Dawn: Beginning Approach to Ceres

by Paul Gilster on December 31, 2014

Speaking of spacecraft that do remarkable things, as we did yesterday in looking at the ingenious methods being used to lengthen the Messenger mission, I might also mention what is happening with Dawn. When the probe enters orbit around Ceres — now considered a ‘dwarf planet’ rather than an asteroid — in 2015, it will mark the first time the same spacecraft has ever orbited two targets in the Solar System. Dawn’s Vesta visit lasted for 14 months in 2011-2012.

We have the supple ion propulsion system of Dawn to thank for the dual nature of the mission. In the Dawn version of the technology, xenon gas is bombarded by an electron beam. The resulting xenon ions are accelerated through charged metal grids out of the thruster. JPL’s Marc Rayman, chief engineer and mission director for the mission, explained thruster design in one of the earliest of his Dawn Journal entries:

Because it is electrically charged, the xenon ion can feel the effect of an electrical field, which is simply a voltage. So the thruster applies more than 1000 volts to accelerate the xenon ions, expelling them at speeds as high as 40 kilometers/second… Each ion, tiny though it is, pushes back on the thruster as it leaves, and this reaction force is what propels the spacecraft. The ions are shot from the thruster at roughly 10 times the speed of the propellants expelled by rockets on typical spacecraft, and this is the source of ion propulsion’s extraordinary efficacy.

Slow but steady wins the race. For the same amount of propellant, a craft equipped with an ion propulsion system can achieve ten times the speed of a probe boosted by today’s conventional rocketry, says Rayman, but on the other hand, an ion-powered spacecraft can manage to carry far less propellant to accomplish the same job, which is how missions like Dawn can be executed. It’s also true that one of Dawn’s thrusters pushes on the spacecraft with about the force of a piece of paper pushing on a human hand on Earth. Dawn isn’t exactly the spacecraft equivalent of a Ferrari — at full power, the vehicle would go from 0 to 60 miles per hour in a stately four days.

Fortunately, space is a zero-g environment without friction, so the minuscule thrust has a chance to build up. ‘Acceleration with patience’ is Rayman’s term. In addition to enhancing maneuverability, ion thrusters are also durable. Dawn’s three thrusters have completed five years of accumulated thrust time, more than any other spacecraft. If all goes well at Ceres, we can’t rule out an extended mission that might include other asteroid targets, just as we hope for a Kuiper Belt object encounter for New Horizons after its 2015 flyby of Pluto/Charon.

dawn_ceres

Image: An artist’s concept shows NASA’s Dawn spacecraft heading toward the dwarf planet Ceres. Dawn spent nearly 14 months orbiting Vesta, the second most massive object in the main asteroid belt between Mars and Jupiter, from 2011 to 2012. It is heading towards Ceres, the largest member of the asteroid belt. When Dawn arrives, it will be the first spacecraft to go into orbit around two destinations in our Solar System beyond Earth. Credit: NASA/JPL-Caltech.

Keep an eye on Rayman’s Dawn Journal as the Ceres encounter approaches. His latest entry goes through the historical background on the dwarf planet’s discovery, and includes the fact that the Dawn team has been working with the International Astronomical Union (IAU) to formalize a plan for names on Ceres that builds upon the name given to it by its discoverer. Astronomer Giuseppe Piazzi found Ceres in 1801 and named it after the Roman goddess of agriculture. The plan going forward is for surface detail like craters to be named after gods and goddesses of agriculture and vegetation, drawing on worldwide sources of mythology.

Deep space has been yielding unexpected results since the earliest days of our exploration, and with Dawn approaching Ceres it’s instructive to recall some of the discoveries the Voyagers made as they moved into Jupiter space, starting with the surprisingly frequent volcanic activity on Io. Ceres will doubtless yield data just as intriguing, says Christopher Russell (UCLA), principal investigator for the Dawn mission:

“Ceres is almost a complete mystery to us. Ceres, unlike Vesta, has no meteorites linked to it to help reveal its secrets. All we can predict with confidence is that we will be surprised.”

Not quite twice as large as Vesta, Ceres (diameter 950 kilometers) is the largest object in the asteroid belt, and unlike Vesta, it apparently has a cooler interior, one that may even include an ocean beneath a crust of surface ice. We’ll know more soon, for Dawn has emerged from solar conjunction and is communicating with Earth controllers, who have programmed the maneuvers for the next stage of operations, which includes the Ceres approach phase. At present, the spacecraft is 640,000 kilometers from the dwarf world, approaching it at 725 kilometers per hour.

tzf_img_post

{ 57 comments }

Long-Distance Spacecraft Engineering

by Paul Gilster on December 30, 2014

I find few things more fascinating than remote fixes to distant spacecraft. We’ve used them surprisingly often, an outstanding case in point being the Galileo mission to Jupiter, launched in 1989. The failure of the craft’s high-gain antenna demanded that controllers maximize what they had left, using the low-gain antenna along with data compression and receiver upgrades on Earth to perform outstanding science. Galileo’s four-track tape recorder, critical for storing data for later playback, also caused problems that required study and intervention from the ground.

But as we saw yesterday, Galileo was hardly the first spacecraft to run into difficulties. The K2 mission, reviving Kepler by using sophisticated computer algorithms and photon pressure from the Sun, is a story in progress, with the discovery of super-Earth HIP 116454 b its first success. Or think all the way back to Mariner 10, launched in 1973 and afflicted with problems including flaking paint that caused its star-tracker to lose its lock on the guide star Canopus. The result: A long roll that burned hydrazine as thrusters tried to compensate for the motion. Controllers were able to use the pressure of solar photons on the spacecraft’s solar panels to create the torque necessary to counter the roll and re-acquire the necessary control.

The Messenger spacecraft also used pressure from solar photons as part of needed course adjustment on the way to Mercury, and now comes news of yet another inspired fix involving the same craft. Messenger was on course to impact Mercury’s surface by the end of March, 2015, having in the course of its four years in Mercury orbit (and six previous years enroute) used up most of its propellant. But controllers will now use pressurization gas in the spacecraft’s propulsion system to raise Messenger’s orbit enough to allow another month of operation.

The helium in question was used to pressurize the propellant tanks aboard the spacecraft. Let me quote Stewart Bushman (JHU/APL), lead propulsion engineer for the mission, on just what is going on here:

“The team continues to find inventive ways to keep MESSENGER going, all while providing an unprecedented vantage point for studying Mercury. To my knowledge this is the first time that helium pressurant has been intentionally used as a cold-gas propellant through hydrazine thrusters. These engines are not optimized to use pressurized gas as a propellant source. They have flow restrictors and orifices for hydrazine that reduce the feed pressure, hampering performance compared with actual cold-gas engines, which are little more than valves with a nozzle.”

