by Paul Gilster | Nov 8, 2010 | Outer Solar System |
The Hartley 2 flyby was an outstanding event, and the only thing I regret about my recent travels was that I was unable to follow the action as the images first streamed in. By now, sights like the one at left have made their way all over the Net, so I won’t dwell on them other than to say that if you haven’t seen the short video clip of the EPOXI flyby, you should definitely give it a look. You’re getting the view from a spacecraft that closed to 700 kilometers or so of the surface, during an encounter taking place at a speed of 12.3 kilometers per second.
Image: Comet Hartley 2 can be seen in glorious detail in this image from NASA’s EPOXI mission. It was taken as the spacecraft flew by around 6:59 a.m. PDT (9:59 a.m. EDT), from a distance of about 700 kilometers (435 miles). Jets can be seen streaming out of the nucleus. Image credit: NASA/JPL-Caltech/UMD.
I’ll go along with EPOXI principal investigator Michael A’Hearn’s description of the comet’s “stark, majestic beauty.” Interesting to learn that Hartley 2 has a hundred times less volume than comet Tempel 1, which was Deep Impact’s first target, but there’s plenty still to learn from the data that streamed in from 37 million kilometers away. Adds A’Hearn (University of Maryland, College Park):
“Early observations of the comet show that, for the first time, we may be able to connect activity to individual features on the nucleus. We certainly have our hands full. The images are full of great cometary data, and that’s what we hoped for.”
Thus we get the most extensive observations of a comet in history. This is the first time that two comets have been imaged by the same spacecraft and at the same spatial resolution. What next for Deep Impact and the EPOXI mission? Interestingly, the spacecraft has enough usable fuel, about four kilograms, to support continued science observations even after the Hartley 2 flyby. We’ll have to see whether tasking it with a new assignment is in the cards.
Hartley 2 has been under scrutiny from other sources as well, including the Odin satellite, a Swedish spacecraft created in collaboration with Canada, Finland and France. Odin studied Hartley 2 at the end of October and was able to detect its water signature line at 556.9 GHz with ease, as shown in the image. The rapid variations in water production are thought to be related to the rotation of the comet’s nucleus. Water vapor is released as the cometary nucleus is heated by the Sun — recall that Hartley 2 passed perihelion as recently as 28 October.
Image: A map of water in Comet Hartley 2, observed by Odin on 29 October 2010. Copyright 2010 Swedish Space Corporation/Centre National d’Etudes Spatiales/Observatoire de Paris.
With Hartley 2 creating a celestial display as cometary dust burns up in the Earth’s atmosphere (the comet passed within 19 million kilometers of our planet on 20 October), this is a comet we’ll long remember. Unfortunately, for me it’s associated with a long airline flight and a developing cold that settled in halfway home. It’s amazing how someone who writes so much about going to the stars dislikes all aspects of travel, but it’s the truth, and that reminds me of a story.
Some years back I talked with Bob Forward, Robert Forward’s son, about his late father. The elder Forward was, of course, a towering figure in interstellar studies, changing our perception of what is possible in terms of delivering payloads to other star systems. Bob told me that one night he asked his father this question: If an alien spacecraft landed on the front yard and you were invited to go out into the universe, with the proviso that you could never come back, would you go? And Robert Forward answered instantly (to the dismay of wife Martha), “Of course!”
Consternation reigned. Would Forward really leave his family behind to go to the stars? Well, yes. “You have to understand,” he went on to say. “This is what I have dreamed about all my life.”
I love the story, but I can only say that I’m on a different page. If the spacecraft lands, I’m all for somebody getting on board, but it won’t be me. What I do hope is that whoever goes has some way to get data back to Earth so I can write the story here. This morning, fighting a fever and a sore throat, nursing a lemon-and-honey hot toddy, I’m thinking that home has its charms, and I’m about ready to get back into bed.
by Paul Gilster | Nov 4, 2010 | Exoplanetary Science |
I had hoped to be able to cover the Hartley 2 flyby today, but I’m traveling on Tau Zero business and have to write this entry early. Instead, I’ll at least keep the EPOXI mission focus by talking about the other half of this unique venture, an investigation of exoplanet systems. We can always talk about what the Hartley 2 encounter produced next week, but not before, as the schedule is crowded and I doubt I’ll be able to get an entry posted here at all on Friday.
