Space Sunday: stars, a rover, a planet and a round-up

A Cataclysmic variable star, with the large main sequence “donor” star to the left, and the white dwarf “primary” taking mass from it, on the right. Credit: Mark A. Garlick

At the end of 2019, I wrote about the speculation around Betelgeuse and its recent behaviour (see: Space Sunday: a look at Betelgeuse). However, when it comes to a star going nova or supernova, there is a candidate out there that will more than likely do so before the end of this century.

V Sagittae, a variable star in the constellation Sagitta that is actually a binary system some 1100 light years away. In particular, it is in a class of stars called cataclysmic variables (CVs). These comprise a large, ordinary star orbiting a much smaller white dwarf at a distance where the white dwarf (referred to as the primary) distorts the larger (called the donor), drawing off mass to form an accretion disk around itself.

Usually, material at the inner edge of this disk heats as a result of both increasing density and proximity to the primary, causing dwarf nova outbursts, resulting in the pair to suddenly brighten. In some cases the material drawn into the primary can trigger a nova explosion or even a Type Ia supernova explosion, which would completely destroy the white dwarf.

Sagitta (the Arrow) is a constellation on the edge of the “Summer Triangle” defined by Vega (in Lyra), Deneb (in Cygnus), and Altair (in Aquila). The Arrow consists of five stars in an arrow-shape, pointing right at V Sge. Image Credit: Schaefer et al 2019.

However, the V Sagittae is amongst the most extreme group of CV stars, with the donor star almost 4 times as massive as its companion, and an extremely hot star. The material being drawn off of it is correspondingly hot (around 12,000ºK), and on contact with the white dwarf is either being accreted, building up the white dwarf’s mass and heat, or violently accelerated away in an extreme solar wind. This wind irradiates the inner hemisphere of the donor, further heating it, fuelling a circle of activity that is seeing mass transferred between the two at an ever-faster rate.

Checks back through archival images show the binary has been steadily brightening for around a century – and the rate of brightening has been accelerating. The net result of this is that astronomers believe V Sagittae is now in the closing decades of this life: over the next several decades, it will continue to brighten and the mass transfer accelerate further. Around the 2080s this will reach a point where the donor will spiral into the primary, triggering a catastrophic nova – one so powerful it could, with the assistance of the extreme solar wind, border on a supernova event.

Astronomers studying the system are so confident of their findings, they are prepared to to put a year on when the collision and subsequent explosion will occur: 2083 ±16 years.

When it happens, the normally faint V Sagittae will be between Venus and Sirius in brightness – not as bright as any supernova from Betelgeuse by any means, but it will be bright enough for even casual observers of the night sky to notice it – although it will be somewhat short-lived as a “new star”; after a month or so, it will dim down once more. The explosion itself will totally destroy whatever was left of the donor star at the time it occurred, with the primary likely converted into a red giant with a core of degenerate electron matter, surrounded by a hydrogen burning shell in turn layer surrounded by a vast halo of mostly hydrogen.

Happy Anniversary Yutu-2 and Chang’e 4

China greeted the New Year with some impressive lunar milestones. January 3rd, 2020 the Chang’e 4 mission to the lunar far side achieved its first anniversary of surface operations, while its Yutu-2 mini-rover completed its 13th lunar day of science operations.

Yutu-2 in particular has proven impressive. Designed to have a primary mission of 90 days, it has survived a full year of operations, which include the little rover having to put itself to sleep for 14 out of the 28 days of a lunar day in order to survive the cold lunar night, and it is still going strong. In that time, it has travelled 357.7m, the longest distance travelled by any vehicle on the lunar surface.

January 3rd, 2019: a shot of the Yutu 2 rover’s wheels ahead of the rover being commanded to roll off the Chang’e 4 lander (l) and as it slowly drives away from the lander (r). Credit: China National Space Administration via Xinhua News Agency

In during so, the rover has revealed a great deal about the composition of lunar soil – regolith – within the South Pole Aitken-Basin, including about materials that are believed to have originated from deep inside the lunar mantle. This has helped scientists understand more about the composition, formation, and evolution of the Earth-Moon system. It’s anticipated that Yutu-2 will continue these  explorations, helping to better understand the Moon’s composition and locate accessible resources that might be used in establishing and operating a permanent lunar base.

