by Paul Gilster | Feb 6, 2020 | Outer Solar System |
The dataflow from New Horizons has been abundant enough that we are now drilling down to atmospheric models that may explain the dwarf planet’s topography. Mention topography on Pluto and the first thing that leaps to mind is Tombaugh Regio, and a new paper in the Journal of Geophysical Research Planets actually takes us into its role in the formation of regional weather patterns on the icy orb. For at the heart of Pluto’s weather appears to be the terrain often called Pluto’s ‘heart,’ from the distinctive shape it imposes upon the landscape.
Pluto’s atmosphere, 100,000 times thinner than ours, is primarily nitrogen, with but small amounts of carbon monoxide and methane. Tombaugh Regio is covered by nitrogen ice, which warms during the day, turning to vapor that condenses in Pluto’s night to once again form ice. The researchers, led by Tanguy Bertrand, an astrophysicist and planetary scientist at NASA’s Ames Research Center in California and the study’s lead author, liken the process to a heartbeat that drives nitrogen winds around Pluto. And here we run into a distinct oddity.
For the paper suggests that this cycle actually produces retro-rotation in Pluto’s atmosphere, so that it circulates in a direction opposite to the dwarf planet’s spin. Air moving past the surface shifts not just heat but grains of ice and haze particles, resulting in the dark wind streaks and plains found across the north and northwestern regions of Tombaugh Regio. Here’s Bertrand on the matter:
“This highlights the fact that Pluto’s atmosphere and winds – even if the density of the atmosphere is very low – can impact the surface. Before New Horizons, everyone thought Pluto was going to be a netball – completely flat, almost no diversity. But it’s completely different. It has a lot of different landscapes and we are trying to understand what’s going on there.”
Image: Four images from NASA’s New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this global view of Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
We learn that most of the nitrogen ice on Pluto is found in Tombaugh Regio, where the 3 kilometer deep basin called Sputnik Planitia (the left lobe of the ‘heart’) contains a 1,000 kilometer ice sheet. Next to this, in the right lobe of the ‘heart,’ we find nitrogen-rich glaciers and highlands. The range of landscapes was one of the most intriguing things found when the first close-up images from New Horizons began to come in. Few had expected to find Pluto an active, geologically interesting world.
The team’s work proceeded through the use of simulations of the nitrogen cycle with a weather forecast model that could be analyzed to study the movement of wind across the surface. The paper discusses the methods at work here:
We used the GCM [Global Climate Model] to investigate different surface-atmosphere interactions involving the near-surface winds, such as the effect of the conductive heat flux from the atmosphere, the erosion of the ice, and the transport of ice grains and dark materials. We find that the cumulative effect of these mechanisms could induce significant contrasts in ice sublimation rate and color, and could explain the formation of the bright and dark plains in Sputnik Planitia.
Winds above 4 kilometers in altitude are found to blow to the west, opposite to the planet’s eastern spin, for most of the dwarf planet’s year. Vaporizing nitrogen in the north of Tombaugh Regio turns to ice in the south, triggering these winds. Along the western boundary of Sputnik Planitia, a current of fast-moving air pushes near the surface, the result of the atmospheric nitrogen condensing back into ice, and the trapping of cold air inside the basin, where its circulation allows it to strengthen as it moves through high cliffs in the area. The western boundary current is strong and is clearly related to the specific terrain it flows across.
As to retro-rotation, it’s unusual, although perhaps not unique. From the paper:
This retro-rotation of Pluto’s atmosphere is a unique circulation regime in the Solar system, except maybe on Triton, where pole-to-pole transport of N2 could also lead to a similar regime. We find that the retro-rotation is maintained during most of Pluto’s year. It could be responsible for many longitudinal asymmetries and geological features observed on Pluto’s surface, such as the depletion of Bladed Terrains at eastern longitudes and the formation of bright pits in eastern Tombaugh regio, although this remains to be explored. Our work confirms that despite a frozen surface and a tenuous atmosphere, Pluto’s climate is remarkably active.
We can only imagine how different Pluto’s surface would look with different wind patterns as we consider Sputnik Planitia and contrast the dark plains and wind streaks to its west. What a curious place Pluto turns out to be, and what a role Sputnik Planitia plays. Says Bertrand:
“Sputnik Planitia may be as important for Pluto’s climate as the ocean is for Earth’s climate. If you remove Sputnik Planitia – if you remove the heart of Pluto – you won’t have the same circulation.”
The New Horizons dataset will be producing papers for many years to come. Oh for a similar mine of information about Triton!