Remember that the Deep Impact spacecraft that visited Tempel 1 in 2005 is now on an adventuresome extended mission called EPOXI (although the spacecraft, confusingly enough, still retains the original ‘Deep Impact’ name). The spacecraft was reawakened in the fall of 2007 and directed to a flyby of the Earth for a gravitational assist that would put it into a heliocentric orbit for the Hartley 2 encounter.
On the cruise portion of that journey, the extrasolar component of the mission kicked in. EPOCh (Extrasolar Planet Observation and Characterization) is the name of that investigation, the observing phase of which began in January of 2008 and ran until August of that year. A deep search of a nearby red dwarf star looking for a planet comparable in size to the Earth has been part of the larger EPOCh study.
33 light years from Earth, GJ 436 is known to host a Neptune-sized planet that transits every 2.6 days. The fact that GJ 436b’s orbit is slightly eccentric has drawn the attention of researchers interested in whether an unseen planet is behind the eccentricity, perhaps one as small as the Earth. Three weeks of continuous observations produced an excellent lightcurve of the Neptune-class world, but despite the fact that the potential planet was expected to orbit nearly edge-on to our line of sight, no transit or other data signature for it was found.
Image: EPOCh data for the transit of the Neptune-sized planet orbiting the red dwarf star GJ436. Using data of this quality, EPOCh was able to attain sensitivity to planets as small as 1.5 Earth radii. Credit: Ballard et al. 2010, Astrophysical Journal, Vol. 716, p. 1047.
The team is able to rule out transiting planets over 1.5 Earth radii to a high degree of confidence. Even in the absence of a transit, variations in the transits of the known planet could signal the existence of further planets. So is there a GJ 436c, or is there some other explanation for the eccentricity of GJ 436b’s orbit? Drake Deming, deputy principal investigator for EPOCh), has this to say in a post on the EPOXI site:
[The] negative result has provided motivation for theorists to consider that GJ436c may not exist. Theorists are re-examining their original conclusion that the orbital eccentricity of GJ436b requires the presence of a second planet. It is possible that tidal forces from the star are not sufficient — even in the absence of a second planet — to quickly drive the orbit of GJ436b to a circular state, as had been previously believed. That would obviate the need for GJ436c, but would require the interior structure of GJ436b to be unlike the gaseous planets of our solar system.
GJ 436 is one of eight stars chosen for the original EPOCh survey, all known to have other transiting planets. The other transiting planet systems included HAT-P-4, TrES-2, TrES-3, XO-2, GJ436, WASP-3, and HAT-P-7. In addition to finding new worlds, intense study of the known transiting planets could reveal the existence of moons or rings around the planet. The EPOCh team has now moved from processing the data to writing papers about the results.
But EPOCh isn’t through yet. Using the same data analysis technique developed for the mission and employed with GJ 436 data, researchers will soon be working with data from the Spitzer Space Telescope. The Deep Impact spacecraft may have Hartley 2 in its sights, but the EPOCh team will be looking to Spitzer’s investigation of the red dwarf system GJ 1214, some 42 light years from Earth. The Spitzer search, to be conducted in the spring of 2011, targets the habitable zone around this star, and the analytical tools developed for EPOCh coupled with Spitzer’s results could theoretically detect a planet here that is even smaller than the Earth.
Thus a spacecraft designed to drive an impactor into a comet — and one that has been re-purposed for a second comet encounter — produces exoplanet data and new analytical techniques that will now mesh with results from a space-based telescope (not to mention its studies of the Earth from space, that provide information about how to observe Earth-like planets at long range). No question about it, Deep Impact has given us plenty of bang for the buck.
You can find more about the EPOCh targets here, and ponder as I do what may turn up in the subsequent analysis of EPOXI and Spitzer data. The paper is Ballard et al., “A Search for Additional Planets in the NASA EPOXI Observations of the Exoplanet System GJ 436,” The Astrophysical Journal Vol. 716, Number 2 (20 June, 2010). Abstract available.
by Paul Gilster | Nov 3, 2010 | Outer Solar System |
With NASA’s EPOXI mission closing on comet Hartley 2 at 12.5 kilometers per second, be aware that live coverage of the close encounter will begin at 1330 UTC (0930 EDT) on Nov. 4 from mission control at the Jet Propulsion Laboratory. NASA TV streaming video will be available, and you should also be able to watch the action on a JPL Ustream channel. Finally, NASA’s Eyes on the Solar System Web tool, a 3-D environment for Solar System exploration, is available for viewing a real-time animation of the cometary flyby on your PC.
