An abrupt change: I’m holding today’s post (about halfway done, on a stellar flyby that may have produced Sedna and other such objects long in our system’s past) to turn to New Horizons’ latest imagery, which is provocative indeed. We’ll cover the Sedna story tomorrow.
What we have from New Horizons is the work of the spacecraft’s Long Range Reconnaissance Imager (LORRI) in a series of images that show Pluto and its largest moon Charon as they more than double in size between May 29 and June 19. There’s plenty here to marvel at, but what stands out for me is the mysterious dark region that NASA’s latest release refers to as ‘a kind of anti-polar cap’ on Charon. Have a look:
Image: These recent images show the discovery of significant surface details on Pluto’s largest moon, Charon. They were taken by the New Horizons Long Range Reconnaissance Imager (LORRI) on June 18, 2015. The image on the left is the original image, displayed at four times the native LORRI image size. After applying a technique that sharpens an image called deconvolution, details become visible on Charon, including a distinct dark pole. Deconvolution can occasionally introduce “false” details, so the finest details in these pictures will need to be confirmed by images taken from closer range in the next few weeks. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
No wonder principal investigator Alan Stern is beside himself:
“This system is just amazing. The science team is just ecstatic with what we see on Pluto’s close approach hemisphere: Every terrain type we see on the planet—including both the brightest and darkest surface areas — is represented there, it’s a wonderland! And about Charon—wow—I don’t think anyone expected Charon to reveal a mystery like dark terrains at its pole. Who ordered that?”
Moreover, we have these images of Pluto itself, with the encouraging news that the hemisphere over which New Horizons will make its closest approach is also the one with the greatest variety of terrain types we’ve yet seen on the dwarf planet. Here’s the Pluto imagery:
Image: These images, taken by New Horizons’ Long Range Reconnaissance Imager (LORRI), show numerous large-scale features on Pluto’s surface. When various large, dark and bright regions appear near limbs, they give Pluto a distinct, but false, non-spherical appearance. Pluto is known to be almost perfectly spherical from previous data. These images are displayed at four times the native LORRI image size, and have been processed using a method called deconvolution, which sharpens the original images to enhance features on Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
At left is Pluto/Charon as viewed in 1978 when USNO astronomer James Christy noticed the ‘bump’ that seemed to emerge from Pluto. This was how Charon was discovered, through imagery that, like Pluto itself in 1930, was studied on photographic plates taken in Flagstaff, AZ. Weighing only twelve percent as much as Pluto, Charon may be as much as half ice, while Pluto seems to be about 70 percent rock by mass. As this JHU//APL news release notes, an astronaut standing on Pluto’s surface would see Charon always in the same part of the sky, but it would appear seven times larger than the Earth’s moon, spanning 3.5 degrees. Now we have imagery like that above to bring both Pluto and Charon into ever tighter focus. Will we find ice volcanoes on Charon, and perhaps a trace of atmosphere?
Centauri Dreams regular James Jason Wentworth wrote recently with some musings about Bracewell probes, proposed by Ronald Bracewell in a 1960 paper. Bracewell conceived the idea of autonomous craft that could monitor developments in a distant solar system, perhaps communicating with any local species that developed technology. Pondering how such a craft might manage station-keeping over the aeons, Jason hit on the idea of using a natural effect that would draw little attention to itself, one he explains below. An amateur astronomer and interstellar travel enthusiast who worked at the Miami Space Transit Planetarium and volunteered at the Weintraub Observatory atop the adjacent Miami Museum of Science, Jason now makes his home in Fairbanks (AK). He was the historian for the Poker Flat Research Range sounding rocket launch facility near Fairbanks. His space history and advocacy articles have appeared in Quest: The History of Spaceflight magazine and Space News.
