by Paul Gilster | Aug 18, 2006 | Missions |
One of the great missions for the 21st century could be FOCAL — a space probe sent to the Sun’s gravity lens some 550 AU out. Gravitational lensing is becoming a major tool for astronomers, and we’ve even seen planetary detections using microlensing, looking at targets in the direction of galactic center and the faint changes in light that indicate a planet’s passage. The gravity lens concept, harking back to a 1936 Einstein paper, came to the fore in 1978, when Dennis Walsh and team spotted a twin quasar image, the result of the lensing caused by an intervening galaxy as it bends light around it.
So we know that lensing works. As far as I know, the first person to apply the notion to spacecraft was Von Eshleman (Stanford University), who considered a space probe to 550 AU to exploit the potential magnifications available there. And such missions have also been considered, by Frank Drake among others, as SETI experiments, using the Sun’s ability to magnify the hydrogen line at 1420 MHz, the so-called ‘waterhole’ frequency for interstellar communications.
But no one has put more thought into a FOCAL mission than Claudio Maccone. The Italian physicist led a 1992 conference that investigated mission concepts, and submitted a proposal to the European Space Agency the following year. Since then, he has followed up this work with a series of papers in Acta Astronautica and the Journal of the British Interplanetary Society, investigating among other things the uses of the gravity lens for cosmology (detailed imaging of a small slice of sky to study the cosmic microwave background), communications (using gravity lenses around nearby stars to boost signals from interstellar probes) and astronomy. His 1997 book The Sun As a Gravitational Lens: Proposed Space Missions (Colorado Springs: IPI Press) is an exhaustive analysis of the topic now in its 3rd edition.
Image: Gravitational lensing at work. A space probe at 550 AU and beyond could exploit such effects to make detailed studies of other solar systems, among numerous other scientific targets. Credit: Martin Kornmesser & Lars Lindberg Christensen, ST-ECF.
In a presentation he will make today at the New Trends in Astrodynamics and Applications conference in Princeton, Maccone notes that the use of stars as gravitational lenses is a logical next step for astronomers. “As each civilization becomes more knowledgeable, they will recognize, as we now have recognized, that each civilization has been given a single great gift: a lens of such power that no reasonable technology could ever duplicate or surpass its power. This lens is the civilization’s star. In our case, our Sun.”
A fascinating aspect of the Sun’s gravity lens is that we do not need to park a spacecraft at 550 AU to utilize it. As the spacecraft pushes past this distance, effects created by the Sun’s corona diminish and imaging only becomes better. We have an opportunity to see images the likes of which could not be produced by ground-based or conventional space-based telescopes, assuming we can find a way to propel a spacecraft to the needed distance in a reasonable amount of time.
If we try to sketch out a rational pattern for exploration beyond Pluto, FOCAL should be front and center in our thinking. New Horizons reaches the Pluto/Charon system in 2015. After that there is active work on Innovative Interstellar Explorer, a probe that would carry instrumentation beyond the heliosphere and become the first mission specifically targeting the interstellar medium. A well-equipped FOCAL probe, driven perhaps by solar sail with close solar flyby, is a logical goal after IIE or as part of a combined mission concept.
But such a mission, the groundbreaker for space-based gravity lens studies, would be the first of many. In particular, as we learn to push spacecraft to truly interstellar speeds, FOCAL becomes a needed precursor that can tell us much about target solar systems. As Maccone notes in The Sun As a Gravitational Lens:
I anticipate that there will be a host of FOCAL space missions launched in all directions around the Sun, each probe launched in the direction exactly opposite to the star to explore with respect to the Sun position….A FOCAL space mission could be used to magnify anything of interest outside the Solar System. One should then say that FOCAL will be used to magnify the nearby planetary systems, meaning not just the nearby stars themselves, but also their planets, halo disks, Oort clouds, etc.
The range of targets is vast if FOCAL-style missions become routine through breakthroughs in our propulsion technologies. In the interim, deep space probes to the required distances offer the possibility of numerous scientific investigations, many of which were first examined by NASA in its studies for the TAU (Thousand Astronomical Units) mission in the 1980s. What Maccone continues to argue forcefully is that any probe into regions beyond the heliosphere will be in position to exploit the gravity lens, and that our designs for such probes should incorporate the needed instrumentation to show us how best to use this tremendous natural tool.
by Paul Gilster | Aug 17, 2006 | Exoplanetary Science |
The Spitzer Space Telescope has peered into the Orion nebula with striking results: nearly 2300 planet-forming disks in the overall Orion cloud complex, a star-forming region some 1450 light years from Earth. This is where infrared truly shines, for such disks are too small to be seen with visible-light telescopes. But Spitzer is made to order for picking up the infrared signature of warm dust, giving us an unprecedented look at solar system formation in the aggregate. The image below gives a glimpse, but be sure to click to enlarge the photograph for a bit more detail.
