by Paul Gilster | Jul 12, 2010 | Outer Solar System |
Centauri Dreams readers should know the name Stuart Atkinson, whose excellent Cumbrian Sky site I’ve linked to before. I don’t have many occasions to reproduce poetry in these pages (although I did quote some lines from Coleridge’s ‘Kubla Khan’ in honor of Huygens’ landing), but when I saw what Stuart had sent in to ESA’s Rosetta team, I knew I had to highlight it here. Rosetta’s encounter with the asteroid 21 Lutetia should bring out a bit of the poet in all of us, but Stuart nails what I felt:
For all these years you were merely
A smear of light through our telescopes
On the clearest, coldest night; a hint
Of a glint, just a few pixels wide
On even your most perfectly-framed portraits.
But now, now we see you!
Swimming out of the dark – a great
Stone shark, your star-tanned skin pitted
And pocked, scarred after aeons of drifting
Silently through the endless ocean of space.
Here on Earth our faces lit up as we saw
You clearly for the first time; eyes wide
With wonder we traced the strangely familiar
Grooves raked across your sides,
Wondering if Rosetta had doubled back to Mars
And raced past Phobos by mistake –
Then you were gone, falling back into the black,
Not to be seen by human eyes again for a thousand
Blue Moons or more. But we know you now,
We know you; you’ll never be just a speck of light again.
Nice. You’ll also find this poem at the Rosetta blog, where various photos of the encounter are reproduced, including this sequence of images before closest approach.
(Credit on all images: ESA 2010 MPS for OSIRIS Team MPS / UPD / LAM / IAA / RSSD / INTA / UPM / DASP / IDA).
Stuart’s own take on the Lutetia imagery is here. See this ESA news release for still more imagery. But the image I like best is one taken by the OSIRIS Narrow Angle Camera from a distance of 36,000 kilometers. You can catch Saturn in the background, its rings faint but clearly visible. There is something magical about seeing one deep space destination in the background of a photograph of another.
Rosetta passed the asteroid at a distance of some 3160 kilometers while moving 15 kilometers per second, something to keep in mind as you examine the clarity of these images. No wonder everyone associated with Rosetta is in a celebratory mood:
“Wunderbar!’ says David Southwood, ESA Director of Science and Robotic Exploration, “It has been a great day for exploration, a great day for European science. The clockwork precision is a great tribute to the scientists and engineers in our Member States in our industry and, not least, in ESA itself. Roll on 2014 and our comet rendezvous.”
Out of the encounter we should get a better estimate of the asteroid’s mass, take readings of the solar wind in its vicinity and better understand the properties of its surface crust. Thus the largest asteroid ever to be visited by a spacecraft, heavily cratered and marked by a deep bowl-shaped depression, will offer us insights into the rest of the main belt population while tantalizing us with the glimpse it provides of a remant from the Solar System’s earliest days. Analysis of Rosetta data should help us determine whether Lutetia is an ancient C-type asteroid or an M-type, the latter thought to be fragments of the cores of larger objects.
So it’s on to comet Churyumov-Gerasimenko for Rosetta, with rendezvous in 2014 and an eventual touchdown (on the 10th of November of that year) by the Philae lander. Splendid work by the Rosetta team. And splendid work, too, by Emily Lakdawalla, who got up an analysis of the close encounter photographs in a hurry on the Planetary Society blog. Emily congratulates the ESA operations and OSIRIS teams for their quick release of images, as do I.
I’ll close with another quote from Stuart Atkinson on the meaning of the Lutetia encounter:
…when you look at these images, you’re looking at pictures that are every bit as important as the first images of Yosemite Valley, taken by Charles Leander Weed in 1859, or the first pictures ever taken of Antarctica, or Everest or the ocean floor. They’re not pictures of something, they’re portarits of somewhere, somewhere new, proof that the desire, the need to explore, and see new places, is etched into our DNA like petroglyphs on a canyon wall.