by James Jason Wentworth
Dreams, daydreams, and flights of fancy have far greater value than most modern people realize. (Centauri Dreams are *not* just two nice-sounding words, but instead constitute a vital and necessary prelude to, and continuing inspiration for, interstellar space flight. Without Centauri dreams, there will be no “Centauri do’s,” as in visiting that stellar system via robotic probes or crewed starships!) Besides being pleasant forms of mental play, dreams can also bring insights that are of great practical importance. Such activities are usually considered the province of poets, storytellers, and songwriters, but scientists have also been helped by them. The most well-known example of this involved Friedrich August Kekulé, a 19th century German chemist, who gained answers he was seeking about molecular configurations from two dreams that he had [1]. The more famous of these two dreams–which involved a snake-like string of atoms that formed a circle, which then transformed into a snake eating its tail–led him to the ring structure of the benzene molecule.
In January of last year, such an insight came to me regarding a new form of fuel-less spacecraft propulsion and attitude control – one that, to my knowledge, no one has suggested before. It would be something that used the forces of nature, and it would also be something subtle and non-polluting. A light sail would fit these preferences well, but it occurred to me that there was another alternative (particularly for use within star systems) which would also employ starlight but would be more subtle than a sail, not blazing forth in the skies of nearby planets. Moreover, it would have other advantages, which would be useful to long-life spacecraft of all kinds, from unmanned Earth satellites to mobile space colonies to Bracewell interstellar messenger probes. I shall explore these advantages below.
The Power of Emitted Photons
The Yarkovsky effect [2] was discovered by Ivan Yarkovsky (1844-1902), a Russian civil engineer who worked on scientific problems in his spare time. The effect imparts a very small but constant thrust to small, rotating bodies in orbit around the Sun, via the heating of the bodies’ surfaces by sunlight. As such an object rotates, its “afternoon” quadrant emits infrared photons as it cools, and this photon emission imparts an asymmetrical thrust force to the object. The Yarkovsky effect affects the orbits of meteoroids and asteroids between about 10 cm and 10 km across. (Smaller objects are heated more uniformly via internal heat transfer, which precludes the asymmetrical infrared photon emission, and larger asteroids are too massive to be affected appreciably by the infrared photon thrust.) A prograde-rotating meteoroid or asteroid (one that is rotating in the same direction that it is orbiting the Sun, counter-clockwise in the case of our solar system) gradually spirals outward away from the Sun due to the Yarkovsky effect, while a retrograde-rotating body spirals inward toward the Sun.
A related phenomenon, the Yarkovsky-O’Keefe-Radzievskii-Paddack effect (YORP effect) [3], affects the rotation rate, the rotational axis tilt, and the rotational axis precession rate in small asymmetric meteoroids and asteroids. These two effects could also be utilized by spacecraft, for fuel-less propulsion as well as attitude control.
Image: The Yarkovsky Effect: An asteroid is warmed by sunlight, its afternoon side becoming hottest. As a result, that face of the asteroid re-radiates most thermal radiation, creating a recoil force on the asteroid and causing it to drift a little. The direction of the radiation depends on whether the asteroid is rotating in a prograde (anticlockwise) manner (a) or in a retrograde (clockwise) manner (b). Credit: “Planetary science: Spin control for asteroids,” by Richard Binzel in Nature 425 (11 September 2003), 131-132.