Bushman adds that stretching propellant use is not the norm:

“Propellant, though a consumable, is usually not the limiting life factor on a spacecraft, as generally something else goes wrong first. As such, we had to become creative with what we had available. Helium, with its low atomic weight, is preferred as a pressurant because it’s light, but rarely as a cold gas propellant, because its low mass doesn’t get you much bang for your buck.”

MESSENGER_Year_Review

Image: A compilation of Messenger images from Mercury in 2014. Next April, Messenger’s operational mission will come to an end, as the spacecraft depletes its fuel and impacts the surface. However, the last few months of operations should be rich, including science data obtained closer to the planet’s surface than ever previously accomplished. Credit: JHU/APL.

So we gain an extra month to add to Messenger’s already impressive data on the closest planet to the Sun. The spacecraft’s most recent studies, begun this past summer, have involved a low altitude observation campaign looking for volcanic flow fronts, small scale tectonic effects, layering in crater walls and other features explained in this JHU/APL news release. Growing out of this effort will be the highest resolution images ever obtained of Mercury’s surface.

The additional month of operations will allow a closer look at Mercury’s magnetic field. “During the additional period of operations, up to four weeks, MESSENGER will measure variations in Mercury’s internal magnetic field at shorter horizontal scales than ever before, scales comparable to the anticipated periapsis altitude between 7 km and 15 km above the planetary surface,” says APL’s Haje Korth, the instrument scientist for the Magnetometer. Korth also says that at these lower altitudes, Messenger’s Neutron Spectrometer will be able to resolve water ice deposits inside individual impact craters at the high northern latitudes of the planet.

That’s a useful outcome and it grows out of sheer ingenuity in using existing resources. What’s fascinating in all these stories is that when we send a spacecraft out, we have frozen its technology level while our own continues to expand and accelerate. Think of the Voyagers, still operational after their 1977 launches, and imagine the kind of components we would use to build them today. The trick in resolving spacecraft problems and extending their missions is to keep the interface between our latest technology and their older tools as robust as possible. That involves, it’s clear, not just hardware and software, but the power of the human imagination.

tzf_img_post

{ 12 comments }

Kepler: Thoughts on K2

by Paul Gilster on December 29, 2014

As we start thinking ahead to the TESS mission (Transiting Exoplanet Survey Satellite), currently scheduled for launch in 2017, the exoplanet focus sharpens on stars closer to home. The Kepler mission was designed to look at a whole field of stars, 156,000 of them extending over portions of the constellations Cygnus, Lyra and Draco. Most of the Kepler stars are from 600 to 3000 light years away. In fact, fewer than one percent of these stars are closer than 600 light years, while stars beyond 3000 light years are too faint for effective transit signatures.

Kepler has proven enormously useful in helping us develop statistical models on how common planets are, with the ultimate goal, still quite a way off, of calculating the value of ηEarth (Eta_Earth) — the fraction of stars orbited by planets like our own. Looking closer to home will be the mandate of TESS, which will be performing an all-sky survey rather than the ‘long stare’ Kepler has used so effectively. We should wind up with a growing catalog of nearby main-sequence stars hosting planets, a catalog that future missions will exploit.

But what of Kepler itself? It’s heartening to see a mission rescued when onboard problems have threatened its survival, and in the case of this doughty space telescope, the revival is almost as interesting as the latest results. The Kepler primary mission ended with the failure of the second of four reaction wheels that were used to stabilize the spacecraft. Kepler needed three reaction wheels for effective pointing accuracy, but ground controllers found a way to use the pressure of sunlight as a kind of ‘virtual’ reaction wheel to control the instrument.

Thus we get K2, the extended Kepler mission. Software to correct for drift in the field of view allows the telescope to achieve about half the photometric precision of the original Kepler mission, according to this University of Hawaii news release. A new exoplanet find — HIP 116454 b — demonstrates that the method works and that Kepler is, at least to some extent, back in business. Unlike most of the stars in the original Kepler field of view, HIP 116454 is relatively nearby, at 180 light years from Earth in the constellation Pisces.

K2_planet

Image: This artist’s conception portrays the first planet discovered by the Kepler spacecraft during its K2 mission. A transit of the planet was teased out of K2’s noisier data using ingenious computer algorithms developed by a researcher at the Harvard-Smithsonian Center for Astrophysics (CfA). The newfound planet, HIP 116454 b, has a diameter of 32,000 kilometers. It orbits its star once every 9.1 days. Credit: CfA.

Working with K2 is exceedingly tricky business, with NASA likening the use of solar photons to stabilize the instrument to the act of balancing a pencil on a finger, a manageable but demanding task. Nor was HIP 116454 b an easy catch. For one thing, only a single transit was detected in the engineering data that was being used to prepare the spacecraft for the full K2 mission. That called for confirmation from ground-based observatories, which came from the Robo-AO instrument mounted on the Palomar 1.5-meter telescope and additional follow-up from the Keck II adaptive optics system on Mauna Kea.

We learn that HIP 116454 b is about two and a half times the size of Earth, with a diameter of about 32000 kilometers and a mass twelve times that of our planet. The paper on this work speculates that we are either dealing with a water world or a mini-Neptune with a gaseous envelope. The planet’s orbital period around the K-class host is 9.1 days at a distance of 13.5 million kilometers. As a ‘super-Earth,’ it will be a high-value target for future observation:

HIP 116454 b could be important in the era of the James Webb Space Telescope (JWST) to probe the transition between ice giants and rocky planets. In the Solar system, there are no planets with radii between 1–3 R while population studies with Kepler data have shown these planets to be nearly ubiquitous… Atmospheric studies with transit transmission spectroscopy can help determine whether these planets are in fact solid or have a gaseous envelope, and give a better understanding on how these planets form and why they are so common in the Galaxy.

The planet will also be useful in relation to another similar world:

Also of interest is the fact that HIP 116454 b is very similar to HD 97658 b, in terms of its orbital characteristics (both are in ∼ 10 day low-eccentricity orbits), mass and radius (within 10% in radius, and within 25% in mass), and stellar hosts (both orbit K–dwarfs). Comparative studies of these two super–Earths will be valuable for understanding the diversity and possible origins of close–in Super–Earths around Sun–like stars.

We know from this work that K2 can find transiting planets despite the loss of precision the telescope has suffered, and the paper points out that the engineering test field in which HIP 116454 b was found as well as other potential fields of view will enable observatories in both hemispheres to view the stars for follow-up, something that was not possible in the original Kepler mission. So with K2 capability, we can see HIP 116454 b, relatively nearby, as a kind of bridge between the original Kepler statistical studies and the nearby targets of the TESS mission to follow.