Thomas Megeath (University of Toledo, OH) likens the research to a census of potential solar systems, saying “…we want to know how many are born in the cities, how many in small towns, and how many out in the countryside.” Megeath and colleagues discovered that 60 percent of the disk-bearing stars in the Orion cloud complex are found in clusters of hundreds of stars, while 15 percent exist in much smaller groupings, with a surprisingly high 25 percent in relative isolation. That may force an adjustment to earlier theories that most young stars would be found in relatively crowded locales.
Image: A Spitzer Space Telescope view of the Orion nebula, our closest massive star-making factory. The nebula is close enough to appear to the naked eye as a fuzzy star in the sword of the popular hunter constellation. Credit: NASA/JPL-Caltech/ T. Megeath (University of Toledo).
Another key issue: If 60 to 70 percent of the stars in the Orion complex have disks, what constraints keep the other stars from developing them? Understanding how disks form in these environments may tell us much about how planets come to be, and perhaps help us examine our own Sun’s birth. Did Sol come from a crowded city of stars or a relatively sparse stellar environment? Because stars drift away from their place of origin, answering that question can be tricky, but this Spitzer study gives us plenty of new material to work with.
Centauri Dreams‘ take: Stepping back from the details, what stands out here is the sheer fecundity of planetary formation. 2300 budding solar systems in this study alone — we seem to live in a universe that will make planets whenever and wherever it can. The implications for astrobiology are both obvious and heartening.
by Paul Gilster | Aug 16, 2006 | Outer Solar System |
Someone with more cultural insight than Centauri Dreams will have to explain why the designation of Pluto as a planet has captivated so large an audience. The issue is front page on my local newspaper this morning and I’m being asked about it by people who have never shown the slightest interest in space exploration. Perhaps it’s the overturning of things memorized long ago, as if someone had changed the multiplication tables, or decided to modify what makes up the letters of the alphabet.
Whatever the case, the IAU’s draft definition seems to lead to a de facto loss of planetary dignity for Pluto even while maintaining its tenuous identification as one of the tribe. For ‘plutons’ — that category to describe planets whose orbits take more than 200 years to complete, with large orbital inclination and eccentricity — will no doubt soon comprise the vast majority of planets as we discover more and more material in the Kuiper Belt. Which will inevitably lead to an accurate sense that the ‘classical’ planets and these other objects are fundamentally different things.
Image: The world’s astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the lower end of the planet scale – what defines the difference between “planets” and “solar system bodies”. If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, three of the bodies in the Solar System will be assigned new status as planets: Ceres, Charon (Pluto’s companion) and 2003 UB313. There is no change in the planetary status of Pluto. In this artist’s impression the planets are drawn to scale, but without correct relative distances. Credit: The International Astronomical Union/Martin Kornmesser.
And that brings us to Ceres and Charon, and the useful fact that the proposed IAU definition would rest upon twin conditions for planetary status, neither of which accepts Pluto’s diameter as a criterion:
The object must orbit a star while not being one itself; neither can it be the satellite of a planet
The object must be massive enough for its own gravity to form it into a spherical shape. That implies objects of sufficient mass with a diameter larger than 800 kilometers or so are planets, though borderline cases will have to be determined as they arise
Which strikes this observer as emminently sensible because it relies on the way objects behave in a solar system environment. As Richard Binzel, a member of the Planet Definition Committee puts it, “Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet.” More discussions to follow and perhaps touch-ups to the draft resolution, with a vote to be taken on the 24th.