And this final Lutetia image:
Spectacular.
by Paul Gilster | Jul 9, 2010 | Exoplanetary Science |
Learning about planets through inference is a necessary procedure, given the state of our technology. We do have a few direct images of exoplanets now, but when relying on radial velocity data or transits, we’re looking at the effects planets cause upon our measurements of their stars. With CoRoT and Kepler now yielding high-quality transit data, it’s encouraging to see how we can go to work on this information to learn even more about the systems they study. Thus the announcement of WASP-3c, a second planet found around a star in the constellation Lyra, whose existence was pegged by its effect on the previously known planet.
WASP-3b was discovered by the Wide Angle Search for Planets project (SuperWASP), a British extrasolar planet detection program that uses robotic observatories that monitor stars for transit events. Eight wide-angle cameras monitor millions of stars, with 26 exoplanets now discovered. The new work, led by Gracjan Maciejewski (Jena University, Germany) went to work on WASP-3b using a method called Transit Timing Variation (TTV), which studies whether any variation in time between planetary transits of a known world can be detected.
The timing of known transiting exoplanets may prove to be an important tool, one that has already been studied on several transiting planet systems. If this work proves out, it will be the first planetary detection using the method. From the paper:
In a single-planet extrasolar system a planet orbits its host star on a Keplerian orbit. If one assumes that the inclination of its orbit plane is close to 90?, transits occur at equal intervals. If there is another (not necessarily transiting) planet in the system it interacts gravitationally with the transiting planet and generates deviations from the strictly Keplerian case. These perturbations result in a quasi-periodic signal in an O?C diagram [analysis of observations minus calculations] of the transiting planet.
In this case, WASP-3b, a planet of 1.76 Jupiter masses in a 1.85 day orbit around a F7-class star, is shown to be perturbed by another body. Gracjan Maciejewski comments on the find:
“We detected periodic variations in the transit timing of WASP-3b. These variations can be explained by an additional planet in the system, with a mass of 15 Earth-mass (i.e., one Uranus mass) and a period of 3.75 days. In line with international rules, we called this new planet WASP-3c.”
Image: This is the so-called O-C diagram. We plot the difference between observed (O) transit time and calculated (C) expected transit time on the y-axis in minutes versus the time given as orbital periods of the known planet WASP-3b. We plot the previously published transit times as blue dots and our own new measurements as red dots. If there would be only one planet around the star WASP-3, then all points should be on one straight line. If there would be a second planet with 15 Earth masses and 3.75 day orbital period (called WASP-3c), then this second planet would modify the orbital period of the first known planet (WASP-3b, 2 Jupiter masses in 2 day orbit) in such a way as shown by the black line, which we have calculated. This is the best fitting configuration, i.e. indirect evidence for such a new planet WASP-3c. Credit: G. Maciejewski/Jena University.
The detection is now being followed up via radial velocity studies with the 10-meter Hobby-Eberly telescope in Texas, where the new planet’s existence can be confirmed. The gravitational interactions that make Transit Timing Variation so useful are studied in terms of period and amplitude to derive the parameters of the perturbing planet, a matter of intense computer work analyzing possible configurations in a given planetary system. As we refine TTV, it’s worth noting that the method is sensitive to small perturbing planets down to Earth mass. In fact, a ‘hot Jupiter’ whose orbit is affected by a one Earth mass planet will show a definite TTV signal of up to a minute, detectable by 1-meter class telescopes.
The paper is Maciejewski et al., “Transit timing variation in exoplanet WASP-3b,” accepted at Monthly Notices of the Royal Astronomical Society and available as a preprint.
by Paul Gilster | Jul 8, 2010 | Astrobiology and SETI |
Take a look at the frequency range of our SETI searches and you’ll see that we are probing into new territory. Project Phoenix, which ran from 1995 to 2004, used radio telescopes at Arecibo, Parkes (NSW, Australia) and Green Bank (WV, USA), working in a frequency range of 1.2 to 3 GHz. The BETA project used a 26-meter radio telescope to examine the so-called ‘waterhole’ frequencies between 1400 and 1720 MHz, which seemed a likely place to look for an extraterrestrial beacon because this range covers an unusually quiet band of the electromagnetic spectrum between the hydrogen spectral line and the strongest hydroxyl line.
