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Astro Update, March 2019

Selected Summaries of Space News

More New Horizons Results

More data has been received from the New Horizons spacecraft since its recent flyby of the Kuiper Belt object informally named Ultima Thule. One new result is that the larger of the object’s two lobes is not so much spherical, as thick-pancake-shaped. This shape was determined from examining what stars were blocked by that lobe in images taken from the object’s night side.

Both lobes display similar reflectivity and color, implying that they formed in the same way and then gently collided. No moons, rings or atmosphere were detected. The rotation is slower than expected for such a contact binary. Because of the slow transmission rate of the tiny spacecraft radio, we can expect new data until at least summer next year and new discoveries beyond that.

The flyby was within 1% of its aim point and 19 seconds of the arrival time predicted a billion miles earlier.

Opportunity Lost

On February 13, NASA declared the Mars rover Opportunity lost. It had not responded since last June, in spite of hundreds of attempts to raise it by radio. The massive Martian dust storm of last May and June had reduced the sunlight to the point the rover’s solar panels could not charge its batteries, and so Opportunity’s computer shut the rover down. There was hope that after the storm passed, the returning sunlight would restore the rover to operation, but apparently the temperature dropped too low during the power outage, ruining some vital part.

Though its mission was designed for just 90 Martian days (about 92 Earth days), Opportunity operated for more than 14 years. The rover took over 200,000 pictures and traveled 28 miles (721 feet on its best day), all records. Opportunity’s achievements include finding evidence of past flowing water. NASA plans to send another rover to Mars in 2020, which is designed to search for signs of ancient life.

Curiosity Gravimeter

Mars rover Curiosity has accelerometers on it to allow precise navigation. Scientists figured out that they could use the accelerometer data as a gravimeter, measuring the gravitational pull of the mountain at Gale Crater. The mountain is less dense than it should be, if the theory of its formation is correct.

It had been thought that sometime after Gale Crater formed from an impact, it filled with water and sediment all the way to its rim. Then over billions of years the softer parts of the sediment eroded by wind, leaving the mountain. However, the weight of a crater completely full of sediment would have compressed the rock to greater density than the rover measured. So it appears the crater could not have filled more than halfway. Now the theorists have to tell us how the mountain grew to about the height of the crater rim without compressing the rock to greater density.

Mars Lander

The InSight spacecraft has finished testing and adjusting its seismometer, which it previously set on the Martian ground with its robotic arm. InSight’s drill has also been placed on the ground, about a yard from the seismometer. It will hammer a heat-sensing probe as much as 16 feet into the soil to measure its heat conductivity and the heat flowing out of the planet. Together with seismometer data, the heat results will help characterize Mars’ interior.

The hammering and calibration is expected to take about 40 days. When this is complete, InSight will never again move any of its parts, to avoid disturbing the seismometer. Even the radio antenna and weather station are designed to work without moving parts.

Venusian Cloud Streaks

Using infrared data from the Japanese Venus orbiter Akatsuki, researchers have found a pair of streaks that persist in the clouds of Venus. Computer simulations of the planet’s atmosphere suggest that polar jet streams converge at the location of the streaks, which forces downflow and creates the streaks.

Sub-Saturn Found

The OGLE survey looks for gravitational-lensing events caused by stars and planets passing in front of more-distant stars. OGLE just found another sub-Saturn-sized exoplanet—that is, one more massive than Neptune, but less than Saturn.

At least 30 sub-Saturns have been found by gravitational lensing, but the core-accretion theory of how gas giant and ice giant planets form says we should not have found so many. It is particularly problematic that the newly found OGLE planet lies beyond the snow line (the distance from its star beyond which starlight is too weak to melt ice), where current theory says sub-Saturns should not form.

Small Kuiper Belt Object Found

The OASES project has been using small telescopes for the past year to monitor hundreds of stars, hoping to detect dimming from outer solar system objects passing in front of them. They have spotted an object in the Kuiper Belt with a diameter somewhere in the range of 1-2 km. (Such a body is far too small for any telescope to see directly.) Theorists tell us that there ought to be lots of objects there, so now we may have found a way to find them.

