THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 482 2018 Dec 16

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    THE SOCIETY FOR POPULAR ASTRONOMY
    Electronic News Bulletin No. 482 2018 December 16

    Here is the latest round-up of news from the Society for Popular
    Astronomy. The SPA is arguably Britain's liveliest astronomical
    society, with members all over the world. We accept subscription
    payments online at our secure site and can take credit and debit
    cards. You can join or renew via a secure server or just see how
    much we have to offer by visiting http://www.popastro.com/

    OVER 100 NEW EXOPLANETS FOUND
    National Institutes of Natural Sciences

    An international team of astronomers using a combination of ground- and
    space-based telescopes has reported more than 100 exoplanets in only three
    months. The planets are quite diverse and expected to play a large role
    in developing the research field of exoplanets and life in the Universe.
    Exoplanets, planets that revolve around stars other than the Sun, have been
    actively researched in recent years. One of the reasons is the success of
    the Kepler Space Telescope, which was launched in 2009 on purpose to search
    for exoplanets. If a planet transits in front of its parent star, then the
    observed brightness of the star drops by a small amount. The Kepler Space
    Telescope detected many exoplanets by that method. However, such dimming
    phenomena could be caused by other reasons. Therefore, confirmation that
    the phenomena are really caused by exoplanets is very important. Kepler
    experienced mechanical trouble in 2013, which led to a successor mission
    called K2. An international research team investigated 227 K2 exoplanet
    candidates with other space telescopes and ground-based telescopes. They
    confirmed that 104 of them really are exoplanets. Seven of the confirmed
    exoplanets have ultra-short orbital periods of less than 24 hours. The
    formation process of exoplanets with such short orbital periods is still
    not clear. Further study of those ultra-short-period planets will help
    to advance research into the processes behind their formation. The team
    also confirmed the existence of many low-mass rocky exoplanets with masses
    less than twice that of the Earth, as well as some planetary systems with
    multiple exoplanets.

    SUPERNOVA COMPLICATES ORIGIN THEORIES
    Carnegie Institution for Science

    A supernova discovered by an international group of astronomers provides an
    unprecedented look at the first moments of a violent stellar explosion. The
    light from the explosion's first hours showed an unexpected pattern which
    reveals that the genesis of the phenomenon is even more mysterious than
    previously thought. Type-Ia supernovae are fundamental to our understanding
    of the cosmos. Their nuclear furnaces are crucial for generating many of
    the elements around us, and they are used as cosmic rulers to measure dist-
    ances across the Universe. Despite their importance, the actual mechanism
    that triggers a Type-Ia supernova explosion has remained elusive for
    decades. That is why catching them in the act is crucial. Astronomers have
    long tried to get detailed data at the initial moments of such explosions,
    with the hope of understanding how those phenomena are triggered. That
    finally happened in February of this year with the discovery of a Type-Ia
    supernova called ASASSN-18bt (also known as SN 2018oh). ASASSN-18bt was
    discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN), an
    international network of telescopes, with headquarters at the Ohio State
    University, that routinely scans the sky for supernovae and other cosmic
    explosions. The Kepler space telescope was simultaneously able to take
    complementary data of the event. Kepler was designed to be very sensitive
    to small changes in light for its mission of detecting extrasolar planets,
    so it was able to obtain especially detailed information about the
    explosion's genesis. Type-Ia supernovae originate from the thermonuclear
    explosion of a white dwarf star — the dead core left by a Sun-like star
    after it exhausts its nuclear fuel. Material must be added to the white
    dwarf from a companion star to trigger the explosion, but the nature of the
    companion star and how the fuel is transferred has long been debated. One
    possibility is that the additional light seen during a supernova's early
    times could be from the exploding white dwarf colliding with its companion
    star. Although that was the initial hypothesis, detailed comparisons with
    theoretical models demonstrated that the additional light may have a
    different, unexplained origin. While the steep increase in ASASSN-18bt's
    early brightness could indicate that the explosion collides with another
    star, follow-up data do not fit predictions for how that should look.
    Other possibilities, such as an unusual distribution of radioactive
    material in the exploded star, are a better explanation for what was
    seen. More observations of ASASSN-18bt and more early discoveries like it
    may help to differentiate between different models and to understand the
    origins of such explosions. Thanks to ASAS-SN and the next generation of
    surveys that are now monitoring the sky every night, astronomers will find
    even more new supernovae and catch them at the moment of explosion. As more
    such events are found and studied, it may be possible to solve the long-
    standing problem of how such stellar explosions originate.