With the Allen Telescope Array coming online, we can look forward to a search of 250,000 stars in the ‘waterhole’ region, but new facilities like LOFAR (Low-Frequency Array) are pushing into the megahertz area in pursuit not only of SETI but also astrophysical studies of the early universe. LOFAR makes me think back to my shortwave radio days, tuning around these frequencies looking for hard to catch transmitters in places like the Falkland Islands (extremely difficult) or Tristan da Cunha (impossible unless you were based in South Africa). I often wondered about SETI at these frequencies and dismissed it as absurd. But that was then.
From Interference to Silence
The problem with LOFAR’s frequency range is that it runs into massive interference from many of the things our civilization does to emit radio signals, from radar to television stations. New technologies have come along that allow us to filter out this interference with great efficiency. But it’s undeniably true that we are radiating strongly in these wavelengths, so it’s an interesting question whether we might be able to pick up a civilization doing the same things by using LOFAR or the upcoming Square Kilometer Array (SKA), which some studies tell us could detect signals like those we produce at a distance of up to 300 light years.
A new paper by Duncan Forgan (University of Edinburgh) and Robert Nichol (Institute of Cosmology and Gravitation, Portsmouth, UK) looks at these possibilities in the context not only of technological capability but the likelihood of running into such radio leakage in the first place. The duo worked with Monte Carlo realization techniques, generating a catalog of planets which can develop life that evolves toward intelligence, and creating statistical populations of ETIs that develop at various times and locations. The resulting realizations are run multiple times and the resulting distributions averaged to quantify the uncertainty in the modeling process.
The connectivity calculations are approached in two ways, the first setting no maximum distance or time limits on communication — imagine a civilization that puts out detectable radio signals for the duration of its existence. The second, however, is more like us and it is deeply constrained. It tends to go quiet at these wavelengths rather quickly, just as our own civilization is doing. Here the distance assumption is 100 parsecs and the time that a civilization leaks radiation into space is reduced to about 100 years. As the authors note:
…we should think carefully about what we might expect the SKA to see. While humans are still leaking radio emission into the Galaxy, the extent of this emission has diminished. Technological improvements have reduced the transmission power required to broadcast, and the dawn of the digital age has begun to supersede traditional radio entirely. These events have occurred in just over 100 years, putting us on the path to becoming a “radio quiet” civilisation. If the Biological Copernican Principle is true (i.e. humans are not atypical as intelligent species), then what happens if all civilisations rapidly become radio quiet?
Human-like Cultures Hard to Find
The first scenario is easy to analyze, with cultures around various stars exchanging messages at the speed of light for the duration of their technological maturity. But the second scenario is grim. Connectivity reduces to virtually zero: Human-like civilizations have a probability of communication in the area of 10-7 under these constraints. We can play with the parameters by extending the observation period of a SKA-like installation from the 30 days used in these calculations to 10 years, in which case the probability of detection becomes 10-4. Assume 105 civilizations and a small number of detections become possible, although the likelihood of using a facility like SKA for 10 years of constant observations on a SETI project is slim.
If other civilizations behave like we do in terms of radio communications, then, we are not likely to find them with SKA. These considerations are not new, but the authors have updated the discussion by using the most recent predictions for the possible distribution of habitable planets and the sensitivity of the latest radio detection technologies. As the paper notes:
While the SKA remains an important instrument for SETI researchers, its abilities are limited to detecting civilisations that are at a less advanced state than Mankind, i.e. they must develop radio technology that remains radio-loud for a significant period of time. This strengthens the argument for a multi-wavelength approach to SETI, as radio-quiet civilisations may be optically loud, or detectable at some other energy scale…
The last point is the most important conclusion of the paper. Because the detection of radiation leakage from other civilizations at our current level of technology is unlikely, the multi-wavelength approach cited above is the best way to proceed. If accidental communications are off the table, we are probably looking for beacons, or else for signals deliberately beamed at us. Such a signal would demand a civilization far more advanced than our own, and in the latter case, one that was aware of our existence because it had developed a capability for optical or radio detections that we have yet to achieve.