Putting Yarkovsky to Work
The now-solved Pioneer anomaly was an unintentional demonstration of the Yarkovsky Effect’s ability to impart measurable thrust to a spacecraft. The Pioneer 10 and 11 spacecrafts’ Radioisotope Thermoelectric Generators (RTGs), rather than the Sun, supplied the infrared photons, which produced a tiny thrust toward the Sun by bouncing off the back of the probes’ dish antennas. A spacecraft that was purposely designed to utilize the Yarkovsky effect (and also the YORP effect, if desired) could move (and maneuver) much more quickly than either massive, rock/metal asteroids or the “accidentally-propelled” Pioneer spacecraft. The rate of acceleration of such a spacecraft would likely be comparable to that of a solar sail, although a higher thrust/mass ratio would increase its possible acceleration rate. A spacecraft of this type might be designed as follows:
Picture a black, rotating, drum-shaped vehicle, whose spin axis is perpendicular to the plane of its orbit around the Sun. (The drum could be a “stand-off” cylinder, like Skylab’s lost meteoroid shield, which could be deployed from a central spacecraft via centrifugal force.) The vehicle would spiral away from the Sun if it rotated in a prograde direction, and it would spiral inward toward the Sun if it rotated in a retrograde direction, just as asteroids (those which are small enough to be affected by the Yarkovsky effect) do. It could also change the plane of its orbit, by tilting its spin axis to inclinations other than perpendicular to its orbit plane. Changing its spin rate and spin direction would alter the magnitude and direction of its infrared photon thrust. Reversing the vehicle’s spin direction could be accomplished either by stopping the spin and re-starting it in the opposite direction or by precessing the spin axis 180 degrees around (the latter method would be preferable for large spacecraft). Like a solar sail, a Yarkovsky/YORP effect propelled spacecraft would have a low rate of acceleration, but it could achieve very high velocities over time.
The YORP effect could be utilized, if desired, to control the spacecraft’s spin rate, spin axis tilt, and spin axis precession rate (using no moving parts) by equipping the drum-shaped vehicle with short, wedge-shaped “blades” (which could, optionally, be made retractable) that would protrude from its sides. The blades could also have electronically-variable light reflectivity and absorption, like the variable-reflectivity liquid crystal steering panels on JAXA’s IKAROS solar sail. These blades would create an asymmetrical total vehicle solar illumination, which is the cause of the multiple YORP effects. As an alternative, the spacecraft’s spin rate control and spin axis pointing might be handled – again without any moving parts – by using selectively-charged wires (or other vehicle parts) to interact with the local planetary or solar magnetic fields. Or the vehicle might use magnetically-levitated, internal torque flywheels to control its spin rate and direction.
The black drum could be a soft (“quilted” quartz cloth, optionally rigidized by a vacuum-hardening pre-impregnated resin) or rigid (a folding metal or composite) outer cylinder standing off from the surface of the spacecraft, held there either by rigid struts or by tensioned cables or cords, in concert with centrifugal force. Either type could contain thermovoltaic cells to generate electricity for the spacecraft’s systems.
Photovoltaic solar cells could, however, be utilized by such a vehicle if desired. Its instruments, imaging system (if any – perhaps a spin-scan camera), and solar cells could be mounted on parts of the spacecraft “bus” that protrude above and below the ends of the black drum. Or, by using angled circumferential mirrors on the exposed ends of the bus (and metallized Kapton or other such material on the inside of the black drum), solar cells on the drum-obscured parts of the bus could be illuminated by sunlight. If a soft fabric drum were used, it would absorb some of the solar and cosmic radiation that degrades solar cells, and so would enable them to last longer.
Such a spacecraft could even use thermocouples in order to utilize the solar heat on the black drum (and the cold in the shadowed areas at its ends, by placing circumferential “heat shades” between the inside wall of the black cylinder and the cold sides of the thermocouples) to generate electricity for its onboard systems. Thermocouples made of dissimilar refractory metals might be very long-lived electricity generating devices for spacecraft of this type.
Station-Keeping for the Long Haul
Such a capability, combined with the ability to change orbits, maintain orbits, and perform Lagrangian point station-keeping without using any propellant (and with no moving parts), would enable Yarkovsky/YORP effect-utilizing spacecraft to operate for very long periods, whether in orbit around the Earth, other planets, the Sun, or other stars. The black drums used by these spacecraft would likely also have at least three advantages over solar sails. Over long periods of time, it is more difficult for a reflective object to remain reflective than for a black object to remain black. Unlike a sail, the drum could be more compact as well as have greater thickness and strength, and its rotation would increase its stiffness. Spacecraft using this method of propulsion should also be able to maneuver more effectively closer to a planet (especially one possessing an atmosphere) than a sail could.