    FOUR NEW GRAVITATIONAL-WAVE DETECTIONS
    Massachusetts Institute of Technology

    Scientists have presented new results from the National Science Foundation's
    LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-
    based VIRGO gravitational-wave detector regarding their searches for
    coalescing cosmic objects, such as pairs of black holes and pairs of neutron
    stars. The LIGO and Virgo collaborations have now confidently detected
    gravitational waves from a total of 10 stellar-mass binary black-hole
    mergers and one merger of neutron stars, which are the dense, spherical
    remains of stellar explosions. Six of the black-hole merger events had been
    reported before, while four are newly announced. From 2015 September 12 to
    2016 January 19, during the first observing run with LIGO since its upgrade
    in a programme called Advanced LIGO, gravitational waves from three binary
    black-hole mergers were detected. The second observing run, which lasted
    from 2016 November 30 to 2017 August 25, yielded one binary neutron-star
    merger and seven additional binary black-hole mergers, including the four
    new gravitational-wave events being reported now. The new events are known
    as GW170729, GW170809, GW170818, and GW170823, in reference to the dates they were detected. All of the events are included in a new catalogue, with
    some of the events breaking records. For instance, the new event GW170729,
    detected in the second observing run on 2017 July 29, is the most massive
    and distant gravitational-wave source ever observed. In that coalescence,
    which happened roughly 5 billion years ago, an equivalent energy of almost
    five solar masses was converted into gravitational radiation.
    GW170814 was the first binary black hole merger measured by the three-
    detector network, and allowed the first tests for gravitational-wave
    polarization (analogous to light polarization). The event GW170817,
    detected three days after GW170814, represented the first time that
    gravitational waves were ever observed from the merger of a binary neutron-
    star system. Moreover, that collision was seen in gravitational waves
    and light, marking an exciting new chapter in multi-messenger astronomy,
    in which cosmic objects are observed simultaneously in different forms of
    radiation. The position in the sky of one of the new events, GW170818,
    which was detected by the global network formed by the LIGO and Virgo
    observatories, was fairly well determined. The position of the binary black
    holes, located 2.5 billion light-years away, was identified in the sky
    within an area of 39 square degrees. That makes it the next-best-localized
    gravitational-wave source after the GW170817 neutron-star merger. The next
    observing run, starting in Spring 2019, is expected to yield many more
    gravitational-wave candidates. The scientific papers describing the new
    findings, which are being initially published on the arXiv repository of
    electronic preprints, present detailed information in the form of a
    catalogue of all the gravitational-wave detections and candidate events of
    the two observing runs as well as describing the characteristics of the
    merging-black-hole population. Most notably, almost all black holes formed
    from stars are less than 45 times the mass of the Sun. Thanks to more
    advanced data processing and better calibration of the instruments, the
    accuracy of the astrophysical parameters of the previously announced events
    has been increased considerably.

    BLACK HOLE 'DOUGHNUTS' ACTUALLY 'FOUNTAINS'
    National Institutes of Natural Sciences

    On the basis of computer simulations and new observations from the Atacama
    Large Millimeter/submillimeter Array (ALMA), researchers have found that the
    rings of gas surrounding active supermassive black holes are not simple
    doughnut shapes. Instead, gas expelled from the centre interacts with
    infalling gas to create a dynamic circulation pattern. Most galaxies host
    a supermassive black hole, millions or billions of times the mass of the
    Sun, in their centres. Some of them swallow material quite actively. But
    astronomers have believed that rather than falling directly into the black
    hole, matter instead builds up around the active black hole, forming a
    doughnut structure. A team of astronomers used ALMA to observe the
    supermassive black hole in the Circinus Galaxy located 14 million
    light-years away. The team then compared its observations to a computer
    simulation made with the Cray XC30 ATERUI super-computer operated by NAOJ. The comparison revealed that the presumptive 'doughnut' is not actually a rigid structure, but is instead a complex collection of highly dynamic
    gaseous components. First, cold molecular gas falling towards the black
    hole forms a disc near the plane of rotation. As it approaches the black
    hole, the gas is heated until the molecules break down into their component
    atoms and ions. Some of the atoms are then expelled above and below the
    disc, rather than being absorbed by the black hole. The hot atomic gas
    falls back onto the disc, creating a turbulent three-dimensional structure.
    The three components circulate continuously, as (in a rather far-fetched
    analogy) the water may be supposed to do in fountains in city parks.

