by Paul Gilster | Jan 12, 2008 | Asteroid and Comet Deflection |
The 36th Carnival of Space is up at Steinn Sigurðsson’s Dynamics of Cats site. Standing out this week are the items flowing in from the American Astronomical Society meeting in Austin, which ran until Friday. The best place to get the overview is Universe Today, but both Random thoughts of an astro major and Bad Astronomy have tracked events closely. Also noteworthy this time around is the news that the asteroid strike on Mars is now effectively ruled out, the odds falling to one in 10,000.
Image: 2007 WD5 from the University of Hawaii 2.2-meter telescope on Mauna Kea, Hawaii. The circled dot is the asteroid (click to enlarge; it’s at dead center, in a green circle). Other dots are artifacts from cosmic rays. The stars are trailed because the telescope is tracking the asteroid as it moves among the stars. Credit: Tholen, Bernardi, Micheli with support from the National Science Foundation.
Too bad, as the opportunities for close observation of such a hit would have taught us much about planetary impacts, and perhaps provided a wake-up call to the budget cutting at Arecibo’s planetary radar. How these things are calculated is interesting in itself, and the Near Earth Object Program site explains how the numbers can seem to vary so wildly:
The sequence of updates over the last few weeks has been typical of past potential impact scenarios, with the odds of impact initially surging and later plummeting towards zero. Early on, the uncertainty region is very large and the probability of impact is rather low. As the uncertainty narrows, but still includes the planet, the probability initially increases. But eventually, as in this case, the uncertainty region shrinks to the point that it no longer overlaps the planet, and the probability of impact begins a precipitous decline. This rise and fall of the computed hazard was most notably seen in Dec. 2004 when asteroid 99942 Apophis briefly reached a 2.7% chance of impact with Earth in April 2029. In every case, the height and the timing of the peak probability – and the subsequent decline – cannot be known until the uncertainty region has shrunk to the point where it no longer intersects the planet.
All of which is useful to keep in mind as we continue to scan the skies for Earth-crossing asteroids. NASA’s Spaceguard Survey is aimed at finding 90 percent of such objects larger than one kilometer in size, a goal that the Near Earth Object Program site says will be met within several years. 2007 WD5 is now said to pose no threat to either Mars or the Earth for the next century. The best estimate is that it will pass 26,000 kilometers from the Martian planetary center on January 30, and almost certainly no closer than 4000 kilometers from the surface (thanks to Hans Bausewein for a correction on the distance).
by Paul Gilster | Jan 11, 2008 | Antimatter |
Although I had planned to push straight on to look at instrumentation for a true interstellar mission (using Mike Gruntman’s landmark paper on the topic), I want to revise that schedule because of the recently announced antimatter news. We’ll return to the instrumentation issue on Monday, including the tricky question of how a probe designed to reach 400 AU can make effective measurements given its speed (75 km/s in the best case scenario Gruntman looks at). Because that question just gets trickier as speeds ramp up, it’s a major one for planning.
But on to antimatter, a cloud of which has been known to exist around the galactic center since the 1970s, when balloon-based gamma-ray detectors first located it. Gamma rays are significant in terms of antimatter because electrons encountering positrons (their antimatter equivalent) annihilate each other, with their mass converted into high energy gamma rays. So the cloud’s presence is well established. The question since its detection is what could have caused it.
Now a new paper in Nature may offer an answer, noting the asymmetric distribution of the antimatter cloud, which extends further on one side of galactic center than on the other. We’re talking about a cloud some 10,000 light years across, generating the energy of 10,000 Suns. The research team used data from the European Space Agency’s Integral satellite (INTErnational Gamma-Ray Astrophysics Laboratory) to detect the asymmetry. Their paper notes that it matches the distribution of a certain type of binary star systems, the latter thought to contain neutron stars and black holes.
Image: Integral mapped the glow of 511 keV gamma rays from electron-positron annihilation. The map shows the whole sky, with the galactic center in the middle. The emission extends to the right. Credit: ESA/Integral/MPE/G. Weidenspointner.
