by Paul Gilster | Jun 15, 2005 | Outer Solar System
New Horizons, the doughty spacecraft soon to be sent to Pluto, Charon and on into the Kuiper Belt, has been shipped from its birthplace — the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD — to NASA’s Goodard Space Flight Center in nearby Greenbelt. At Goddard, New Horizons will begin another round of pre-launch tests, with liftoff scheduled for next January.
What I hear from people associated with this project is that New Horizons is ready to go. As mentioned here earlier, APL’s David Dunham gave a talk about upcoming missions at the recent New Trends in Astrodynamics conference in Princeton. I had been concerned about the review process that New Horizons must undergo, required because it carries a radioisotope thermoelectric generator (RTG) that produces energy from fissionable materials. The final environmental impact statement produced by this process is due in the summer, with final NASA decision on the mission in the fall. But when I talked to him at the conference banquet that night, Dunham seemed to have no apprehensions about New Horizons receiving final launch approval.
Meanwhile, the spacecraft has been through a rough week in the APL vibration test lab where it was shaken aboard a large table to simulate the liftoff ride, which will be on an Atlas V launch vehicle. Having passed these tests, the probe will be subjected to deep space conditions, receive balance and alignment testing in a series of spin situations, and made to undergo noise simulations that look for vibration problems during the liftoff phase.
Image: The excellent quality of its 8.3-meter primary mirror and the stability of the atmosphere above Mauna Kea, Hawaii, allowed the Subaru Telescope to provide clearly separated images of Pluto and Charon using its Cooled Infrared Spectrograph/Camera. This image is produced from three 2-second exposures taken through infrared filters on June 9, 1999. Credit: Dale P. Cruikshank, Catherine de Bergh, Sylvain Dout, Thomas R. Geballe, Tobias C. Owen, Eric Quirico, Ted L. Roush and Bernard Schmitt; published in Science (vol. 285, p.1355), 1999.
The launch window is 35 days, opening on January 11. This is designed to be a fast mission; in fact, New Horizons, boosted by the Atlas V and a STAR-48B kick motor, will reach the orbit of the moon in less than nine hours after launch, entering Jupiter space for a flyby just 13 months later. When it encounters Pluto in 2015, New Horizons will be about 3.1 billion miles from Earth, and its extended mission would take it into the Kuiper Belt for a possible encounter with one or more of the icy worlds in this trans-Neptunian region.
We don’t know exactly what we’ll find on Pluto, but APL has an interesting feature comparing Pluto to Triton, Neptune’s enigmatic moon. Both objects are thought to have interiors composed of water ice over a rocky core, with non-water ices like methane, carbon dioxide and nitrogen present on their surfaces. Like Pluto, Triton is seen by many astronomers as a Kuiper Belt object, one disrupted by a giant impact in the past that formed the moon Charon (just as Triton may have struck another Neptunian satellite as it wandered into the planet’s gravity well). In both cases, the ensuing redistribution of mass may have produced some exceedingly interesting surface features.
by Paul Gilster | Jun 14, 2005 | Exoplanetary Science |
‘Normal’ is a tricky word when you’re talking about extrasolar objects. As in ‘normal star,’ a phrase used during yesterday’s news briefing about the new planet detected around Gliese 876, and in much of the press coverage since. The planet’s low mass (about 5.9 times the mass of Earth) rules out the possibility that it is in any sense Jupiter-like, and the natural assumption is that this is a rocky world in a tight orbit around an M-class red dwarf.
Now it is true that M-class stars are normal, if by normal we mean abundant; in fact, some 70 percent of the stars in the Milky Way seem to be red dwarfs (maybe 15 percent are K-class, 3 percent G class, like our Sun, and 3 percent F class, all categories that might support life-bearing planets).
Image: An enlarged view of the rocky planet orbiting Gliese 876. Copyright 2005 by Lynette Cook (http://extrasolar.spaceart.org/), and used with her permission.
But what the scientists studying this object mean by a ‘normal star’ is a star that is still undergoing the processes of stellar burning. They’re making this distinction because until yesterday, the only rocky worlds found orbiting another star had been found around a pulsar, the savaged remnant of an exploded star. M-class red dwarfs are still burning, but they are not Sun-like, and we should keep that caveat in mind. These stars are small, cool, and quite long lived.
And any life found on planets around them might be exotic indeed. Take Proxima Centauri. Like our newfound planet around Gliese 876, a rocky world around Proxima would have to orbit close in to stay warm enough for possible life to form (admittedly, the Gliese 876 planet seems a bit too close in — its surface temperatures would be high enough to make life as we know it hard to imagine). Well inside the orbit of Mercury in our own Solar System is where the habitable zone is going to be around a red dwarf like this. In Proxima’s case, the frequent x-ray laden flares it emits might prove fatal to life, or they might serve as an evolutionary stimulus, depending on their severity.
