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Archives

Astro Update, January 2019

Selected Summaries of Space News

by Don Lynn

Phobos Grooves Explained

Since the first close-up images were taken of the Martian moon Phobos decades ago, scientists have argued about why there are long grooves on its surface. Leading theories were that the moon cracked from an impact or from tidal forces. But there were problems applying any of the theories to the observed grooves. A new computer simulation of an impact on Phobos suggests that the grooves were formed by boulders rolling across the surface after they were thrown out by the impact that formed the largest crater on that moon.

Sun’s Sibling Found

Stars generally form in a cluster, then scatter over time. Astronomers would like to find the stars that formed in a cluster with our Sun, though they may now be spread around our galaxy. So a team of astronomers searched archived observations for stars that are both the same age as the Sun, and that have spectra showing the same distribution of elements.

As it happens, out of hundreds of thousands of stars, only one was found to match the Sun, a star known as HD 186302. By chance, this star has nearly the same mass and size as the Sun. A search for planets orbiting this star will be made.

Another Strange Variable Star

A star that varied in brightness irregularly in a manner unlike any other star was announced in 2015. Dubbed Tabby’s Star after Tabetha Boyajian, the lead author, the star’s characteristics prompted a wide variety of explanations, including a gigantic structure built by aliens. The best current theory is that multiple dust clouds orbiting the star cause the variations.

Any theory for Tabby’s Star now has to explain another star recently found to behave similarly. A search of existing data found a star, VVV-WIT-07, that showed dimming in 2012 somewhat similar to Tabby’s Star. Observations of both will continue to try to pin down the cause with certainty.

Rogue Planets

Astronomers have found what are probably two more rogue planets, that is, ones that are not orbiting any star. Theory predicts that billions of planets should escape from planetary systems in our galaxy, but only about a dozen rogue planets are known. That’s not surprising, because such planets lie far from any stars that might light them, and are thus extremely dim and very difficult to find.

The suspected new rogue planets were discovered by examining microlensing data from the OGLE project. A planet that happens to pass in front of a star causes a small gravitational lens effect, brightening the star. A rough estimate of mass can then be calculated from the amount of lensing. The first of the newly found objects is somewhere in the mass range between Earth and Neptune, while the second is between Jupiter and a brown dwarf star.

Helium Atmospheres Found

Two recent studies have found evidence of helium atmospheres on exo-planets. A trail of helium escaping from planet WASP-69b was detected; it extends roughly 110,000 miles into space. The Saturn-sized planet lies about 160 light-years away and orbits its star about every four Earth days.

A huge, puffed-up atmosphere (perhaps due to heat from its star) was found surrounding a roughly Neptune-sized planet known as HAT-P-11b, which orbits a star about 120 light-years from us. Only a few exo-planets have been found to have helium atmospheres, probably because they’re difficult to detect with current technology. The first such detection was just last May.

Voyager 2 Leaves Heliosphere

On November 5, instruments aboard Voyager 2 indicated that it had become the second spacecraft (after Voyager 1) to leave the heliosphere, the bubble in which the Sun’s magnetic field and particle wind overcome interstellar particles and fields. Voyager 2 is currently over 11 billion miles away; from that distance, its radio signal takes over 16 hours to reach us.

The spacecraft’s plasma instrument should provide unique data about interstellar space, since Voyager 1’s instrument broke before the craft left the heliosphere. Both Voyagers were launched in 1977 to explore the outer planets, and finished visiting those planets in 1989. The Oort Cloud, composed of comets orbiting the Sun, stretches much farther, so the Voyagers may take as much as 30,000 years to leave the Oort Cloud.

Osiris-REX

The Osiris-REX spacecraft arrived at its target, tiny (0.3 miles across) near-Earth asteroid Bennu on December 3rd. It then spent 20 days holding at a distance of 12 miles to survey Bennu from all sides. By the time you read this, the spacecraft should be mapping the asteroid in great detail from only 2400 feet away. Bennu has a rare shape, somewhat like a pyramid and its reflection; strangely enough, a similar shape has just been seen for Ryugu, the asteroid currently being explored by the Japanese spacecraft Hayabusa-2.

About July 2020, Osiris-REX will make a series of gentle falls to Bennu’s surface to grab samples, to be returned to Earth in September 2023. The spacecraft has already made a major discovery: There are hydrated minerals on Bennu’s surface, which were likely formed in the presence of liquid water long ago. Bennu is too small to have ever held liquid water on its surface, so the creation of hydrated minerals probably occurred when the asteroid was part of a far larger body that subsequently broke apart.

Lunar Rover Launched

The Chinese probe Chang’e 4, the first-ever spacecraft designed to soft-land on the far side of our Moon, has successfully touched down on the South Pole-Aitken Basin, one of the few lava-filled plains on the far side.

The problem with maintaining radio contact with a far-side mission has been solved by the early-2018 launch of a radio-relay satellite, named Quequiao. It was placed so as to circulate about the lunar L2 Lagrange point, which is located behind the Moon.

Chang’e 4 will study lunar seismology, surface geology, the solar wind, low-frequency radio astronomy (there is no radio interference on the far side), and cosmic rays. The next Chinese space mission will be a lunar sample-return craft launching in late 2019. [Updated 1-5-19.  —Ed.]

Astronauts Launched

Since the failure of a Soyuz rocket to reach orbit while attempting to take astronauts to the International Space Station (ISS) in October, the ISS has been under-staffed, with only three occupants instead of the usual five or six. Soyuz return capsules have a limited life, so the remaining three astronauts were scheduled to return to Earth December 20th.

Space officials were worried that this meant ISS would go empty if the Russian rocket agency could not complete their investigation and fix Soyuz by that time. However, they finished their work early, and three more astronauts (an American, a Russian, and a Canadian) were successfully sent to ISS on a Soyuz on December 3rd. ISS will still have only three occupants after the December 20th departure, until another crew of three can be launched on February 28th.

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