The paper is Vanderburg et al., “Characterizing K2 Planet Discoveries: A super-Earth transiting the bright K-dwarf HIP 116454,” accepted for publication in The Astrophysical Journal (preprint).

tzf_img_post

{ 11 comments }

Have a Wonderful Holiday

by Paul Gilster on December 24, 2014

I’m cooking all afternoon in anticipation of a family dinner tonight. The first fruits of my labors are in the photo below. I cultivated the sourdough starter I use for this bread three years ago — over the years, it has really developed some punch, and produces a fine, aromatic loaf. My afternoon now turns to large poultry, a country-sausage stuffing (with some of the sourdough bread as a key ingredient), various greens, beans and a chipotle-laden sweet potato dish I discovered last year. I leave it to my daughter to bring her usual spectacular salad and dessert.

I want to wish all of you the best, and hope your day is going as well as mine. It’s always a privilege to write for this audience.

sourdough

tzf_img_post

{ 22 comments }

An Internal Source for Earth’s Water?

by Paul Gilster on December 23, 2014

The last time we caught up with Wendy Panero’s work, the Ohio State scientist was investigating, with grad student Cayman Unterborn, a possible way to widen the habitable zone. Slow radioactive decay in elements like potassium, uranium and thorium helps to heat planets from within and is perhaps a factor in plate tectonics. In 2012, Unterborn argued that planets with higher thorium content than the Sun would generate much more heat than the Earth, allowing a habitable zone with liquid water on the surface correspondingly farther out from the star.

You can read about that work and its implications in Widening the Habitable Zone. I was reminded of it because Panero reported at the recent American Geophysical Union meeting on her latest direction, a study involving the formation of the Earth’s water. Recall that analysis of data from the Rosetta probe implicated asteroids rather than comets as the main delivery mechanism for Earth’s oceans (see Rosetta: New Findings on Cometary Water). Panero’s new work indicates a substantial part of Earth’s water was made right here.

Hydrogen atoms trapped inside crystal defects and voids within minerals can bond with the oxygen already plentiful in these substances, which is how rock that appears dry can contain water. The potential for a great deal of water exists in mantle rock, given that the mantle makes up more than 80 percent of the total volume of the planet, according to this Ohio State news release. Working with doctoral student Jeff Pigott, Panero subjected mantle minerals to high pressures and temperatures to create conditions like those deep inside the Earth.

Changes to the crystal structures of these minerals as a result of compression helped the team gauge how much hydrogen the minerals can store. The minerals in play here include bridgmanite (a high pressure form of olivine), which is abundant in the lower mantle, and ringwoodite (another form of olivine). 525 to 800 kilometers below the surface, both minerals exist in a ‘transition zone’ that can hold large amounts of water, which could be carried to the surface by convection of mantle rock, the same process that produces plate tectonics. The researchers’ tests indicated that bridgmanite contained too little hydrogen to be significant for water delivery, but ringwoodite emerged as a candidate for deep-earth water storage.

2mantle_convection_cell

Image: This plate tectonics diagram from the Byrd Polar and Climate Research Center shows how mantle circulation delivers new rock to the crust via mid-ocean ridges. New research suggests that mantle circulation also delivers water to the oceans.

Panero and Piggott’s computer calculations helped to reveal the geochemical processes that would allow the minerals to rise through the mantle to the surface, the prerequisite for release of water into the oceans. Panero calls the relationship between surface water and plate tectonics “one of the great mysteries in the geosciences.” Her calculations with Pigott indicate that another mineral, garnet, may also play a role in delivering water from ringwoodite back down into the lower mantle, a circulation cycle enabled by plate tectonics that could maintain half as much water below the Earth as is currently flowing in today’s surface oceans.

A cycle like this would keep mantle water replenished even as it fed the oceans — the mantle would never exhaust its water supply. Says Panero:

“If all of the Earth’s water is on the surface, that gives us one interpretation of the water cycle, where we can think of water cycling from oceans into the atmosphere and into the groundwater over millions of years. But if mantle circulation is also part of the water cycle, the total cycle time for our planet’s water has to be billions of years.”

Such a process also relaxes the need for accounting for all of Earth’s water through bombardment from asteroids or comets. The latter surely played a role, but perhaps supplemented water that has been reaching the surface through plate tectonics ever since.

tzf_img_post

{ 29 comments }

Are Europa’s Plumes Really There?

by Paul Gilster on December 22, 2014

A new study of data from the Cassini Saturn orbiter has turned up useful information about, of all places, Europa. Cassini’s 2001 flyby of Jupiter en route to Saturn produced the Europa data that were recently analyzed by members of the probe’s ultraviolet imaging spectrograph (UVIS) team. We learn something striking: Most of the plasma around Europa is not coming from internal activity being vented through geysers, but from volcanoes on Jupiter’s moon Io. Europa actually contributes 40 times less oxygen to its surrounding environment than previously thought.

These findings cast one Europa mission possibility in a new light. In 2013, researchers using the Hubble Space Telescope reported signs of plume activity, which immediately called the example of Enceladus to mind. If Europa were venting materials from an internal ocean, a possible mission scenario would be to fly a probe through the plume, just as the Cassini team has done with its probe at Enceladus. The latter also has strong signs of an internal ocean, but its geysers are far more substantial than what Hubble has observed at Europa. The new Cassini data indicate that at least in 2001, the UVIS instrument could detect no plume activity on Europa.

Europa_hemisphere

Image: Europa is a high value target in any case, but plumes would offer useful options for future spacecraft to study. Credit: NASA/JPL-Caltech/SETI Institute.

In the mix here is the kind of plasma observed in Europa’s orbit. The UVIS instrument saw hot plasma there, a stark contrast to the cold, dense gas that marks out the orbit of Enceladus. The Enceladan plumes account for this gas, which slows down electrons being dragged through it by Saturn’s magnetic field, lowering the temperature of the plasma. If something similar is happening at Europa, there is little sign of it. Cassini could find no evidence that large amounts of water vapor were being injected into Europa’s orbit, so any Europan plume activity is minimal. Amanda Hendrix (Planetary Science Institute) is a Cassini UVIS team member who co-authored the new study:

“It is certainly still possible that plume activity occurs, but that it is infrequent or the plumes are smaller than we see at Enceladus. If eruptive activity was occurring at the time of Cassini’s flyby, it was at a level too low to be detectable by UVIS.”

Even the tenuous atmosphere around Europa, millions of times thinner than Earth’s atmosphere, turns out to be about 100 times less dense than previously believed, according to these results. So the Hubble findings point to the need for further study, to determine whether there is intermittent plume activity or not. Maybe what had been thought to be 200-kilometer high jets of water are an extreme rarity, or perhaps they were the result of a mistaken original analysis.

