Category: Asteroids & Comets

Voyager 2 Reaches Interstellar Space

 

Researchers at the University of Iowa report that the spacecraft Voyager 2 has entered the interstellar medium (ISM), the region of space outside the bubble-shaped boundary produced by wind streaming outward from the sun. Voyager 2, thus, becomes the second human-made object to journey out of our sun’s influence, following Voyager 1’s solar exit in 2012.

In a new study, the researchers confirm Voyager 2’s passage on Nov. 5, 2018, into the ISM by noting a definitive jump in plasma density detected by an Iowa-led plasma wave instrument on the spacecraft. The marked increase in plasma density is evidence of Voyager 2 journeying from the hot, lower-density plasma characteristic of the solar wind to the cool, higher-density plasma of interstellar space. It’s also similar to the plasma density jump experienced by Voyager 1 when it crossed into interstellar space.

“In a historical sense, the old idea that the solar wind will just be gradually whittled away as you go further into interstellar space is simply not true,” says Iowa’s Don Gurnett, corresponding author on the study, published in the journal Nature Astronomy. “We show with Voyager 2 — and previously with Voyager 1 — that there’s a distinct boundary out there. It’s just astonishing how fluids, including plasmas, form boundaries.”

Gurnett, professor emeritus in the UI Department of Physics and Astronomy, is the principal investigator on the plasma wave instrument aboard Voyager 2. He is also the principal investigator on the plasma wave instrument aboard Voyager 1 and authored the 2013 study published in Science that confirmed Voyager 1 had entered the ISM.

Voyager 2’s entry into the ISM occurred at 119.7 astronomical units (AU), or more than 11 billion miles from the sun. Voyager 1 passed into the ISM at 122.6 AU. The spacecraft were launched within weeks of each other in 1977, with different mission goals and trajectories through space. Yet they crossed into the ISM at basically the same distances from the sun.

That gives valuable clues to the structure of the heliosphere — the bubble, shaped much like a wind sock, created by the sun’s wind as it extends to the boundary of the solar system.

“It implies that the heliosphere is symmetric, at least at the two points where the Voyager spacecraft crossed,” says Bill Kurth, University of Iowa research scientist and a co-author on the study. “That says that these two points on the surface are almost at the same distance.”

“There’s almost a spherical front to this,” adds Gurnett. “It’s like a blunt bullet.”

Data from the Iowa instrument on Voyager 2 also gives additional clues to the thickness of the heliosheath, the outer region of the heliosphere and the point where the solar wind piles up against the approaching wind in interstellar space, which Gurnett likens to the effect of a snowplow on a city street.

The Iowa researchers say the heliosheath has varied thickness, based on data showing Voyager 1 sailed 10 AU farther than its twin to reach the heliopause, a boundary where the solar wind and the interstellar wind are in balance and considered the crossing point to interstellar space. Some had thought Voyager 2 would make that crossing first, based on models of the heliosphere.

“It’s kind of like looking at an elephant with a microscope,” Kurth says. “Two people go up to an elephant with a microscope, and they come up with two different measurements. You have no idea what’s going on in between. What the models do is try to take information that we have from those two points and what we’ve learned through the flight and put together a global model of the heliosphere that matches those observations.”

The last measurement obtained from Voyager 1 was when the spacecraft was at 146 AU, or more than 13.5 billion miles from the sun. The plasma wave instrument is recording that the plasma density is rising, in data feeds from a spacecraft now so far away that it takes more than 19 hours for information to travel from the spacecraft to Earth.

“The two Voyagers will outlast Earth,” Kurth says. “They’re in their own orbits around the galaxy for five billion years or longer. And the probability of them running into anything is almost zero.”

“They might look a little worn by then,” Gurnett adds with a smile.

The Iowa study is one of five papers on Voyager 2 published in Nature Astronomy. These papers confirm the passage of Voyager 2 to interstellar space and provide details on the characteristics of the heliopause.

Gurnett and Kurth are the study’s sole authors. Their research was funded by NASA, through a contract with the Jet Propulsion Laboratory.

Gas ‘Waterfalls’ Reveal Infant Planets Around Young Star

 

The birthplaces of planets are disks made out of gas and dust. Astronomers study these so-called protoplanetary disks to understand the processes of planet formation. Beautiful images of disks made with the Atacama Large Millimeter/submillimeter Array (ALMA) how distinct gaps and ring features in dust, which may be caused by infant planets.