    THOUSANDS OF GLOBULAR CLUSTERS SCATTERED AMONG GALAXIES
    NASA/Goddard Space Flight Center

    Gazing across 300 million light-years into a monstrous city of galaxies,
    astronomers have used the Hubble Space Telescope to do a comprehensive
    census of some of its most diminutive members: a whopping 22,426 globular
    star clusters found to date. The survey will allow for astronomers to use
    the globular-cluster field to map the distribution of matter and dark matter
    in the Coma galaxy cluster, which holds over 1,000 galaxies that are packed
    together. Because globular clusters are much smaller than entire galaxies
    — and much more abundant — they are a much better tracer of how the fabric
    of space is distorted by the Coma cluster's gravity. In fact, the Coma
    cluster is one of the first places where observed gravitational anomalies
    were considered to be indicative of a lot of unseen mass in the Universe —
    later to be called 'dark matter'. Among the earliest denizens of the
    Universe, globular star clusters are quasi-spherical islands of several
    hundred thousand ancient stars. They are integral to the birth and growth of
    a galaxy. About 150 globular clusters exist in our Milky Way galaxy, and,
    because they contain the oldest known stars in the Universe, must have been
    present in the early formative years of our galaxy. Some of the Milky Way's
    globular clusters are visible to the naked eye as fuzzy-looking 'stars'.
    But at the distance of the Coma cluster, its globular clusters appear as
    dots of light even to Hubble's super-sharp vision. The survey found the
    globular clusters to be scattered in the space between the galaxies. They
    have been orphaned from their home galaxies by galaxy near-collisions inside
    the cluster. Hubble revealed that some globular clusters line up along
    bridge-like patterns — evidence for gravitational interactions between the
    galaxies.
    The team of astronomers first thought about the distribution of globular
    clusters in Coma when examining Hubble images that show the globular
    clusters extending all the way to the edge of any given photograph of
    galaxies in the Coma cluster. The team was looking forward to more data
    from one of the legacy surveys of Hubble that was designed to obtain data of
    the entire Coma cluster, called the Coma Cluster Treasury Survey. However,
    halfway through the program, in 2006, Hubble's Advanced Camera for Surveys
    (ACS) had an electronic failure. (It was later repaired by astronauts
    during a 2009 servicing mission.) To fill in the survey gaps, the team
    pulled numerous Hubble images of the galaxy cluster taken from other Hubble
    observing programmes. A mosaic was assembled of the central region of the
    cluster. The team developed algorithms to sift through the Coma mosaic
    images that contain at least 100,000 potential sources. The programme used
    the globular clusters' colour (dominated by the glow of ageing red stars)
    and spherical shape to eliminate extraneous objects — mostly background
    galaxies not associated with the Coma cluster. Though Hubble has superb
    detectors with unmatched sensitivity and resolution, their main drawback
    is that they have tiny fields of view. The research showcases the science
    that may be possible with the planned Wide Field Infrared Survey Telescope
    (WFIRST) that will have a much larger field of view than Hubble, when entire
    galaxy clusters can be imaged at once.