The paper is Bertrand et al., “Pluto’s beating heart regulates the atmospheric circulation: results from high resolution and multi?year numerical climate simulations,” Journal of Geophysical Research Planets 04 February 2020 (abstract).
by Paul Gilster | Feb 4, 2020 | Sail Concepts |
When we consider pushing a sail to 20 percent of lightspeed, which is the target velocity for Breakthrough Starshot, it’s interesting to think about how laser propulsion differs from sunlight. After all, while constructing a huge laser array presents numerous challenges on Earth, we already have a star to work with, and sail technology that is beginning to be tested in space. Consider, too, that we have operational spacecraft like the Parker Solar Probe that are exploring regions close to the Sun, helping us learn more about heat shields, even as we plan missions like Solar Cruiser, whose sail would enable interesting non-Keplerian orbits near the Sun.
Wouldn’t it be easier to find ways to use our Sun’s own energies to drive our starship by getting a boost from gravitational effects? A closer look reveals the power of solar sails in nearby space (i.e., within the system), while illuminating the problems at interstellar distances.
For getting a solar sail up to the highest possible speeds, we will probably need an Oberth maneuver, which means falling deep into a gravitational well and applying propulsion at the time when velocity in that well peaks, which squeezes maximum effect out of our boost. With a sail, that would mean a so-called ‘Sundiver’ mission, bringing a furled sail shielded by an occulter (perhaps a small asteroid) breathtakingly close to the Sun and then opening the sail at perihelion for the best possible kick.
Depending on shielding and thus how close we can get to our star, we do get a substantial boost over the velocities of our two interstellar Voyagers, one of which moves at just over 17 kilometers per second. We might squeeze 100 kilometers per second out of the Sundiver maneuver, and perhaps a bit more, with future craft using sails made out of new metamaterials achieving even better. But we’re nowhere near 20 percent of the speed of light.
Solar sails certainly have broad applications as we build a Solar System infrastructure, and we can use them with Sundiver maneuvers to get into nearby interstellar space. But if we find long mission times unacceptable, we need to be thinking of alternatives when we mount a true star mission.
Other civilizations won’t necessarily have to work with the same constraints, depending on the kind of star they orbit. I’m always interested in how intelligence might exploit natural objects to achieve interstellar goals, so I took note when I saw Avi Loeb’s latest piece in Scientific American. Loeb (Harvard University), who chairs Breakthrough Starshot, is not one to avoid speculation, without a healthy dose of which we can hardly work with concepts involving SETI and highly evolved civilizations.
So let’s leave the realm of what humans can do with our small G-class star far behind as we consider what sufficiently advanced technologies might attempt. If a culture were to be a billion or more years old, how would we know where to look for it? One way into the problem is to consider the need for energy useful to Kardashev Type II and III civilizations, energy which is available in abundance around certain natural phenomena.
And given the recent attention to Betelguese and the question of when it might become a supernova, Loeb has been moved to consider the power such an event would unleash. The results make working with the flux from a G-class star seem trivial indeed. Even the best Sundiver maneuver at Sol yields velocities that would take hundreds of years to reach the nearest stars. A similar maneuver around the most luminous stars we know might reach 10 percent of lightspeed (not too shabby!). But for maximum kick, a lightsail riding the shockwave of a supernova from a star like Betelguese or Eta Carinae could be pushed to a high percentage of c.
Image: Hubble Space Telescope-Image of Supernova 1994D (SN1994D) in galaxy NGC 4526 (SN 1994D is the bright spot on the lower left). Credit: NASA/ESA.
Could a civilization time a supernova explosion accurately enough to ride the shockwave? That’s a question we can’t answer, nor can we say what kind of timeframes such a culture might operate within. Loeb imagines numerous lightsails parked around a star nearing the end of its life, perhaps placed by a civilization nearby, and perhaps left there indefinitely. If this civilization wanted to use the supernova for propulsion, it would still face numerous problems:
First, as in Starshot, the sails must be highly reflective so as not to absorb too much heat and burn up. Second,once the sails are placed in orbit around the massive star, they will be pushed away by the bright starlight or mass loss prior to the explosion. To avoid this danger, one could deploy the sails in a folded configuration and equip them with a switch that would open them up like umbrellas as soon as the explosion flash begins to rise. Third, even though the launch can start from a distance that is a hundred times larger than the size of the exploding star, care must be taken in selecting particularly empty acceleration paths – clear of any stellar debris.
Indeed, given dust particles with a relative speed close to the speed of light, such a sail would have to be folded to reduce the area it presents in the direction of flight as soon as it reached peak velocity. An even wilder prospect: A massive enough star (Loeb mentions Eta Carinae) collapsing into a black hole could produce gamma-ray bursts (GRBs) that would drive the Lorentz factor to extreme levels. Now we’re in range of Poul Anderson’s ‘Leonora Christine,’ the runaway starship in the novel Tau Zero that crosses galaxies in far less than a human lifetime as measured by the crew, while aeons pass outside their rest frame.
