Messier 87

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Messier 87

Messier 87 as seen by the Hubble Space Telescope
Observation data Epoch J2000
Constellation	Virgo
Right ascension	12h 30m 49.42338s
Declination	+12° 23′ 28.0439″
Apparent dimension (V)	7.2 × 6.8 moa
Apparent magnitude (V)	9.59
Characteristics
Type	E+0-1 pec, NLRG Sy
Astrometry
Heliocentric radial velocity	1307 ± 7 km/s
Redshift	0.004360 ± 0.000022
Galactocentric velocity	1254 ± 7 km/s
Distance	53.5 ± 1.63 Mly (16.40 ± 0.50 Mpc)
Other designations
Virgo A, Virgo X-1, NGC 4486, UGC 7654, PGC 41361, VCC 1316, Arp 152, 3C 274, 3U 1228+12.
Database references
SIMBAD	data

Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the local Universe, it is notable for its large population of globular clusters—about 12,000 compared to the 150–200 orbiting the Milky Way—and its jet of energetic plasma that originates at the core and extends at least 1,500 parsecs (4,900 light-years), traveling at relativistic speed. It is one of the brightest radio sources in the sky, and a popular target for both amateur and professional astronomers.

The French astronomer Charles Messier discovered M87 in 1781, and catalogued it as a nebulous feature while searching for objects that would otherwise confuse comet hunters. M87 is located about 16.4 million parsecs (53 million light-years) from Earth and is the second-brightest galaxy within the northern Virgo Cluster, having many satellite galaxies. Unlike a disk-shaped spiral galaxy, M87 has no distinctive dust lanes. Instead, it has an almost featureless, ellipsoidal shape typical of most giant elliptical galaxies, diminishing in luminosity with distance from the center. Forming around one sixth of its mass, M87's stars have a nearly spherically symmetric distribution. Their population density decreases with increasing distance from the core. It has an active supermassive black hole at its core, which forms the primary component of an active galactic nucleus.

The galaxy is a strong source of multiwavelength radiation, particularly radio waves. Its galactic envelope extends to a radius of about 150 kiloparsecs (490 thousand light-years), where it is truncated—possibly by an encounter with another galaxy. Its interstellar medium consists of diffuse gas enriched by elements emitted from evolved stars.

Observation history

In 1781, the French astronomer Charles Messier published a catalogue of 103 objects that had a nebulous appearance as part of a list intended to identify objects that might otherwise be confused with comets. In subsequent use, each catalogue entry was prefixed with an "M". Thus, M87 was the eighty-seventh object listed in Messier's catalogue. During the 1880s, the object was included as NGC 4486, in the New General Catalogue of nebulae and star clusters assembled by the Danish-Irish astronomer John Dreyer, which he based primarily on the observations of the English astronomer John Herschel.

In 1918, the American astronomer Heber Curtis of Lick Observatory noted M87's lack of a spiral structure and observed a "curious straight ray ... apparently connected with the nucleus by a thin line of matter." The ray appeared brightest near the galactic center. The following year, a supernova within M87 reached a peak photographic magnitude of 21.5, although this event was not reported until photographic plates were examined by the Russian astronomer Innokentii A. Balanowski in 1922.

Identification as galaxy

In Hubble's galaxy classification scheme, M87 is an E0 galaxy

 

In 1922, the American astronomer Edwin Hubble categorized M87 as one of the brighter globular nebulae, as it lacked any spiral structure, but like spiral nebulae, appeared to belong to the family of non-galactic nebulae. In 1926 he produced a new categorization, distinguishing extragalactic from galactic nebulae, the former being independent star systems. M87 was classified as a type of elliptical extragalactic nebula with no apparent elongation (class E0).

In 1931, Hubble described M87 as a member of the Virgo Cluster, and gave a provisional estimate of 1.8 million parsecs (5.9 million light-years) from Earth. It was then the only known elliptical nebula for which individual stars could be resolved, although it was pointed out that globular clusters would be indistinguishable from individual stars at such distances. In his 1936 The Realm of the Nebulae, Hubble examines the terminology of the day; some astronomers labeled extragalactic nebulae as external galaxies on the basis that they were stellar systems at far distances from our own galaxy, while others preferred the conventional term extragalactic nebulae, as galaxy then was synonym for the Milky Way. M87 continued to be labelled as an extragalactic nebula at least until 1954.

Spiral flow of the black-hole-powered jet, imaged with the Hubble Space Telescope

Modern research

In 1947, a prominent radio source, Virgo A, was identified overlapping the location of M87. The source was confirmed to be M87 by 1953, and the linear relativistic jet emerging from the core of the galaxy was suggested as the cause. This jet extended from the core at a position angle of 260° to an angular distance of 20″ with an angular width of 2″. In 1969–70, a strong component of the radio emission was found to closely align with the optical source of the jet.

In 1966, the United States Naval Research Laboratory's Aerobee 150 rocket identified Virgo X-1, the first X-ray source in Virgo. The Aerobee rocket launched from White Sands Missile Range on 7 July 1967, yielded further evidence that the source of Virgo X-1 was the radio galaxy M87. Subsequent X-ray observations by the HEAO 1 and Einstein Observatory showed a complex source that included the active galactic nucleus of M87. However, there is little central concentration of the X-ray emission.

M87 has been an important testing ground for techniques that measure the masses of central supermassive black holes in galaxies. In 1978, stellar-dynamical modeling of the mass distribution in M87 gave evidence for a central mass of five billion solar masses. After the installation of the COSTAR corrective-optics module in the Hubble Space Telescope in 1993, the Hubble Faint Object Spectrograph (FOS) was used to measure the rotation velocity of the ionized gas disk at the center of M87, as an "early release observation" designed to test the scientific performance of the post-repair Hubble instruments. The FOS data indicated a central black hole mass of 2.4 billion solar masses, with 30% uncertainty.

Visibility

Area in constellation Virgo around M87

 

M87 is located near the high declination border of the Virgo constellation, next to the constellation of Coma Berenices. It lies along the line between the stars Epsilon Virginis and Denebola. At an apparent magnitude of 9.59, the galaxy can be observed using a small telescope with a 6 cm (2.4 in) aperture, extending across an angular area of 7.2 × 6.8 arcminutes with a bright, 45-arcsecond core. Viewing the jet is a challenge without the aid of photography. Before 1991, the Russian-American astronomer Otto Struve was the only person known to have seen the jet visually, using the 254 cm (100 in) Hooker telescope. In more recent years it has been observed in larger amateur telescopes under excellent conditions.