To get more certainty that these gaps are actually caused by planets, and to get a more complete view of planet formation, scientists study the gas in the disks in addition to dust. 99 percent of a protoplanetary disk’s mass is gas, of which carbon monoxide (CO) gas is the brightest component, emitting at a very distinctive millimeter-wavelength light that ALMA can observe.

Last year, two teams of astronomers demonstrated a new planet-hunting technique using this gas. They measured the velocity of CO gas rotating in the disk around the young star HD 163296. Localized disturbances in the movements of the gas revealed three planet-like patterns in the disk.

In this new study, lead author Richard Teague from the University of Michigan and his team used new high-resolution ALMA data from the Disk Substructures at High Angular Resolution Project (DSHARP) to study the gas’s velocity in more detail. “With the high fidelity data from this program, we were able to measure the gas’s velocity in three directions instead of just one,” said Teague. “For the first time, we measured the motion of the gas rotating around the star, towards or away from the star, and up- or downwards in the disk.”

Unique gas flows

Teague and his colleagues saw the gas moving from the upper layers towards the middle of the disk at three different locations. “What most likely happens is that a planet in orbit around the star pushes the gas and dust aside, opening a gap,” Teague explained. “The gas above the gap then collapses into it like a waterfall, causing a rotational flow of gas in the disk.”

This is the best evidence to date that there are indeed planets being formed around HD 163296. But astronomers cannot say with one hundred percent certainty that the gas flows are caused by planets. For example, the star’s magnetic field could also cause disturbances in the gas. “Right now, only a direct observation of the planets could rule out the other options. But the patterns of these gas flows are unique and it is very likely that they can only be caused by planets,” said co-author Jaehan Bae of the Carnegie Institution for Science, who tested this theory with a computer simulation of the disk.

The location of the three predicted planets in this study correspond to the results from last year: they are likely located at 87, 140 and 237 AU. (An astronomical unit — AU — is the average distance from the Earth to the Sun.) The closest planet to HD 163296 is calculated to be half the mass of Jupiter, the middle planet is Jupiter-mass, and the farthest planet is twice as massive as Jupiter.

Planet atmospheres

Gas flows from the surface towards the midplane of the protoplanetary disk have been predicted by theoretical models to exist since the late ’90s, but this is the first time that they have been observed. Not only can they be used to detect infant planets, they also shape our understanding of how gas giant planets obtain their atmospheres.

“Planets form in the middle layer of the disk, the so-called midplane. This is a cold place, shielded from radiation from the star,” Teague explained. “We think that the gaps caused by planets bring in warmer gas from the more chemically active outer layers of the disk, and that this gas will form the atmosphere of the planet.”

Teague and his team did not expect that they would be able to see this phenomenon. “The disk around HD 163296 is the brightest and biggest disk we can see with ALMA,” said Teague. “But it was a big surprise to actually see these gas flows so clearly. The disks appears to be much more dynamic than we thought.”

“This gives us a much more complete picture of planet formation than we ever dreamed,” said co-author Ted Bergin of the University of Michigan. “By characterizing these flows we can determine how planets like Jupiter are born and characterize their chemical composition at birth. We might be able to use this to trace the birth location of these planets, as they can move during formation.”

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Meteor Magnets In Outer Space: Finding Elusive Giant Planets

Astronomers believe planets like Jupiter shield us from space objects that would otherwise slam into Earth. Now they’re closer to learning whether giant planets act as guardians of solar systems elsewhere in the galaxy.

A UCR-led team has discovered two Jupiter-sized planets about 150 light years away from Earth that could reveal whether life is likely on the smaller planets in other solar systems.

“We believe planets like Jupiter have profoundly impacted the progression of life on Earth. Without them, humans might not be here to have this conversation,” said Stephen Kane, lead study author and UCR associate professor of planetary astrophysics. “Understanding how many other stars have planets like Jupiter could be very important for learning about the habitability of planets in those systems.”

Along with liquid water oceans, Kane said astronomers believe such planets have the ability to act as ‘slingshots,’ pulling objects like meteors, comets, and asteroids out of their trajectories en route to impact with small, rocky planets.