The paper is Forgan and Nichol, “A Failure of Serendipity: The Square Kilometre Array will struggle to eavesdrop on Human-like ETI,” accepted for publication in the International Journal of Astrobiology and available as a preprint.
by Paul Gilster | Jul 7, 2010 | Asteroid and Comet Deflection |
Asteroids are much in the news these days, with Japanese and European missions returning outstanding photos and information about them. While we await testing on what may be fragments of the asteroid Itokawa from the Hayabusa team, we now prepare for another asteroid flyby on the part of the European Space Agency’s Rosetta spacecraft, which carries DLR’s Philae lander, a craft destined for eventual touchdown on the comet 67P/Churyumov-Gerasimenko.
But that’s not until 2014, when the most detailed study of a comet ever attempted reaches its destination. Along the way, Rosetta has delivered interesting asteroid results, including a 2008 flyby of the ‘diamond in the sky’ asteroid called Steins. We can now look forward to a flyby of the main belt asteroid 21 Lutetia, which will occur on July 10. Three instruments on the lander — a magnetometer and plasma monitor (ROMAP), and two gas analyzers — will be switched on during the flyby. More in this DLR news release, which notes that scientists will be looking for signs of a possible exosphere, examining the thermal history of the object and checking for the presence of magnetic minerals.
Image: The Earth as seen by Rosetta’s OSIRIS narrow-angle camera from a distance of 633,000 kilometers on 12 November 2009. This was during Rosetta’s last Earth swingby, one of four gravity assist maneuvers performed by the spacecraft. Credit: ESA/MPS.
Rosetta will hustle past Lutetia at 15 kilometers per second, a tricky enough problem in data acquisition even if it weren’t being performed 25 light minutes away, resulting in commands taking almost an hour after being sent before their receipt can be confirmed. This classic problem in deep space communications is handled by storing the needed commands aboard the spacecraft well before the flyby, for automatic execution at the appropriate time. Both the Rosetta site and DLR should offer updates as the encounter approaches.
We wait with great interest, meanwhile, for further news about Hayabusa as the Japan Aerospace Exploration Agency (JAXA) works with the return capsule’s sample container. The tiny particles that show up in this image may well be from the near-Earth asteroid Itokawa, captured aboard the craft even though the crucial mechanism designed to collect asteroid fragments failed. The idea is that dust from Hayabusa’s landing may still have found its way into the probe.
Image: Particles found in Hayabusa’s sample canister may be from the asteroid Itokawa. Credit: JAXA.
With all this good science in progress, it’s important to note as well that the committee drawing up plans for NASA’s mission goals and priorities for 2013 to 2022 will meet again on July 13, a Washington DC gathering that will examine, among other things, the biggest ticket items being considered, a three-part Mars sample return effort and a mission to Europa, both conceived as joint efforts with the European Space Agency. These and 24 other missions are now receiving their cost estimates for ranking by the committee.
Keep your eye on this process, for with funding as constrained as it is, attempting Mars and Europa at the same time could be a problem for smaller missions destined for other targets. This is from an article by Eric Hand on the proceedings that ran this week in Nature:
…attempting a Europa mission and the Mars sample return at the same time could crowd out smaller missions to other parts of the Solar System, says Alfred McEwen, principal investigator for the HiRISE camera on the Mars Reconnaissance Orbiter, which is currently imaging Mars. “If both go forward, can NASA — and ESA for that matter — do much of anything else?” he asks.
Or will cost and complexity bias the committee toward smaller missions?
What [Steve] Squyres has called “sticker shock” for the biggest missions could bias the survey in favour of small- and medium-cost mission lines known as Discovery and New Frontiers. “I could put together a spectacular programme without either one of those [flagship missions]. There are many ways to slice this,” says Squyres. Or, as [Fran] Bagenal puts it: “They could say, ‘A pox on both your houses. Let’s just continue with New Frontiers and Discovery until you come up with an easier, cheaper way to do these missions.'”