Properties

Huge halo around Messier 87

 

In the modified Hubble sequence galaxy morphological classification scheme of the French astronomer Gérard de Vaucouleurs, M87 is categorized as an E0p galaxy. "E0" designates an elliptical galaxy that displays no flattening—that is, it appears spherical. A "p" suffix indicates a peculiar galaxy that does not fit cleanly into the classification scheme; in this case, the peculiarity is the presence of the jet emerging from the core. In the Yerkes (Morgan) scheme, M87 is classified as a type-cD galaxy. A D galaxy has an elliptical-like nucleus surrounded by an extensive, dustless, diffuse envelope. A D type supergiant is called a cD galaxy.

The distance to M87 has been estimated using several independent techniques. These include measurement of the luminosity of planetary nebulae, comparison with nearby galaxies whose distance is estimated using standard candles such as cepheid variables, the linear size distribution of globular clusters, and the tip of the red-giant branch method using individually resolved red giant stars. These measurements are consistent with each other, and their weighted average yields a distance estimate of 16.4 ± 0.5 megaparsecs (53.5 ± 1.63 million light-years).

M87 is one of the most massive galaxies in the local Universe. It spans a diameter of 120 thousand light-years, which is slightly lower than that of the Milky Way, but M87 is a spheroid, not a flat spiral. The mass of M87 within a radius (r) of 9–40 kiloparsecs (29–130 thousand light-years) from the core steadily increases roughly in proportion to r^1.7. Within a radius of 32 kiloparsecs (100 thousand light-years), the mass is (2.4 ± 0.6) × 10¹² times the mass of the Sun, which is double the mass of the Milky Way galaxy. Only a fraction of this mass is in the form of stars. M87 has an estimated mass to luminosity ratio of 6.3 ± 0.8; that is, about one part in six of the galaxy's mass is in the form of stars that radiate energy. The total mass of M87 may be 200 times that of the Milky Way.

Stellar velocity map of the central region of M87, showing the motion of stars relative to Earth. Blue patches represent motion towards the Earth and red ones away from the Earth, while yellow and green are in between the two extremes. The map signifies random motion of the stars.

 

Gas is infalling into the galaxy at the rate of two to three solar masses per year, most of which may be accreted onto the core region. The extended stellar envelope of this galaxy reaches a radius of about 150 kiloparsecs (490 thousand light-years), compared to about 100 kiloparsecs (330 thousand light-years) for the Milky Way. Beyond that distance the outer edge of the galaxy has been truncated by some means; possibly by an earlier encounter with another galaxy. There is evidence of linear streams of stars to the northwest of the galaxy, which may have been created by tidal stripping of orbiting galaxies or by small satellite galaxies falling in toward M87. Moreover, a filament of hot, ionized gas in the northeastern outer part of the galaxy may be the remnant of a small, gas-rich galaxy that was disrupted by M87 and could be feeding its active nucleus. M87 is estimated to have at least 50 satellite galaxies, including NGC 4486B and NGC 4478.

The spectrum of the nuclear region of M87 shows the emission lines of various ions, including hydrogen (HI, HII), helium (HeI), oxygen (OI, OII, OIII), nitrogen (NI), magnesium (MgII) and sulfur (SII). The line intensities for weakly ionized atoms (such as neutral atomic oxygen, OI) are stronger than those of strongly ionized atoms (such as doubly ionized oxygen, OIII). A galactic nucleus with such spectral properties is termed a LINER, for "low-ionization nuclear emission-line region". The mechanism and source of weak-line-dominated ionization in LINERs and M87 are under debate. Possible causes include shock-induced excitation in the outer parts of the disk or photoionization in the inner region powered by the jet.

Elliptical galaxies such as M87 are believed to form as the result of one or more mergers of smaller galaxies. They generally contain relatively little cold interstellar gas (in comparison with spiral galaxies) and they are populated mostly by old stars, with little or no ongoing star formation. M87's elliptical shape is maintained by the random orbital motions of its constituent stars, in contrast to the more orderly rotational motions found in a spiral galaxy such as the Milky Way. Using the Very Large Telescope to study the motions of about 300 planetary nebulae, astronomers have determined that M87 absorbed a medium-sized star-forming spiral galaxy over the last billion years. This has resulted in the addition of some younger, bluer stars to M87. The distinctive spectral properties of the planetary nebulae allowed astronomers to discover a chevron-like structure in M87's halo which was produced by the incomplete phase-space mixing of a disrupted galaxy.

Components

Supermassive black hole

Artist's concept of a supermassive black hole and its accretion disk

 

The core contains a supermassive black hole that weighs billions of times the Sun's mass: estimates have ranged from (3.5 ± 0.8) × 10⁹ M_☉ to (6.6 ± 0.4) × 10⁹ M_☉, with the latest measurement being 7.22+0.34
−0.40×10⁹ M_☉. This mass is one of the highest known for such an object. A rotating disk of ionized gas surrounds the black hole, and is roughly perpendicular to the relativistic jet. The disk rotates at velocities of up to roughly 1,000 km/s, and spans a maximum diameter of 0.12 parsecs (0.39 light-years). Gas accretes onto the black hole at an estimated rate of one solar mass every ten years (about 90 earth masses per day).

Observation suggests that the black hole may be displaced from the galactic center by about seven parsecs (23 light-years). The displacement is in the opposite direction of the one-sided jet, which may indicate that the black hole was accelerated away by the jet. Another possibility is that the change in location occurred during the merger of two supermassive black holes. A 2011 study did not find any statistically significant displacement.

Interstellar medium

The space between the stars in M87 is filled with a diffuse interstellar medium of gas that has been chemically enriched by the elements ejected from stars as they passed beyond their main sequence lifetime. Carbon and nitrogen are continuously supplied by stars of intermediate mass as they pass through the asymptotic giant branch. The heavier elements from oxygen to iron are produced largely by supernova explosions within the galaxy. Of the heavy elements, about 60% were produced by core-collapse supernovae, while the remainder came from type Ia supernovae. The distribution of oxygen is roughly uniform throughout, at about half of the solar value (i.e., oxygen abundance in the Sun), while iron distribution peaks near the center where it approaches the solar iron value. Since oxygen is produced mainly by core-collapse supernovae, which occur during the early stages of galaxies and mostly in outer star-forming regions, the distribution of these elements suggests an early enrichment of the interstellar medium from core-collapse supernovae and a continuous contribution from Type Ia supernovae throughout the history of M87. The contribution of elements from these sources was much lower than in the Milky Way.