A further possible advantage – for human-dispatched Bracewell probes sent to “loiter” in their target star systems for decades, centuries, or even millennia – would be that such black spacecraft wouldn’t attract visual attention as a sail-equipped probe would. An infrared search could find such a probe, but with dark asteroids and black, extinct comet nuclei likely being as common in other stellar systems as in our own, it might escape positive identification as an alien visitor, at least for some time.
Image: One of many science fiction treatments of Bracewell probes occurs in Michael McCollum’s Life Probe (Del Rey, 1983).
As Robert Freitas [4] has written, any civilization – perhaps even our own – might consider an alien Bracewell probe in its star system to be a threat, at least initially. Providing such probes with a measure of protection would “buy them time to explain themselves” by making them less-than-easy to find. This, and their ability to move between broadcasts, would better enable them to establish contact and demonstrate their peaceful purposes before they might otherwise be attacked by a wary race.
While brute-force methods got humanity into space, it is increasingly obvious that for far journeys and long sojourns there, harnessing the subtle natural forces that are freely available just above our heads is the only way that humanity can truly thrive and prosper in that realm.
It’s hard to consider a place with surface temperatures of -180°C ‘Earthlike,’ but there are reasons why we see the term so often applied to Titan. The most striking of these is the presence of surface lakes and seas, a phenomenon found nowhere else in the Solar System. The temperatures are cold enough to make the circulating fluid liquid methane and ethane rather than water, but we see things in Cassini imagery that are strikingly familiar, including seas fed by river-like channels and large numbers of shallow lakes that appear in flatter areas.
The European Space Agency’s Thomas Cornet has been leading a team investigating Titan’s surface features in greater detail. In particular, the lakes of Titan do not appear to be fed by rivers, making it likely that they are filled either by rainfall or by liquids welling up from below. Empty depressions can be found where lakes may once have been, and it is believed that some of the lakes dry out during Titan’s thirty-year cycle of seasons. Cornet and team wanted to learn how the depressions in which the lakes are found formed in the first place.
Image: A radar image of Titan’s north polar regions (centre), with close ups of numerous lakes (left) and a large sea (right). The sea, Ligeia Mare, measures roughly 420 x 350 km and is the second largest known body of liquid hydrocarbons on Titan. Its shorelines extend for some 2000 km and many rivers can be seen draining into the sea. By contrast, the numerous lakes are typically less than 100 km across and have more rounded shapes with steep sides. The radar images were created using data collected by the international Cassini spacecraft. Credit: NASA/JPL-Caltech/ASI/USGS; left and right: NASA/ESA. Acknowledgement: T. Cornet, ESA.
Here again we find a terrestrial analog for Titan, the ‘karstic’ landforms seen on Earth when rocks like limestone and gypsum erode because of the effects of rainfall and groundwater. Erosion depends on many factors, temperature, rainfall and chemistry among them, but what develops over time are sinkholes and caves in areas where there is abundant moisture, and salt-pans where conditions are more arid. The ESA team studied how long it would take for areas on Titan to go through similar processes, adjusting for the differences in terrain.
Rather than water, Cornet and colleagues assumed liquid hydrocarbons as the primary dissolving agent, while modeling a surface replete with solid organic material. Given what we know about Titan’s climate, the result was a period of 50 million years to create a 100-meter deep depression at Titan’s rainy high polar latitudes, where we see so many lakes. The team believes this time scale is compatible with what appears to be a relatively youthful surface.
Cornet describes the work and its results:
“We compared the erosion rates of organics in liquid hydrocarbons on Titan with those of carbonate and evaporite minerals in liquid water on Earth. We found that the dissolution process occurs on Titan some 30 times slower than on Earth due to the longer length of Titan’s year and the fact it only rains during Titan summer. Nevertheless, we believe that dissolution is a major cause of landscape evolution on Titan, and could be the origin of its lakes.”