Many larger planets have been found close to their stars. However, those aren’t as useful for learning about the architecture of our own solar system, where the giant planets including Saturn, Uranus and Neptune are all farther from the Sun. Big planets far from their stars have, until now, been harder to find.

A study recently accepted for publication in the Astronomical Journal details how Kane’s team found success in a novel approach combining traditional detection methods with the latest technologies.

One popular method of searching for exoplanets — planets in other solar systems — involves monitoring stars for “wobble,” in which a star moves toward and away from Earth. The wobble is likely caused by the gravitational pull a nearby planet is exerting on it. When a star wobbles, it’s a clue there may be an exoplanet nearby.

When the planet is far from its star, the gravitational pull is weaker, making the wobble smaller and harder to detect. The other problem with using the wobble detection method, Kane said, is that it just takes a long time. Earth only takes a year to orbit the sun. Jupiter takes 12, Saturn takes 30, and Neptune takes an astonishing 164 years.

The larger exoplanets also take many years to circle their stars, which means observing a complete orbit could engulf an astronomer’s entire career. To accelerate the process, Kane and his team combined the wobble method with direct imaging. This way, if the team thought a planet might be causing wobble, they could confirm it by sight.

Obtaining a direct image of a planet quadrillions of miles away is no simple task. It requires the largest possible telescope, one that is at least 32 feet long and highly sensitive. Even from this distance, the light of the stars can overexpose the image, obscuring the target planets.

The team overcame this challenge by learning to recognize and eliminate the patterns in their images created by starlight. Removing the starlight allowed Kane’s team to see what remained.

“Direct imaging has come a long way both in terms of understanding the patterns we find, and in terms of the instruments used to create the images, which are much higher resolution than they’ve ever been,” Kane said. “You see this every time a new smartphone is released — the camera detectors are always being improved and that’s true in astronomy as well.”

In this project, the team applied the combination of wobble and imaging method to 20 stars. In addition to the two being orbited by giant Jupiter-like planets that had not been previously discovered, the team also detected a third, previously observed star with a giant planet in its system.

Going forward, the team will continue to monitor 10 of the stars where planetary companions could not be ruled out. In addition, Kane is planning a new project to measure how long it takes these exoplanets to complete rotations toward and away from their stars, which cannot currently be measured.

Kane’s team is international, with members at the Australian Astronomical Observatory, University of Southern Queensland, University of New South Wales and Macquarie University in Australia, as well as at the University of Hertfordshire in the United Kingdom. They are also spread across the U.S. at the National Optical Astronomy Observatory in Tucson, AZ, Southern Connecticut State University, NASA Ames Research Center and Stanford University in California and the Carnegie Institution of Washington in D.C.

“This discovery is an important piece of the puzzle because it helps us understand the factors that make a planet habitable and whether that’s common or not,” said Kane. “We are converging rapidly on answers to this question that the past 3,000 recorded years of history could only wish they had available to them.”

Iron Volcanoes May Have Erupted on Metal Asteroids

Metallic asteroids are thought to have started out as blobs of molten iron floating in space. As if that’s not strange enough, scientists now think that as the metal cooled and solidified, volcanoes spewing liquid iron could have erupted through a solid iron crust onto the surface of the asteroid.

This scenario emerged from an analysis by planetary scientists at UC Santa Cruz whose investigation was prompted in part by NASA’s plans to launch a probe to Psyche, the largest metallic asteroid in the solar system. Francis Nimmo, professor of Earth and planetary sciences, said he was interested in the composition of metallic asteroids indicated by analyses of iron meteorites, so he had graduate student Jacob Abrahams work on some simple models of how the asteroids cooled and solidified.

“One day he turned to me and said, ‘I think these things are going to erupt,'” Nimmo said. “I’d never thought about it before, but it makes sense because you have a buoyant liquid beneath a dense crust, so the liquid wants to come up to the top.”

The researchers described their findings in a paper that has been accepted for publication in Geophysical Research Letters.

Metallic asteroids originated early in the history of the solar system when planets were beginning to form. A protoplanet or “planetesimal” involved in a catastrophic collision could be stripped of its rocky outer layers, exposing a molten, iron-rich core. In the cold of space, this blob of liquid metal would quickly begin to cool and solidify.