Titan Rain

Even though the Cassini mission at Saturn has ended, its data continues to feed research. Images of Saturn’s moon Titan from June 2016 show what appears to be a reflection from wet ground in its northern hemisphere, presumably because methane rain just fell there. Rain has been expected in the north, since southern Titan had its methane rainy season about half a Saturn-year ago. The reflection was rough-looking, because the surface there is probably pebbly.

Saturn Day Measured

There are no permanent markings on Saturn to time the planet’s rotation, and attempts since Voyager (1980) to measure the rotation by the motion of the magnetic field have yielded varying results. (Apparently, the Saturnian magnetic field is too symmetric about the rotational axis for this to work.) The planet’s internal mass distribution was recently found to make waves in the rings, though this effect had been predicted theoretically, as long ago as 1982. Results of the new study of ring waves (from Cassini data) say Saturn’s day is 10 hours, 33 minutes, 38 seconds—several minutes faster than previous measurements.

Jupiter Cloud Cycle

Scientists studying long-term infrared observations of Jupiter have found a recurring cycle of 6-7 years in the clouds near the planet’s equator. In visible light, this cycle results in a darkening of the bright white, near-equatorial clouds for 12-18 months, roughly every seven years. The darkening is caused by a dissipation of ammonia clouds, which are bright white, allowing us to see lower, darker cloud decks. The scientists expect the next visible-light darkening this year, and it may have already started. The Juno spacecraft, currently in Jovian orbit, will monitor this.

More Impacts

A new study of the ages and sizes of meteorite craters on both the Moon and Earth shows that the rate of impacts rose about 300 million years ago. Since then, the pace of impacts has been about 2.6 times the rate during the previous hundreds of millions of years. (Lunar crater ages were measured by how fast their surface temperature changes at sundown, which depends on how long that surface has been pulverized by tiny meteorite impacts.) The increase may have been the result of a collision in the asteroid belt, but this is just one possibility. The study showed no change in the distribution of sizes of meteorites impacting as far back in time as could be measured.

Possibly Oldest Earth Rock

Where would you look to find the oldest rock on Earth? Weather and plate tectonics do a pretty good job of getting rid of old rocks here on Earth, so scientists looked through rocks brought back from the Moon by Apollo astronauts. Since we have found dozens of meteorites that fell to Earth that match the composition of Moon rocks, it seems likely that rocks get blasted the other direction when large meteorites crater Earth. (In fact, it probably happened more often billions of years ago, because the Moon orbited the Earth more closely, and so would sweep up more of Earth’s debris.) The researchers found one rock that analysis shows is extremely likely to have come from Earth.

The pressure and temperature required to have created the rock’s minerals indicate that it took shape about 12 miles below Earth’s surface 4-4.1 billion years ago, and so may be older than anything else we have found that formed on our planet. It was partially melted by an impact event on the Moon about 3.9 billion years ago, which buried it under lunar soil. The rock was then excavated by another impact about 26 million years ago, and then an astronaut picked it up in 1971.

Planet-Forming Disk

V883, a young star in Orion, is undergoing an outburst. This is causing icy objects in the planet-forming disk surrounding the star to sublimate, unleashing various chemicals that had been trapped in ice. This affords a great opportunity to observe what those chemicals are, adding to our knowledge of how planets form.

Astronomers are using ALMA (radio-telescope array in Chile) to do just that. They are finding chemicals similar to those found in comets in our solar system, including methanol, acetaldehyde, acetone, methyl formate, and acetonitrile. The distribution of the first two of those chemicals was mapped to be a ring twice the size of Neptune’s orbit. Those chemicals were undetectable at a greater radius from the star because they were still frozen inside ice, and probably not seen inside the ring due to obscuring dust.

Iron Planet Found

Astronomers have discovered a pair of super-Earth-sized exoplanets orbiting a star in Cygnus. One has a density similar to Earth’s, but the other is more than twice as dense. The discoverers think that the only way this could happen is if the now-very-dense one suffered a collision with some other planet sometime in the past, which blasted off most of the previous rocky crust and mantle, leaving little more than a dense iron core. If so, this is the first evidence that any exoplanet has survived a catastrophic collision.

Cryovolcano (Almost) Explained

More than three years ago, the Dawn spacecraft found salt deposits on the asteroid Ceres that are likely the result of geologically recent water/ice volcano (cryovolcanic) activity. Ceres, however, is too small to have retained enough heat since its formation to melt ice for such activity.