Examination of M87 at far infrared wavelengths shows an excess emission at wavelengths longer than 25 μm. Normally, this may be an indication of thermal emission by warm dust. In the case of M87, the emission can be fully explained by synchrotron radiation from the jet; within the galaxy, silicate grains are expected to survive for no more than 46 million years because of the X-ray emission from the core. This dust may be destroyed by the hostile environment or expelled from the galaxy. The combined mass of dust in M87 is no more than 70,000 times the mass of the Sun. By comparison, the Milky Way's dust equals about a hundred million (10⁸) solar masses.

Although M87 is an elliptical galaxy and therefore lacks the dust lanes of a spiral galaxy, optical filaments have been observed in it, which arise from gas falling towards the core. Emission probably comes from shock-induced excitation as the falling gas streams encounter X-rays from the core region. These filaments have an estimated mass of about 10,000 solar masses. Surrounding the galaxy is an extended corona with hot, low-density gas.

Globular clusters

M87 has an abnormally large population of globular clusters. A 2006 survey out to an angular distance of 25′ from the core estimates that there are 12,000 ± 800 globular clusters in orbit around M87, compared to 150–200 in and around the Milky Way. The clusters are similar in size distribution to those of the Milky Way, most having an effective radius of 1 to 6 parsecs. The size of the M87 clusters gradually increases with distance from the galactic center. Within a four-kiloparsec (13,000-light-year) radius of the core, the cluster metallicity—the abundance of elements other than hydrogen and helium—is about half the abundance in the Sun. Outside this radius, metallicity steadily declines as the cluster distance from the core increases. Clusters with low metallicity are somewhat larger than metal-rich clusters. In 2014, HVGC-1, the first hypervelocity globular cluster, was discovered escaping from M87 at 2,300 km/s. The escape of the cluster with such a high velocity may have been the result of a close encounter with, and subsequent gravitational kick from, a supermassive black hole binary. If this interpretation is correct, the core of M87 would have two supermassive black holes, the result of an ancient collision between two galaxies which merged into a single giant galaxy.

Almost a hundred ultra-compact dwarfs have been identified in M87. They resemble globular clusters but have a diameter of ten parsecs (33 light-years) or more, much larger than the three-parsec (9.8-light-year) maximum of globular clusters. It is unclear whether they are dwarf galaxies captured by M87 or a new class of massive globular cluster.

Jet

This Hubble Space Telescope photograph shows the jet of matter ejected from M87 at nearly the speed of light, as it stretches 1.5 kpc (5 kly) from the galactic core

 

In this X-ray (Chandra) and radio (VLA) composite image, hot matter (blue in X-ray) from the Virgo cluster falls toward the core of M87 and cools, where it is met by the relativistic jet (orange in radio), producing shock waves in the galaxy's interstellar medium

 

The relativistic jet of matter emerging from the core extends at least 1.5 kiloparsecs (5,000 light-years) from the nucleus and consists of matter ejected from a supermassive black hole. The jet is highly collimated, appearing constrained to an angle of 60° within 0.8 parsecs (2.6 light-years) of the core, to about 16° at two parsecs (6.5 light-years), and to 6–7° at twelve parsecs (39 light-years). Its base has the diameter of 5.5 ± 0.4 Schwarzschild radii, and is probably powered by a prograde accretion disk around a spinning supermassive black hole. The German-American astronomer Walter Baade found that light from the jet was plane polarized, which suggests that the energy is generated by the acceleration of electrons moving at relativistic velocities in a magnetic field. The total energy of these electrons is estimated at 5.1 × 10⁵⁶ ergs (5.1 × 10⁴⁹ joules or 3.2 × 10⁶⁸ eV). This is roughly 10¹³ times the energy produced by the Milky Way in one second, which is estimated at 5 × 10³⁶ joules. The jet is surrounded by a lower-velocity non-relativistic component. There is evidence of a counter jet, but it remains unseen from the Earth due to relativistic beaming. The jet is precessing, causing the outflow to form a helical pattern out to 1.6 parsecs (5.2 light-years). Lobes of expelled matter extend out to 80 kiloparsecs (260 thousand light-years).

In pictures taken by the Hubble Space Telescope in 1999, the motion of M87's jet was measured at four to six times the speed of light. This phenomenon, called superluminal motion, is an illusion caused by the relativistic velocity of the jet. The time interval between any two light pulses emitted by the jet is, as registered by the observer, less than the actual interval due to the relativistic speed of the jet moving in the direction of the observer. This results in perceived faster-than-light speeds. Detection of such motion is used to support the theory that quasars, BL Lacertae objects and radio galaxies may all be the same phenomenon, known as active galaxies, viewed from different perspectives. It is proposed that M87 is a BL Lacertae object (with a low-luminosity nucleus compared with the brightness of its host galaxy) seen from a relatively large angle. Flux variations, characteristic of the BL Lacertae objects, have been observed in M87.

Radio wavelength image of M87 showing strong radio emission from the core

 

Observations indicate that the rate at which material is ejected from the supermassive black hole is variable. These variations produce pressure waves in the hot gas surrounding M87. The Chandra X-ray Observatory has detected loops and rings in the gas. Their distribution suggests that minor eruptions occur every few million years. One of the rings, caused by a major eruption, is a shock wave 26 kiloparsecs (85 thousand light-years) in diameter around the black hole. Other features observed include narrow X-ray-emitting filaments up to 31 kiloparsecs (100 thousand light-years) long, and a large cavity in the hot gas caused by a major eruption 70 million years ago. The regular eruptions prevent a huge reservoir of gas from cooling and forming stars, implying that M87's evolution may have been seriously affected, preventing it from becoming a large spiral galaxy. These observations also indicate that the variable eruptions produce sound waves of about 56 to 59 octaves below middle C in the medium.