We’re in Dyson sphere country here, a reference Loeb himself makes, looking into ways advanced civilizations could harvest high energy sources around them. A Dyson sphere or array, gathering the maximum amount of stellar light, might be found by its infrared signature, so Dysonian SETI, which looks for evidence of ETI in our astronomical records, has a target.
Clément Vidal has also worked this notion in his book The Beginning and the End (Springer, 2014), looking into questions like extracting energy from the accretion disk around a rotating black hole or tapping the power of X-ray binaries. We are also in Olaf Stapledon territory, asking about extraterrestrial engineering that is inconceivable to ourselves, but possibly visible as a SETI signature in the ‘watering holes’ for energy the universe makes available.
It’s hard for me to imagine a civilization with the patience to wait out a supernova explosion to drive a lightsail, but when we’re dealing with technologies that may be several billion years beyond our own, we have no business imposing our own limitations on the cosmos. Anomalies in our observations of supernova remnants would be worth investigating, although Loeb admits to the difficulty of isolating artificial components within them. Even so, dying stars conceivably have reason to be pondered by civilizations advanced enough to make use of their energy.
by Paul Gilster | Feb 3, 2020 | Deep Sky Astronomy & Telescopes |
In a few short months, New Horizons will be almost 8 billion kilometers out, a distance that still boggles the mind until we remember that Voyager 1 has reached 22.2 billion kilometers (over 148 AU). Then, of course, we’re humbled again with the thought that the inner Oort Cloud is thought to be between 2,000 and 5,000 AU from the Sun, with an outer edge that could extend as far as halfway to the nearest star. That star, Proxima Centauri, is 268,770 AU from us.
As New Horizons hunts Kuiper Belt objects for the next flyby, the spacecraft is now being used to perform parallax studies to detect the apparent ‘shift’ in the relative position of nearby stars as compared with what we see on Earth. Earth’s orbit is about 300 million kilometers in diameter, so we see that apparent shift by comparing observations taken half a year apart. That’s a pretty decent baseline, but if we extend the baseline, as now with New Horizons, we can see better parallax effects, and thus tighten the distance measurements we make from them.
The most celebrated name in parallax studies is German astronomer and mathematician Friedrich Bessel, who in 1838 obtained the first stellar parallax measurements to determine the distance of the star 61 Cygni. Here again it’s humbling to reflect that the distances to the nearest stars are almost half a million times greater than the baseline offered by Earth’s orbit, which makes Bessel’s feat all the more impressive. Bessell worked out a distance of about 10 light years, not all that far off the 11.4 light year distance modern astronomers have estimated for 61 Cygni.
I might mention that Bessell (1784-1846) was also the first astronomer to predict the existence of an unseen companion around another star, announcing from his study of Sirius that deviations in its motion indicated a companion that we now know as the white dwarf Sirius B.
But back to New Horizons. On April 22 and 23, the spacecraft will take images of Proxima Centauri and Wolf 359. These can be used in combination with Earth-based images taken on the same dates to yield what the New Horizons team is calling ‘a record-setting parallax measurement,’ one that will be made in coordination with Earth-based observatories and a public observing campaign. Amateur astronomers with a camera-equipped, 6-inch or larger telescope are invited to participate. Says New Horizons principal investigator Alan Stern:
“These exciting 3D images, which we’ll release in May, will be as if you had eyes as wide as the solar system and could detect the distance of these stars yourself. It’ll be a truly vivid demonstration of the immense distance New Horizons has traveled, and a cool way to take advantage of the spacecraft’s unique vantage point out on the very frontier of our solar system!”
Image: Color images of the Wolf 359 (top) and Proxima Centauri star fields, obtained in late 2019. The large proper motions of both stars (at the center of each image) will cause them to shift by over an arcsecond by April 2020, when NASA’s New Horizons spacecraft, nearly five billion miles (8 billion kilometers) from Earth, will image them. A green circle provides a rough estimate of where both stars will appear in the New Horizons images. Credit: William Keel/University of Alabama/SARA Observatory.
A new way to find our way around nearby interstellar space? New Horizons science team member Tod Lauer (National Science Foundation Optical-Infrared Astronomy Research Laboratory) points out the method’s history and its future promise:
“For all of history, the fixed stars in the night sky have served as navigation markers. As we voyage out of the solar system and into interstellar space, how the nearer stars shift can serve as a new way to navigate. We will see this for the first time with New Horizons.”
More details are available here for those interested in participating in the New Horizons Parallax program. I’m anxious to see the results, and also reminded how limited parallax measurements have historically been when confined to Earth’s orbit around the Sun. New Horizons points the way to a future in which we extend the baseline even further. Imagine a mission like Claudio Maccone’s FOCAL, an observing instrument collecting light at distances greater than 550 AU, where the gravitational lensing effects of the Sun become observable. And who knows, perhaps one day we’ll have a baseline as far as the Alpha Centauri stars to help map the Orion Arm.