One explanation suggested that an impacting asteroid heated up Ceres to cause the vulcanism. But the 20-million-year age of the crater in which the activity took place was old enough that Ceres should have cooled off from the impact before the volcanic activity. A new study, though, concluded that material found in the crust of Ceres is acting to insulate heat almost enough to make the impact-causes-volcanic-activity theory work. More study is needed to eliminate that “almost.”

Dark Energy May Be Growing

A new study of distant quasars in both visible light and X-rays claims that scientists can now determine the absolute brightness of many quasars. (This would allow distance measurements to be calculated from apparent brightnesses, the same way astronomers do with individual stars.) Researchers applied their brightness formula to 1,600 quasars and then analyzed the expansion of the Universe at various distances, concluding that dark energy has increased in strength over billions of years. However, similar analysis using the brightness of Type Ia supernovas does not show the dark energy change. More work is needed to resolve this discrepancy.

Black Hole Outburst

A year ago, a stellar-mass black hole, MAXI J1820+070, suddenly started consuming material from its close companion star, becoming the brightest X-ray source in the sky. The X-rays are emitted by both the accretion disk (where infalling material orbits before making it into the black hole) and an even hotter corona surrounding the disk. (The disk and corona of this black hole are too small to be resolved by any current technology, but their emissions can be distinguished by their different energies.) In addition, X-rays are reflecting off material farther out from the black hole, arriving later than X-rays that take a direct path to us.

Two X-ray instruments mounted on the International Space Station have been monitoring the situation for the past year. Analysis of the data shows that the corona shrunk during the outburst from about 100 miles across to about 10 miles. One configuration that fits the data is that the corona is located at the base of a pair of jets, whose activity varies with the amount of material falling in. The details of how material falls into black holes and produces X-rays are not well known, so study of such events will help solve this.

Huge Data Release

Pan-STARRS is a 1.8-meter telescope with a 1.4-billion-pixel camera, which has been in full operation for four years. It shoots the entire sky available from Maui about 15 times per year (three repeats in five colors). Computer processing finds anything that moves or changes from the baseline survey, such as asteroids, variable stars, and supernovas. The Pan-STARRS team just released its second data set, consisting of 1.6 billion megabytes. It contains a catalog of three billion sources, a composite picture of all the sky available there, and all the individual images. It’s the largest astronomical data release ever made. The release can be accessed by anyone, even amateurs.

Warped Galaxy Map

In an effort to define the shape of the Milky Way’s arms, a team of astronomers mapped out over 1,300 Cepheid-variable stars in three dimensions. They found the map did a better job of defining the warps in our galaxy’s disk. (Causes of galaxy warpage are thought to include gravitational pull of passing satellite galaxies, magnetic fields, interstellar winds, and misalignment with the galaxy’s halo.)

Computer modeling showed that it would take multiple pushes on the Milky Way to attain the newly measured shape. Limits on the dimness of Cepheids that can be observed allowed only about 2/3 of the Milky Way to be mapped. The astronomers hope to eventually extend their map to the whole galaxy.

Andromeda Collision

You have probably heard that the Andromeda galaxy (M31) is heading toward us and is going to collide with our Milky Way galaxy in a few billion years. Also, you have probably heard different time estimates for that collision. That’s because the relative motion of Andromeda is very difficult to measure in the right-left and up-down dimensions. (We have a very accurate number for forward-back motion, because that can be measured by redshift.)

However, the recent release of data from the Gaia spacecraft includes extremely accurate motions in all three dimensions for over a billion stars, including scads of them in M31. A team of astronomers used the Gaia data and distinguished what part of more than 1,000 Andromeda stars’ motions is rotation, and what part is motion of the galaxy as a whole. They also did the same for the Triangulum galaxy (M33). Then they computer-simulated what the current galaxy motions and gravity are going to do for the next several billion years.

Their result is that Andromeda and the Milky Way will merge about 4.5 billion years from now. Because of the just-measured sideways motion of Andromeda of about 30 miles per second, it will be a glancing blow, not head-on. Also determined from this study is that M33 is making its first approach to M31, contrary to earlier theories that had it orbiting. This may mean that the Local Group of galaxies (of which the Milky Way, M31, and M33 are the heavyweight members) may have come together more recently than thought.

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