Thoughts on a Spacecraft’s Rebirth

According to a recent NASA news release, the agency has never before signed the kind of agreement it has made with Skycorp, Inc., a Los Gatos, CA-based firm that will now attempt contact with the International Sun-Earth Explorer-3 (ISEE-3) spacecraft. You’ll recall that this is the vehicle that scientists and space activists alike have been talking about resurrecting now that, having completed its studies of the solar wind in 1981 and later comet observations, it is making its closest approach to the Earth in more than thirty years (see ISEE-3: The Challenge of the Long Duration Flight).

According to its website, Skycorp is in the business of bringing “…new technologies, new approaches, and reduced cost to the manufacture of spacecraft and space systems.” Founded in 1998, the company signed a Space Act Agreement with NASA for the use of the International Space Station in 1999, and qualified the first commercial payload used in the filming of a television commercial (for Radio Shack) in 2001. In addition to its ISEE-3 involvement, Skycorp is now working on an orbit servicing system (for Orbital Recovery Corporation) and the design of lunar surface systems with NASA.

The document NASA has now signed is a Non-Reimbursable Space Act Agreement (NRSAA) with Skycorp that involves not just contact with the ISEE-3 spacecraft but, possibly, command and control over it. ISEE-3 will near the Earth this August, and the agreement lays out the variety of what NASA describes as “technical, safety, legal and proprietary issues” that will need to be addressed before contacting and re-purposing the spacecraft can be attempted.

“The intrepid ISEE-3 spacecraft was sent away from its primary mission to study the physics of the solar wind extending its mission of discovery to study two comets.” said John Grunsfeld, astronaut and associate administrator for the Science Mission Directorate at NASA headquarters in Washington. “We have a chance to engage a new generation of citizen scientists through this creative effort to recapture the ISEE-3 spacecraft as it zips by the Earth this summer.”

It’s hard not to get excited about the prospects here. The ISEE-3 Reboot Project works with a spacecraft that, although inactive for many years, still contains fuel and probably functional instruments. Of course, ISEE-3’s reactivation will be handled remotely, but in the 1960s this would have made a great scenario for a short story in one of the science fiction magazines. In that era, ideas like in-space repair of satellites and salvage and re-use of older equipment by human crews were concepts made fresh by the sudden progress of the manned space program. After all, we were doing space walks!


I’m remembering “The Trouble with Telstar,” a 1963 story by John Berryman (the SF writer, not the poet) that brought home to readers what would be involved in maintaining a space infrastructure. In the editorial squib introducing it, John Campbell wrote: “The real trouble with communications satellites is the enormous difficulty of repairing even the simplest little trouble. You need such a loooong screwdriver.” It was a lesson we’d learn again in spades with the Hubble repairs. Berryman, a writer and engineer who died in 1988, followed up with “Stuck,” another tale of space repair that inspired the gorgeous John Schoenherr cover at the right.

Fortunately, the reactivation of ISEE-3 isn’t a hands-on repair job and we can attempt to salvage this bird from Earth. Current thinking is to insert the spacecraft into an orbit at the L1 Lagrangian point, at which time the probe would be put back into operations. In this sense, ISEE-3 is an interesting measure of our ability to build long-term hardware. Like Voyager, the diminutive spacecraft was never intended for activities over this kind of time-frame, but new operations do appear possible. Everything depends, of course, upon the satellite’s close approach this summer, for if communications cannot be established, it will simply continue its orbit of the Sun.

So we have a “citizen science” program hard at work on a novel problem, with the help of the agency that put the spacecraft into motion all those years ago. Any new data from a re-born ISEE-3 is to be broadly shared within the science community and the public, offering a useful educational tool showing how we gather data in space and disseminate the results. We’ll also learn a good deal about how spacecraft endure the space environment over a span of decades, information that will contribute to our thinking about future probes on long missions and potentially extendable observation windows.

Not bad for a satellite sent out over three decades ago to study how the solar wind can affect satellites in Earth orbit and possibly disrupt our sensitive technological infrastructure. I’m now wondering whether there are other spacecraft out there that might be brought back to life, and reminded that when we build things to last, we can discover uses that the original designers may not have dreamed of. That’s a lesson we’ll want to remember as we create mission concepts around any new space hardware.