M87 is a very strong source of gamma rays, the most energetic rays of the electromagnetic spectrum. Gamma rays emitted by M87 have been observed since the late 1990s. In 2006, using the High Energy Stereoscopic System Cherenkov telescopes, scientists measured the variations of the gamma ray flux coming from M87, and found that the flux changes over a matter of days. This short period indicates that the most likely source of the gamma rays is a supermassive black hole. In general, the smaller the diameter of the emission source, the faster the variation in flux, and vice versa.

A knot of matter in the jet (designated HST-1), about 65 parsecs (210 light-years) from the core, has been tracked by the Hubble Space Telescope and the Chandra X-ray Observatory. By 2006, the X-ray intensity of this knot had increased by a factor of 50 over a four-year period, while the X-ray emission has since been decaying in a variable manner.

The interaction of relativistic jets of plasma emanating from the core with the surrounding medium gives rise to radio lobes in active galaxies. The lobes occur in pairs and are often symmetrical. The two radio lobes of M87 together span about 80 kiloparsecs; the inner parts, extending up to two kiloparsecs, emit strongly at radio wavelengths. Two flows of material emerge from this region, one aligned with the jet itself and the other in the opposite direction. The flows are asymmetrical and deformed, implying that they encounter a dense intracluster medium. At greater distances, both flows diffuse into two lobes. The lobes are surrounded by a fainter halo of radio-emitting gas.

Environment

Messier 87 is a member of the Virgo Cluster, and can be seen to lower left of this cluster image.

 

M87 is near the center of the Virgo Cluster, a closely compacted structure of about 2,000 members. It forms the core of the larger Virgo Supercluster, of which the Local Group (including the Milky Way) is an outlying member. It is organized into at least three distinct subsystems associated with the three large galaxies—M87, M49 and M86—with the subgroup centered around M87 (Virgo A) and M49 (Virgo B). There is a preponderance of elliptical and S0 galaxies around M87, with a chain of elliptical galaxies aligned with the jet. In terms of mass, M87 is a dominant member of the cluster, and hence appears to be moving very little relative to the cluster as a whole. It is defined as the cluster center. The cluster has a sparse gaseous atmosphere that emits X-rays that decrease in temperature toward the middle, where M87 is located. The combined mass of the cluster is estimated to be (0.15–1.5) × 10¹⁵ solar masses.

Measurements of the motion of intracluster planetary nebulae between M87 and M86 suggest that the two galaxies are moving toward each other and that this may be their first encounter. M87 may have interacted with M84 in the past, as evidenced by the truncation of M87's outer halo by tidal interactions. The truncated halo may also have been caused by contraction due to an unseen mass falling into M87 from the rest of the cluster, which may be the hypothesized dark matter. A third possibility is that the halo's formation was truncated by early feedback from the active galactic nucleus at the core of M87.

Nuclear goes retro — with a much greener outlook

Returning to designs abandoned in the 1970s, start-ups are developing a new kind of reactor that promises to be much safer and cleaner than current ones.

By M. Mitchell Waldrop 02.22.2019

Original link:  https://www.knowablemagazine.org/article/technology/2019/nuclear-goes-retro-much-greener-outlook?fbclid=IwAR3nXEUQeXwAgjqO8DEDWjApmJBuDHrIfRt-2Ii4TfxflYj4niQpwZ-3uT0

Troels Schönfeldt can trace his path to becoming a nuclear energy entrepreneur back to 2009, when he and other young physicists at the Niels Bohr Institute in Copenhagen started getting together for an occasional “beer and nuclear” meetup.

The beer was an India pale ale that they brewed themselves in an old, junk-filled lab space in the institute’s basement. The “nuclear” part was usually a bull session about their options for fighting two of humanity’s biggest problems: global poverty and climate change. “If you want poor countries to become richer,” says Schönfeldt, “you need a cheap and abundant power source.” But if you want to avoid spewing out enough extra carbon dioxide to fry the planet, you need to provide that power without using coal and gas.

It seemed clear to Schönfeldt and the others that the standard alternatives simply wouldn’t be sufficient. Wind and solar power by themselves couldn’t offer nearly enough energy, not with billions of poor people trying to join the global middle class. Yet conventional nuclear reactors — which could meet the need, in principle — were massively expensive, potentially dangerous and anathema to much of the public. And if anyone needed a reminder of why, the catastrophic meltdown at Japan’s Fukushima Daiichi plant came along to provide it in March 2011.

On the other hand, says Schönfeldt, the worldwide nuclear engineering community was beginning to get fired up about unconventional reactor designs — technologies that had been sidelined 40 or 50 years before, but that might have a lot fewer problems than existing reactors. And the beer-and-nuclear group found that one such design, the molten salt reactor, had a simplicity, elegance and, well, weirdness that especially appealed.

Molten salt reactors might just turn nuclear power into the greenest energy source on the planet.

The weird bit was that word “molten,” says Schönfeldt: Every other reactor design in history had used fuel that’s solid, not liquid. This thing was basically a pot of hot nuclear soup. The recipe called for taking a mix of salts — compounds whose molecules are held together electrostatically, the way sodium and chloride ions are in table salt — and heating them up until they melted. This gave you a clear, hot liquid that was about the consistency of water. Then you stirred in a salt such as uranium tetrafluoride, which produced a lovely green tint, and let the uranium undergo nuclear fission right there in the melt — a reaction that would not only keep the salts nice and hot, but could power a city or two besides.

A compound combining various salts (including uranium tetrafluoride), shown as a solid at left and a liquid at right, is one example of a compound that could be used in a molten salt reactor. CREDIT: ORNL / US DOE 

Weird or not, molten salt technology was viable; the Oak Ridge National Laboratory in Tennessee had successfully operated a demonstration reactor back in the 1960s. And more to the point, the beer-and-nuclear group realized, the liquid nature of the fuel meant that they could potentially build molten salt reactors that were cheap enough for poor countries to buy; compact enough to deliver on a flatbed truck; green enough to burn our existing stockpiles of nuclear waste instead of generating more — and safe enough to put in cities and factories. That’s because Fukushima-style meltdowns would be physically impossible in a mix that’s molten already. Better still, these reactors would be proliferation resistant, because their hot, liquid contents would be very hard for rogue states or terrorists to hijack for making nuclear weapons.

Molten salt reactors might just turn nuclear power into the greenest energy source on the planet.

