Category Archives: Planetary and space science

The 2013–22 decadal survey in solar and space physics

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Last Wednesday the National Academy of Sciences held a press conference in Washington, DC, to introduce its newly completed report on priorities for the coming decade in solar and space physics. Daniel Baker of the University of Colorado chaired the committee that wrote the report. Thomas Zurbuchen of the University of Michigan was the vice chair. Together, they summarized the report’s highlights for the assembled reporters, scientists, and bureaucrats.

Like its counterparts in astronomy and planetary science, the latest solar and space physics decadal survey is more than just a shopping list of missions and facilities. Its authors begin by defining their field in a broad and inspiring way:

We live on a planet whose orbit traverses the tenuous outer atmosphere of a variable magnetic star, the Sun. This stellar atmosphere is a rapidly flowing plasma—the solar wind—that envelops Earth as it rushes outward, creating a cavity in the galaxy that extends to some 140 astronomical units (AU). There, the inward pressure from the interstellar medium balances the outward pressure of the solar plasma forming the heliopause, the boundary of our home in the universe. Earth and the other planets of our solar system are embedded deep in this extended stellar atmosphere or “heliosphere,” the domain of solar and space physics.

The report goes on to review past and present accomplishments in solar and space physics before defining the four overarching goals that guided the committee members as they drew up their final recommendations:

  1. Determine the origins of the Sun’s activity and predict the variations in the space environment.
  2. Determine the dynamics and coupling of Earth’s magnetosphere, ionosphere, and atmosphere and their response to solar and terrestrial inputs.
  3. Determine the interaction of the Sun with the solar system and the interstellar medium.
  4. Discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe.

As I listened to Baker and Zurbuchen’s presentation, it became clear that two other overarching considerations informed the report. The first is a conceptual emphasis on viewing Earth’s aurorae, the solar wind, coronal mass ejections, and other heliospheric phenomena as part of a single system. It will be interesting to see whether this systemic view becomes manifest in journals, conferences, and courses. I, for one, have tended to think of solar physics as belonging more to astronomy than to heliospheric physics.

The second consideration is a realistic and—to use Baker’s word—responsible approach to costs. The committee retained Aerospace Corp, a nonprofit consultancy based in El Segundo, California, to carry out an independent cost appraisal and technical evaluation (CATE) of potential missions. For the most part, the total cost of the committee’s recommended suite of programs lies within the budget envelope that NASA provided the committee for the years 2013–22.

Physicists who remember chuckling when they first encountered the zeroth law of thermodynamics might be amused to learn that the committee’s first recommendation is also numbered zero—for good reason. As NASA and NSF, the other principal sponsor of heliospheric research, look to future missions and facilities, the committee recommends that they first complete their current program.

Among the lineup is Solar Probe Plus (shown here in an artist’s impression). The ambitious mission, whose price tag is $1.4 billion, aims to fly as close as possible to the Sun to determine how the solar corona is heated and how the solar wind is accelerated.

Diversify, realize, integrate, venture, educate

The committee’s second recommendation, numbered 1.0, is to implement an initiative that goes by the acronym DRIVE (for “diversify, realize, integrate, venture, educate”). As far as I can tell, DRIVE aims to reorganize and reinvigorate the way researchers and their students practice heliospheric science.

Surprisingly, given its high priority, DRIVE is not expensive. The committee projects that the initiative will cost at most about $50 million a year. To fulfill the goals embodied by its name, DRIVE seeks to make research opportunities more accessible to universities through small and mid-sized missions, including the shoebox-sized spacecraft called CubeSats.

Funding the analysis and interpretation of data adequately is a key element of DRIVE, as is fostering interdisciplinary approaches to heliospheric research. Indeed, the committee urges NASA and NSF to establish heliospheric science centers, where observers, theorists, and modelers can work together to solve the grand challenges of solar and space physics.

When Baker and Zurbuchen introduced DRIVE, it sounded somewhat woolly to me. Now, having read the DRIVE section of the report, I think it’s a bold and worthwhile model that could be profitably emulated in other fields, such as green energy or neuroscience. But to be effective, DRIVE will probably need a light administrative structure.

Accelerate and expand the Heliophysics Explorer program!

Recommendation 2.0 seeks to revitalize NASA’s Explorer program of modestly sized and priced spacecraft. Begun in 1958, the program, according to the committee, is “arguably the most storied scientific spaceflight program in NASA’s history.” Despite its success, which includes three Nobel prizes, funding for the Explorer program fell in 2004 and has languished since. To quote the report:

The medium-class (MIDEX) and small-class (SMEX) missions of the Explorer program are ideally suited to advancing heliophysics science and have a superb track record for cost-effectiveness. Since 2001, 15 heliophysics Explorer mission proposals have received the highest category of ranking in competition selection reviews, but only 5 have been selected for flight. Thus there is an extensive reservoir of excellent heliophysics science to be accomplished by Explorers.