Image: Close-up radar image showing both empty and liquid-filled depressions (coloured blue) on Saturn’s largest moon, Titan. Credit: NASA/ESA. Acknowledgement: T. Cornet, ESA.
Those arid regions at lower latitudes, as you might expect, take much longer to develop the same kind of depressions, up to 375 million years according to the team’s modeling. Here again the time scale is consistent with the observed scarcity of depressions in these regions of the moon. Thus the comparison with Earth remains surprisingly useful, although the processes at work continue under entirely different climate and chemical conditions.
A Polar Wind Like Earth’s
An atmospheric phenomenon on Titan is likewise replicated in its own way on our own world. Seven years of data from the Cassini probe have helped a team at University College London reach conclusions about the so-called ‘polar wind’ on Titan that is driving gas from its atmosphere. Data from the Cassini Plasma Spectrometer (CAPS) show that about seven tons of hydrocarbons and nitriles are being lost into space every day.
The question the study answers is why this is happening. Titan’s atmosphere is primarily nitrogen and methane. Nitriles are molecules with tight nitrogen/carbon bonds. Andrew Coats (UCL), who led the investigation of the polar wind, explains their interactions:
“Although Titan is ten times further from the Sun than Earth is, its upper atmosphere is still bathed in light. When the light hits molecules in Titan’s ionosphere, it ejects negatively charged electrons out of the hydrocarbon and nitrile molecules, leaving a positively charged particle behind. These electrons, known as photoelectrons, have a very specific energy of 24.1 electron volts, which means they can be traced by the CAPS instrument, and easily distinguished from other electrons, as they propagate through the surrounding magnetic field.”
A major difference between Titan and Earth is that the former has no magnetic field. It is, however, surrounded by Saturn’s rotating magnetic field, which forms a comet-like tail around the moon. The CAPS instrument, in 23 flybys in which Cassini moved through Titan’s ionosphere or its magnetic tail, detected photoelectrons up to 6.8 Titan radii out, showing how efficiently they move along magnetic field lines. What Coates and team have done is to show how the negatively charged photoelectrons create an electrical field that continues to pull hydrocarbon and nitrile particles out of the atmosphere wherever it is lit by the Sun.
Image: Schematic illustration of the magnetic connection between the sunlit ionosphere (left) and Cassini in the tail. Credit: UCL/Andrew Coates.
The same phenomenon occurs on Earth but only in the polar regions, where the magnetic field is open. Titan is not similarly constrained, which is why we see much more widespread effects. As the paper notes: “…we have presented new observations of the photoelectron spectra in Titan’s sunlit ionosphere and in the tail. The results again confirm a magnetic connection between the sunlit ionosphere and tail, along the draped magnetic field lines.”
In both cases, we generate a polar wind, and similar processes may exist on Mars and Venus, making Titan, if not ‘Earthlike,’ a world with intriguing similarities to terrestrial-class planets.
The Coates paper is “A New Upper Limit to the Field-Aligned Potential Near Titan,” published in Geophysical Research Letters 18 June 2015 (abstract). The Cornet paper on Titan’s surface is “Dissolution on Titan and on Earth: Towards the age of Titan’s karstic landscapes,” accepted for publication in Journal of Geophysical Research – Planets.
Astronomers at Penn State, NASA Ames, the University of Chicago and the SETI Institute are publishing news of an exoplanetary first: A planet smaller than Earth whose mass and size have both been measured. Kepler-138b is a Mars-sized world orbiting a red dwarf about 200 light years from Sol in the constellation Lyra. This is transit work, focusing on a system with two other transiting worlds, all three of which are too close to their parent star to make life a likely possibility.