Crazy? “We had to try,” says Schönfeldt. So in 2014 he and his colleagues launched Seaborg Technologies, a Copenhagen-based start-up named in honor of the late Glenn Seaborg, a Manhattan Project veteran who helped pioneer the peaceful uses of nuclear energy. With Schönfeldt as chief executive officer, they set about turning their vision into an ultracompact molten salt reactor that could serve the developed and developing world alike.

“It will be exceedingly hard, but that is significantly better than impossible.” Troels Schönfeldt

They weren’t alone: Efforts to revive older nuclear designs had been bubbling up elsewhere, and dozens of start-ups were trying to commercialize them. At least half a dozen of these start-ups were focused on molten salt reactors specifically, since they were arguably the cleanest and safest of the lot. Research funding agencies around the world had begun to pour millions of dollars per year into developing molten salt technology. Even power companies were starting to make investments. A prime example was the Southern Company, a utility conglomerate headquartered in Atlanta, Georgia. In 2016, the company started an ambitious molten salt development program in collaboration with Oak Ridge and TerraPower, a nuclear research company in Bellevue, Washington.

“In the next 20 to 30 years, the energy environment is going to undergo a major transformation to a low- to no-carbon future,” says Nick Irvin, Southern’s director of research and development. There will be far fewer centralized power plants and many more distributed sources like wind and solar, he says. Molten salt reactors fit ideally into this future, he adds, because of both their inherent safety and their ability to consume spent nuclear fuel from traditional nuclear reactors.

A molten salt reactor differs from a conventional nuclear reactor in a number of ways, starting with the fact that it uses nuclear fuel that’s liquid instead of solid. This has profound implications for safety. For example, meltdowns would be a non-issue: The fuel is already molten. And if temperatures in the fuel mix get too high for any reason, a plug of frozen salt below the reactor will melt and allow everything to drain into an underground holding tank for safekeeping. Long-lived nuclear waste would also be a non-issue: A chemical system would continuously extract reaction-slowing fission products from the molten fuel, which would allow plutonium and all the other long-half-life fissile isotopes to be completely consumed.

Getting there won’t be easy — not least because hot molten salts can be just as corrosive as they sound. Every component that comes into contact with the brew will have to be made of a specialized, high-tech alloy that can resist them. “You dissolve the uranium in the salt,” says Nathan Myhrvold, a venture capitalist who serves as vice chairman of TerraPower’s board. “What you have to make sure is that you don’t dissolve your reactor in it!”

Certainly no one expects to have a prototype power plant operating before the mid-2020s, or to field full-scale commercial reactors until the 2030s. Still, says Schönfeldt, “it will be exceedingly hard, but that is significantly better than impossible.”

Rethinking nuclear

To Rachel Slaybaugh, today’s surge of entrepreneurial focus on nuclear technology is astonishing. “It feels like we’re at the beginning of a movement, with an explosion of ideas,” says Slaybaugh, a nuclear engineer at the University of California, Berkeley, who has written about green energy options in the Annual Review of Environment and Resources.

But, as nuclear engineer Leslie Dewan points out, this explosion is also something of a throwback to the post–World War II era. “Nuclear power technology was incredibly new,” says Dewan, who in 2011 cofounded one of the first of the molten salt start-ups, Transatomic Power in Cambridge, Massachusetts. It was a time of blue-sky thinking, she says, “where they were trying many, many different types of technologies, running experiments, building and prototyping.”

Atoms may be the smallest unit of matter, but they are made up of even smaller subunits, including protons, neutrons and electrons. The energy that binds the protons and neutrons together in the atom’s nucleus is enormous, and can be released and put to use during a nuclear reaction.

The basics had been known since 1938, when German scientists discovered that firing a neutron into certain heavy atomic nuclei would cause the nucleus to fission, or split into two pieces. The rupture of such a “fissile” nucleus would release an enormous amount of energy, plus at least two new neutrons. These neutrons could then slam into nearby nuclei and trigger the release of more energy, plus 4 neutrons — then 8, 16, 32 and so on in an exponentially growing chain reaction.

This runaway energy release could produce a very powerful bomb, as the wartime Manhattan Project demonstrated. But taming it, and turning the chain reaction into a safe, steady-state heat source for power production, was a lot trickier.

Fission is at the heart of nuclear energy production. Fission happens when a free neutron slams into an unstable atomic nucleus and shatters it into two or more “fission products”: lighter elements such as krypton and barium that cluster around the middle of the periodic table. Thanks to Einstein’s famous equation E = mc², this process also transforms a tiny bit of the original nucleus’s mass into an immense amount of energy.

That’s where all the postwar experimentation came in. There were reactors fueled by uranium, which comes out of the ground containing virtually the only fissile isotope found in nature, uranium-235. There were reactors known as breeders, which could accomplish the magical-sounding feat of producing more fuel than they consumed. (Actually, they relied on the fact that uranium is “fertile,” meaning that its most abundant isotope, uranium-238, almost never undergoes fission by itself — but it can absorb a neutron and turn into highly fissile plutonium-239.) And there were reactors fueled with thorium, a fertile element that sits two slots to the left of uranium in the periodic table, and is about three times more abundant in the Earth’s crust. A neutron will turn its dominant isotope, thorium-232, into fissile uranium-233.

An element, such as carbon or uranium, is defined by the number of protons in its nucleus: Carbon has 6, uranium 92. But atoms may vary in the number of neutrons, producing slightly different flavors, or isotopes, of an element. For example, the most common form of carbon is carbon-12, which has 6 protons and 6 neutrons, and is stable. But other isotopes include carbon-13, which is also stable, and carbon-14, which is unstable and radioactive. Uranium’s most common form is uranium-238, which has 92 protons and 146 neutrons, and decays only after billions of years. Other, less stable isotopes include uranium-235 (92 protons and 143 neutrons) and uranium-233 (92 protons and 141 neutrons), both of which can undergo nuclear fission if they are struck by a neutron.

At the same time, designers were trying out different types of coolant: the fluid that circulates through the reactor core, absorbs the heat being produced by the fission reactions, and carries it out to where the heat can do something useful like running a standard steam turbine to generate electricity. Some opted for ordinary water: an abundant, familiar substance that carries a lot of heat per unit of volume. But others went with high-temperature substances such as liquid sodium metal, helium gas or even molten lead. “Coolants” like these could keep a reactor running at 700 degrees Celsius or more, which would make it substantially more efficient at generating power.