The committee will draft its recommendations in late September, intending to publish its report by April. This is a treacherous time for mission science — only so many projects can be chosen — but as Rosetta closes on Lutetia enroute to a comet, and as we wait for news from JAXA’s doughty Hayabusa team, we should remember the value of smaller-ticket missions that often don’t get the press of the biggest projects. In choosing our targets, keeping a focus on asteroids is a prudent course as we weigh planetary security and analyze the dangers.
by Paul Gilster | Jul 6, 2010 | Uncategorized |
The planet known as TrES-2b is an interesting and useful place. Just over Jupiter mass, it orbits a solar mass star some 717 light years from Earth, a ‘hot Jupiter’ in a tight 2.47-day orbit. It’s also a transiting planet, discovered by the Trans-Atlantic Exoplanet Survey, which uses small, automated equipment and off-the-shelf technology to get the job done, feeding planet candidates to larger installations like the Keck Observatory and Palomar Observatory. But TrES-2b has a new and important distinction: It’s in the field of view of the space-based Kepler telescope.
Now we’re really in business. Exomoon-hunter David Kipping (University of London) said in a recent email that when this planet is viewed in ‘short-cadence mode’ with Kepler, it’s like seeing transits in High Definition. And indeed, that seems to be the case, as you can see in the diagram below. Kepler offers two measurement cadences: 1 minute cadence for up to 512 targets and a 30 minute cadence for up to 170,000 stars. In the early phases of the mission, short-cadence observations are reserved largely for astroseismology targets, but as planet candidates turn up through long-cadence observations, they can be put on short-cadence for better transit coverage.
Have a look at the image to see the difference between the short- and long-cadence lightcurves for TrES-2b. Click to enlarge the image:
Image: Short-cadence folded transit lightcurve of TrES-2b (circles) with model fit overlaid. We also show the long-cadence folded transit lightcurve (triangles) with an overlaid model. The long-cadence curve is a smeared out version of the short-cadence data due to the long integration times. Image and caption: David Kipping.
Working with Gáspár Bakos (Harvard-Smithsonian Center for Astrophysics), Kipping has produced a paper analyzing 18 short-cadence lightcurves of TrES-2b from Kepler, with that ‘HD’ quality he talks about producing ‘the most accurate determination of the transit parameters yet obtained for this system.’ Indeed, the photometry is exquisite, and the precision of the data may be the most outstanding result of our TrES-2b investigations to this point. I want to quote from Kipping’s email on this:
Although TrES-2b is found to be a fairly static system, the real result is the quality of the data which exhibits unprecedented quality – 230 parts per million (ppm) per minute – which is enough to detect a 6.5 hour transit (typical for a habitable-zone planet) with a noise level of 11.6ppm (so for signal-to-noise = 3 you could detect a 34.8ppm transit). This is actually above the design specification which was expected to reach 15.2ppm over the same time-scale for a star of this brightness. It translates to Kepler being able to detect planets about 13% smaller than the design specification.
TrES-2b now becomes the planet most closely studied for the possible presence of a satellite. The new work excludes moons between 0.25 to 3 Earth masses depending on the orbital configuration, a finding displayed in the chart below. Make no mistake about where this points — Kepler is now proven to produce data of sufficient detail to detect exomoons below Earth mass. And while TrES-2b hasn’t yielded one, this exciting paper tells us that we should be able to study habitable-zone Jupiters for exomoons that could potentially hold life.
Image: Excluded exomoon masses for TrES-2b, as a function of the orbital distance of the moon around TrES-2b and the orbital inclination with respect to the observer’s line-of-sight. Contours are given in units of Earth masses. The Kepler data is able to easily probe down to sub-Earth mass exomoons. Diagram and caption: David Kipping/Gáspár Bakos.
The paper is Kipping and Bakos, “Analysis of Kepler’s Short-Cadence Photometry for TrES-2b,” now available as a preprint.
