Pressure runs high at edge of solar system

Out at the boundary of our solar system, pressure runs high. This pressure, the force plasma, magnetic fields and particles like ions, cosmic rays and electrons exert on one another when they flow and collide, was recently measured by scientists in totality for the first time - and it was found to be greater than expected. Using observations of galactic cosmic rays - a type of highly energetic particle - from NASA's Voyager spacecraft scientists calculated the total pressure from particles in the outer region of the solar system, known as the heliosheath. At nearly 9 billion miles away, this region is hard to study. But the unique positioning of the Voyager spacecraft and the opportune timing of a solar event made measurements of the heliosheath possible. And the results are helping scientists understand how the Sun interacts with its surroundings. "In adding up the pieces known from previous studies, we found our new value is still larger than what's been measured so far," said Jamie Rankin, lead author on the new study and astronomer at Princeton University in New Jersey. "It says that there are some other parts to the pressure that aren't being considered right now that could contribute." On Earth we have air pressure, created by air molecules drawn down by gravity. In space there's also a pressure created by particles like ions and electrons. These particles, heated and accelerated by the Sun create a giant balloon known as the heliosphere extending millions of miles out past Pluto. The edge of this region, where the Sun's influence is overcome by the pressures of particles from other stars and interstellar space, is where the Sun's magnetic influence ends. (Its gravitational influence extends much farther, so the solar system itself extends farther, as well.)

In order to measure the pressure in the heliosheath, the scientists used the Voyager spacecraft, which have been travelling steadily out of the solar system since 1977. At the time of the observations, Voyager 1 was already outside of the heliosphere in interstellar space, while Voyager 2 still remained in the heliosheath.

"There was really unique timing for this event because we saw it right after Voyager 1 crossed into the local interstellar space," Rankin said. "And while this is the first event that Voyager saw, there are more in the data that we can continue to look at to see how things in the heliosheath and interstellar space are changing over time."

The scientists used an event known as a global merged interaction region, which is caused by activity on the Sun. The Sun periodically flares up and releases enormous bursts of particles, like in coronal mass ejections. As a series of these events travel out into space, they can merge into a giant front, creating a wave of plasma pushed by magnetic fields.

When one such wave reached the heliosheath in 2012, it was spotted by Voyager 2. The wave caused the number of galactic cosmic rays to temporarily decrease. Four months later, the scientists saw a similar decrease in observations from Voyager 1, just across the solar system's boundary in interstellar space.

Knowing the distance between the spacecraft allowed them to calculate the pressure in the heliosheath as well as the speed of sound. In the heliosheath sound travels at around 300 kilometers per second - a thousand times faster than it moves through air.

The scientists noted that the change in galactic cosmic rays wasn't exactly identical at both spacecraft. At Voyager 2 inside the heliosheath, the number of cosmic rays decreased in all directions around the spacecraft. But at Voyager 1, outside the solar system, only the galactic cosmic rays that were traveling perpendicular to the magnetic field in the region decreased. This asymmetry suggests that something happens as the wave transmits across the solar system's boundary.

"Trying to understand why the change in the cosmic rays is different inside and outside of the heliosheath remains an open question," Rankin said.

Studying the pressure and sound speeds in this region at the boundary of the solar system can help scientists understand how the Sun influences interstellar space. This not only informs us about our own solar system, but also about the dynamics around other stars and planetary systems.

Mercury mission to explore origin of Solar System

Is Mercury's core liquid or solid, and why -- on the smallest planet in our solar system -- is it so big? What can the planet closest to the Sun tell us about how our solar system came into being? An unmanned European-Japanese space mission, dubbed BepiColombo, blasted off early Saturday morning from French Guiana, to probe these and other mysteries. "BepiColombo is coming like a white knight with better and more precise data," said Alain Doressoundiram, an astronomer at the Paris Observatory. "To understand how Earth was formed, we need to understand how all rocky planets formed," including Venus and Mars, he told AFP. "Mercury stands apart and we don't know why." First, however, the suite of instruments on board the Ariane 5 rocket will have to travel seven years and nine million kilometres (5.6 million miles) to reach their destination. In a statement after the launch, ArianeGroup said the satellite had successfully escaped Earth's gravity field and was beginning its long journey where it will reach speeds of up to 40,000 kilometres (25,000 miles) an hour. According to Pierre Bousquet, an engineer at France's National Centre for Space Research and head of the French team contributing to the mission, Mercury is "abnormally small," leading to speculation that it survived a massive collision in its youth.