EPOXI is a great instance of re-purposing a spacecraft to extract maximum value. This is the Deep Impact vehicle that gave us such a spectacular view of the impactor smash-up on comet Tempel 1 back in the summer of 2005. Under the name EPOXI, the mission has pressed ahead with two separate objectives, the first being EPOCh, or Extrasolar Planet Observation and Characterization, which has involved a number of nearby stars known to have transiting exoplanets. The idea here was to observe the stars to see if other worlds might be detectable around them. More on this analysis and its potential for finding Earth-like worlds in a day or two.
The spacecraft has also given us useful studies of Earth as seen from a distance, information that should provide perspective as we learn to use spectral information to map continents and oceans on distant worlds. But Hartley 2 is part of the other extended mission, known as the Deep Impact Extended Investigation (DIXI). The mission will use the two Deep Impact telescopes with digital imagers that were used for Tempel 1, along with an infrared spectrometer, in making this fifth close encounter of a comet. Closest approach at Hartley 2 should be at about 700 kilometers.
Meanwhile, we’re already getting interesting information, including the fact that Hartley 2 is quite active. The spacecraft caught two jets firing off the comet’s surface over a 16-hour period on October 26, viewing these from a distance of 8 million kilometers. The video below shows the jets in action as seen by the High-Resolution Imager and the Medium Resolution Imager.
The jets are thought to originate from similar latitudes on the comet’s nucleus. Says EPOXI principal investigator Michael A’Hearn (University of Maryland):
“These movies are excellent complements of one another and really provide some excellent detail of how a comet’s jets operate. Observing these jets from EPOXI provides an entirely different viewpoint from what is available for Earth-based observers and will ultimately allow a proper three-dimensional reconstruction of the environment surrounding the nucleus.”
Arecibo’s planetary radar has also been focused on Hartley 2, with observations that began on October 24 and continued through the 29th. The comet came within 17.7 million kilometers of Earth on the 20th, its closest approach since discovery in 1986. The comet in these images appears to EPOXI project manager Tim Larson (JPL) as ‘a cross between a bowling pin and a pickle,’ a vegetable analogy quickly picked up by mission scientist Jon Giorgini (JPL):
“Observing comet Hartley 2 from the Earth with radar was like imaging a 6-inch spinning cucumber from 836 miles away. Even without all the data in, we can still make some basic assertions about Hartley 2. Its nucleus is highly elongated and about 2.2 kilometers long, and it rotates around itself about once every 18 hours. In addition we now know the size, speed and direction of particles being blown off the comet, and we immediately forwarded all this information to the EPOXI team.”
Below is the pickle in situ:
Image: Twelve radar images of the nucleus of comet Hartley 2 were obtained by the Arecibo Observatory’s planetary radar from Oct 25 to 27, 2010. Image Credit: NAIC-Arecibo/Harmon-Nolan.
Hartley 2 is offering up the best extended view of a comet in history as it makes its pass through the inner Solar System, and the creative recycled use of the spacecraft offers savings of up to 90 percent over the cost of a similar mission built from scratch. We’re going to be learning a lot more about Hartley 2 in short order, although expect images of closest approach to be delayed after the encounter as Deep Impact reorients its high gain antenna on Earth, at which point the downloading of cometary closeups can begin in earnest.
by Paul Gilster | Nov 2, 2010 | Missions |
by Marc Millis
When Pete Worden (NASA Ames) spoke to the Long Now Foundation recently, he surely didn’t realize how much confusion his announcement of a ‘100-Year Starship’ study would create. The news coverage has been all over the map and frequently incorrect, ranging from intimations of a coverup (Fox News) to mistaken linkages between the study and competely unrelated talk about one-way missions to Mars (the Telegraph and many other papers). What’s really going on in this collaboration between NASA and DARPA? Marc Millis has some thoughts on that based on his own talks with the principals. Millis, former head of NASA’s Breakthrough Propulsion Physics project and founding architect of the Tau Zero Foundation, here puts some of the myths to rest and explains where the 100-Year Starship fits into our future.