Are these binary stars the cause of the antimatter cloud? They’re what’s known as ‘hard’ low-mass X-ray binaries. The mechanism at play is that gas from a low-mass star spirals into a black hole or neutron star nearby, with high-energy (hard) X-rays resulting. That and the relative similarity between the distributions of cloud and stars makes the case that the binaries are producing these interesting positrons. In fact, says lead author Georg Weidenspointer (Max Planck Institute for Extraterrestrial Physics), “Simple estimates suggest that about half and possibly all the antimatter is coming from X-ray binaries.”
Of course, what comes immediately to mind at this end is James Bickford’s interesting work on antimatter collection here in the Solar System. As we saw in several earlier posts, Bickford has been advocating collection strategies that would mine the antimatter being formed naturally not only near the Earth but also in abundance further out in the Solar System, especially around Saturn. So the idea of antimatter farming again comes to the front with this renewed reminder that the exotic stuff occurs as a result of astrophysical processes and not just in particle accelerators.
Not that we’re able to tap a cloud like this one, so vast and so much further from Earth. But on a theoretical level, it’s useful to learn more about antimatter production even while we’re discovering the limitations in our existing theories. For the questions the antimatter cloud poses are themselves vast. The low-mass binaries seem associated with the antimatter cloud but we lack knowledge of how they could produce enough positrons to account for it. That probably targets particle jets as the necessary area for investigation, something NASA’s GLAST (Gamma-ray Large Area Space Telescope) may be able to shed further light on. And GLAST is helpfully ready for a 2008 launch.
The paper is Weidenspointner et al., “An asymmetric distribution of positrons in the Galactic disk revealed by big gamma-rays,” Nature 451 (10 January 2008), pp. 159-162 (abstract).
by Paul Gilster | Jan 10, 2008 | Missions |
Imagine yourself aboard a spacecraft pushing into interstellar space. At what point would the Sun cease to be the brightest object in your sky? We’re already looking at missions designed to study the local interstellar medium (LISM), with the goal of reaching anywhere from 300 to 400 AU, a region believed to be undisturbed by the Sun. From that range, the Sun still shows an apparent visual magnitude of -13.7, making it brighter than any other star we see from Earth (Sirius comes in at magnitude -1.46).
So it’s a long push. In fact, an early interstellar probe moving at 75 kilometers per second would have to travel six thousand years to reach the point where the Sun is no longer the brightest star. At 100,000 AU, which is 1.61 light years, our imaginary probe occupant would finally see a sky where the Sun was just another bright star.
I get this information from a fascinating paper by Mike Gruntman (USC), who was kind enough to forward links not only to it but several other papers we’ll look at in the near future. This one is a 2004 examination of the instrumentation we’ll want to have aboard the first true interstellar probe. Note the word ‘true.’ We do have two functional probes that are pushing toward interstellar space right now, but neither Voyager craft was designed from the start with an interstellar science package in mind. Nor was New Horizons, even though it may tell us much not only about Pluto/Charon but also the Edgeworth/Kuiper Belt.
No, Gruntman is talking about a specific mission indeed. You could say that such a mission is modest compared to what we hope to achieve in interstellar flight. But pushing out to 400 AU is no easy business, especially if you want to do it within the working lifetime of the people who built it. Chemical propulsion is out — even the most efficient chemical systems (specific impulse 450 seconds) demand a mass ratio of 340,000 to reach the necessary velocity. Nuclear electric propulsion and solar sail technologies, however, are in the mix, and both are not that far from the technology readiness level demanded of this mission.