Because MIDEX and SMEX missions are comparatively cheap, developing and launching more of them would not require a big outlay. The committee recommends that NASA augment the current Explorer program for solar and space physics by $70 million per year.

In addition to more money for the Explorer program, the committee also recommends establishing a faster, more nimble way of accommodating missions of opportunity—that is, missions that are conceived in response to new technologies, new scientific knowledge, or new partnership opportunities with other space agencies.

NASA: Let academia lead space science

Perhaps by coincidence, a commentary by Baker appeared in Nature two weeks before his committee released its report. Entitled “NASA: Let academia lead space science,” the commentary urged the space agency to fund more missions that are small enough in scope that university-based principal investigators (PIs) can develop and lead them.

Whether Baker’s fellow committee members endorsed his commentary is not clear. They do, however, evidently share his belief in the merits of PI-led missions. Recommendation 3.0 calls for NASA to transform its Solar Terrestrial Probes program from a large, centrally directed program to “a moderate-sized, competed, PI-led mission line that is cost-capped at approximately $520-million per mission.”

The STP program aims to elucidate the physics of the Sun’s influence on Earth, on the other bodies in the solar system, and on the interstellar medium. To avoid the risk that a competitive free-for-all would omit important aspects of STP science, the committee outlined three kinds of missions that it would like to see fly:

  1. IMAP (Interstellar Mapping and Acceleration Probe) to characterize the zone where the Sun’s magnetohydrodynamic influence ceases to prevail in the solar neighborhood.
  2. DYNAMIC (Dynamical Neutral Atmosphere) to study how Earth’s ionosphere and thermosphere influence, and are influenced by, processes that occur at lower and higher altitudes.
  3. MEDICI (Magnetosphere Energetics, Dynamics, and Ionospheric Coupling) to determine how the magnetosphere-ionosphere-thermosphere system responds to solar and magnetospheric forcing.

The committee’s enthusiasm for modest missions is not unbridled, however. In the committee’s view, tackling the problem of how and why the Sun varies is a job for large, integrated missions. NASA’s Living with a Star program already includes the Solar Probe Plus and the Radiation Belt Storm Probes missions. Recommendation 4.0 is for Geospace Dynamics Constellation, a set of six formation-flying spacecraft that will characterize how the energy of geomagnetic storms is deposited and transformed in Earth’s atmosphere.

Recharter the National Space Weather Program

In March 1989 a geomagnetic storm caused the collapse of Hydro-Québec’s electricity grid. Five months later another geomagnetic storm shut down electronic trading on Toronto’s stock exchange.

Anticipating such storms—or space weather—and predicting their effects is more important, now that the world’s electrical infrastructure has expanded, the number of Earth-orbiting satellites has increased, and telecommunications have become economically and socially more important.

The current solar cycle, the 24th since records began in 1755, is set to peak next year. To monitor the cycle’s activity, the US relies on a set of spacecraft, such as the Solar and Heliospheric Observatory, whose principal purpose is basic research and whose engineering lifetimes are coming to an end.

To avoid gaps in coverage, the committee recommends that NASA, the National Oceanic and Atmospheric Administration, and the Department of Defense should plan ahead and plan together. Of particular importance, the committee says, is maintaining a permanent monitoring capability at L1, the first Lagrange point of the Sun–Earth system. Lying between the two bodies 1.5 million km from Earth, L1 is an ideal vantage for tracking solar activity.

The US has a comprehensive plan, the National Space Weather Program, for dealing with space weather. The trouble is, as the committee puts it, “implementation of such a program would require funding well above what the survey committee assumes to be currently available.” Accordingly, the committee recommends that the NSWP

should be rechartered under the auspices of the National Science and Technology Council and should include the active participation of the Office of Science and Technology Policy and the Office of Management and Budget. The plan should build on current agency efforts, leverage the new capabilities and knowledge that will arise from implementation of the programs recommended in this report, and develop additional capabilities, on the ground and in space, that are specifically tailored to space weather monitoring and prediction.

I haven’t read all 455 pages of the committee’s report. In venturing to summarize it, I have no doubt missed some important points and emphases. But what I have read has impressed me. Here is a plan to study the heliosphere as a system in a comprehensive, multidisciplinary, and cost-effective way. I hope its recommendations are heeded.

Five hundred small details

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Among aficionados of men’s fashion, Cary Grant is as revered for his meticulous style as for his acting. His most celebrated suit—the lightweight woolen one he wore throughout North by Northwest—was made at his request by his Savile Row tailor, Kilgour, French and Stanbury. “It takes five hundred small details to make one favorable impression,” he once said.