If you have not yet heard, there’s been a bit of news flurry over the announcement that DARPA is funding NASA Ames to the tune of $1M for a one-year study for a “100-Year Starship.” I was as surprised as anyone when I heard. First, DARPA (Defense Advanced Research Projects Agency) is a defense agency, not one concerned yet with things beyond Earth. Next, NASA’s Ames Research Center does not specialize in the advanced sciences and technologies of star flight, but rather in information technology, air traffic safety, astrobiology, and human factors.
If anything, I would have expected NASA Glenn Research Center (advanced propulsion and power for air and space flight) or NASA’s Jet Propulsion Lab (planetary and deep space probe missions) to have been selected as the NASA partner. And the third surprise is that NASA, in general, has been indifferent (since around 2003) to any goals beyond the von Braun visions for humans on the Moon and Mars. What is less surprising is that this confusion is wide spread. Quoting from Michael Braukus, a NASA spokesman at HQ, DC: “This is not a NASA program, there’s no money for it.”
So, what is the real story behind this and what does this mean for the Tau Zero Foundation?
First, the news stories mixed things up. During a lecture that Pete Worden (head of NASA-Ames) gave at a Long Now Foundation event, several different items were mentioned: The 100-year starship, microwave power beaming for launch assist, one-way missions to Mars, etc. The latter are separate items, not part of the starship study. Also, not all of this information was ready for disclosure, so it was jumping the gun a bit.
DARPA’s press release actually deals with HOW starships should be studied, rather than studying the starships themselves. They want help from Ames to consider the business case for a non-government organization to provide such services that would use philanthropic donations to make it happen. Quoting from DARPA’s news release: “The 100-Year Starship study looks to develop the business case for an enduring organization designed to incentivize breakthrough technologies enabling future spaceflight.” DARPA’s Paul Eremenko adds this:
“We endeavor to excite several generations to commit to the research and development of breakthrough technologies and cross-cutting innovations across a myriad of disciplines such as physics, mathematics, biology, economics, and psychological, social, political and cultural sciences, as well as the full range of engineering disciplines to advance the goal of long-distance space travel, but also to benefit mankind.”
Accordingly, I am one of the folks they contacted with whom to discuss this further. Even though this is an ideal match for the Tau Zero Foundation, there are other organizations and other implementation options that Ames and DARPA want to look at. Beyond that, I’m not supposed to go into details since it’s all “pre-decisional” kind of stuff. Rest assured that when things can be discussed you’ll get the most reliable reports right here on Centauri Dreams.
That said, I’ve also been writing a status update about the Tau Zero Foundation. Now that this latest interruption has ebbed, I’ll get back to that and will fill you in on all the things Tau Zero has been doing and where we stand today.
Ad astra incrementis!
by Paul Gilster | Nov 1, 2010 | Outer Solar System |
It’s staggering how much our view of the Solar System has changed over the past few decades. The system I grew up with seemed a stable place. The planets were in well-defined orbits out to Pluto and, even if it were always possible another might be found, it surely couldn’t pose any great surprise in that great emptiness that was the outer system. But today we routinely track trans-Neptunian objects with diameters over 500 kilometers — about 50 of these have now been found, and some 122 TNOs at least 300 kilometers in diameter. We know about well over a thousand objects in that ring of early system debris called the Kuiper Belt.
It’s an increasingly messy place, this outer Solar System, and it has its own terminology. We have centaurs and plutinos, resonance objects, cubewanos, scattered disk objects (SDOs), Neptune trojans, damocloids, apollos and, perhaps, inner Oort cloud objects.
Nope, this isn’t the Solar System I grew up with, and every new discovery adds to the enchantment. Its burgeoning population of outer objects tells us much about its history, assuming we can make the right deductions from what we see. Orbital trajectories are a kind of history written in motion. The reason that a belt of objects beyond Neptune was first suspected was that Jupiter-family comets have orbital inclinations too low to be consistent with an origin in the Oort Cloud, that spherical cloud of comets thought to stretch a light year or more from the Sun. Advances in CCD technology soon made it possible to track down Kuiper Belt objects, and it’s now believed that 100,000 KBOs with diameters larger than 100 kilometers could exist, and perhaps as many as 800 million objects with diameters larger than five kilometers.
Image: Views of the Kuiper Belt and the Oort Cloud. Credit: Donald K. Yeoman/NASA/JPL.