By the 1960s, researchers had tested reactors featuring combinations of all these options and more. But the approach that won out for commercial power production — and that is still used in virtually all of the 454 nuclear plants operating around the world — was the water-cooled uranium reactor. This wasn’t necessarily the best nuclear design, but it was one of the first: Water-cooled reactors were originally developed in the 1940s to power submarines. So in the 1950s, when the Eisenhower administration launched a high-profile push to harness nuclear energy for peaceful purposes, the technology was adapted for civilian use and scaled up enormously. Other designs were left for later, if ever. By the 1960s and 1970s, second-generation water-cooled reactors were being deployed globally.

“It feels like we’re at the beginning of a movement.” Rachel Slaybaugh

Even then, however, there were many in the field who were uneasy with that choice. Among the most notable was nuclear physicist Alvin Weinberg, a Manhattan Project veteran and director of the Oak Ridge National Laboratory. Weinberg had participated in the development of water-cooled reactors, and knew that they had some key vulnerabilities — including water’s low boiling point, just 100°C at normal atmospheric pressure.

Commercial nuclear plants could get that up to 325°C or so by pressurizing the reactor vessel. But as Weinberg and others knew very well, that was not enough to rule out the nightmare of nightmares: a meltdown. All it would take was some freak accident that interrupted the flow of water through the core and trapped all the heat inside. You could shut down power production by dropping rods of boron or cadmium into the reactor core to soak up neutrons and stop the chain reaction. But nothing could stop the heat produced by the decay of fission products — the melange of short-lived but fiercely radioactive elements that inevitably build up inside an active reactor as nuclei split in two.

When the nuclei of certain “fissile” isotopes such as uranium-235 are struck by a neutron, they don’t just split apart. They also release at least two additional neutrons, which can then go on to split two more nuclei. This produces at least 4 additional neutrons – then 8, 16, 32, 64 and so on. The result is an exponentially growing chain reaction, which can produce a nuclear explosion if it’s allowed to run away — or useful nuclear power if it’s contained and controlled inside a reactor.

Unless the operators managed to restore the coolant flow within a few hours, that trapped fission-product heat would send temperatures soaring past the 325°C mark, turn the water into high-pressure steam, and reduce the solid fuel to a radioactive puddle melting its way through the reactor vessel floor. Soon after, the vessel would likely rupture and send a pressurized plume of fission products into the atmosphere. Included would be radioactive strontium-90, iodine-131 and caesium-137 — extremely dangerous isotopes that can easily enter the food chain and end up in the body.

To forestall such a catastrophe, designers had equipped the commercial water-cooled reactors with all manner of redundancies and emergency backup cooling systems. But to Weinberg’s mind, that was a bit like installing fire alarms and sprinkler systems in a house built of papier-mâché. What you really wanted was the nuclear equivalent of a house built of fireproof brick — a reactor that based its safety on the laws of physics, with no need for operators or backup systems to do anything.

Weinberg and his team at Oak Ridge believed that they could come very close to that ideal with the molten salt reactor, which they had been working on since 1954. Such a reactor couldn’t possibly suffer a meltdown, even in an accident: The molten salt core was liquid already. The fission-product heat would simply cause the salt mix to expand and move the fuel nuclei farther apart, which would dampen the chain reaction.

Pressure would be a non-issue as well: The salts would have a boiling point far higher than any temperature the fission products could produce. (One common choice for nuclear applications is FLiBe, a mix of lithium fluorides and beryllium fluorides that doesn’t boil until 1,400°C, about the temperature of a butane blowtorch.) So the reactor vessel would never be in danger of rupture from molten salt “steam.” In fact, the reactor would barely shift from its normal operating pressure of one atmosphere.

Better still, the molten core would trap fission products far more securely than in solid-fueled reactors. Cesium, iodine and all the rest would chemically bind with the salts the instant they were created. And since the salts could not boil away in even the worst accident, these fission products would be held in place instead of being free to drift off and take up radioactive residence in people’s bones and thyroid glands.

And just in case, the liquid nature of the fuel allowed for a simple fail-safe known as a freeze plug. This involved connecting the bottom of the reactor vessel to a drain pipe, which would be plugged with a lump of solid fuel salt kept frozen by a jet of cool gas. If the power failed and the gas flow stopped, or if the reactor got too hot, the plug would melt and gravity would drain the contents into an underground holding tank. The mix would then cool, solidify and remain in the tank until the crisis was over — salts, fuel, fission products and all.

Molten salt success, then a detour

Weinberg and his team successfully demonstrated all this in the Molten Salt Reactor Experiment, an 8-megawatt prototype that ran at Oak Ridge from 1965 to 1969. The corrosiveness of the salts was a potential threat to the long-term integrity of pipes, pumps and other parts, but the researchers had identified a number of corrosion-resistant materials they thought might solve the problem. By the early 1970s, the group was well into development of an even more ambitious prototype that would allow them to test those materials as well as to demonstrate the use of thorium fuel salts instead of uranium.

Hundreds of nuclear isotopes have been observed in the laboratory, but only three of them seem to be a practical source of fission energy. Uranium-235, which comprises 0.7 percent of natural uranium ore, can sustain a chain reaction all by itself; it is the primary energy source for nuclear power reactors operating in the world today. Uranium-238, which comprises virtually all the other atoms in mined uranium, is “fertile”: it can’t sustain a chain reaction by itself, but when it’s hit by a neutron it can transform into the highly fissile isotope plutonium-239. Likewise with thorium-232: Slamming it with a neutron can turn it into the fissile isotope uranium-233.

The Oak Ridge physicists were also eager to try out a new system for dealing with the waste fission products — one that again took advantage of the fuel’s liquid nature, but had been tested only in the laboratory. The idea was to siphon off a little of the reactor’s fuel mix each day and run it through a nearby purification system, which would chemically extract the fission products in much the same way that the kidneys remove toxins from the bloodstream. The cleaned-up fuel would then be circulated back into the reactor, which could continue running at full power the whole time. This process would not only keep the fission products from building up until they snuffed out the chain reaction — a problem for any reactor, since these elements tend to absorb a lot of neutrons — but it would also enhance the safety of molten salt still further. Not even the worst accident can contaminate the countryside with fission products that aren’t there.

But none of it was to be. Officials in the US nuclear program terminated the Oak Ridge molten salt program in January 1973 — and fired Weinberg.

