a flight rate of one every 18 months,
2) start of a New Frontiers line of medium-cost (less than $650M) competitively procured missions to be implemented as in the Discovery program, but selected from a prioritized list provided by the Survey, with a flight rate of about one every 3 years, and
3) one large-cost mission (greater than $650M) per decade.
The recommended large-cost mission is the Europa Geophysical Explorer, a version of the JPL Europa Orbiter concept. The recommended medium-cost New Frontiers missions are in priority order 1) KBO/Pluto Explorer, 2) Lunar South Polar Aitken Basin Sample Return, 3) Jupiter Polar Orbiter with Probes, 4) Venus In-Situ Explorer, and 5) Comet Surface Sample Return. The prioritized list of New Frontiers missions includes more than three missions to provide flexibility for technology and budgetary developments over the next ten years. In addition to the Discovery program of low-cost missions, the Survey recommends extension of the Cassini mission beyond its prime mission termination in 2007.
The Survey contains a separate set of prioritized recommendations for the Mars Exploration Program. After the launch of the Mars Reconnaissance Orbiter in 2005, there are two recommendations for the low-cost category of missions 1) a Mars Scout program of competitively procured missions implemented in the same manner as Discovery, with a flight rate of one Scout launch at every other Mars opportunity (one every 52 months) beginning in 2007, and 2) a Mars upper atmosphere orbiter. In the medium class category, the recommendations are for a Mars Smart Lander launch in 2009 and a Mars Long-lived Lander Network that could be implemented by international cooperation. The Survey recommends that these missions be implemented in a manner to build towards a Mars Sample Return mission early in the next decade. The Lunar South Polar Aitken Basin Sample Return mission should also be implemented in a manner to provide appropriate technological development for a Mars Sample Return.
There are also recommendations on fundamental research and analysis including a gradual increase in grant programs, recommendations on mission data analysis, the Deep Space Network, and technology development with an endorsement of the nuclear power and propulsion technologies initiative, and recommendations on ground-based support programs including a recommendation to share development and operations of a Large Synoptic Survey Telescope with the NSF. Implementation of the recommendations in this Survey would provide for a broad, integrated program of scientific exploration throughout the Solar System and enable new scientific discoveries addressing some of the most compelling scientific questions in planetary science.
The Division for Planetary Sciences of the American Astronomical Society endorses this report and looks forward to seeing its provisions implemented.
The full report is posted at http://www.nap.edu and has been posted at the community decadal website: http://www.aas.org/dps/decadal/
The DPS is the world's largest professional organization dedicated to the exploration of the Solar System.
On pp. 128-129, I discuss the question of whether the formation of our Solar System was caused by a nearby supernova. Such a catastrophic event was invoked because of the presence of aluminum-26, a radioactive isotope that had to have been formed not long before our Solar System's formation. The book mentions that this isotope has been found, in the 1990s, to permeate space, making a supernova unnecessary. At the September 2001 symposium held to mark the 2nd anniversary of the Chandra X-ray Observatory, a team of Penn State and MIT scientists reported that an image of Orion shows 31 solar-size stars emitting x-ray flares at a prodigious rate. The protons omitted from these flares could have caused fission of heavy elements to form the radioactive isotopes under question, not only in Orion but also long ago in our Solar System. Thus our model of the Solar System forming gently from the collapse of a gas cloud is fine, since the radioactive isotopes would have formed subsequently.
SwRI Press Release
August 15, 2001
The "giant impact" theory, first proposed in the mid-1970s to explain how the Moon formed, has received a major boost as new results demonstrate for the first time that a single impact could yield the current Earth-Moon system.
Simulations performed by researchers at Southwest Research Institute (SwRI) and the University of California at Santa Cruz (UCSC) show that a single impact by a Mars-sized object in the late stages of Earth's formation could account for an iron-depleted Moon and the masses and angular momentum of the Earth-Moon system. This is the first model that can simultaneously explain these characteristics without requiring that the Earth-Moon system be substantially modified after the lunar forming impact. The findings appear in the August 16 issue of Nature.
The Earth-Moon system is unusual in several respects. The Moon has an abnormally low density compared to the terrestrial planets (Mercury, Venus, Earth, and Mars), indicating that it lacks high-density iron. If the Moon has an iron core, it constitutes only a few percent of its total mass compared to Earth's core, which is about 30 percent of its mass. The angular momentum of the Earth-Moon system, contained in both the Earth's spin and the Moon's orbit, is quite large and implies that the terrestrial day was only about five hours long when the Moon first formed close to the Earth. This characteristic provides a strong constraint for giant impact models.