The Outer System Poker Game
All of which is intriguing in its own right, but sometimes it takes a wild card to drive the story forward. That wild card came in the form of Sedna, discovered in 2003 by Mike Brown (Caltech). Brown has been ruminating over the discovery on his Mike Brown’s Planets site, where he notes the fact that the orbit of every object in the Solar System can be explained, at least in principle, by interactions with the known planets. Every object except Sedna:
Seven years ago, the moment I first calculated the odd orbit of Sedna and realized it never came anywhere close to any of the planets, it instantly became clear that we astronomers had been missing something all along. Either something large once passed through the outer parts of our solar system and is now long gone, or something large still lurks in a distant corner out there and we haven’t found it yet.
The possibilities are fascinating, one being the existence of an unknown planet of approximately Earth’s size at roughly 60 AU. Another possibility: A star that passed close to the Solar System at some point in the remote past, perhaps as close as 500 or 600 AU. In both cases, gravitational interactions would have interfered with what would otherwise have been a routine Kuiper Belt object, kicking it into its present orbit. Brown pegs the chances of a rogue star encounter at around one percent, but in any case, finding the culprit star would be impossible. The Sun has orbited the Milky Way 18 times in our Solar System’s history. “Everything is now so mixed up,” he adds, “that there is no way to know for sure what was where back when.”
The View from a Cluster
The third possibility? A kick from not one passing star but from many relatively nearby stars, a kick dating back to the Sun’s presence in the cluster in which the Sun was born. Brown’s description of the process and the place in which it might have occurred is worth repeating:
In the cluster of stars in which the sun might have been born there would have been thousands or even tens to hundreds of thousands of stars in this same volume, all held together by the gravitational pull of the massive amounts of gas between the still-forming stars. I firmly believe that the view from the inside of one of these clusters must be one of the most awesome sights in the universe, but I suspect no life form has ever seen it, because it is so short-lived that there might not even be time to make solid planets, much less evolve life.
A striking view indeed, and the poets among us can muse on its transience. Brown continues:
For as the still-forming stars finally pull in enough of the gas to become massive enough to ignite their nuclear-fusion-powered cores they quickly blow the remaining gas holding everything together away and then drift off solitary into interstellar space. Today we have no way of ever finding our solar siblings again. And, while we see these processes occurring out in space as other stars are being born, we really have no way to see back 4.5 billion years ago and see this happening as the sun itself formed.
But Sedna may help, because its orbit should be a record of what was going on when the Sun and our Solar System were in their infancy, a key to unlocking a 4.5 billion year old puzzle. The problem is that with only a single object of this kind, we wouldn’t have enough information on which to build the bigger picture, which is why researchers like Brown continue to look for other Sednas. It’s also why numerous other theories have sprung up, including the possibility that Sedna once orbited a different star and is actually an extra-solar dwarf planet. Or (an old favorite) that a brown dwarf somewhere in the Oort Cloud could have given it its nudge.
Of Dust and the Disk
All this reminds me of Mark Kuchner’s work on Kuiper Belt dust. Kuchner (NASA GSFC) has been running supercomputer simulations tracking the interactions of dust grains, and points to the Kuiper Belt as not only the home of countless small objects, but of dust and debris that model, though in a much older and developed way, the debris disks around Vega and Fomalhaut. At stake is how dust travels through the Solar System, affected by the solar wind and pushed by sunlight, not to mention the effects of collisions between icy grains themselves.
Kuchner’s team has been able to create infrared simulations of the Solar System as it might be seen from another star, using models of dust generation that could reflect what the condition of the Kuiper Belt was in a series of time frames going back in steps to 15 million years ago. The simulations show that a broad dusty disk like today’s collapses into a dense ring as we go back in time, producing something similar to the rings we’ve found around other stars. But today’s belt is still active. “[E}ven in the present-day solar system,” says Christopher Stark (Carnegie Institution for Science), “collisions play an important role in the Kuiper Belt’s structure.”
Interestingly for our model of dust in the outer system, Neptune’s gravitational effects push nearby particles into preferred orbits, creating a clear zone near the planet and dust enhancements that precede and follow it around the Sun. Kuchner calls this ‘carving a little gap in the dust.’ Our picture of dust in planetary systems is developing, but it’s worth noting how much work we have to do to anticipate the effects of dust on fast-moving spacecraft as we push past the heliopause and into true interstellar space. And Sedna’s odd orbit reminds us how much awaits discovery in our own systems’ furthest reaches.