This story isn’t over, because New Scientist is reporting that scientists who discovered the original Europa plume are working on an rebuttal of the Cassini findings. The magazine quotes Kurt Retherford (Southwest Research Institute), one of the plume discoverers, as saying that Cassini was simply too far from Jupiter to get an adequate reading of Europa’s plume activity, adding “We would say using their technique, they couldn’t possibly find water.”

A future mission may be what it takes to make sense of observations that were at the outside of Hubble’s envelope and may not be repeated until we can get a closer look. If Europa isn’t venting water into space, its status as a highly interesting astrobiology target is hardly compromised, but a flyby through a plume will disappear from our list of mission possibilities. Can Hubble make a repeat, definitive observation to settle this?

The Cassini findings were presented on December 18 at the American Geophysical Union fall meeting in San Francisco. The paper is Shemansky et al., “A New Understanding of the Europa Atmosphere and Limits on Geophysical Activity,” Astrophysical Journal Vol. 797, No. 2 (2014), p. 80 ff. (abstract).

tzf_img_post

{ 17 comments }

Interstellar: Herald to the Stars or a Siren’s Song?

by Paul Gilster on December 19, 2014

Not long after I published my thoughts on Chris Nolan’s film Interstellar, Centauri Dreams regular Larry Klaes weighed in with his own take. Views on Interstellar have been all over the map, no surprise given how personal film criticism can be (take a look at the critical reception of Bladerunner over the years). I like the point/counterpoint aspect of what Larry does here, and while I imagine most readers have seen the movie by now, his criticisms may provoke a few more viewings and, I hope, a look at Kip Thorne’s excellent book on science in the making of the film.

By Larry Klaes

When I first heard about the existence of the film Interstellar, I was initially hopeful yet cautious. Most science fiction, especially these days, is some variation on Star Wars, which is often about as scientific and science fictional as the Harry Potter series. Yet Christopher Nolan and his team insisted they were striving hard to stick to REAL science with their production: They even had the famous Caltech astrophysicist Kip Thorne on their side, the very man who convinced none other than Carl Sagan to go with a wormhole rather than a black hole as the means to propel Ellie Arroway across space and time to meet the ETI in his 1985 novel (and 1997 film) titled Contact.

My real hope was that Interstellar would both portray a realistic method of travel among the stars based on currently known and plausible science and technology (no ambiguous hyper drives or nearly magical wormholes) and ignite the public’s passion for true interstellar exploration – along with overall space exploration and colonization in the process. A decent and maybe even original story with characters I genuinely cared about would be nice, too. Something to counterbalance the last three decades of fantasy and soap opera that Star Wars and Star Trek had done to science fiction after the golden age of the cinematic genre in the 1960s and early 1970s.

Interstellar-2014-Movie-Poster

The cry and hue that the story of Interstellar was based on real science and was very similar to the classic 2001: A Space Odyssey thrust upon the world by Stanley Kubrick and Arthur C. Clarke 46 years earlier only grew as the date of its general release approached. My gut feeling that Interstellar was not going to be a straightforward science film of fiction also grew, but I kept hoping to be wrong, that somehow we were going to have a cinematic creation of the genre worthy of 2001 or at least in the same room as Kubrick’s masterpiece. Heck, I would be happy if Interstellar was in the same building or at least on the same street as the original 2001: A Space Odyssey.

When I finally saw Interstellar on the big screen I sadly realized my underlying fears were true. In a number of ways the film was even worse than I predicted and not just because it felt like I was watching something created by an amateur filmmaker whom the studios had given a big budget to and told to do as he pleased. My hopes of giving the general nonscientific public an anchor to see and appreciate how we might really send our species to the stars one day were also dashed.

Now I may be wrong, for Interstellar did do a lot of positive cheerleading for the cause of science and expansion into space and that could be enough to tip the scales or at least contribute to humanity getting off this rock permanently. Nevertheless I feel the way Nolan and company went about showing how people might spread into the Cosmos could ultimately undermine the enterprise with unrealistic and even damaging expectations.

Is Interstellar a cinematic herald that will turn the public’s interest and support towards a destiny in space? Or is it an unwitting siren’s song that will lure in unwary viewers to expect our journeys and colonization of the Cosmos to be like the fantastical paths in the film, only to lead them to unfulfilled dreams, disappointment, and ultimately turn away from what has long been called the Final Frontier?

Now to elaborate….

Let’s Put On a Show!

First the film itself. I am not a big fan of Christopher Nolan’s work even though I know he has a rather rabid fan base. I feel most of his films, while nice looking and not lacking in action, are rather heavy-handed in their messages, which themselves are given a gravity I find undeserving overall. About the only film of Nolan’s that did do more than just temporarily entertain me was The Dark Knight from his Batman trilogy and that was mainly due to the late actor Heath Ledger’s amazing performance as the psychopathic villain The Joker.

nolan_working

In any event I expected a director and producer of Nolan’s experience to come up with something better than what I witnessed on the big screen. Yes Nolan was clearly influenced by 2001 and many parts of Interstellar were both emulations and tributes to the 1968 cinematic landmark, but ultimately that is essentially what Interstellar felt like, a tribute by someone who thought they could make a 2001 level science fiction film for this generation but ultimately fell short. It was both a bit surprising and disappointing.

Across the Internet many people were quite vocal in praising and defending Interstellar and that of course is their right. I noted in particular how many were adamant both that it was “just a film” as if this were some deep revelation and that any poor or inaccurate portrayals of science should not matter, usually reverting back to the “just a film” reason or Interstellar not being a documentary as the most common excuses. No, Interstellar was not a terrible film and it was obvious that Nolan et al tried very hard to make good science fiction cinema. Perhaps that is what makes it all the sadder that for all their talent and budget they could not match what they tried to emulate or even as a lesser science fiction film, not even in the three hours they had to tell their story. And when one has spent hundreds of millions of dollars and years on a project and loaded it with important messages for a wide audience, then Interstellar is NOT “just a film.”

To me it felt like Nolan said to himself “Hey, I can write science fiction!” and then proceeded to show why and how making a good creation of the genre is not as easy as the fluff the public has been fed since 1977. I also blame this on why certain segments of the public think Interstellar is so wonderful: When you have been fed a near steady diet of hamburger for decades, suddenly being given a better made hamburger (yet a hamburger nonetheless) makes you think you are dining on a porterhouse steak. Or cooking up one, apparently.

I am not going to delve much into the science portrayed in Interstellar as many have already done so (Google the words ‘interstellar science’) and amazingly even a science book by Kip Thorne himself was produced on the very subject, which perhaps may be the best thing to come out of this whole effort. I do not want to detract from the main points I really want to make about Interstellar next in this piece. Besides, there will still be enough of my comments on the science of Interstellar in the process.