Exomoons: A New Technique for Detection

A friend asked me the other day whether my interest in exomoons — moons around exoplanets — wasn’t just a fascination with the technology of planet hunting. After all, we’ve finally gotten to the point where we can detect and confirm planets around other stars. An exomoon represents the next step at pushing our methods, and a detection would be an affirmation of just how far new technology and ingenious analysis can take us. So was there really any scientific value in finding exomoons, or was the hunt little more than an exercise in refining our tools?

I’ve written about technology for a long time, but the case for exomoons goes well beyond what my friend describes. We’ve found not just gas giants but ‘super-Earths’ in the habitable zones of other stars, and it’s a natural suggestion that around one or both classes of planet, an exomoon might he habitable even if the parent world were not. It’s a natural assumption that moons exist around other planets elsewhere as readily as they do around the planets of our own Solar System, and given the sheer number of planets out there, the prospect of adding potential sites for life — perhaps more common sites than habitable planets themselves — is irresistible.

Then in catching up with work after my recent trip, I noticed René Heller’s work on the exomoon question, as reported in Astrobiology Magazine. Heller (McMaster University, Ontario) is proposing a new method of detection that involves the particular eclipsing effect of moons during the kind of transit studies that Kepler has provided so much data on. What is striking here is that if Heller is right, we may be able to detect not just moons several times larger than Ganymede — this is where the current state of the art seems to be — but moons much smaller still, on the scale of the moons we find in our own Solar System.

Heller’s paper, recently published in The Astrophysical Journal, picks up on further scientific advantages in studying exomoons. Such objects can offer us insights into how exoplanets formed. Consider this (internal references omitted for brevity):

The satellite systems around Jupiter and Saturn, for example, show different architectures with Jupiter hosting four massive moons and Saturn hosting only one. Intriguingly, the total mass of these major satellites is about 10-4 times their planet’s mass, which can be explained by their common formation in the circumplanetary gas and debris disk…, and by Jupiter opening up a gap in the heliocentric disk during its own formation… The formation of Earth is inextricably linked with the formation of the Moon…, and Uranus’ natural satellites indicate a successive ‘collisional tilting scenario’, thereby explaining the planet’s unusual spin-orbit misalignment.

And so on. Between astrobiology and planet formation, I’d say there is plenty of reason to be interested in exomoons, though my friend is right that I still get jazzed by the ability of skilled researchers to develop the strategies we need for their detection. We’ve followed the fortunes of the Hunt for Exomoons with Kepler (HEK) for some time in these pages, and it’s fascinating to me that what Heller is proposing would also make use of the abundant Kepler data. But while HEK works through the study of variations in transit timings and durations, Heller’s method, which he calls the ‘Orbital Sampling Effect,’ relies on a different kind of analysis.

Addendum: My mistake. A note from exomoon hunter David Kipping, who heads up the Hunt for Exomoons with Kepler, sets this straight. Let me quote it:

“This is not true, actually HEK uses dynamics and photometric effects in combination. We model both the moon transit effects (which Heller is talking about) plus the dynamical perturbations, such as transit timing and duration variations, simultaneously. The difference is that Heller stacks all of the transit data on top of itself and we do not. An important point to bear in mind is that this stacking process does not give you any more signal-to-noise than using the entire time series and fitting it globally, as HEK does. Since one does not actually gain any greater sensitivity by using the OSE method, one cannot finds moons smaller than the sensitivity limits we’ve achieved thus far in HEK and I can tell you that Galilean satellites are only detectable in very exceptional cases- generally Earth-like radius sensitivity is the norm.

“Let me say though, the OSE method is fast and computationally cheap but lacks the dynamics of our models, which ultimately allow for an exomoon confirmation. So I think this method could be useful tool for quickly scanning the Kepler data to identify interesting anomalies worthy of further analysis.”