"A huge crater visible on its surface could be the scar left over from that encounter," Bousquet told AFP. Finding out if this is true is on BepiColombo's "to do" list.

- Going hot and cold -

This scenario would explain why Mercury's core accounts for a whopping 55 percent of its mass, compared to 30 percent for Earth.

Mercury is also the only rocky planet orbiting the Sun beside our own to have a magnetic field.

Magnetic fields are generated by a liquid core but given its size, Mercury's should have grown cold and solid by now, as did Mars.

This anomaly might be due to some feature of the core's composition, something BepiColombo's instruments will measure with much greater precision than has been possible so far.

On its surface, Mercury is a planet of extremes, vacillating between hot days of about 430 degrees Celsius (more than 800 degrees Fahrenheit) to super-frosty nights of minus 180C (minus 290F).

Those days and nights last nearly three Earth months each.

Earlier missions have detected evidence of ice in the deepest recesses of the planet's polar craters.

Scientists speculate that this may have accumulated from comets crashing onto Mercury's surface.

"If the presence of ice is confirmed, it means that some of those water samples date back nearly to the origin of the solar system," Doressoundiram said.

- Lashed by solar winds -

Mercury is 58 million kilometres (36 million miles) from the Sun, nearly three times closer than Earth.

"The planet is whipped by solar winds," a constant torrent of ionised particles bombarding the surface at 500 kilometres per second, said Bousquet.

The scientists will be able to study the impact of these winds -- 10 times stronger than the ones hitting Earth's atmosphere -- on Mercury's magnetic field.

The BepiColombo mission will deploy two spacecraft. The Mercury Planet Orbiter, built by ESA, will investigate planet's surface and interior composition.

The Mercury Magnetospheric Orbiter, made by the Japan Aerospace Exploration Agency, will study the region of space around the planet that is influenced by its magnetic field.

The mission will also look for tectonic activity, and seek to understand why spectroscopic observations show no iron even if it is thought to be one of the planet's major component elements.

Compared to Mars, Venus, and Saturn, Mercury has barely been explored. Only two spacecraft have ever paid it a visit.

NASA's Mariner 10 did three flybys in 1974 and 1975, providing the first up-close images. More than 30 years later, NASA's Messenger did the same, before settling into orbit around Mercury in 2011.

The new mission is named after Giuseppe (Bepi) Colombo, a brilliant Italian mathematician and engineer who first understood the relationship between Mercury's rotation and orbit.

Practising for BepiColombo's epic escape to Mercury

The international BepiColombo spacecraft will soon take flight, on a complex journey to the innermost planet of the Solar System, Mercury. Encompassing nine planetary flybys and travelling a total distance of nine billion km over a period of seven years, this will be one of the most intricate journeys ever flown by mission teams at ESA's ESOC mission control centre. With launch set for 20 October, flight controllers led by Operations Manager Elsa Montagnon are now busily preparing for the start of what will be Europe's first mission to Mercury - the smallest and least explored terrestrial planet of the Solar System. "Mission teams have spent months simulating BepiColombo's unique and complex journey," explains Elsa. "Taking turns, in 12-hour shifts, we have been practising the spacecraft's various launch and early mission processes and manoeuvres in real-time so we are prepared for every possible eventuality." BepiColombo is a joint mission between ESA and the Japan Aerospace Exploration Agency (JAXA). The mission comprises two science orbiters: ESA's Mercury Planetary Orbiter (MPO) and JAXA's Mercury Magnetospheric Orbiter (MMO). The ESA-built Mercury Transfer Module (MTM) will carry the orbiters to Mercury using a combination of solar electric propulsion and gravity assist flybys.

After its arrival at the planet of extremes in 2025, it will spend at least a year in orbit gathering data on Mercury's composition, density, magnetic field and exosphere, as well as probing the planet's interaction with solar wind.

Before the science begins, however, the multi-module spacecraft has to safely escape Earth, switch on, and receive instructions from mission control on where to go next.

A rocky road

Europe's space scientists have identified BepiColombo as one of the most challenging long-term planetary projects ever flown, as Mercury's proximity to the Sun makes it difficult for spacecraft to reach without being pulled into the star's enormous gravity.

"To get to Mercury without being subsumed by our giant star, the spacecraft will make a series of nine planetary flybys; circling Earth once, Venus twice, and Mercury itself six times," explains Andrea Accomazzo, Flight Director for BepiColombo.