When You Wish Upon a Wormhole…

I was honestly quite bothered by the way Interstellar went about saying and showing how humanity may one day achieve the stars. The first issue is their reason for spreading into deep space: In the film it is not due to humanity’s desire for knowledge or adventure, it is primarily one of survival. Now while needing to evacuate Earth and the Sol system is one legitimate reason for developing a means of interstellar transportation and colonization, there are two problems with this scenario: One is that it is often considered to be the ONLY legitimate reason for sending humans to the stars. The other is that if humanity and our planet are in some kind of dire trouble where the only alternative is to evacuate, the odds are probably rather high that humanity will already be in a state where building any kind of interstellar vessel, even a slow-moving multigenerational starship, may no longer be an option. So if we are ever to develop a real interstellar capability, perhaps we should start conducting one while our civilization is not in a major state of crisis or outright impending doom.

In the near future, a disease called only the Blight has decimated most crops across Earth and is steadily increasing the amount of nitrogen in our planet’s atmosphere. This in turn has led to many aspects of the society falling towards doom, including space travel in general and NASA in particular. The slow march to extinction for the human species appears to be inevitable and the efforts at preservation shown in Interstellar are only buying time.

Then suddenly it is revealed (by methods which feel like nothing less than the supernatural) that NASA isn’t gone but merely hiding underground (literally) to escape a skeptical and increasingly ignorant and panicky public while what is left of the United States government secretly funds the space agency for several plans it has to save the human race, or at least some of it. One part of the plan involves flying up to a spaceship in Earth orbit which will then take a crew of brave astronauts (and a collection of frozen and fertilized human eggs as the backup part of the plan) to travel to a wormhole which appeared near the planet Saturn some decades ago. From there the astronauts will use the wormhole to journey to a solar system in another galaxy and check out three worlds circling a black hole to see if initial reports beamed back from earlier expeditions there do indeed prove these alien planets to be viable places for human colonization.

Interstellar_ship

It has already been stated multiple times elsewhere about the concerns one might understandably have about venturing to planets in the gravitational grip of a black hole. We have also been informed more than once how Thorne made some actual new scientific revelations about this type of celestial object while helping the filmmakers create a realistic black hole. What bothers me deeply is that the public will have even more firmly entrenched in their minds that cosmic wormholes are probably the only way humanity will ever achieve interstellar travel (the other faster than light speed contenders are of course warp drives and hyperdrives).

What may be even more harmful is that the wormhole was apparently not a natural cosmic formation but placed deliberately near the ringed gas giant world by some unknown advanced entities from the future in our Sol system for use by their distant human ancestors from the struggling Earth. This will merely add reinforcement to those who think humans both in the past and present are not bright enough to solve their own problems and build wondrous devices in the process, that some outside force like advanced aliens or future humans must intervene or all is lost. This is an insult to the intelligence and ingenuity of our ancestors and present ourselves, who have done and learned some amazing things without any external assistance and beaten some very strong odds against us and our societies.

Anyone who has done more than a very casual read of Centauri Dreams knows there are multiple methods for interstellar travel which are plausible and do not invoke help from remote human descendants or superior ETI and do not require abilities that seem to be almost magical in their powers. Ironically the new television miniseries called Ascension, which I thought would be little more than an imitation of Mad Men set in space for the novelty, involves a multigenerational starship with Orion nuclear pulse propulsion – both forms of star voyaging which we could build either now or in the rather near future.

It is obvious that Nolan, feeling he had to have a means for star travel grand enough to match his visions and huge film budget ($165 million, which does not even include marketing costs), while lacking a strong background in science and technology (he even says as much in Thorne’s science companion book), went for what he thought would invoke the wonder and magic of the Cosmos for his audience. Maybe it did, but maybe it also left the general public thinking there is only one real way to attain the vast realm beyond the Sol system. This implies that we should let the smart people of the future do all the grunt work to hopefully come up with a device that seems to solve the physics and technical issues resident with wormholes with little effort.

This concern of mine has a real world basis in the reactions of the press and public to some of the more recent interstellar workshops and conferences. While these wonderful and groundbreaking events have many real experts discussing about and working upon a wide range of interstellar methods of propulsion and exploration which involve real physics and technology, the media instead tends to focus on those presentations about warp drives and wormholes, both of which suffer from some major obstacles in reality.

However, thanks to certain popular genres, this does not stop the general public from overly attaching their attention and excitement onto hypothetical means of reaching the stars which may not come to fruition for a very long time, if ever. Witness the hype about the belief that NASA is developing a warp drive, when in reality there is just one person working on the subject on a very limited budget and has so far produced a few academic papers from it all. Meanwhile the more plausible methods such as the aforementioned Orion, fusion propulsion, laser sails, and antimatter, are left standing off to the side, diminished by the very artificial glare of the flashier warp drives and wormholes. Yet Orion and its brethren, while certainly having some issues of their own, give us much better chances of actually getting us to Alpha Centauri and beyond, even if it will take years to centuries rather than minutes or seconds.

Despite what may seem at first to be incongruent to common sense, many people do get much of their “education” about the world from films and television whether they think so or not (or like It or not); whether it is accurate or not is another matter, yet they still absorb it and take what the film shows as the way something is, especially if it is a place or object unfamiliar to their everyday experiences. This certainly includes space science and astronomy, as these subjects are often either taught sparingly in the public school systems or not at all. This is why the general public and the media by extension focus so much on using warp drives or hyperdrives or wormholes as the most popular and expected means of deep space propulsion. These are not only the most complex methods but also perhaps the most unrealistic, but when your education comes from Hollywood, that hardly matters – except when it comes to supporting groups that are trying to make interstellar travel a reality.

interstellar_water

Just So Long as We Look Really Cool

Then there is the issue of where the expedition in Interstellar was going to start a new civilization for humanity. Forget for a moment that the best candidate planets are circling a giant black hole, perhaps one of the worst places to be near in the Universe between the constant threat of being torn apart and then crushed into a singular dot and the massive amounts of radiation from all the x-rays being generated as celestial debris is constantly being pulled into the black hole and heating up immensely in the process. There is also the general lack of any light from an object with a mass strong enough to keep any photons from escaping but why quibble at this point? The worlds around the black hole named Gargantua seem to get enough illumination somehow so that the visiting astronauts can see what they are doing and getting into.

No, my even greater issue with the scenario is that apparently there was not a single world of the 400 billion star systems in our own Milky Way galaxy which were good enough to resettle what was left of our species. This despite the fact that even in our early stages of knowledge about real exoplanets we can now comfortably estimate there are billions of habitable worlds in the Milky Way: With those kinds of odds at least some of them should be good enough for terrestrial life to settle upon, or at the least be made viable for colonization. Instead the superior future humans (or whatever they really are exactly) chose a galaxy so remote that the people doing the initial exploring do not even know where it is In the Universe. So if something went wrong, the people on either side of the wormhole are literally stuck with no other apparent options.