The Innovative Interstellar Explorer concept shows how the idea of a dedicated interstellar probe has evolved. Gruntman is a member of this team, and much of what he brings to IIE in terms of instrumentation and methods of measurement can be seen in this paper. IIE re-examines the concept in significant ways, as can be seen from this quote on the site:
We have provided a first cut of a self-consistent design for such a mission using Radioisotope Electric Propulsion (REP), existing launch vehicle hardware, and a Jupiter gravity assist. While the final speed of the probe is not as high as might be wished, it is sufficiently high to provided new – and potentially transformational – knowledge of our surroundings in interstellar space. More importantly, the required technology advances are evolutionary such that the probe could be built – and launched – as soon as the next launch window opens in late 2014.
Whatever the final design, the thinking within NASA on mission concepts like this goes back to a 1976 conference at the Jet Propulsion Laboratory that looked at the engineering problems it would face and the scientific gains it might accomplish. Ever since then, precursor missions pointing to the current Innovative Interstellar Explorer and beyond have continued to be discussed and refined both within NASA and without.
But just what is the mission? The primary goal is to reach the true interstellar medium and examine its properties. But ponder this: The Sun moves at a velocity of 26 km/s with respect to the surrounding interstellar medium. The motion is called the ‘interstellar wind,’ and as Gruntman points out, its direction is close to the plane of the ecliptic. We consider the heliosphere the place where the Sun controls the local plasma environment, but our data on its interaction with the LISM are scarce, despite the intriguing exploits of the Voyagers. So along with studies of the LISM itself, the second goal is to put the best instruments to work on what happens as the Sun’s influence effectively ends.
Gruntman sees a dedicated probe to this area as a natural next step as we go about the lengthy process of building an interstellar future. From the paper:
The lack of the direct experimental data and the resulting uncertainties in understanding of fundamental processes of the sun–LISM interaction severely limit our ability to develop a self-consistent concept of the heliosphere. Exploring in situ the nearby galactic environment and the region of the solar system frontier is thus an essential, logical, and unavoidable step in our quest for understanding our star, its interaction with the Galaxy, and laying out the foundation for the truly interstellar ?ight of the distant future.
The Voyagers, of course, are immensely helpful already as they push through the termination shock, and their expected lifetimes for transmitting data may see them through the heliopause, that region where the Sun’s influence becomes negligible. And as you can see, the powers that be now speak of the Voyager Interstellar Mission. But Voyager was never designed for this kind of work, and a dedicated mission to explore the heliosphere and its interactions with what lies beyond, as well as evaluating the nature of the interstellar medium itself, would clearly upgrade our fundamental knowledge.
Figuring out what kind of instrumentation such a probe will need is crucial. Interstellar plasma flows around the heliopause and is thus inaccessible to us. Large interstellar dust grains make it to the inner heliosphere but small grains are pushed out by the solar wind. Even the larger grains are heated by the Sun, with the possibility of destroying organic molecules of the kind we need to study. For these and other reasons, the local interstellar medium is largely unknown to us. We’ll talk tomorrow about how a fast moving interstellar probe will examine that medium, and what other opportunities a mission like this might open for wide-ranging study.
The paper is Gruntman, “Instrumentation for Interstellar Exploration,” Advances in Space Research 34 (2004), pp. 204-212 (available online).
by Paul Gilster | Jan 9, 2008 | Deep Sky Astronomy & Telescopes |
Is the image at left an accurate depiction of what triggers at least some of the gamma-ray bursts we’re now detecting? Or is it a model now in need of serious revision? We’re looking at an artist’s conception of the merger of two neutron stars, an event that produces gamma rays (note the jets emanating from the center). Such a scenario may be the cause of short gamma ray bursts (GRBs). But NASA’s Swift satellite and the Gemini Observatory (Hawaii) have detected one such burst that takes us farther back in time than ever before, some 7.4 billion years. And therein lies a tale.
GRB 070714B was detected last July 14, the second burst of the day (note the terminal B in the designation). Short bursts are those lasting less than three seconds, the most popular theory for their formation being neutron star merger and collapse into a black hole, with consequent ejection of energy. Such bursts are obviously tricky to study because their short duration calls for immediate follow-up with optical telescopes. In this case, the 4-meter William Herschel Telescope was able to locate the optical afterglow at the burst location.