Chang’e 4 is part of a broader Chinese Lunar Exploration Programme, which includes the Chang’e 5 sample return mission scheduled for launch towards the end of 2020. It will be followed by a second sample return mission in 2024, (Chang’e 6), while Chang’e 7 will continue and extend the work of Chang’e 4, and Chang’e 8, scheduled for a 2027 launch, will test technologies and lay the groundwork for a crewed mission.

Circumbinary Planets

We’re all probably familiar with the scene: Luke Skywalker walks from his igloo-abode, kicks the sand in frustration and then stops to look out beyond the horizon to where twin suns, white and yellow, dip towards the horizon, their light reflected in Luke’s moody gaze as the music swells…

At the time when Star Wars: A New Hope was released, the idea of circumbinary planets – those that orbit not one, but two stars – appeared to be a thing of pure science fiction, hence the exotic look to Tatooine’s sky. It wasn’t until 2003 that the first such planet was confirmed, in the system PSR B1620-26, a millisecond pulsar and a white dwarf, both orbited by a planet some 2.5 times bigger than Jupiter.

Perhaps the most famous (if fictional) circumbinary planet we know… Credit: Lucasfilm ; 20th Century Fox

Since then, more than 20 such planets have been located – although they are hard to detect, because while they are orbiting two companion stars, the stars are also orbiting one another, making any planetary transits. It’s now been confirmed that TESS – the Transiting Exoplanet Survey Satellite – has found its first circumbinary planet.

Called TOI 1338 b, the planet is estimated to be between Neptune and Saturn in size; it is orbiting a binary star system called TOI 1338, in the constellation Pictor and about 1300 light years away. One of the stars it orbits is about 10% more massive than our Sun, while the other is cooler and dimmer, and only one third of the Sun’s mass. One of the interesting aspects of TOI 1338 b is that it is on almost the same plane as the two stars, so it experiences a solar eclipse every 15 days terrestrial days.

These eclipses have also made finding the planet that much harder – the regular changes in the amount of light being received from the binary system makes detecting any dips in brightness caused by the planet’s passage in front of the larger of the two stars harder to confirm (the smaller star is too faint for planetary transits in front of it to be readily detected from Earth). In the end, it was down to a high school intern – Wolf Cukier – to realise a planet was present. Working at NASA’s Goddard Space Centre, he spent a considerable amount of time pouring over the TESS TOI 1338 data in order to confirm planetary transits across the larger star.

A further complication in pinning down TOI 1338 b’s presence is that its orbit is variable, so its transits of the larger star are non-periodic, varying between every 93 and 95 days. Further, a certain amount of luck was involved in confirming its presence. Whilst the planet is in the same plane at the two stars, its orbit is such that there are 8-year periods when transits cannot be seen from Earth, and the next is due to start in November 2023.

Since 2015, the number of circumbinary planets has been increasingly steadily as astronomers have learned how to better identify them in data gathered in observing distant stars and star systems, while systems like TOI 1338 and Kepler 413 suggesting such planets could be a lot more common in our galaxy.

A Spaceflight Rapid Round-up

NASA Artemis: SLS Core Stage Rolls Out and “the Turtles” are Ready to Fly

January 8th, 2020 marked the roll-out of the first core stage of NASA’s massive Space Launch System (SLS) booster. Measuring 65m (212 ft) in length, the core stage is intended to power the first Artemis flight, an uncrewed Orion vehicle, on its way around the Moon and back. The stage was rolled out of its assembly building at NASA’s Michoud Assembly Facility, New Orleans, accompanied by workers who helped build it, together with a jazz band. It then took a 2km road journey to the Pegasus barge that will transport it along the Mississippi for delivery to the Stennis Space Centre.