And back to the original post:

What more can we tease out of an exoplanetary transit? Observation of a moon orbiting an exoplanet around its equatorial midline and passing in front of the planet, then behind it, should — over the course of time and numerous observations — build up a series of dots representing its position at any given moment in its orbit. With enough observations, the effect is of two ‘wings’ coming out of the sides of the planet, an effect that will appear lighter at the inner edges and darker at the outer edges. Astrobiology Magazine explains:

That’s because when the moon reaches the extent of its orbit and then starts circling back around the planet, its positions overlap more in a tighter space. As such, the “wingtips” look darker; that is, there is increased eclipsing of background starlight at the moon’s farthest apparent positions from the planet.

And of course to spot this effect requires constant observation of the star over a long period of time, allowing the moon to complete a large number of orbits “in order for its light-blocking effect to preferentially stack up at the wingtips.” And voila, what we have with the Kepler spacecraft is an observatory that gave us precisely this, observations of about 150,000 stars through its long stare of data gathering, some four years before equipment failure laid it low. Heller’s point is that we need wait for no future technology to hunt today for moons like those in our Solar System.


Image: This figure from Heller’s paper depicts the effects of a transiting exoplanet with exomoon averaged over time. We would expect a drop in starlight as the exoplanet moves in front of the host star, but Heller’s method focuses on the change to the lightcurve caused by the exomoon before and after it crosses the stellar disk, an effect which repeats on the other side. Credit: René Heller.

The ‘stacking up’ effect of these transit observations over time as the moon and planet transit the star in different configurations allows the detection. Because we need plentiful statistical data to make all this work, red dwarf stars are ideal candidates because habitable zone planets there have extremely short years and thus make many transits. According to Heller, moons down to Ganymede size should be detectable around M-dwarfs, while around warmer orange dwarf stars, exomoons about ten times Ganymede’s mass would be within range. G-class stars like the Sun are not represented well enough in the Kepler data because they lack sufficient transits.

The minute effects discernible in an exoplanet’s regular transit are what make the exomoon detection possible. Heller notes that the Orbital Sampling Effect (OSE) yields data indicative of the moons’ radii and planetary distances, while study of the planet’s transit timing variations (TTV) and transit duration variations (TDV), in conjunction with Orbital Sampling Effect, allow measurements of the moon’s mass. More complex transit signatures could, using this method, even allow the detection of multiple exomoon candidates.

The paper is Heller, “Detecting extrasolar moons akin to solar system satellites with an orbital sampling effect,” The Astrophysical Journal, Vol. 787, No. 1 (2014), abstract and preprint available. Thanks to Dave Moore for the pointer to this work.


A New Marker for Planet Formation

Given how many planet-hosting stars we’re finding, any markers that can tell us which are most likely to have terrestrial worlds would be welcome. New work out of Vanderbilt University is now providing us with an interesting possibility. Working with the university’s Keivan Stassun, graduate student Trey Mack has developed a model that studies the chemical composition of a given star and relates it to the amount of rocky material it has ingested during the course of its lifetime. Stars with a high amount of such material may be places where small, terrestrial worlds are rare.

What Mack has done is to look at the relative abundance of fifteen specific elements. According to this Vanderbilt news release, he was most interested in elements with high condensation temperatures like aluminum, silicon, calcium and iron, the kind of materials that become building blocks for planets like the Earth. In this context, it’s important to remember that stars are 98 percent hydrogen and helium, with all elements heavier than these being referred to as ‘metals.’

Bear in mind how stellar metallicity may be affected during planet formation, as discussed in the paper:

There are at least two planet formation processes that may alter stellar surface abundances: (1) the accretion of hydrogen-depleted rocky material (Gonzalez, 1997), which would result in the enrichment of the stellar atmosphere, and (2) H-depleted rocky material in terrestrial planets may be withheld from the star during their formation, which would result in the depletion of heavy elements relative to H in the stellar atmosphere (Melendez et al. 2009).

All of this can lead to scenarios involving planetary migration:

For the enrichment scenario, Schuler et al. (2011a) suggest that stars with close-in giant planets (?0.05 AU) may be more enriched with elements of high condensation temperature (TC). This is thought to be a result of giant planets which form in the outer planetary system migrating inward to their present close-in positions. As they migrate, they can push rocky material into the host star (e.g., Ida & Lin 2008; Raymond et al. 2011). For the depletion scenario, Melendez et al. (2009) and Ram?rez et al. (2009) propose that the depletion of refractory elements in Sun-like stars may correlate with the presence of terrestrial planets.