    ALL STARLIGHT IN OBSERVABLE UNIVERSE MEASURED
    Clemson University

    Astrophysicists believe that our Universe, which is about 13.7 billion years
    old, began forming the first stars when it was a few hundred million years
    old. Since then, the Universe has become a star-making tour de force.
    Using new methods of starlight measurement, a team of astrophysicists
    analyzed data from the Fermi Gamma-ray Space Telescope to determine the
    history of star formation over most of the Universe's lifetime. Putting a
    number on the amount of starlight ever produced has several variables that
    make it difficult to quantify in simple terms. But according to the new
    measurement, the number of photons that escaped into space after being
    emitted by stars is of order 4 x 10^84. Despite that stupendously large
    number, it is interesting to note that, with the exception of the light that
    comes from our own Sun and galaxy, the rest of the starlight that reaches
    Earth is exceedingly dim — equivalent to a 60-watt light bulb viewed in
    complete darkness from about 2.5 miles away. That is because the Universe
    is almost incomprehensibly huge. It is also why the sky is dark at night,
    other than light from the Moon, visible stars and the faint glow of the
    Milky Way. The Fermi Gamma-ray Space Telescope was launched into low orbit
    on 2008 June 11. It is a powerful observatory that has provided enormous
    amounts of data on gamma rays (the most energetic form of light) and their
    interaction with the extragalactic background light (EBL), which is a cosmic
    fog composed of all the ultraviolet, visible and infrared light emitted by
    stars or from dust in their vicinity. The team analyzed almost nine years
    of data pertaining to gamma-ray signals from 739 blazars. Blazars are
    galaxies containing supermassive black holes that are able to release
    narrowly collimated jets of energetic particles that leap out of their
    galaxies and streak across the cosmos at nearly the speed of light. When
    one of the jets happens to be pointed directly at us, it is detectable
    even when originating from extremely far away. Gamma-ray photons produced
    within the jets eventually collide with the cosmic fog, leaving an observ-
    able imprint. That enabled the team to measure the density of the fog
    not just at a given place but also at a given time in the history of the
    Universe. Gamma-ray photons travelling through a fog of starlight have a
    large probability of being absorbed. By measuring how many photons have
    been absorbed, they were able to measure how thick the fog was and also
    measure, as a function of time, how much light there was in the entire range
    of wavelengths.
    By means of galaxy surveys, the star-formation history of the Universe has
    been studied for decades. But one obstacle faced by previous research was
    that some galaxies were too far away, or too faint, for any present-day
    telescopes to detect. That obliged scientists to estimate the starlight
    produced by such distant galaxies rather than record it directly. The team
    was able to circumvent that by using Fermi's Large Area Telescope data to
    analyze the extragalactic background light. Starlight that escapes
    galaxies, including the most distant ones, eventually becomes part of the
    extragalactic background light (EBL). Therefore, accurate measurements of
    that cosmic fog, which have only recently become possible, eliminated the
    need to estimate light emissions from ultra-distant galaxies. The team
    performed the gamma-ray analysis of all 739 blazars, whose black holes are
    millions to billions of times more massive than the Sun. By using blazars
    at different distances from us, they measured the total starlight at
    different time periods. They measured the total starlight of each epoch —
    one billion years ago, two billion years ago, six billion years ago,
    etc. — all the way back to when stars were first formed. That allowed the
    team to reconstruct the EBL and determine the star-formation history of the
    Universe in a more effective manner than had been achieved before. When
    high-energy gamma rays collide with low-energy visible light, they transform
    into pairs of electrons and positrons. According to NASA, Fermi's ability
    to detect gamma rays across a wide range of energies makes it uniquely
    suited for mapping the cosmic fog. Those particle interactions occur over
    immense cosmic distances, which enabled the group to probe deeper than ever
    into the Universe's star-forming productivity. Scientists have tried to
    measure the EBL for a long time. However, very bright foregrounds like
    the zodiacal light (which is light scattered by dust in the Solar System)
    rendered such measurements very challenging. Star formation, which occurs
    when dense regions of molecular clouds collapse and form stars, peaked
    around 11 billion years ago. But though the forming of new stars has since
    slowed down, it has never stopped. For instance, about seven new stars are
    created in our Milky Way galaxy every year. Understanding star formation
    also has ramifications for other areas of astronomical study, including
    research regarding cosmic dust, galaxy evolution and dark matter. The
    team's analysis will provide future missions with a guideline to explore the
    earliest days of stellar evolution — such as the upcoming James Webb Space
    Telescope, which will be launched in 2021 and will enable scientists to hunt
    for the formation of primordial galaxies. The first billion years of our
    Universe's history are a very interesting epoch that has not yet been probed
    by current satellites. Perhaps one day we will find a way to look all the
    way back to the Big Bang; that must be the ultimate goal.

    NEW THEORY EXPLAINS THE MISSING 95 PER CENT OF THE COSMOS
    University of Oxford