As for what the iron volcanoes would look like, Abrahams said it depends on the composition of the melt. “If it’s mostly pure iron, then you would have eruptions of low-viscosity surface flows spreading out in thin sheets, so nothing like the thick, viscous lava flows you see on Hawaii,” he said. “At the other extreme, if there are light elements mixed in and gases that expand rapidly, you could have explosive volcanism that might leave pits in the surface.”

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Japan Probe Prepares to Blast Asteroid from Hayabusa2

A Japanese probe began descending towards an asteroid on Thursday on a mission to blast a crater into its surface and collect material that could shed light on the solar system’s evolution.

The mission will be the latest in a series of explorations carried out by the Japanese space agency’s Hayabusa2 probe and could reveal more about the origin of life on Earth.

But the task scheduled for Friday will be the riskiest yet of Hayabusa2’s investigations, and involves the release of a device filled with explosives.

The so-called “small carry-on impactor”, a cone-shaped device capped with a copper bottom, will emerge from Hayabusa2 on Friday, after the probe has arrived just 500 meters above the asteroid Ryugu.

The probe will then depart the area, and the impactor is programmed to explode 40 minutes later, propelling the copper bottom towards Ryugu, where it should gouge a crater into the surface of the asteroid that sits 300 million kilometers from Earth.

Hayabusa2 will move away from the area to avoid being damaged by debris from the explosion or the collision with Ryugu.

As it does so it will release a camera slightly above the site of the detonation that should be able to capture images of the event.

Hubble Watches Asteroid Coming Apart

A small asteroid has been caught in the process of spinning so fast it’s throwing off material, according to new data from NASA’s Hubble Space Telescope and other observatories.

Images from Hubble show two narrow, comet-like tails of dusty debris streaming from the asteroid (6478) Gault. Each tail represents an episode in which the asteroid gently shed its material—key evidence that Gault is beginning to come apart.

Discovered in 1988, the 2.5-mile-wide (4-kilometer-wide) asteroid has been observed repeatedly, but the debris tails are the first evidence of disintegration. Gault is located 214 million miles (344 million kilometers) from the Sun. Of the roughly 800,000 known asteroids between Mars and Jupiter, astronomers estimate that this type of event in the asteroid belt is rare, occurring roughly once a year.

Watching an asteroid become unglued gives astronomers the opportunity to study the makeup of these space rocks without sending a spacecraft to sample them.

“We didn’t have to go to Gault,” explained Olivier Hainaut of the European Southern Observatory in Germany, a member of the Gault observing team. “We just had to look at the image of the streamers, and we can see all of the dust grains well-sorted by size. All the large grains (about the size of sand particles) are close to the object and the smallest grains (about the size of flour grains) are the farthest away because they are being pushed fastest by pressure from sunlight.”

Gault is only the second asteroid whose disintegration has been strongly linked to a process known as a YORP effect. (YORP stands for “Yarkovsky-O’Keefe-Radzievskii-Paddack,” the names of four scientists who contributed to the concept.) When sunlight heats an asteroid, infrared radiation escaping from its warmed surface carries off angular momentum as well as heat. This process creates a tiny torque that can cause the asteroid to continually spin faster. When the resulting centrifugal force starts to overcome gravity, the asteroid’s surface becomes unstable, and landslides may send dust and rubble drifting into space at a couple miles per hour, or the speed of a strolling human. The researchers estimate that Gault could have been slowly spinning up for more than 100 million years.

Piecing together Gault’s recent activity is an astronomical forensics investigation involving telescopes and astronomers around the world. All-sky surveys, ground-based telescopes, and space-based facilities like the Hubble Space Telescope pooled their efforts to make this discovery possible.

The initial clue was the fortuitous detection of the first debris tail, observed on Jan. 5, 2019, by the NASA-funded Asteroid Terrestrial-Impact Last Alert System (ATLAS) telescope in Hawaii. The tail also turned up in archival data from December 2018 from ATLAS and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) telescopes in Hawaii. In mid-January, a second shorter tail was spied by the Canada-France-Hawaii Telescope in Hawaii and the Isaac Newton Telescope in Spain, as well as by other observers. An analysis of both tails suggests the two dust events occurred around Oct. 28 and Dec. 30, 2018.