For the Vanderbilt work, the wide binary pair HD 20782/81 proved a useful study, chosen by Mack because both stars have planets and both evidently condensed out of the same cloud of dust and gas, thereby beginning their lives with the same chemical compositions. To my knowledge, this is the only known wide binary in which both stars host detected planets. The two stars are G-class dwarfs like the Sun, one being orbited by two Neptune-class planets, the other by a single Jupiter-size world in a highly eccentric orbit. The spectra of the stars indicates that both show an abundance of refractory materials significantly higher than the Sun.

The abundances of these metals is high enough, in fact, to indicate that each of the two stars would have had to consume an amount of rocky material equal to 10-20 Earth masses to produce the observed spectra. On that score it is significant that both stars host giant planets on eccentric orbits closing as tightly as 0.2 AU. Mack summarizes the finding:

“Imagine that the star originally formed rocky planets like Earth. Further, imagine that it also formed gas giant planets like Jupiter. The rocky planets form in the region close to the star where it is hot and the gas giants form in the outer part of the planetary system where it is cold. However, once the gas giants are fully formed, they begin to migrate inward and, as they do, their gravity begins to pull and tug on the inner rocky planets. With the right amount of pulling and tugging, a gas giant can easily force a rocky planet to plunge into the star. If enough rocky planets fall into the star, they will stamp it with a particular chemical signature that we can detect.”


Image: What if we could determine if a given star is likely to host a planetary system like our own by breaking down its light into a single high-resolution spectrum and analyzing it? A spectrum taken of the Sun is shown above. The dark bands result from specific chemical elements in the star’s outer layer, like hydrogen or iron, absorbing specific frequencies of light. By carefully measuring the width of each dark band, astronomers can determine just how much hydrogen, iron, calcium and other elements are present in a distant star. The new model suggests that a G-class star with levels of refractory elements like aluminum, silicon and iron significantly higher than those in the Sun may not have any Earth-like planets because it has swallowed them. Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF.

With this in mind, the possibility that either of the binary twins hosts terrestrial planets is sharply reduced. The star orbited by two Neptune-class planets seems to have ingested more rocky material than its twin, with Mack and Stassun speculating that the two planets proved to be more efficient orbital disruptors for any terrestrial worlds that may have once been there. Even so, the other star, orbited by a Jupiter-class world, evidently pushed a large amount of rocky material into its star as well. The chemical composition is telling us that G-class stars with such high metallicities probably lack the kind of inner rocky planets many astronomers are searching for.

If these signatures are born out by subsequent study, we may have found a way to quickly determine whether a given G-class star is likely to have terrestrial planets, simply by analyzing its chemical composition. Several previous studies have linked star metallicity with planet formation, with one concluding that gas giants are found around high-metallicity stars, while terrestrial planets can be found around stars with a wide range of metal content. This work extends the use of metals as a marker in interesting new directions, playing off the link between a G-class star’s chemical composition and the kind of solar system it is likely to produce.

The paper is Mack et al., “Detailed Abundances of Planet-Hosting Wide Binaries. I. Did Planet Formation Imprint Chemical Signatures in the Atmospheres of HD 20782/81?” The Astrophysical Journal Vol. 787, No. 2 (2014), p. 98 (abstract / preprint).


GU Piscium b: Tuning Up our Imaging

How do you go about characterizing a directly imaged planet around another star? In the absence of a transit, one way is to apply theoretical models of planetary formation and evolution to the light spectrum you’re working with. When a team of researchers led by Marie-Ève Naud (a graduate student at the Université de Montréal) used these methods on direct imaging data from four different observatories to characterize a planet 155 light years from the Earth, they arrived at a temperature of some 800 degrees Celsius. The work drew inferences based upon the location of the newly detected world. For the planet, GU Piscium b, orbits a star that is a member of the AB Doradus moving group, some 30 stars that move together with the star AB Doradus.