In Alfred Hitchcock’s 1959 thriller North by Northwest Cary Grant plays an advertising executive who becomes enmeshed in an espionage plot that takes him from Manhattan to Mount Rushmore.

NASA’s Curiosity rover certainly made a favorable impression on 6 August when it landed safely on Aeolis Palus in Gale Crater on Mars. The nuclear-powered, car-sized vehicle was built by three principal contractors, Boeing, Lockheed Martin, and MacDonald Dettwiler, but many other organizations—universities, national labs, and companies—contributed their expertise.

Siemens, the German engineering conglomerate, supplied software that engineers used to help design the rover and to manage the more than one terabyte of data that the mission generated even before a physical prototype was built. Siemens was so proud of its role that it took out a full-color, full-page ad in Wednesday’s Wall Street Journal in celebration.

Though far smaller than Siemens, Ocean Optics is just as proud of its contribution to Curiosity. The company, which is based in Dunedin, Florida, sent me a press release about its three compact, high-resolution spectrometers that form part of the rover’s ChemCam instrument. Developed by Los Alamos National Laboratory and France’s Centre d’Etude Spatiale des Rayonnements, ChemCam will fire its pulsed laser at Martian rocks to vaporize their surfaces. By analyzing the vapor, the Ocean Optics spectrometers will help determine the rocks’ chemical composition.

I’m not sure how many individuals have contributed to Curiosity. By Nature‘s count, the number of scientists—just scientists—is 400. They and their colleagues in other professions have pulled off a remarkable coup.

Among Cary Grant’s films, my favorites are three he made under the direction of Alfred Hitchcock: Suspicion (1941), Notorious (1946), and North by Northwest (1959). Like most of Hitchcock’s films, all three had modestly sized casts, but Grant did star in a full-blown, cast-of-thousands historical epic. Directed by Stanley Kramer and set amid Napoleon’s struggle to conquer Spain and Portugal, the 1957 film bears a title that could be justifiably applied to a documentary about the making of the Curiosity rover: The Pride and the Passion.

A game of stones

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This spring Jupiter and Venus shine close together in the evening sky. To naked-eye observers, the two planets appear as they did to our prehistoric ancestors—as bright stars that change slowly in position and barely in intensity.

The planets’ apparent inertness belies their energetic histories. Mercury, Venus, and the other rocky planets formed by accretion. Small dust grains coalesced to make bigger grains, then pebbles, stones, boulders—all the way up to objects the size of Mars. After crossing that threshold—about 1/10 of Earth’s mass—rocky protoplanets ran out of local material to consume. To grow further, Mars-sized protoplanets had to collide with each other and merge.

That scenario comes from theory and simulation, but evidence exists in our solar system for impacts on the scale needed to complete Earth’s formation. The rotation axes of Venus and Uranus have been knocked far out of alignment with those of the other planets. Mars is distinctly asymmetric: Its southern hemisphere features high mountains and ancient impact craters; its northern hemisphere has been scoured flat by massive lava flows that followed an impact. And given that lunar and terrestrial crusts have identical isotopic compositions, the Moon almost certainly formed when a Mars-sized object smashed into Earth.

Mercury, too, appears to have suffered a violent impact. As new results from NASA’s MESSENGER spacecraft have confirmed, the planet’s core occupies a staggering 61% of the total volume. By contrast, Earth’s inner and outer cores together amount to 16% of the total volume. According to prevailing theory, the way to make a core-heavy planet like Mercury is to start with a normally proportioned planet like Earth and smash it with a protoplanetary projectile that ablates most of the crust and mantle.

A song of ice and fire

In 2007 I interviewed Peter Grünberg after he and Albert Fert had been awarded the Nobel Prize in Physics for their independent discoveries of giant magnetoresistance. “When and why did you decided to become a physicist?” I asked him. “In high school,” he replied. “I was fascinated that the motions of the planets could be explained by simple physics.”

For young Grünberg, Newton’s rarefied laws were enough to spark his interest in physics, but other students might need something more awe-inspiring—something, perhaps, like the formation of the planets.

The plot begins with a vast swirling doughnut-shaped cloud of gas and dust surrounding the young Sun. Rocky planets begin to form in the hot zone close to the Sun; the gas giants start farther out, beyond the point at which sunlight is sill strong enough to melt water. As the formation process plays out, protoplanets compete for material, they smash into each other, they change orbits, and they suffer cratering bombardments from asteroids—a world of strife not unlike the one inhabited by Tyrion Lannister (shown here) and the other characters in HBO’s series Game of Thrones.