"Unlike missions that take spacecraft to the outer regions of the Solar System, the Mercury Transfer Module will use the gravity of these inner planets, in combination with the thrust provided by electric propulsion, to slow the spacecraft down."

The Sun's huge gravity field acts as an enormous gravity 'well'. Getting a spacecraft to Mercury, and therefore close to the Sun, means dropping it into this steep well - the difficulty comes in ensuring the spacecraft ends up at Mercury and not at our gigantic star.

"The closer we get to the Sun the more we are constricted in our path," explains Frank Budnik from the Flight Dynamics team.

"For example, BepiColombo's large solar arrays need to be tilted at just the right angle to get enough sunlight to power the high-energy demand of the propulsion system and keeping the spacecraft running. At the same time, they mustn't get too much sunlight, or they could be beyond their limits."

"There is only a small corridor in which the solar arrays can be operated to fulfil both of these constraints."

BepiColombo will launch at 01:45 GMT (03:45 CEST) on 20 October, on board an Ariane 5 rocket. After the spacecraft separates from the rocket's 'upper stage', teams at ESOC will take control, sending commands to the spacecraft to get it into normal operational mode - a process that is expected to take about four days.

This period, dubbed the 'Launch and Early Orbit Phase' (LEOP), sees the control systems and instruments switched on, and their health and proper functioning assessed.

This is a risky time when the satellite is unusually vulnerable - not yet fully functional but still exposed to the hazards of space.

Simulating the scene

In preparation for this vital period, mission control teams have spent months simulating every expected scenario - the perfect LEOP, launch and separation of the satellite from the launcher, as well as a whole host of scenarios in which something goes wrong.

Establishing contact between the spacecraft and mission controllers has also been rehearsed.

Deep space ground stations across three continents will support this mission, with ESA's global antenna network maintaining links to and from BepiColombo throughout the journey.

"BepiColombo is one of the world's most ambitious interplanetary missions, and it could not be in safer hands," says Rolf Densing, Director of Operations at ESOC.

"With decades of collective experience and hundreds of hours of simulation practice, teams at ESA's mission control are ready to set out for the rocky planet."

Extremely distant Solar System object found

Carnegie's Scott Sheppard and his colleagues - Northern Arizona University's Chad Trujillo, and the University of Hawaii's David Tholen - are once again redefining our solar system's edge. They discovered a new extremely distant object far beyond Pluto with an orbit that supports the presence of an even-farther-out, super-Earth or larger Planet X. The newly found object, called 2015 TG387, will be announced Tuesday by the International Astronomical Union's Minor Planet Center. A paper with the full details of the discovery has also been submitted to The Astronomical Journal. 2015 TG387 was discovered about 80 astronomical units (AU) from the Sun, a measurement defined as the distance between the Earth and Sun. For context, Pluto is around 34 AU, so 2015 TG387 is about two and a half times further away from the Sun than Pluto is right now. The new object is on a very elongated orbit and never comes closer to the Sun, a point called perihelion, than about 65 AU. Only 2012 VP113 and Sedna at 80 and 76 AU respectively have more-distant perihelia than 2015 TG387. Though 2015 TG387 has the third-most-distant perihelion, its orbital semi-major axis is larger than 2012 VP113 and Sedna's, meaning it travels much farther from the Sun than they do. At its furthest point, it reaches all the way out to about 2,300 AU. 2015 TG387 is one of the few known objects that never comes close enough to the solar system's giant planets, like Neptune and Jupiter, to have significant gravitational interactions with them.

"These so-called Inner Oort Cloud objects like 2015 TG387, 2012 VP113, and Sedna are isolated from most of the solar system's known mass, which makes them immensely interesting," Sheppard explained. "They can be used as probes to understand what is happening at the edge of our solar system."

The object with the most-distant orbit at perihelion, 2012 VP113, was also discovered by Sheppard and Trujillo, who announced that find in 2014. The discovery of 2012 VP113 led Sheppard and Trujillo to notice similarities of the orbits of several extremely distant solar system objects, and they proposed the presence of an unknown planet several times larger than Earth - sometimes called Planet X or Planet 9 - orbiting the Sun well beyond Pluto at hundreds of AU.

"We think there could be thousands of small bodies like 2015 TG387 out on the solar system's fringes, but their distance makes finding them very difficult," Tholen said. "Currently we would only detect 2015 TG387 when it is near its closest approach to the Sun. For some 99 percent of its 40,000-year orbit, it would be too faint to see."