And those temperatures, even the high ones on the surface of the new Gliese 876 planet, are still provocative. Any planet this close to a red dwarf is probably going to be tidally locked to it, so that one side remains in perpetual light, the other in darkness. Assuming an atmosphere, the hottest spot on the planet would be at the center of the day side; the coldest would be directly opposite, on the night side. Temperatures would drop steadily as one moved toward the dividing line between light and darkness. Ken Croswell paints an interesting scenario involving such a planet in the article “Red, Willing and Able,” which originally ran in New Scientist (January 27, 2001) and is now available online. The recent National Geographic TV program Extraterrestrial also showed an intriguing red dwarf planet with bizarre though perfectly feasible life-forms.
Scientists at NASA’s Ames Research Center studied a hypothetical planet around a red dwarf in the 1990s and concluded that even a sparse atmosphere might be enough to circulate heat to the dark side of the planet, keeping its atmospheric gases from freezing out. If water existed on such a world, the possibility of oceans underneath crustal ice, heated by geothermal activity from below, could mean life might be found even on the dark side.
On the day side, of course, most light would be in the infrared, and the sun would be a stationary red disk — no axial tilt, no seasons. And here’s the kicker when it comes to possible life around red dwarf stars: these are exceedingly long-lived objects. If our Sun has another eight to ten billion years left, a red dwarf might live 10 times as long, 100 billion years in which to let evolution work out the quirks of existing in such an unusual ecosphere. Gliese 876 doesn’t seem like a candidate for biology, but we’re getting to where we’ll be able to detect smaller rocky worlds around similar stars, and that makes the planet hunt get more and more interesting.
NOTE: This post originally mis-stated the expected life span of a red dwarf as 100 times as long as the Sun. The correct figure is ten times as long, or 100 billion years, as now corrected above.
At Princeton two weeks ago, I asked the university’s Jeremy Kasdin, who had just delivered a fine presentation on imaging extrasolar planets from space (with obvious reference to the Terrestrial Planet Finder mission), whether TPF would examine red dwarfs among its list of Sun-like stars in nearby space. And Kasdin said yes, though they wouldn’t be primary targets — “…they’re simply too cool,” he added. I can see his point, since resources will be limited, and it makes sense to concentrate on stars more or less like our own. But wouldn’t it be fascinating if we find that unusual forms of terrestrial biospheres do develop around some M-class stars, and that they may be places where intelligences far older than our own have found a way to survive?
The Ames work is described in Manoj Joshi, Robert Haberle, and R. Reynolds, “Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability,” Icarus 129 (1997), pages 450-65. See also Martin J. Heath et al., “Habitability of Planets Around Red Dwarf Stars,” Origins of Life and Evolution of the Biosphere 29 (1999), pages 405-24.
by Paul Gilster | Jun 13, 2005 | Exoplanetary Science
Finding a planet that resembles the Earth is the ultimate goal of our exoplanetary explorations. It implies the possibility of life on a world not so different from our own, and encourages the speculation that Earth-like worlds are out there in huge numbers.
We certainly haven’t found such a place yet, but we’re getting closer. Which is why Gliese 876, a red dwarf some 15 light years from our Solar System, made news today at a National Science Foundation briefing. No planet yet found — and that includes roughly 155 extrasolar planets to date — is as similar to Earth as the one found here. Gliese 876 is in the direction of the constellation Aquarius, and it is known to possess two larger gas-giant worlds as well as this much smaller neighbor.
Not that conditions on the newfound world would be exactly habitable by our standards. The planet, some seven and a half times the mass of Earth, orbits its host star once every 1.94 days, and it’s only two million miles from it (about one-tenth the size of Mercury’s orbit in our own Solar System). Its radius is about twice that of Earth.
Scientists working on the find estimate the planet’s surface is somewhere between 200 to 400 degrees Celsius (400 to 700 degrees Fahrenheit). Even so, that’s closer to an Earth-like world than what we’re used to finding; it’s certainly not a ‘hot Jupiter,’ and the fact that it is detectable at all tells us that we’ll soon be able to find even smaller rocky planets in the actual habitable zone of nearby stars.
“This is the smallest extrasolar planet yet detected and the first of a new class of rocky terrestrial planets,” said team member Paul Butler of the Carnegie Institution of Washington. “It’s like Earth’s bigger cousin.”
Image: Centered in this unremarkable, 1/4 degree wide patch of sky in the constellation Aquarius is the star Gliese 876. Gliese 876 is smaller than the Sun, only about 1/3 as massive, and too faint to be seen without a telescope. But it is known to be one of the nearest stars, only 15 light-years distant. Today’s announcement brings the number of known planets around this star to three. Credit: NASA/SkyView Digitized Sky Survey.
The first planet found around Gliese 876 was detected in 1998 by Butler and Geoff Marcy (University of California at Berkeley), who determined that it was a gas giant about twice the mass of Jupiter. Another planet was found in 2001, this one about half the mass of Jupiter. These two planets have been the subject of considerable scrutiny because they’re locked into resonant orbits, the outer planet orbiting the star in 60 days, which is twice the period of the inner planet. It was the study of this orbital resonance, much of it performed by Jack Lissauer (NASA Ames) and Eugenio Rivera (Lick Observatory) that led eventually to the discovery of the world announced today.