If we look back at how far exoplanet research has come in the last fifteen years, it’s startling to realize that Kepler-138b, with a mass of about 6.7 percent that of the Earth, is 3000 times less massive than the first planet whose density was measured. That’s the word from Eric Ford (Penn State), a co-author on the study, which is being published today in Nature, and I assume he’s talking about HD 209458 b, whose size and density were first measured in 1999.
Previous work on the Kepler-138 system had measured the masses of the two outer planets, but the new work used additional Kepler data that not only allowed measurement of the small inner planet but tightened up mass measurements of the outer planets as well. Daniel Jontof-Hutter, a colleague of Ford’s at Penn State who led the study, comments on the methods used:
“Each time a planet transits the star, it blocks a small fraction of the star’s light, allowing us to measure the size of the planet. We also measured the gravity of all three planets, using data from NASA’s Kepler mission, by precisely observing the times of each transit. Each planet periodically slows down and accelerates ever so slightly from the gravity of its neighboring planets. This slight change in time between transits allowed us to measure the masses of the planets.”
Out of knowledge of both mass and size, exoplanet researchers can calculate the planet’s density. A low density planet is assumed to be primarily made of hydrogen and helium, while planets whose density is intermediate are thought to contain a high proportion of water. High density worlds are likely to have the same rocky composition as the Earth itself.
Image: This plot shows the masses and sizes of the smallest exoplanets for which both quantities have been measured. The solar system planets (shown in red) are for comparison. The three Kepler-138 planets (shown in orange) are among the four smallest exoplanets with both size and mass measurements. Kepler-138b is the first exoplanet smaller than Earth to have both its mass and size measured. This significantly extends the range of planets with measured densities. Credit: NASA Ames/W Stenzel.
Given its mass and size, Kepler-138b is probably a rocky world, but further observations will be needed to confirm this. The two outer planets in this system, Kepler-138c and Kepler-138d, are both approximately the size of the Earth, as this NASA Ames news release notes. The researchers believe Kepler-138c, whose density is similar to Earth, is rocky, while Kepler-138d, although roughly the same size, is less than half as dense, implying the presence of a greater proportion of low-density constituents like water and hydrogen.
“The substantial difference between the densities of the two larger planets tells us that not all planets similar to Earth in size are rocky,” said Jack Lissauer, co-author and planetary scientist at NASA’s Ames Research Center in Moffett Field, Calif. “Further study of small planets will help provide more understanding of the diversity that exists in nature, and will help determine if rocky planets like Earth are common or rare.”
Image: This animation shows the mass-radius diagram based on measurements of 127 exoplanets. The video begins by showing planets with masses similar to Jupiter and slowly zooms towards small masses and radii planets to display a comparison of the physical properties of the Kepler-138 planets relative to Earth, Venus, Mars and Mercury. The planet Kepler-138b is the first exoplanet smaller than the Earth to have both its mass and its size measured, and is one of three planets that orbit the star Kepler-138, and pass in front of it, or transit, every orbit. Each time a planet transits the star, it blocks a small fraction of the star’s light, allowing astronomers to measure the size of the planet. All three planets were identified by NASA’s Kepler mission that has discovered over a thousand planets around other stars. Credits: Jason Rowe, NASA Ames/SETI Institute.
The paper is Jontof-Hutter et al., “The mass of the Mars-sized exoplanet Kepler-138 b from transit timing,” Nature 522 (18 June 2015), 321-323 (abstract).
Given that the Philae lander has just come to life after seven months without communicating, it’s no wonder that the mood among everyone involved with Rosetta’s mission to comet 67P/Churyumov-Gerasimenko is exuberant. On the surface of the comet, conditions have been improving for Philae since March, meaning that with higher temperatures and better illumination, it was hoped that the lander might reactivate. That hope was realized on June 13 when Rosetta picked up 330 data packets from an earlier segment of the lander’s mission.