In many science fiction films and television series which deal with deep space, Hollywood has often made no distinction between a solar system and a galaxy. While the makers of Interstellar were aware of these two very different kinds of celestial objects, the fact that they still had our main characters whiz off not to another solar system in the Milky Way but another entire galaxy many millions if not perhaps billions of light years away probably blurred the distinction for many in the audience, who sadly have little better knowledge of astronomy than a typical Hollywood producer.

For that matter, this film should have properly been called Intergalactic rather than Interstellar, since any travel outside the Sol system by the astronauts led not to another star system in our galaxy but another entire stellar island. I see this as a missed educational opportunity for a cinematic production team which boasted how scientific their film is and hoped it would inspire millions of viewers.

What is Love? Baby Don’t Hurt Me

Perhaps there is one thing more derailing about Interstellar than a near magical wormhole sent from the future and that is the film’s take on the concept of love. And the fifth dimension. Of the many instances where Nolan tried to channel Kubrick’s 2001, perhaps the biggest example was near the end of the film, when our hero astronaut named Cooper rides into the black hole called Gargantua and somehow ends up inside an infinite bookcase where he attempts to communicate with his daughter named Murphy using Morse code. She in turn goes on to create what I can only surmise is some form of cavorite – the antigravity material written about in the 1901 science fiction novel The First Men in the Moon by H. G. Wells – which somehow saves what is left of the human race by getting them off Earth and into giant space colonies, one of which ends up around the ever-photogenic Saturn.

This then begs the question: Why did we spend the vast majority of the film focused on trying to save humanity by colonizing some really distant alien worlds when we ended up living in massive space cities in our Sol system after all?

Much of the film’s latter parts can only be described as a metaphysical mess which were actually made worse when Nolan tried to throw in explanations to make it all seem somehow plausible along with dramatic and heartwarming. Sure, Kubrick also went seemingly transcendent with the famous Stargate sequence in 2001, but at least he had the good sense to speak only with images and let the viewers decide what was going on. Nolan fell into the trap that most cinematic and television producers make these days and that is they feel they have to explain EVERYTHING. Even if it is a bunch of metaphysical technobabble.

All Nolan really did was reinforce my concern that the public will think we can reach the stars only if we do the future technological equivalent of clicking our heels together three times and wishing really hard.

ship_scene

And this thing called love. I literally started to sink into my theater chair when I heard the character of Brand played by Anne Hathaway declare with dead seriousness that love is not just a chemical reaction or a genetic drive to continue the species but a physical, tangible force that can transcend space and time and unite two people despite any and all odds including deep, deep space. This is how she knew her lover was still alive on one of the worlds circling that black hole.

I know love can make people do amazing, crazy, and stupid things, but Nolan really went off into the deep end of the pseudoscientific swimming pool here. This notion about love is the kind of thing one expects from a Lifetime or Hallmark Channel production, not an expensive epic wannabe that continually boasts about how scientific it is – and then immediately dances down the magical mystery tour path. If they had stuck with the melodramatic message that love can drive and unite two people even if they are very far apart in space and time in the metaphorical sense, that would have worked. But then they went the pseudoscience route, which really undermined Nolan’s repeated claims about how science-oriented Interstellar is.

How Not to Buy the Farm

For a film that I assumed wanted to inspire the average Joe and Jane to support space exploration, I was left rather wondering about their treatment of farmers and farming in general. Cooper’s son, Tom, is designated by the local school system – the same one that said the Apollo lunar landings were a hoax, please note – as being smart enough for farming. Cooper takes this pretty much as a given and for the rest of the film we see our hero astronaut bonding over and over with his daughter, the smart one who grows up to become the scientist who solves the “gravity problem” – whatever the heck that was anyway. As an astronaut who desperately wants to get back into space, everything else, including working in the terrestrial dirt, is second rate.

Not too many generations ago, most people were farmers. And even though the number of participants in this occupation are much less these days, there are still plenty more whose jobs are much closer to farming than those of science. So why does Interstellar implicitly put down the average Joe and Jane even if it thinks it doesn’t mean to? Son Tom grows up to become the farmer he was more or less assigned to be, one who is so focused on his trade that even when his sister and her boyfriend warn Tom that he and his family have to consider leaving their home and fields due to health problems from all the dust caused from the Blight, his reaction is a very negative one tinged with growing hostility.

In the end we don’t know what becomes of Tom or his family in the later years, because Cooper’s bond is mainly with his scientist daughter and not the son he pretty much wrote off the day the school determined he was good enough for farming.

While I for one was very happy to see a film which verbally promoted and elevated space and those who want to explore it, I was also surprised at all the underlying negativity towards what I guess can best be described as Middle America. If anything Nolan should have been trying to find ways to show folks who were not science minded or ever considered the possibilities that space holds for our species and our planet that they too could participate in the Great Adventure; that space truly does hold the keys to our future. Instead we get Cooper making comments such as: “We used to look up at the sky and wonder at our place in the stars, now we just look down and worry about our place in the dirt.” He may have been largely right both for his world and ours, but putting up a further divide between the two modes of thought that also treats Earth as something separate from the rest of the Universe, which as a big ball of rock circling a star through space certainly means it is not, is not going to help the cause in our world to get off that dirt – or use space to help it, for that matter.

Oh Look, O’Neil’s Space Colonies – Hey, Wait, Come Back!

Despite the fact that it kind of felt like defeating the whole point of spending so much time and effort with those astronauts trying to find a new world to colonize on the other side of that mysterious wormhole, I was rather pleased to see the scenes near the end taking place aboard an honest-to-goodness O’Neil space colony.

In the 1970s we were presented with huge artificial space cities looking like either long cylinders or wheels with spokes. Thousands of people were going to live on them and construction would begin by the start of the 21st Century: The Bicentennial issue of National Geographic Magazine devoted an article to the concept, complete with very nice artwork showing how we could all be living in these floating space colonies fifty years hence – that is how serious and widespread the idea was becoming, at least in the minds of space fans
.
Of course just like the manned missions to Mars, we are still waiting for them to happen, but the ideas have taken on a form of reality in science fiction cinema. The Stanford torus version of the O’Neil space colony is a key player in the 2013 film Elysium, although it isn’t doing space utilization any favors by showing that only the rich and powerful get to live way up in Earth orbit while everyone else gets to struggle for existence on a dying planet.