The object was discovered as part of the team's ongoing hunt for unknown dwarf planets and Planet X. It is the largest and deepest survey ever conducted for distant solar system objects.

"These distant objects are like breadcrumbs leading us to Planet X. The more of them we can find, the better we can understand the outer solar system and the possible planet that we think is shaping their orbits - a discovery that would redefine our knowledge of the solar system's evolution," Sheppard added.

It took the team a few years of observations to obtain a good orbit for 2015 TG387 because it moves so slowly and has such a long orbital period. They first observed 2015 TG387 in October of 2015 at the Japanese Subaru 8-meter telescope located atop Mauna Kea in Hawaii.

Follow-up observations at the Magellan telescope at Carnegie's Las Campanas Observatory in Chile and the Discovery Channel Telescope in Arizona were obtained in 2015, 2016, 2017 and 2018 to determine 2015 TG387's orbit.

2015 TG387 is likely on the small end of being a dwarf planet since it has a diameter near 300 kilometers. The location in the sky where 2015 TG387 reaches perihelion is similar to 2012 VP113, Sedna, and most other known extremely distant trans-Neptunian objects, suggesting that something is pushing them into similar types of orbits.

Trujillo and University of Oklahoma's Nathan Kaib ran computer simulations for how different hypothetical Planet X orbits would affect the orbit of 2015 TG387. The simulations included a super-Earth-mass planet at several hundred AU on an elongated orbit as proposed by Caltech's Konstantin Batygin and Michael Brown in 2016.

Most of the simulations showed that not only was 2015 TG387's orbit stable for the age of the solar system, but it was actually shepherded by Planet X's gravity, which keeps the smaller 2015 TG387 away from the massive planet.

This gravitational shepherding could explain why the most-distant objects in our solar system have similar orbits. These orbits keep them from ever approaching the proposed planet too closely, which is similar to how Pluto never gets too close to Neptune even though their orbits cross.

"What makes this result really interesting is that Planet X seems to affect 2015 TG387 the same way as all the other extremely distant solar system objects. These simulations do not prove that there's another massive planet in our solar system, but they are further evidence that something big could be out there," Trujillo concludes.

Mysterious 'lunar swirls' point to moon's volcanic, magnetic past

The mystery behind lunar swirls, one of the solar system's most beautiful optical anomalies, may finally be solved thanks to a joint Rutgers University and University of California Berkeley study. The solution hints at the dynamism of the moon's ancient past as a place with volcanic activity and an internally generated magnetic field. It also challenges our picture of the moon's existing geology. Lunar swirls resemble bright, snaky clouds painted on the moon's dark surface. The most famous, called Reiner Gamma, is about 40 miles long and popular with backyard astronomers. Most lunar swirls share their locations with powerful, localized magnetic fields. The bright-and-dark patterns may result when those magnetic fields deflect particles from the solar wind and cause some parts of the lunar surface to weather more slowly. "But the cause of those magnetic fields, and thus of the swirls themselves, had long been a mystery," said Sonia Tikoo, coauthor of the study recently published in the Journal of Geophysical Research - Planets and an assistant professor in Rutgers University-New Brunswick's Department of Earth and Planetary Sciences. "To solve it, we had to find out what kind of geological feature could produce these magnetic fields - and why their magnetism is so powerful."

Working with what is known about the intricate geometry of lunar swirls, and the strengths of the magnetic fields associated with them, the researchers developed mathematical models for the geological "magnets." They found that each swirl must stand above a magnetic object that is narrow and buried close to the moon's surface.

The picture is consistent with lava tubes, long, narrow structures formed by flowing lava during volcanic eruptions; or with lava dikes, vertical sheets of magma injected into the lunar crust.

But this raised another question: How could lava tubes and dikes be so strongly magnetic? The answer lies in a reaction that may be unique to the moon's environment at the time of those ancient eruptions, over 3 billion years ago.

Past experiments have found that many moon rocks become highly magnetic when heated more than 600 degrees Celsius in an oxygen-free environment. That's because certain minerals break down at high temperatures and release metallic iron. If there happens to be a strong enough magnetic field nearby, the newly formed iron will become magnetized along the direction of that field.

This doesn't normally happen on earth, where free-floating oxygen binds with the iron. And it wouldn't happen today on the moon, where there is no global magnetic field to magnetize the iron.

But in a study published last year, Tikoo found that the moon's ancient magnetic field lasted 1 billion to 2.5 billion years longer than had previously been thought - perhaps concurrent with the creation of lava tubes or dikes whose high iron content would have become strongly magnetic as they cooled.