Image: One possibility for the GRB 070714B event, the merger of two neutron stars. Credit: NASA/Dana Berry.
With the host galaxy now identified, researchers could use the Gemini North telescope to reveal the spectral line of ionized hydrogen in that galaxy, tagging its redshift at 0.92. That makes GRB 070714B half the age of the universe. Thus we can move the era during which we know short GRBs were a factor significantly back in time. In fact, this burst is almost twice as distant as the short GRB previously considered the record holder.
Nor is this burst, which occurred in Taurus, average in any other way. Its energy is about one hundred times the norm for short bursts, a level more typical of a long GRB (duration greater than three seconds). Does it make sense for there to be such variation in the neutron-star merger model? Says Swift lead scientist Neil Gehrels (NASA GSFC):
“It is unclear whether another mechanism is needed to explain this explosion, such as a neutron star-black hole merger. Or it could be that there are a wide range of energies for neutron star-neutron star mergers, but that seems unlikely.”
The take on long GRBs is increasingly that they are the result of the collapse and subsequent explosion of massive stars. But the neutron star smash-up is only one of the possible models designed to explain the short bursts. Did one of the beams generated by the event happen to be aimed directly at our planet? It’s a possibility, and would account for the detection of more powerful energies than would otherwise have been the case. But we know so little about the mechanism at play here that short GRBs should provide fertile ground for research for some time to come.
by Paul Gilster | Jan 8, 2008 | Outer Solar System |
Having looked at Titan and Europa yesterday, we can complete our outer planetary trifecta with a stop at Enceladus, lately of great interest as an active and possibly life-bearing moon. One hot spot detected there by Cassini is ejecting plumes of ice and vapor above the arid world in a cloud so fine that, according to William McKinnon (Washington University, St. Louis), the result is like a smoke made of ice, its particles about one-thousandth of a millimeter across. Enceladus is clearly a geologically active world, far from the inert desolation once expected.
All of which makes the Saturnian moon intriguing in the extreme when you start wondering about the presence of water and the possibilities of life. But McKinnon is quick to dash that hope when asked bluntly whether there is evidence for a subterranean ocean:
“I don’t think so,” McKinnon said. “The strongest piece of evidence against that is measurements made from Earth of the plume don’t show any sodium. If the source of the plumes were fresh water like on Earth, the plumes would contain enough detectable sodium. Fresh water flows through rocks and on streambeds, and so it picks up bits of mineral chemistry. The emerging view is that there’s not obvious evidence for a subterranean ocean in contact with rock, no boiling or venting.”
Image: Enceladus as seen by Cassini. Investigating its hot spots is again on the orbiter’s agenda in March. Credit: NASA/JPL/Space Science Institute.
We thus wind up with what is still the most obvious explanation for the plumes of Enceladus, the tides that cause crustal motion in a set of fault lines near the south pole. On its next pass, in March of this year, Cassini will go again into the plume and examine the venting area in the infrared, along with the cracks, impact craters and various fissures found in this intriguing area. We should also wind up with better measurements of the gases and vapors issuing from this activity.
The tidal motion here is interesting. Enceladus is in dynamic resonance with Dione, so that every time the more distant Dione orbits Saturn, Enceladus makes two revolutions. The continuous squeeze from Saturn does interesting things to the surface, with the temperature at the hot spot at least 100 degrees warmer than the temperature at the actual poles. And the same kind of tidal mechanisms help power Io’s volcanism and put Europa under continual forces that re-shape its surface.
The same American Geophysical Union meeting where McKinnon delivered his Enceladus presentation in December was the venue for his comment that an ocean on Europa is now considered all but a certainty. Radar sounding by a future Europa orbiter could tell us much more about the depth of the ice shell covering that body of water. So we continue to turn to the moons of the outer planets as possible sites for life, with the odds on Enceladus (never high) perhaps fading but Europa more in the picture than ever. Nor (and I hope David Grinspoon is still with us!) should we count out exotic possibilities on Titan itself.