The nuclear engineering community was just too heavily committed to solid fuels, both financially and intellectually. Practitioners already had decades of experience with experimental and commercial solid-fueled reactors, versus that one molten salt experiment at Oak Ridge. A huge infrastructure existed for processing and producing solid fuel. And, not incidentally, the US research program was committed to a grand vision for the global nuclear future that would expand this infrastructure enormously — and that, viewed with 20-20 hindsight, would lead the nuclear industry into a trap.

Key to that vision was a different way of dealing with the buildup of fission products. Since Oak Ridge–style continuous purification wasn’t an option in a solid-fuel reactor, water-cooled or otherwise, standard procedure called for burning fuel until the fission products rendered it useless, or spent. In water-cooled power reactors this took roughly three years, at which point the spent fuel would be switched out for fresh, then stored at the bottom of a pool of water for a few years while the worst of its fission-product radioactivity decayed.

Radioactivity forms when an unstable atomic nucleus sheds its excess energy by firing off a high-speed particle. This allows the nucleus to “decay,” or settle into a more stable form. The type of particle that’s emitted depends on the isotope involved, but primarily is alpha, beta or gamma. The rate of decay depends on the isotope’s half-life: the time it takes for half the original sample of nuclei to decay.

From there, the plan was to recycle it. Counting the remaining uranium, plus the plutonium that had formed from neutrons hitting uranium-238 nuclei, the fuel still contained most of the potential fission energy it had started with. So there was to be a new, global network of reprocessing plants that would chemically extract the fission products for disposal, and turn the uranium and plutonium into fresh fuel. That network, in turn, would ultimately support a new generation of sodium-cooled breeder reactors that would produce plutonium by the ton — thus solving what was then thought to be an acute shortage of the uranium needed to power the all-nuclear global economy of the future.

But that plan started to look considerably less visionary in May 1974, when India tested a nuclear bomb made with plutonium extracted from the spent fuel of a conventional reactor. Governments around the world suddenly realized that global reprocessing would be an invitation to rampant nuclear weapons proliferation: In plants handling large quantities of pure plutonium, it would be entirely too easy for bad actors to secretly divert a few kilograms at a time for bombs. So in April 1977, US President Jimmy Carter banned commercial reprocessing in the United States, and much of the rest of the world followed.

The vision for nuclear energy 50 years ago included a “closed loop” for nuclear fuels. In this scenario, most fuels would be reprocessed and cycled back into reactors to make more energy. The only wastes would be fission products, which are radioactive for a few hundred years at most.

That helped cement the already declining interest in breeder reactors, which made no sense without reprocessing plants to extract the new-made plutonium, and left the world with a nasty disposal problem. Instead of storing spent fuel underwater for a few years, engineers were now supposed to isolate it for something like 240,000 years, thanks to the 24,100-year half-life of plutonium-239. (The rule of thumb for safety is to wait 10 half-lives, which reduces radiation levels more than a thousand-fold.) No one has yet figured out how to guarantee isolation for that span of time. Today, there are nearly 300,000 tons of spent nuclear fuel still piling up at reactors around the world, part of an as yet unresolved long-term storage problem.

In retrospect, those 1970s-era nuclear planners would have done well to put serious money back into Oak Ridge’s molten salt program: As developers there tried to point out at the time, the continuous purification approach could have solved both the spent-fuel and proliferation problems at a stroke.

The proliferation risk would be minimal because — unlike the kind of reprocessing plants envisioned for the breeder program — the Oak Ridge system would never isolate uranium-235, plutonium-239 or any other fissile material. Instead, these isotopes would stay in the cleaned-up fuel salts at concentrations far too low to make a bomb. They would be circulated back into the reactor, where they could continue fissioning until they were completely consumed.

“The nuclear industry was not in an innovation frame of mind for 30 years.” Nathan Myhrvold

The reactor’s purification system would likewise offer a solution to the spent fuel issue. It would strip out the reaction-quenching fission products from the fuel almost as quickly as they formed, which would potentially allow the reactor to run for decades at a stretch with only an occasional injection of fresh fuel to replace what it burned. Some of that fuel could even come from today’s 300,000-ton backlog of spent solid fuel.

Admittedly, it would take centuries for even a large network of molten salt reactors to work through the full backlog. But burning it would eliminate the need to safely store it for thousands of centuries. By consuming the long-lived isotopes like plutonium-239, molten salt reactors could reduce the nuclear waste stream to a comparatively small volume of fission products having half-lives of 30 years or less. By the 10 half-life rule, this waste would then need to be isolated for just 300 years. That’s not trivial, says Schönfeldt, “but it’s something that can be handled” — say, by encasing the waste in concrete and steel, or putting it down a deep borehole.

Unfortunately, the late 1970s was not a good time for reviving any kind of nuclear program, molten salt or otherwise. Public mistrust of nuclear energy was escalating rapidly, thanks to rising concerns over safety, waste and weapons proliferation. The power companies’ patience was wearing thin, thanks to the skyrocketing, multibillion-dollar cost of standard water-cooled reactors. And then in March 1979 came a partial meltdown at Three Mile Island, a conventional nuclear plant near Harrisburg, Pennsylvania. In April 1986, another catastrophe hit with the fire and meltdown at the Chernobyl plant in Ukraine.

The resulting backlash against nuclear power was so strong that new plant construction effectively ceased — which is why most of the nuclear reactors operating today are at least three to four decades old. And nuclear power research stagnated, as well, with most of the money and effort going into ensuring the safety of those aging plants.

“The nuclear industry was not in an innovation frame of mind for 30 years,” says TerraPower’s Myhrvold.

In the 1970s, the threat that reprocessed fuels could be secretly diverted to make nuclear weapons led the United States and many other nations to reject the closed fuel cycle. Instead, they have opted to dispose of spent nuclear fuel after just one pass through the reactor. But because the fuel now contains long-half-life isotopes like plutonium-239, the wastes must be managed and stored for more than 200,000 years before they will be considered safe. No one knows how to guarantee that spent fuel will remain undisturbed for that span of time — one of the many safety concerns that have dogged nuclear energy programs.

Old tech revival

This defensive crouch lasted well into the new century, while the molten salt concept fell further and further into obscurity. That began to change only in 2002, when Kirk Sorensen came across a book describing what the molten salt program had accomplished at Oak Ridge.

“Why didn’t we do it this way in the first place?” he remembers wondering.