Once at Stennis, the stage will be installed on a test stand for months of tests, including a crucial “green run”. This will still the stage fully loaded with propellants and the four shuttle-derived RS-25 engines fired for about eight minutes, the length of time they’re used on an actual launch. The outcome of these tests and the checks and refurbishment that follow, will go a long way to determining whether or not the Artemis-1 mission will take place in late 2020 or slip to 2021.

Following this, on January 10th, NASA formally introduced the latest group of 13 Astronaut Candidates (“ascans”) to gain operational status. Accepted for training in June 2017 from 18,000 applications, the 13 candidates are now available for crew rotations aboard the space station, with some among them also having the opportunity to fly Artemis missions to the Moon.

Nicknamed The Turtles, on account of them commencing their training at the Johnson Space Centre, near Houston, Texas, just before major parts of that city were flooded by hurricane Harvey, they bring the total number of active NASA astronauts up to 48. The group comprises seven men and six women among their number, including 2 Canadians, with six of them from non-military backgrounds.

The Turtles are ready to fly (l to r): Jonny Kim, Joshua Kutryk, Jessica Watkins, Jenni Sidey-Gibbon, Francisco Rubio, Kayla Barron, Jasmin Moghbeli, Loral O’Hara, Zena Cardman, Raja Chari, Matthew Dominick, Bob Hines, and Warren Hoburg Credit: NASA

NASA commercial Crew: Starliner Awaits Review; Crew Dragon Test Delayed

The next steps in the Boeing CST-100 Starliner programme await the results of a joint NASA / Boeing investigation into the anomaly that caused the first uncrewed orbital flight test (OFT) to forego a planned rendezvous and docking with the International Space Station, and instead make an early return to Earth (see: Space Sunday: Starliner’s first orbital flight).

Boeing’s CST-100 on the ground at the end of its foreshortened OFT-1 flight, December 20th, 2019. Credit: Boeing

While the anomaly would not have been life-threatening had a crew been on the vehicle (in fact, a crew would likely have been able to intervene and correct the problem), NASA and Boeing want to understand exactly how the software error that caused it came to be, and ensure it is correctly mitigated. In addition, NASA wants to thoroughly evaluate the data that was gathered during the foreshortened flight – particularly that obtained while the craft was in orbit – to determine whether or not a further uncrewed test flight might be warranted.

In the meantime, an in-flight abort (IFA) test for SpaceX’s Crew Dragon vehicle, had been delayed by NASA, and will now not take place before at least January 18th, 2020, to give SpaceX additional preparation time. The flight will test the ability of the Crew Dragon’s launch abort system to get a crew safely clear of a malfunctioning booster during launch. If successful, the test will potentially clear Crew Dragon for crewed flights to / from the ISS.

A Crew Dragon capsule at the SpaceX processing facility at Kennedy Space Centre. Four of the 8 paired SuperDraco engines that are critical to crew aborts can be seen to the left and front centre of the capsule. Credit: SpaceX

Virgin’s Second SpaceShipTwo Passes Milestone

Virgin Galactic’s next SpaceShipTwo vehicle reached a construction milestone on January 8th, 2020, when the six-passenger spaceliner was set on its own undercarriage for the first time.

The move marked the end of the major assembly process for the vehicle at Virgin Galactic’s subsidiary, The Spaceship Company, based in Mojave, California. Work will now proceed with connecting the vehicle’s integrated systems, including the flight control systems, prior to fitting-out the cabin. There will then be a period of integrated vehicle ground testing to verify the integrity of the vehicle and its system, prior to flight tests commencing.

The new Virgin Galactic SpaceShipTwo vehicle under assembly and integration at The Spaceship Company, Mojave, California. The completed VSS Unity, undergoing touch-up, can be partially seen on the left of the photo. Credit: Virgin Galactic

At the same time, the only remaining flight-ready SpaceShipTwo, VSS Unity, is undergoing “touch-up” in readiness for transfer to Virgin Galactic’s commercial flight operations centre at Spaceport America, New Mexico. Once there, it will complete its final test flights prior to starting on passenger-carrying flights, potentially before the end of 2020.