Follow-up observations with the William Herschel Telescope and ESA’s (European Space Agency) Optical Ground Station in La Palma and Tenerife, Spain, and the Himalayan Chandra Telescope in India measured a two-hour rotation period for the object, close to the critical speed at which a loose “rubble-pile” asteroid begins to break up.

Astronomers Discover 83 Supermassive Black Holes In The Early Universe

Astronomers from Japan, Taiwan and Princeton University have discovered 83 quasars powered by supermassive black holes in the distant universe, from a time when the universe was less than 10 percent of its present age.

“It is remarkable that such massive dense objects were able to form so soon after the Big Bang,” said Michael Strauss, a professor of astrophysical sciences at Princeton University who is one of the co-authors of the study. “Understanding how black holes can form in the early universe, and just how common they are, is a challenge for our cosmological models.”

This finding increases the number of black holes known at that epoch considerably, and reveals, for the first time, how common they are early in the universe’s history. In addition, it provides new insight into the effect of black holes on the physical state of gas in the early universe in its first billion years. The research appears in a series of five papers published in The Astrophysical Journal and the Publications of the Astronomical Observatory of Japan.

Supermassive black holes, found at the centers of galaxies, can be millions or even billions of times more massive than the sun. While they are prevalent today, it is unclear when they first formed, and how many existed in the distant early universe. A supermassive black hole becomes visible when gas accretes onto it, causing it to shine as a “quasar.” Previous studies have been sensitive only to the very rare, most luminous quasars, and thus the most massive black holes. The new discoveries probe the population of fainter quasars, powered by black holes with masses comparable to most black holes seen in the present-day universe.

The research team used data taken with a cutting-edge instrument, “Hyper Suprime-Cam” (HSC), mounted on the Subaru Telescope of the National Astronomical Observatory of Japan, which is located on the summit of Maunakea in Hawaii. HSC has a gigantic field-of-view — 1.77 degrees across, or seven times the area of the full moon — mounted on one of the largest telescopes in the world. The HSC team is surveying the sky over the course of 300 nights of telescope time, spread over five years.

The team selected distant quasar candidates from the sensitive HSC survey data. They then carried out an intensive observational campaign to obtain spectra of those candidates, using three telescopes: the Subaru Telescope; the Gran Telescopio Canarias on the island of La Palma in the Canaries, Spain; and the Gemini South Telescope in Chile. The survey has revealed 83 previously unknown very distant quasars. Together with 17 quasars already known in the survey region, the researchers found that there is roughly one supermassive black hole per cubic giga-light-year — in other words, if you chunked the universe into imaginary cubes that are a billion light-years on a side, each would hold one supermassive black hole.

The sample of quasars in this study are about 13 billion light-years away from the Earth; in other words, we are seeing them as they existed 13 billion years ago. As the Big Bang took place 13.8 billion years ago, we are effectively looking back in time, seeing these quasars and supermassive black holes as they appeared only about 800 million years after the creation of the (known) universe.

It is widely accepted that the hydrogen in the universe was once neutral, but was “reionized” — split into its component protons and electrons — around the time when the first generation of stars, galaxies and supermassive black holes were born, in the first few hundred million years after the Big Bang. This is a milestone of cosmic history, but astronomers still don’t know what provided the incredible amount of energy required to cause the reionization. A compelling hypothesis suggests that there were many more quasars in the early universe than detected previously, and it is their integrated radiation that reionized the universe.

“However, the number of quasars we observed shows that this is not the case,” explained Robert Lupton, a 1985 Princeton Ph.D. alumnus who is a senior research scientist in astrophysical sciences. “The number of quasars seen is significantly less than needed to explain the reionization.” Reionization was therefore caused by another energy source, most likely numerous galaxies that started to form in the young universe.

The present study was made possible by the world-leading survey ability of Subaru and HSC. “The quasars we discovered will be an interesting subject for further follow-up observations with current and future facilities,” said Yoshiki Matsuoka, a former Princeton postdoctoral researcher now at Ehime University in Japan, who led the study. “We will also learn about the formation and early evolution of supermassive black holes, by comparing the measured number density and luminosity distribution with predictions from theoretical models.”

Based on the results achieved so far, the team is looking forward to finding yet more distant black holes and discovering when the first supermassive black hole appeared in the universe.