Sorensen, then a NASA engineer in Huntsville, Alabama, was so intrigued that he tracked down the old Oak Ridge technical reports, which were moldering in file cabinets, and talked NASA into paying to have them scanned. The files filled up five compact discs, which he copied and sent around to leaders in the US energy industry. “I received no response,” he says. So in 2006, in hopes of reaching somebody who would find the concept as compelling as he did, he uploaded the documents to energyfromthorium.com, a website he’d created with his own money.

That strategy worked — slowly. “I would give Kirk Sorensen personal credit,” says Lou Qualls, a nuclear engineer at Oak Ridge who became the Department of Energy’s first national technical director for molten salt reactors in 2017. “So Kirk is one of those voices out in the wilderness, and for a long time people would go, 'We don’t even know what you’re talking about.’” But once the old reports became available online, “people started to look at the technology, to understand it, see that it had a history,” Qualls says. “It started getting more credibility.”

“We … became nuclear engineers because we’re environmentalists.” Leslie Dewan

It helped that rising concerns about climate change — and the ever-growing backlog of spent nuclear fuel — had put many nuclear engineers in the mood for a radical rethink of their field. They could see that incremental improvements in standard reactor technology weren’t getting anywhere. Manufacturers had been hyping their “Generation III” designs for water-cooled reactors with enhanced safety features, but these were proving to be just as slow and expensive to build as their second-generation predecessors from the 1970s.

So instead, there was a move to revive the old reactor concepts and update them into a whole new series of Generation IV reactors: devices that would be considerably smaller and cheaper than their 1,000-megawatt, multibillion-dollar predecessors, with safety and proliferation resistance built in from the start. Among the most prominent symbols of this movement was TerraPower. Launched in 2008 with major funding from Microsoft cofounder Bill Gates, the company immediately started development of a liquid sodium-cooled device called the Traveling Wave Reactor.

The molten salt idea was definitely on the Gen IV list. Schönfeldt remembers getting excited about it as early as 2008. At MIT, Dewan and her fellow graduate student Mark Massie first encountered the idea in 2010, and were intrigued by the reactors’ inherent safety. “We both became nuclear engineers because we’re environmentalists,” says Dewan. Besides, her classmate had grown up watching his native West Virginia being devastated by mountaintop removal mining. “So Mark wanted to design a nuclear reactor that’s good enough to shut down the coal industry.”

Then in March 2011, the dangers of the nuclear status quo were underscored yet again. A tsunami knocked out all the cooling systems and backups at Japan’s Fukushima Daiichi plant and sent its 1970s-vintage power reactors into the worst meltdown since Chernobyl. That April, Sorensen launched the first of the molten salt start-up companies, Huntsville-based Flibe Energy. His goal ever since has been to develop and commercialize a Liquid-Fluoride Thorium Reactor — pretty much the same device that was envisioned at Oak Ridge back in the 1960s.

Dewan and Massie founded Transatomic the same month. And other molten salt start-ups soon followed, each building on the basic concept with a host of different design strategies and fuel choices. When Seaborg launched in 2014, for example, Schönfeldt and his colleagues started designing a molten salt Compact Used fuel BurnEr (called CUBE) that would not only run on a combination of spent nuclear fuel and thorium, but also be really, really small by reactor standards. “The fact that you can transport it to the site on the back of a truck is a major upside,” says Schönfeldt, “especially in remote regions.”

TerraPower, meanwhile, decided in 2015 to develop a much larger molten salt device, the Molten Chloride Fast Reactor, as a complement to the company’s ongoing work on its sodium-cooled Traveling Wave Reactor. The new system retains the latter’s ability to burn the widest possible range of fuels — including not just spent nuclear fuel, but also the ordinarily non-fissile uranium-238. (Both designs take advantage of the fact that a uranium-238 nucleus hit by a neutron has a tiny, but non-zero, probability of fissioning.) But unlike in the Traveling Wave Reactor, explains the company’s chief technical officer, John Gilleland, the molten salts’ 700°C-plus operating temperature will allow it to generate the kind of heat needed for industrial processes such as petroleum cracking and plastics making. Industrial process heat currently accounts for about one-third of total energy usage within the US manufacturing sector.

This industrial heat is now produced almost entirely by burning coal, oil or natural gas, says Gilleland. So if you could replace all that with carbon-free nuclear heat, he says, “you could hit the carbon problem in a very striking way.”

Of course, none of this is going to happen tomorrow. The various molten salt companies are still refining their designs by gathering lab data on liquid fuel chemistry, and running massive computer simulations of how the melt behaves when it’s simultaneously flowing and fissioning. The first prototypes won’t be up and running until the mid-2020s at the earliest.

And not all the companies will be there. Transatomic became the molten salt movement’s first casualty in September 2018, when Dewan shut it down. Her company had simply fallen too far behind in its design work relative to competitors, she explains. So even though investors were willing to keep going, she says, “it wouldn’t feel right for us to continue taking their money when I didn’t see a viable path forward for the business side.”

Still, most of the molten salt pioneers say they see reason for cautious optimism. Since at least 2015, the US Department of Energy has been ramping up its support for advanced reactor research in general, and molten salt reactors in particular.

“We kept telling people the three big advantages of molten salt reactors — no meltdown, no proliferation, burning up nuclear waste.” Troels Schönfeldt

Meanwhile, notes Slaybaugh, licensing agencies such as the US Nuclear Regulatory Commission are gearing up with the computer simulations and evaluation tools they will need when the advanced-reactor companies start seeking approval for constructing their prototypes. “People are looking at these technologies more carefully and more seriously than they have in a long time,” she says.

Perhaps the biggest and most unpredictable barrier is the public’s ingrained fear about almost anything labeled “nuclear.” What happens if people lump in molten salt reactors with older nuclear technologies, and reject them out of hand?

Based on their experience to date, most proponents are cautiously optimistic on this front as well. In Copenhagen, Schönfeldt and his colleagues kept hammering on the why of nuclear power, which was to fight climate change, poverty and pollution. “And we kept telling people the three big advantages of molten salt reactors — no meltdown, no proliferation, burning up nuclear waste,” he says. And slowly, people were willing to listen.

“We’ve moved a long way,” says Schönfeldt. “When we started in 2014, commercial nuclear power was illegal in Denmark. In 2017, we got public funding.”