Previous models had shown two classes of impacts capable of producing an iron-poor Moon, but both were more problematic than the original idea of a single Mars-sized impactor in the last stages of Earth's formation. One model involved an impact with twice the angular momentum of the Earth-Moon system; this would require that a later event (such as a second large impact) alter the Earth's spin after the Moon's formation. The second model proposed that the Moon-forming impact occurred when Earth had only accreted about half its present mass. This required that the Earth accumulated the second half of its mass after the Moon formed. However, if the Moon also accumulated its proportionate share of material during this period, it would have gained too much iron-rich material -- more than can be reconciled with the Moon today.
The models developed by SwRI and UCSC use the modeling technique known as smooth particle hydrodynamics, or SPH, which also has been used in previous formation studies. In SPH simulations, the colliding planetary objects are modeled by a vast multitude of discrete spherical volumes, in which thermodynamic and gravitational interactions are tracked as a function of time.
The new high-resolution simulations show that an oblique impact by an object with 10 percent the mass of the Earth can eject sufficient iron-free material into Earth-orbit to yield the Moon, while also leaving the Earth with its final mass and correct initial rotation rate. This simulation also implies that the Moon formed near the very end of Earth's formation.
"The model we propose is the least restrictive impact scenario, since it involves only a single impact and requires little or no modification of the Earth-Moon system after the Moon-forming event," says the paper's lead author, Dr. Robin M. Canup, assistant director of the SwRI Space Studies Department in Boulder.
UCSC Professor Erik Asphaug adds, "Our model requires a smaller impactor than previous models, making it more statistically probable that the Earth should have a Moon as large as ours."
Modeling lunar formation is important to the overall understanding of the origin of the terrestrial, or Earth-like, planets.
"It is now known that giant collisions are a common aspect of planet formation, and the different types of outcomes from these last big impacts might go a long way toward explaining the puzzling diversity observed among planets," says Asphaug.
The Moon is also believed to play an important role in Earth's habitability because of its stabilizing effect on the tilt of Earth's rotational pole. "Understanding the likelihood of Moon-forming impacts is an important component in how common or rare Earth-like planets may be in extrasolar systems," adds Canup.
An image is available at: http://www.swri.org/press/impact.htm.
An annotated reading and resource list, compiled by Andy Fraknoi, is available for beginners who want to learn more about our solar system is now part of the educational web pages of the non-profit Astronomical Society of the Pacific.
The resource listing can be found
The listing discusses some of the most useful books and articles for teachers and students on:
* the solar system in general, * each planet, * asteroids and comets, * impacts, and * the search for life on other local worlds.
There are also brief lists of useful web sites and places to get slides and posters of solar system images.
This list is part of the work of Project ASTRO, a national program to train professional and amateur astronomers to "adopt" 4th - 9th grade classrooms in partnership with a local teacher in their communities. The project is currently operating in 11 regional sites from Boston to San Francisco.
Dr. Carl Pilcher, Science Director for Exploration of the Solar System in NASA's Office of Space Science, announces the first issue of the new Solar System Exploration newsletter. It is online at http://sse.jpl.nasa.gov/results/newsletter/newslet.html in pdf format. The newsletter will be published several times a year to keep the planetary science community informed about activities and plans at NASA Headquarters. The first issue discusses the Congressional budget status, the Mars reports and replanning efforts, and education and public outreach. We hope to facilitate dialog between the science community and Headquarters, and invite your feedback comments, and suggestions. Send to email@example.com or Ronald.S.Saunders@jpl.nasa.gov
We are pleased to announce the launching of the first installment of a prototype education/public outreach product, "Thursday's Classroom," produced by the NASA/Marshall Science Directorate.
The first installment deals with solar eclipses, and future episodes of this prototype will introduce other solar science topics, specifically, sunspots, the sun-earth connection, solar observing, and the solar cycle.
Our aim is to provide a lasting connection between NASA's latest research and the classroom environment. We welcome your feedback and invite you to explore our product at http://thursdaysclassroom.com
Elizabeth Newton (NASA/Marshall)
Jim Miller (Univ. of AL in Huntsville)
See the location, look, and phase of any of 32 planets and moons on any date, as simulated at JPL. The site includes texture maps of various planets and moons.
Paper models of Voyager, Galileo, Magellan, Hubble, Keck, and Mars Global Surveyor.