    Scientists may have solved one of the biggest questions in modern physics,
    with a new paper unifying dark matter and dark energy into a single
    phenomenon: a fluid which possesses 'negative mass'. If you were to push a
    negative mass, it would accelerate towards you. That astonishing new theory
    may also prove right a prediction that Einstein made 100 years ago. Our
    current, widely-recognized model of the Universe, called Lambda-CDM, tells
    us nothing about what dark matter and dark energy are like physically.
    We only know about them because of the gravitational effects that they
    have on other, observable matter. The new model offers a new explanation.
    Scientists now think that both dark matter and dark energy can be unified
    into a fluid which possesses a type of 'negative gravity', repelling all
    other material around them. Although such matter is peculiar to us, it
    suggests that our cosmos is symmetrical in both positive and negative
    qualities. The existence of negative matter had previously been ruled out
    as it was thought such material would become less dense as the Universe
    expands, which runs contrary to our observations that show dark energy
    does not thin out over time. However, the research applies a 'creation
    tensor', which allows for negative masses to be continuously created.
    It demonstrates that when more and more negative masses are continually
    bursting into existence, that negative-mass fluid does not dilute during
    the expansion of the cosmos. In fact, the fluid appears to be identical to
    dark energy. The theory also provides the first correct predictions of the
    behaviour of dark-matter haloes. Most galaxies are rotating so rapidly
    that they should be tearing themselves apart, which suggests that an
    invisible 'halo' of dark matter must be holding them together. The new
    research features a computer simulation of the properties of negative mass,
    which predicts the formation of dark matter haloes just like the ones
    inferred by observations made by modern radio telescopes.
    Albert Einstein provided the first hint of the dark Universe exactly 100
    years ago, when he discovered a parameter in his equations known as the
    'cosmological constant', which we now know to be synonymous with dark
    energy. Einstein famously called the cosmological constant his 'biggest
    blunder', although modern astrophysical observations prove that it is
    a real phenomenon. In notes dating back to 1918, Einstein described his
    cosmological constant, writing that “a modification of the theory is
    required such that 'empty space' takes the role of gravitating negative
    masses which are distributed all over the interstellar space.” It is
    therefore possible that Einstein himself predicted a negative-mass-filled
    Universe. Previous approaches to combining dark energy and dark matter
    have attempted to modify Einstein's theory of general relativity, which has
    turned out to be incredibly challenging. The new approach takes two old
    ideas that are known to be compatible with Einstein's theory — negative
    masses and matter creation — and combines them together. The outcome seems
    rather beautiful: dark energy and dark matter can be unified into a single
    substance, with both effects being simply explicable as positive-mass
    matter surfing on a sea of negative masses. Proof of the theory will come
    from tests performed with a cutting-edge radio telescope known as the Square
    Kilometre Array (SKA), an international endeavour to build the world's
    largest telescope, in which the University of Oxford is collaborating.
    There are still many theoretical issues and computational simulations to work
    through, and Lambda-CDM has a nearly-30-year head start, but astronomers are
    looking forward to seeing whether the new extended version of Lambda-CDM
    can accurately match other observational evidence of our cosmology. If so,
    it may suggest that the missing 95% of the cosmos had an aesthetic solution:
    we had forgotten to include a simple minus sign.

    FIRST LIGHT FOR SPECULOOS
    ESO

    The SPECULOOS Southern Observatory (SSO) has been successfully installed at
    the Paranal Observatory and has obtained its first light. After finishing
    the commissioning phase, this new array of planet-hunting telescopes will
    begin scientific operations, starting in earnest in 2019 January. SSO is
    the core facility of a new exoplanet-hunting project called Search for
    habitable Planets EClipsing ULtra-cOOl Stars (SPECULOOS) [1], and consists
    of four telescopes equipped with 1-metre primary mirrors. The telescopes,
    named Io, Europa, Ganymede and Callisto after the four Galilean moons of
    Jupiter, will enjoy pristine observing conditions at the Paranal site,
    which is also home to ESO's flagship Very Large Telescope (VLT). Paranal
    provides a near-perfect site for astronomy, with dark skies and a stable,
    arid climate. SPECULOOS aims to search for potentially habitable Earth-
    sized planets surrounding ultra-cool stars or brown dwarfs, whose planetary
    populations are still mostly unexplored. Only a few exoplanets have been
    found orbiting such stars, and even fewer lie within their parent star's
    habitable zone. Even though those dim stars are hard to observe, they are
    abundant — comprising about 15% of the stars in the nearby universe.
    SPECULOOS is designed to explore 1000 such stars, including the nearest,
    brightest, and smallest, in search of Earth-sized habitable planets.
    SPECULOOS will search for exoplanets using the transit method, following
    the example of its prototype TRAPPIST-South telescope at ESO's La Silla
    Observatory. That telescope has been operational since 2011 and detected
    the famous TRAPPIST-1 planetary system. As a planet passes in front of its
    star it blocks some of the star's light, causing a small partial eclipse,
    resulting in a subtle but detectable dimming of the star. Exoplanets with
    smaller host stars block more of their stars' light during a transit, making
    the periodic eclipses much easier to detect than those associated with
    larger stars. Thus far, only a small fraction of the exoplanets detected
    by that method have been Earth-sized or smaller. However, the small size
    of the SPECULOOS target stars combined with the high sensitivity of its
    telescopes allows detection of Earth-sized transiting planets located in
    the habitable zone. Those planets will be ideally suited for follow-up
    observations with large ground- or space-based facilities.