Stephan Ulamec (DLR), Philae lander project manager, has positive things to say:
“We are still examining the housekeeping information at the Lander Control Centre in the DLR German Aerospace Center’s establishment in Cologne, but we can already tell that all lander subsystems are working nominally, with no apparent degradation after more than half a year hiding out on the comet’s frozen surface.”
Image: Processed NAVCAM image of Comet 67P/C-G taken on 5 June 2015. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0.
ESA also reports that a second burst of lander data was received on June 14. Apparently over 8000 packets of additional status data are available in Philae’s memory, but controllers are not clear as to when the data were recorded. We do know that the internal temperature of the lander has risen to -5ºC, an indication that we’re moving toward enough sunlight to reach the operating temperatures needed for normal operations and electricity generation.
67P/Churyumov-Gerasimenko rotates about every twelve hours. Philae needs at least 19 watts to switch on its transmitter, and ESA’s Rosetta blog reports that power levels are at about the 13 W level at comet sunrise and reach 24 W later in the cometary day. We’re learning that the lander’s solar panels are apparently generating power for over 135 minutes in each period of illumination. The downloaded telemetry evidently extends over a full day-night cycle of the comet, which makes it possible to infer how and when the lander is receiving sunlight.
Remember that communications are being relayed through the Rosetta orbiter, which can contact the lander twice every 24 hours. Because Rosetta is currently in a 200 to 235 kilometer orbit that is not optimized for communications with the lander, a trajectory change will be needed. The new orbit, scheduled to begin at 2325 UTC today, would reduce the orbiter’s distance to about 180 kilometers and should enable better contact with Philae. Establishing reliable contact and optimizing it will allow a new phase of science investigations to begin.
New Horizons: Trajectory Tune-Up
We’re now inside the one month mark before arrival at Pluto/Charon, and the plucky New Horizons couldn’t have been sent on a more interesting trajectory. The plan is to take the spacecraft inside the orbits of all five of Pluto’s moons, an approach known to be feasible following multiple sets of observations dedicated to spotting potential hazards. New Horizons’ Long-Range Reconnaissance Imager (LORRI) camera performed the work, returning the long exposure images needed to search for rings, even smaller moons and dust in the system.
The upshot: No new moons and no rings. This update from the New Horizons team notes that if any rings do exist, they would have to be less than 1600 kilometers wide or reflecting less than one five-millionth of the incoming sunlight. Another hazard search begins June 15 — this is an ongoing process — and we’ll get the update on those results by June 25. The final far encounter phase — Approach Phase 3 — begins in the last week of June and ends a week before the spacecraft’s close approach to Pluto. Additional images for final navigation uses are part of this phase, as are measurements of the solar wind and high-energy particles on the approach.
Image: This “movie,” composed of images taken by New Horizons’ Long Range Reconnaissance Imager (LORRI), shows Pluto as it rotates about its axis. The images were taken May 28-June 3, 2015, from distances ranging from approximately 56 million kilometers to 48.5 million kilometers. Visible are dramatic variations in Pluto’s surface features as it rotates. When a very large, dark region near Pluto’s equator appears near the limb, it gives Pluto a distinctly, but false, non-spherical appearance. Pluto is known to be almost perfectly spherical from previous data. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
New Horizons performed a 45-second thruster burn to optimize its trajectory on June 14, a change of a mere 52 centimeters per second based on previous imaging. It’s mind-boggling for those of us who grew up with Pluto as little more than a seemingly unreachable speck at system’s edge to realize that New Horizons is now less than 35 million kilometers out. We’re headed for a close approach at a distance of roughly 12,500 kilometers above the surface.
The second installment of “New Horizons: Countdown to Pluto” airs today at 1130 EDT (1530 UTC) on NASA TV, with repeat showtimes at 1530 EDT (1930 UTC) and 1930 EDT (2330 UTC). In this installment, mission team members Alice Bowman, Cathy Olkin and Chris Hersman will be going through the latest news on mission operations and science.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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