The AB Doradus association is helpful because a moving group is made up of stars of roughly the same age and metallicity, a sign they probably formed in the same location. The fact that these are young stars, perhaps 100 million years old, also helped the team pull together an estimate of the planet’s mass, some 9 to 13 times that of Jupiter. The striking thing about GU Piscium b is that it orbits at about 2000 AU, the farthest exoplanet from its host that I am aware of. A single trip around its star would take GU Piscium b some 80,000 years. That distance makes infrared detection possible, particularly since young planets like this are still cooling and are therefore brighter.


Image: The planet GU Piscium b and its star GU Piscium. This image is composed of visible and infrared images from the Gemini South telescope and an infrared image from the CFHT. Because infrared light is invisible to the naked eye, astronomers use a color code in which infrared light is represented by the color red. GU Piscium b is brighter in infrared than in other filters, which is why it appears red in this image. Credit: Gemini Observatory/OMM/CFHT/W.M. Keck Observatory.

René Doyon, director of the Observatoire Mont-Mégantic (OMM) some 200 kilometers east of Montreal, calls GU Piscium b “a true gift of nature,” noting that its large distance from the star means a number of instruments can study it in depth, including smaller telescopes like the one at OMM. In this case, the work also involved the Gemini Observatory, the Canada-France-Hawaii Telescope (CFHT) and the W.M. Keck Observatory, whose combined resources produced images at a variety of wavelengths. Having such a clearly defined object for direct imaging studies should be helpful as we work on imaging planets closer to their stars.

On that score, a nod to the debut of the Gemini Planet Imager (GPI), whose first image produced the high-quality result shown below. This is Beta Pictoris b, some 63 light years from Earth, seen through a single 60-second exposure. The GPI’s advanced optics can detect planets down to Jupiter mass orbiting stars similar to the Sun, whereas the previous generation of adaptive optics could image planets no smaller than three times Jupiter’s mass, and only planets as far from their star as Saturn or Neptune is from the Sun.


Image: The bright white dot is the planet Beta Pictoris b, glowing in the infrared light from the heat released when it was formed 10 million years ago. The bright star Beta Pictoris is hidden behind a mask at the center of the image. (Image processing by Christian Marois, NRC Canada).

So we’ve had only a few planets directly imaged, but we’re making progress. Enough so that Bruce Macintosh, principal investigator on the GPI project, predicts in the team’s paper on this work that the planet may transit the host star in 2017, based on information gleaned from the GPI dataset, which yielded clues to the planet’s orbit. A transit of a directly imaged planet would open up in-depth investigation of the planet’s characteristics — remember that Kepler, although it has found thousands of transiting worlds, cannot produce direct imagery of any of them.

And no, not even the Gemini Planet Imager, in operation since November at its site in Chile, will let us see planets as small as the Earth. But Macintosh sees a continued and positive evolution in our capabilities. “…[I]t’s a step on the road,” he added. “Some day a future space telescope will use the same technology, and be able to see an Earth around one of these nearby stars.”

The Beta Pictoris b paper is Macintosh et al., “First light of the Gemini Planet Imager,” published online by Proceedings of the National Academy of Sciences (abstract). The GU Piscium b paper is Naud et al., “Discovery of a Wide Planetary-Mass Companion to the Young M3 Star GU Psc,” The Astrophysical Journal Vol. 787, No. 1 (2014). Abstract and preprint available.


Time Out

Over the past months, enough projects have piled up in need of attention that I finally have to decide to get serious about them. That means a short break here. No Centauri Dreams posts this week, therefore, with publication resuming next week on Monday or Tuesday. While I’m putting various things — some space-related, some not — in order, I’ll try to keep up with comment moderation, though it may get sporadic for a time. Meanwhile, do keep plugging into Heath Rezabek’s book survey as we try to isolate not only what books from my shortlist are the most useful, but also search for books you think should be on the list. Please add any titles you think worthwhile in the space provided on the survey form. I look forward to watching this survey grow, and to Heath’s reflections on it once it has grown to sufficient size. See you in a week.