    CHINA'S MISSION TO THE FARSIDE OF THE MOON
    Spaceweather.com

    China is about to go where no one has gone before — the far side of the
    Moon. On the morning of Dec. 8 (Chinese time), a Long March 3B rocket
    blasted off from the Xichang Satellite Launch Centre in Sichuan province,
    propelling a lander and rover towards the lunar farside. If the mission
    succeeds, it will catapult China into the forefront of lunar exploration
    with a landing that no other nation has even dared to attempt. From Earth,
    we can see only one side of the Moon. The other side, the lunar farside,
    is perpetually hidden from view. Apollo astronauts have flown over the
    far side of the Moon, and many satellites have photographed the Moon from
    behind–revealing it to be a rugged, heavily cratered landscape startlingly
    different from the side we normally see. But no one has ever landed there.
    China's Chang'e 4 mission aims to be the first. Reportedly, the lander will
    touch down inside a 186-km-wide crater called Von Karman. The crater is
    part of the South Pole–Aitken basin, the largest known impact structure
    in the Solar System. The Chang'e-4 rover will explore the landing site,
    probing it with ground-penetrating radar and measuring the mineral compo-
    sition with an infrared spectrometer. If water is present, the rover might
    find it. And that's just the beginning. The lander will also conduct
    experiments in lunar gardening. A small climate-controlled greenhouse in
    the lander will test whether potato and thale-cress (Arabidopsis) seeds can
    sprout and photosynthesize in low gravity without the twin protections of a
    thick atmosphere and magnetic field.
    Communicating with the far side of the moon is tricky. There is no direct
    line of sight. To overcome that problem, on 2018 May 21 China launched a
    satellite named Queqiao (Chinese for “Magpie Bridge”) to relay signals
    between the lunar farside and the Earth. Queqiao will be able to talk to
    ground stations in China, Argentina and Namibia, sending back radio signals
    and TV images. However, Chang'e 4 will have to perform the critical landing
    completely autonomously — a daring plan. Landing is expected to occur
    early in the New Year.

    VOYAGER 2 ENTERS INTERSTELLAR SPACE
    NASA

    The Voyager 2 probe has now left the heliosphere — the protective bubble
    of particles and magnetic fields created by the Sun. Mission scientists
    determined that the probe crossed the outer edge of the heliosphere on
    Nov. 5. That boundary, called the heliopause, is where the tenuous, hot
    solar wind meets the cold, dense interstellar medium. The twin, Voyager 1,
    crossed that boundary in 2012, but Voyager 2 carries a working instrument
    that will provide first-of-its-kind observations of the nature of that
    gateway into interstellar space. Voyager 2 is now slightly more than
    18 billion kilometres from the Earth. Mission operators can still
    communicate with it as it enters this new phase of its journey, but inform-
    ation — though moving at the speed of light — takes about 16.5 hours to
    travel from the spacecraft to the Earth. The most compelling evidence of
    Voyager 2's exit from the heliosphere came from its onboard Plasma Science
    Experiment (PLS), an instrument that on Voyager 1 stopped working in 1980,
    long before that probe crossed the heliopause. Until recently, the space
    surrounding Voyager 2 was filled predominantly with plasma flowing out from
    the Sun. That outflow, called the solar wind, creates a bubble — the
    heliosphere — that envelops the planets in the Solar System. The PLS
    uses the electrical current of the plasma to detect the speed, density,
    temperature, pressure and flux of the solar wind. The PLS aboard Voyager 2
    observed a steep decline in the speed of the solar-wind particles on
    Nov. 5. Since that date, the plasma instrument has observed no solar wind
    flow in the environment around Voyager 2, which makes mission scientists
    confident that the probe has left the heliosphere. In addition to the
    plasma data, Voyager's science-team members have seen evidence from three
    other onboard instruments — the cosmic-ray sub-system, the low-energy
    charged-particle instrument and the magnetometer — that is consistent with
    the conclusion that Voyager 2 has crossed the heliopause.

    Bulletin compiled by Clive Down
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