"No one had thought about this reaction in terms of explaining these unusually strong magnetic features on the moon. This was the final piece in the puzzle of understanding the magnetism that underlies these lunar swirls," Tikoo said.

The next step would be to actually visit a lunar swirl and study it directly. Tikoo serves on a committee that is proposing a rover mission to do just that.

Impact of a stellar intruder on our solar system

The solar system was formed from a protoplanetary disk consisting of gas and dust. Since the cumulative mass of all objects beyond Neptune is much smaller than expected and the bodies there have mostly inclined, eccentric orbits it is likely that some process restructured the outer solar system after its formation. Susanne Pfalzner from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and her colleagues present a study showing that a close fly-by of a neighbouring star can simultaneously lead to the observed lower mass density in the outer part of the solar system and excite the bodies there onto eccentric, inclined orbits. Their numerical simulations show that many additional bodies at high inclinations still await discovery, perhaps including a sometimes postulated Planet X. The findings are published in the present issue of "The Astrophysical Journal." A near catastrophe billions of years ago might have shaped the outer parts of the solar system, while leaving the inner regions basically untouched. Researchers from the Max Planck Institute for Radio Astronomy in Bonn and their collaborators found that a close fly-by of another star can explain many of the features observed in the outer solar system.

"Our group has been looking for years at what fly-bys can do to other planetary systems never considering that we actually might live right in such a system," says Susanne Pfalzner, the leading author of the project. "The beauty of this model lies in its simplicity."

The basic scenario of the formation of the solar system has long been known: our Sun was born from a collapsing cloud of gas and dust. In the process a flat disk was formed where not only large planets grew but also smaller objects like the asteroids, dwarf planets, etc. Due to the flatness of the disk one would expect that the planets orbit in a single plane unless something dramatic happened afterwards.

Looking at the solar system right to the orbit of Neptune everything seems fine: most planets move on fairly circular orbits and their orbital inclinations vary only slightly. However, beyond Neptune things become very messy. The biggest puzzle is the dwarf planet Sedna, which moves on an inclined, highly eccentric orbit and is so far outside, that it could not have been scattered by the planets there.

Just outside Neptune's orbit another strange thing happens. The cumulative mass of all the objects dramatically drops by almost three orders of magnitude. This happens at approximately the same distance where everything becomes messy. It might be coincidental, but such coincidences are rare in nature.

Susanne Pfalzner and her co-workers suggest that a star was approaching the Sun at an early stage, 'stealing' most of the outer material from the Sun's protoplanetary disk and throwing what was left over into inclined and eccentric orbits.

Performing thousands of computer simulations they checked what would happen when a star passes very close-by and perturbs the once larger disk. It turned out that the best fit for today's outer solar systems comes from a perturbing star which had the same mass as the Sun or somewhat lighter (0.5-1 solar mass) and flew past at approximately three times the distance of Neptune.

However, the most surprising thing for the researchers was that a fly-by does not only explain the strange orbits of the objects of the outer solar system, but also gives a natural explanation for several unexplained features of our solar system, including the mass ratio between Neptune and Uranus, and the existence of two distinct populations of Kuiper Belt objects.

"It is important to keep exploring all the possible avenues for explaining the structure of the outer solar system. The data are increasing but still too sparse, so theories have a lot of wiggle room to develop," says Pedro Lacerda from the Queen's University in Belfast, a co-author of the paper. "There is a certain danger that one theory crystallises as truth, not because it explains the data better but because of other pressures. Our paper shows that a lot of what we currently know can be explained by something as simple as a stellar fly-by."

The big question is the likelihood for such an event. Nowadays, fly-bys even hundreds of times more distant are luckily rare. However, stars like our Sun are typically born in large groups of stars which are much more densely packed. Therefore, close fly-bys were significantly more common in the distant past. Performing another type of simulation, the team found that there was a 20%-30% chance of experiencing a fly-by over the first billion years of the Sun's life.

This is no final proof that a stellar fly-by caused the messy features of the outer solar system, but it can reproduce many observational facts and seems relatively realistic. So far it is the simplest explanation and if simplicity is a sign for validity this model is the best candidate so far.

"In summary, our close fly-by scenario offers a realistic alternative to present models suggested to explain the unexpected features of the outer solar system," concludes Susanne Pfalzner. "It should be considered as an option for shaping the outer solar system. The strength of the fly-by hypothesis lies in the explanation of several outer solar system features by one single mechanism."