For Interstellar, the space colony is for everyone, or at least all those who could manage to survive long enough to escape Earth and start new lives on these huge artificial worlds thanks to Cooper’s scientist daughter solving the “gravity problem”, whatever that was exactly. Again, why the film did not depict humanity expanding into the Sol system, which would have been a lot easier than even more conventional forms of interstellar travel let alone a wormhole that appears by virtual magic, I still do not quite understand. Just as I do not understand why those future humans let machines, resources, and lives be wasted attempting to colonize a few really distant and rather inhospitable alien planets which being from the future they should have known about already. So that Cooper and Brand could hook up on the one planet that was livable? I know Brand also had all those Plan B frozen fertilized eggs with her as well, so maybe they were supposed to start a new branch of the human race in another galaxy, but I did not see how exactly they were going to gestate all those eggs and then raise the children into successful adulthood on their own? Were those monolithic robots supposed to help?

We the audience did get a more satisfying and plausible answer to solving the dilemma of settling space with those big artificial colony worlds, but it took most of the three hour long film going down some rather murky and even dead end roads to get there. I wondered how many viewing Interstellar could appreciate or even remember what an O’Neil colony was and what it promised humanity by then? And what about settling actual worlds in our Sol system, namely the Moon and Mars? Did they get colonized and Nolan just didn’t bother to have anyone mention it? Are the planetoids and comets colonized too? Why didn’t they get at least a mention considering just how important their roles will be when real space colonies come along? I think a real opportunity to present how we could colonize our celestial neighborhood was missed here.

Teaching Them to Long for the Endless Immensity of the Sea

Ironically, for all my views on and concerns about Intergalactic – I mean Interstellar – my fondest hope is that I am wrong when it comes to the film succeeding in its intentions, that Nolan’s effort does pay off with the public supporting space in their minds, with their words, and with their wallets. Especially those who so vocally defended Interstellar. We have enough pretend space programs and actors portraying astronauts; we need to do our part to show how much more amazing and exciting the real Universe is and can be.

Perhaps the author of The Little Prince, Antoine de Saint-Exupery, has it right about how to get the public interested in settling the stars when he said the following:

“If you want to build a ship, don’t drum up people to collect wood and don’t assign them tasks and work, but rather teach them to long for the endless immensity of the sea.”

And while you are at it, as Interstellar said multiple times, do not go gentle into that good night. Rather, go boldly with a sense of adventure and purpose, all of which space can give us – and which we can do on our own with our minds and tools. We are the ones who will create the future, not the other way around.

tzf_img_post

{ 71 comments }

A New Look at High Obliquity Exoplanets

by Paul Gilster on December 18, 2014

Looking forward from winter into spring in North America — unfortunately still a few months out — I can thank Earth’s obliquity for a seasonal change I enjoy more every year. Obliquity is the angle that our planet’s rotational axis makes as it intersects the orbital plane, which in the case of Earth is 23.5°, so that when we reach the summer solstice, the north pole of the planet tilts toward the Sun by this angle. At winter solstice, the pole is tilted away by the same amount.

Our Solar System’s most extreme case of obliquity is Uranus, where the angle is a whopping 97.8°. Imagine a planet where the north pole points all but directly at the Sun, cycling through a year where the southern pole will eventually do the same. I’m reminded of Stephen Baxter’s novel Ark (Roc, 2011). Here, interstellar travelers come to a planet they hopefully designate Earth II (82 Eridani is its primary). Alas, the obliquity turns out to be 90 degrees, kicking off extreme seasonality. In this passage, one of the characters explains the problem to the rest of the crew:

“Every part of the planet except an equatorial strip will suffer months of perpetual darkness, months of perpetual light. Away from the equator you’ll suffer extreme heat, aridity, followed by months of Arctic cold—we estimate the surface temperature will drop to a hundred degrees below across much of the space-facing hemisphere, and there’ll be one hell of a blanket of snow and ice. Even the equator would be a challenge to inhabit, for even at the height of summer in either hemisphere the sun would be low, the heat budget minimal, the climate wintry.”

Is a planet with 90 degree obliquity in any sense habitable? It’s a question the crew debate and I won’t spoil what they find out. But I will point to a new study out of MIT that looks at planetary obliquity and finds that, depending on the world, habitable conditions may emerge. The work of David Ferreira (University of Reading, UK), Sara Seager (MIT) and colleagues, the paper in Icarus uses a three-dimensional computer model to simulate interactions between atmosphere, ocean and sea ice over a 3000-meter deep ocean. Shallower, more simplified oceans at 200, 50 and 10 meters in depth were also plugged into the mix for comparison.

The model assumes an Earth-sized planet at a similar distance from its star, one that is completely covered in water. Simulating three planetary obliquities — 23 degrees, 54 degrees and 90 degrees — the researchers found that a global ocean as shallow as 50 meters would absorb so much solar energy throughout the polar summer, releasing it back into the atmosphere during the winter, that the climate would remain relatively mild, with temperatures comfortably spring-like year round. It’s a result that no one on the team had anticipated.

“We were expecting that if you put an ocean on the planet, it might be a bit more habitable, but not to this point,” Ferreira says. “It’s really surprising that the temperatures at the poles are still habitable.”

MIT-AquaPlanet-01

Image: A water world at Earth’s distance from the Sun, assuming a deep enough ocean, could maintain habitable temperatures year-round even if its obliquity is high. Credit: Christine Daniloff/MIT.

It’s also a fragile situation, to judge from these results. A ten-meter deep global ocean would not be deep enough. The world would experience a runaway effect in which the first ice that formed would spread quickly onto the dark side. The eventual emergence of the dark side into light would not help, for by this point, the massive ice sheets that had formed would reflect sunlight efficiently enough to allow the ice to continue to spread, until the world became completely encased in ice. With this kind of high obliquity world in an Earth-like orbit — Ferreira and Seager call it an ‘aquaplanet’ — you’re either facing a warm ocean and benign temperatures or a global snowball.

But the potential is there for an all-water planet at high obliquity, one with a sufficiently deep ocean, to offer conditions in which life might develop. Adds Ferreira:

“The expectation was that such a planet would not be habitable: It would basically boil, and freeze, which would be really tough for life. We found that the ocean stores heat during summer and gives it back in winter, so the climate is still pretty mild, even in the heart of the cold polar night. So in the search for habitable exoplanets, we’re saying, don’t discount high-obliquity ones as unsuitable for life.”

The paper is Ferreira et al., “Climate at high-obliquity,” Icarus Vol. 243 (15 November 2014), pp. 236-248 (abstract). An MIT news release is available.

tzf_img_post

{ 10 comments }

Voyager: Shock Waves in Deep Space

by Paul Gilster on December 17, 2014

What exactly is the shock wave that Voyager 1 encountered earlier this year, a wave that is still propagating outward, according to new data from the craft? Researchers at the Jet Propulsion Laboratory refer to it as a ‘tsunami wave,’ a simile that reminds us of the devastating effects of roiled water as it encounters land following an earthquake or an impact in the ocean.