 

Eventually, far into the future, the Sun turns into a red giant and its puffed-out atmosphere engulfs and destroys all the planets. By then, humanity will have left the solar system and settled in a new one that future planetary scientists identified and selected from what I expect will be a catalog full of habitable worlds.

The continuing discovery of new exoplanets adds to our knowledge of planet formation, but because the new planets and suns are not like our own, they confound the quest to identify general principles, at least at first. Nevertheless, the challenging nature of planet formation should not deter us from evoking it to inspire the next generation of physicists.

Grünberg’s and Fert’s discovery led to a great leap forward in the capacity, compactness, and cost of data storage devices. Who knows what a student captivated by the solar system’s tumultuous history might go on to invent.

Bringing bits of an asteroid back to Earth

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Itokawa.jpg

The kidney-shaped asteroid 25143 Itokawa (shown below) has a length of 535 m and a mass of 3.5 × 1010 kg. Its eccentric 556-day orbit takes it just within Earth’s orbit and just beyond Mars’s orbit.

On 25 November 2005 the Japanese space probe Hayabusa touched down on Itokawa, scooped up a sample of the dusty surface, and headed back to Earth. On 13 June last year Hayabusa reentered Earth’s atmosphere. Before the probe burned up, it jettisoned its sample return capsule, which made a safe parachute landing at the Woomera Test Range in the South Australian outback.

Today, in a series of six papers that appear in Science, the Hayabusa science team reports what it learned from subjecting the Itokawa dust grains to a series of physical and chemical tests. The team’s main finding—that asteroids and meteorites are made of the same, ancient material—confirmed prevailing theory. What came as a surprise—at least to me—is how much the asteroid has changed, and continues to change, even as it sits in a seemingly passive orbit.

And of course, bringing back a sample from any solar system object represents a stunning technological feat—and for Japan’s space agency, JAXA, a triumphant milestone in the history of space exploration.

Asteroids and meteorites

Asteroids are rocky objects of various types and sizes that orbit the Sun in a wide belt that lies mostly between Mars and Jupiter. Meteorites are rocky objects of various types and sizes that fall to Earth. Both classes of object are thought to be made up of ancient material that did not end up in Mercury, Venus, Earth, or Mars.

By tracing meteorite trajectories, astronomers had already deduced that meteorites mostly likely come from the asteroid belt. Comparing asteroids’ remotely sensed spectra with meteorites’ chemical compositions had suggested that the two classes of space rock are closely related.

The Hayabusa team has proven directly that Itokawa, which belongs to one of the most common classes of asteroid, the stony S-class, has the same composition as the most common type of meteorite, the chondrite. Evidently, meteorites are asteroids or bits of asteroid that are knocked off their orbital perches and make their way to Earth.

But the team could do more than clinch the case for kinship. From the composition and structure of some of the grains, the researchers deduced that the grains had been heated in the past to 800 °C and had cooled to 600 °C at a rate of 0.5 °C per millennium. If, as seems likely, the heat source was internal and originated in the decay of aluminum-26, then the slow cooling rate implies that Itokawa was once 20 km in diameter—40 times its current size! Writing in one of the papers, Tomoki Nakamura of Tohoku University and his collaborators concluded that

the Itokawa parent S-class asteroid was originally much larger, experienced intense thermal metamorphism, and was then catastrophically disaggregated by one or more impacts into many small pieces, some of which re-accreted into the present greatly diminished, rubble-pile asteroid.

Further evidence of catastrophic impact comes from cracks in some of the grains. But, according to the team’s analysis of the grains’ three-dimensional structure, the impacts, presumably from meteorites, were not energetic enough to melt the grains.

Itokawa also shows evidence of another kind of bombardment: from particles in the solar wind and from cosmic rays. The solar wind contains helium, neon, argon, and other noble gases. When those atoms strike Itokawa, they become implanted at depths of about 1 μm. The more energetic cosmic rays not only penetrate more deeply, they also transmute sodium, potassium, and other elements into noble gases.

The two sources of implanted nobel gases—the solar wind and cosmic rays—have different isotopic compositions. By analyzing the noble gas content of a grain, it’s therefore possible to deduce where a given isotope originated. When the researchers detected in a micron-sized gathered from Itokawa’s surface an isotope created by a cosmic ray at an original depth of 1 m, they reached a remarkable conclusion. To quote from the paper written by Tohoku University’s Keisuke Nagao and his collaborators:

Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.

The Hayabusa team is made up of researchers from about 20 different institutions, mostly in Japan. But the lead authors of the six papers all come from Tohoku University in Sendai. Although Sendai is far enough inland to have escaped the 11 March tsunami, the accompanying earthquake damaged many buildings there. The prompt publication of the Hayabusa results, despite the earthquake and its continuing aftermath, represents another triumph.

Charles Day