A later addition for clarity: Pluto and Charon are considered a double planet rather than a planet and its satellite. Why aren’t the Earth and Moon treated the same way? The reason is found in the IAU’s definition of a double planet, which is this: “A pair of objects, which each independently satisfy the definition of ‘planet’ are considered a ‘double planet’ if they orbit each other around a common point in space that is technically known as the ‘barycentre’. In addition, the definition of ‘double planet’ requires that this ‘barycentre’ point must not be located within the interior of either body.” All of which makes Pluto and Charon a double planet but rules out the Earth and Moon as a double.
by Paul Gilster | Aug 15, 2006 | Deep Sky Astronomy & Telescopes, Research Tools |
When is a galactic grouping ‘compact’? Take a look at the four closely grouped galaxies in the image below; they’re most of the galaxies in Stephan’s Quintet (the fifth is off-image to the lower right). Redshift measurements indicate that the top three of these are at the same distance from us, about 300 million light years away in Pegasus. A group is considered compact when it shares the same gas reservoir, or so I learned while reading about a presentation on the subject made by C. Mendes de Olivera at the ongoing IAU meeting in Prague.
Image: Four of the galaxies of Stephan’s Quintet. The galaxy at bottom left is a foreground object, but the other three are at the same distance from us and engaged in spectacular gravitational interactions. Credit: Jane C. Charlton (Penn State) et al., HST, ESA, NASA.
I owe the opportunity to learn about these matters to Ph.D student Thomas Marquart, who is working in the Galaxy Group at Uppsala Astronomical Observatory in Sweden. Marquart is posting regular entries on the IAU’s General Assembly on a weblog and is inviting (though with little result so far) other participants in the meeting to post their own thoughts as the sessions continue. In any case, his own observations, largely focusing on galactic evolution, make for lively reading and offer a sense of participation in a major scientific event.
One that, it must be said, has more to do than focus on the status of Pluto, despite newspaper stories implying that the subject has conference goers on the edge of their seats. Better by far to read about the IAU’s work at a site like this, and let’s hope that weblogs, which are easy to set up, become a regular part of such scientific gatherings. Sure, they’re unofficial, and that’s their interest. They give you inside thoughts rather than press releases, with comments on the hot issues and insights into the personalities involved.
And yes, as to Pluto, my thinking remains the same. Whatever the IAU decides is fine by me, though if it were my decision, I would leave Pluto with a planetary designation and simply declare its size the lower limit for planets. There are good reasons for acknowledging its properties as a Kuiper Belt object, but it’s also true that we have much to learn about the Kuiper Belt and what it contains. The safer course, keeping us from changing our nomenclature every few decades, would be to defer any change of planetary status until we’ve had a closer look at Pluto and other Kuiper Belt objects, starting with New Horizons in 2015. More on this tomorrow and then on to more pressing things.
by Paul Gilster | Aug 14, 2006 | Research Tools, Tau Zero Foundation |
The annual New Trends in Astrodynamics and Applications conference meets for the third time this week in Princeton, with Ed Belbruno calling the house to order on Wednesday. From an interstellar perspective, this year’s conference is packed — last year we had but three interstellar papers, whereas the 2006 meeting will feature two complete sessions and no fewer than nine papers on topics ranging from collecting antimatter from natural sources in the Solar System (James Bickford) to spacecraft miniaturization (Mason Peck) and antimatter/nuclear hybrids (Gerald Jackson). You can find the list of speakers and their topics at the program site.
This year the focus on near-term precursor concepts is robust. Greg Matloff will report on interim missions as a way to ‘prep for Centaurus,’ while Les Johnson and Sandy Montgomery (NASA MSFC) will present the latest solar sail developments, and Claudio Maccone will examine the FOCAL mission to the Sun’s gravity lens. I had been looking forward to renewing conversations with both Matloff and Maccone and haven’t seen Johnson or Montgomery since researching my book in 2003, but unexpected developments scuttled my travel plans.
Nonetheless, Centauri Dreams should be able to report on many of these papers after the fact, and from what I’ve seen already, they should make for fascinating reading. Marc Millis’ presentation on the “Incessant Obsolescence Postulate and Practical Interstellar Flight” makes shrewd points about mission times and targets that will provide fodder for lengthy discussion, and Jordin Kare will be on hand to talk about his ‘Sailbeam’ concept for probes moving at a tenth of lightspeed.
All this good material reminds me that two years have gone by since I first talked about making the re-creation of a yearly interstellar bibliography a prime goal of the Tau Zero Foundation (which in those days was being developed under a different name). I say ‘re-creating’ because the first interstellar bibliography was produced by Robert Forward and Eugene Mallove over a quarter of a century ago. Its last appearance was in the Journal of the British Interplanetary Society in 1980, with 2700 items in 70 subject categories. That a working bibliography is a basic tool for research in these disciplines is obvious, and my hope is to begin work on the new one before year’s end.