But in this case the cause is a coronal mass ejection (CME), in which the Sun heaves out a magnetic cloud of plasma from its surface, generating a pressure wave. As this JPL news release explains, the outgoing wave runs into charged particles in deep space — interstellar plasma — creating the disturbance. In all, Voyager 1 has experienced three of these shock waves, with the most recent first being observed in February of 2014 and still continuing.

The new data were presented on December 15 at the American Geophysical Union meeting in San Francisco by Don Gurnett (University of Iowa), who is quoted as saying “Most people would have thought the interstellar medium would have been smooth and quiet. But these shock waves seem to be more common than we thought.” Project scientist Ed Stone (Caltech) said the shock wave was causing the ionized interstellar gas to “‘sing’ or vibrate like a bell.”

“The density of the plasma is higher the farther Voyager goes,” Stone added. “Is that because the interstellar medium is denser as Voyager moves away from the heliosphere, or is it from the shock wave itself? We don’t know yet.”

You can hear the ‘singing’ that Stone describes in this JPL video, which includes a color-coded graph showing the frequency of the waves — this indicates the density of the plasma. Detection of the wave came through the spacecraft’s plasma instrument. Previous instances of the phenomenon occurred from October to November of 2012 and April to May of 2013. In fact, it was the shock wave in 2013 that helped scientists determine that Voyager 1 had left the heliosphere, the ‘bubble’ created by the solar wind that extends well past the outer planets. The plasma the spacecraft encountered then was forty times denser than any previously measured.

Voyager_wave

Image: The graphic above shows the frequency of the waves, an indication of the density of the plasma. Colors correspond to the intensity of the wave, with red being the loudest and blue the weakest. Credit: NASA/JPL.

Just how far out should these ‘tsunami waves’ be encountered? Gurnett, who is principal investigator for the plasma instrument on Voyager, thinks the waves may propagate out to twice the distance currently separating Voyager and the Sun. If that’s the case, we’ll be dealing with interesting plasma data as long as the Voyagers stay with us. Somewhere around 2025 is the likely endpoint of communications as the two probes run out of the power needed to transmit.

Ponder this: If it were not for loss of consumables, as seen in Voyager’s diminishing hydrazine and power levels, we could probably continue to track the craft for another century. As of this morning, Voyager 1 is 129.92 AU from the Sun, for a round-trip light time of over 36 hours.

tzf_img_post

{ 9 comments }

The Virtues of Oddly Shaped Planets

by Paul Gilster on December 16, 2014

A new paper out of George Mason University tackles the subject of planets deformed by tidal effects in close proximity to their star. It’s a useful study for reasons I’ll explain in a moment, but first a digression: I once had the chance to talk physics with the late Sheridan Simon, who besides being a popular lecturer on astrophysics at Guilford College (Greensboro, NC) also had a cottage industry designing planets for science fiction writers. Simon loved oddly shaped planets and because the Super Bowl was coming up, he had taken it upon himself to design a planet in the shape of a football, just to see what would happen if a place like this actually existed.

“And you know what? It works,” the bearded, exuberant Simon said with a grin. “But when you model what it looks like from space, the atmosphere is a problem. It looks plaid!”

TheFlightOfTheDragonfly

Simon played around with planets of every description, and if you’d like to see him at work on a planetary system around Tau Ceti, check what he developed back in 1992 for James K. Hoffman. I don’t know if he ever worked with Robert Forward, but of course the ultimate deformed planet in science fiction would be Rocheworld, from the novel of the same name (in an early version, Flight of the Dragonfly), where Forward envisions two worlds close enough to each other that they are deformed into egg-shapes and actually share an atmosphere.

Deformed planets in the realm of hot Jupiters have been studied for some time, for some of these worlds are close enough to their star to experience significant distortion in shape. One that Prabal Saxena and his George Mason University team mention in their paper is WASP-12b, which has been shown in earlier studies to have ellipsoidal variations in its transit depth that suggest a 3:2 ratio between the planet’s longest and its shortest axes. It’s an important effect because misunderstanding the distortion in shape caused by rotational and tidal effects can lead to mistakes in calculating the radius of the planet, and thus parameters such as density.

What Saxena and team are interested in is how rocky worlds orbiting red dwarf stars may experience stresses that can change their shape. Saxena comments:

“Imagine taking a planet like the Earth or Mars, placing it near a cool red star and stretching it out. Analysing the new shape alone will tell us a lot about the otherwise impossible to see internal structure of the planet and how it changes over time.”

Sun_planet

Image: An artist’s impression of a stretched rocky planet in orbit around a red dwarf star. So close to the star, there is a difference in the strength of the gravitational field on each side of the planet, stretching it significantly. Credit: Shivam Sikroria.

The paper, which appears in Monthly Notices of the Royal Astronomical Society, goes to work on how to take both tidal and rotational forces into account. The French astronomer Édouard Roche (1820-1883) was the first to calculate the distance within which an orbiting body will disintegrate because of the tidal forces induced by its primary. Inside what we now call the Roche limit, material in orbit will become dispersed into rings, while outside the limit, it can coalesce. The process varies depending on the innate rigidity of the body in question. A more fluid world deforms gradually, a process that compounds the tidal forces that will destroy it, while a more rigid planet may hold its shape until being broken apart by these same forces.

All of this could be useful as we try to learn more about the planet’s characteristics. From the paper:

The variation of rigidity of a planet may produce a small but detectable signal in the cases that were tested as one gets very close to the fluid Roche limit, and again it is important to remember planets have also been detected interior to the fluid Roche limit (the inner distance bound). Merely the constraining of tidal bulge amplitude along with Roche limit considerations may put meaningful limits on interior structure. The ability to directly constrain the shape of a planet would provide clues towards tidal theory, the orbital configuration of the system and bulk properties of the planet.

Not many M-dwarf planetary systems are likely to show the signature of worlds near the Roche limit, but the paper argues that the general physical principles in play here may also help us interpret the signatures of planets in particular orbital resonances or other configurations. Several dozen ‘hot Jupiters’ have been found that should, by virtue of their proximity to the Roche limit, show observable effects. For solid planets, the large transit depth may make red dwarf planets near the Roche limit an excellent realm for further study as we learn to interpret what any planetary deformations can tell us about their internal characteristics.

The paper is Saxena et al., “The observational effects and signatures of tidally distorted solid exoplanets,” published online by Monthly Notices of the Royal Astronomical Society 14 December 2014 (abstract / preprint).

tzf_img_post

{ 16 comments }