“We had a model for the two planets interacting with one another, but when we looked at the difference between the two-planet model and the actual data, we found a signature that could be interpreted as a third planet,” said Lissauer.
And here’s Gregory Laughlin of the Lick Observatory at the University of California, Santa Cruz:
“The planet’s mass could easily hold onto an atmosphere,” noted Laughlin, an assistant professor of astronomy at UC Santa Cruz. “It would still be considered a rocky planet, probably with an iron core and a silicon mantle. It could even have a dense steamy water layer. I think what we are seeing here is something that’s intermediate between a true terrestrial planet like the Earth and a hot version of the ice giants Uranus and Neptune.”
A paper detailing the results has been submitted to The Astrophysical Journal.
by Paul Gilster | Jun 12, 2005 | Exoplanetary Science
We should have some interesting news about exoplanets tomorrow afternoon. That’s when a media briefing will be given to reporters at the National Science Foundation in Arlington VA. The briefing is titled “Scientists Make New Discovery About Planets Outside Our Solar System,” and although I have a hunch what this one is about, I’m not confident enough to run with it here. But it’s intriguing that Jack Lissauer (NASA Ames), who is participating in the briefing, has done groundbreaking work on planetary formation in binary systems, as discussed in these pages back in December.
Other participants in the briefing include exoplanetary pioneers Geoff Marcy (University of California, Berkeley) and Paul Butler (Carnegie Institution), as well as Eugenio Rivera from Lick Observatory. Rivera has previously worked with Lissauer on the ‘resonant’ orbits of two planets around the red dwarf Gliese 876, some 15 light years from Earth. Michael Turner, who heads NSF’s Directorate of Mathematical and Physical Sciences, will evidently introduce the panel and present initial findings.
An accompanying Webcast will be available at this address. More on the story as it develops.
by Paul Gilster | Jun 11, 2005 | Breakthrough Propulsion, Missions
“Fusion reactors powered by deuterium/helium-3 are a good candidate for a very advanced spacecraft propulsion. The fuel has the highest energy-to-mass ratio of any substance found in nature, and, further, in space the vacuum the reaction needs to run can be had for free in any size desired. A rocket engine based upon controlled fusion could work simply by allowing the plasma to leak out of one end of the magnetic trap, adding ordinary hydrogen to the leaked plasma, and then directing the exhaust mixture away from the ship with a magnetic nozzle. The more hydrogen added, the higher the thrust (since you’re adding mass to the flow), but the lower the exhaust velocity (because the added hydrogen tends to cool the flow a bit). For travel to the outer solar system, the exhaust would be over 95 percent ordinary hydrogen, and the exhaust velocity would be over 250 km/s (a specific impulse of 25,000 s, which compares quite well with the specific impulses of chemical or nuclear thermal rockets of 450 s or 900 s, respectively). Large nuclear electric propulsion (NEP) systems using fission reactors and ion engines, a more near-term possibility than fusion, could also achieve 25,000 s specific impulse. However, because of the complex electric conversion systems such NEP engines require, the engines would probably weigh about eight times as much as fusion systems and, as a result, the trips would take twice as long. If no hydrogen is added, a fusion configuration could theoretically yield exhaust velocities as high as 15,000 km/s, or 5 percent the speed of light! Although the thrust level of such a pure D-He3 rocket would be too low for in-system travel, the terrific exhaust velocity would make possible voyages to nearby stars with trip times of less than a century.”
Robert Zubrin, Entering Space: Creating a Spacefaring Civilization (New York: Jeremy P. Tarcher/Putnam, 1999), p. 161.
Centauri Dreams note: As far as I know, the first suggestion that helium-3 be used in an interstellar mission came from the Project Daedalus team in the 1970’s. Deuterium/tritium is thought to be an easier reaction to initiate, but working with helium-3 offered a significant benefit: the reaction yields protons and alpha particles that can be shaped by a magnetic nozzle, rather than neutrons, which a magnetic field cannot influence. The mission would be composed of two stages, the first burning for two years, the second for almost as long, before the long coast to destination.
Image: The Project Daedalus design. Credit: NASA/Glenn Research Center.
Today, Daedalus stands as a reminder of the era of mammoth starship thinking. The probe’s earliest designs called for a 450 ton probe with a reaction chamber some 330 feet in diameter; even with scaling to a multi-stage design, the entire mission demanded fifty billion fuel pellets, calling for 30,000 tons of helium-3 and 20,000 tons of deuterium. As for helium-3 in such quantities, the designers recommended mining the atmosphere of Jupiter.
Zubrin discusses how such mining might take place in Entering Space. The enabling technology would be a winged craft that Zubrin calls a Nuclear Indigenous Fueled Transatmospheric (NIFT) vehicle. A NIFT would either separate out the helium-3 as it moved through the atmosphere of a gas giant, or else rendezvous in the atmosphere with an aerostat station. It would have the capability of delivering its helium-3 cargo to an orbiting fusion-powered tanker that would, in turn, take it to the inner Solar System.
