Space Sunday: reborn stars, icy worlds and air propulsion

A symbiotic X-ray binary of an ageing red giant (l) and relatively young neutron star (r – not to scale). Interaction between the two may have helped the neutron star to be “come back to life”. 

Astronomers have witnessed an extraordinary stellar event – a star “coming back to life” thanks to its nearby neighbour.

The two stars are from different points in the stellar evolutionary process. The “dead” star is a neutron star, all that remains of a massive star  – possibly with 30 times the mass of the Sun – which ended its life in a violent explosion, leaving whatever matter was left so densely packed, a sphere of the material just 10 km (6.25 mi) in diameter could have a mass 1.5 times that of the Sun.

The “donor” star is a red giant. This is a star similar to the Sun which has reached the latter stages of its life. With the hydrogen in its core exhausted, the star has swollen in size as a result of heat overcoming gravity, and has begun thermonuclear fusion of hydrogen in a shell surrounding the core. When this happens, the star sheds stellar material from its outer layers in a solar wind that travels several hundreds of km/sec.

In this particular case, the two stars – red giant and neutron – form what’s called a symbiotic X-ray binary system – one of one 10 such binaries of this kid so far discovered. There are also some oddities about this particular pairing which makes it somewhat unique. For one thing, while most neutron stars spin at several rotations per second, the neutron star in this pairing takes around 2 hours to complete one rotation. In addition, this star has a much stronger magnetic field than is usual for neutron stars, suggesting it is relatively young.

The ESA INTEGRAL observatory was the first to spot the “re-animation” of the neutron star. Credit; ESA

The “re-animation” of the neutron star occurred in late 2017, and is the subject of a paper published in the Journal of Astronomy and Astrophysics. It was spotted by the European Space Agency’s  INTEGRAL mission on August 13th 2017, which detected high-energy emission from the dead stellar core of the neutron star. These emissions were quickly picked-up by other observatories, such as ESA’s  XMM Newton observatory and NASA’s NuSTAR and Swift space telescopes, and a number of ground-based telescopes, confirming the event.

Its discovery has prompted two main questions: what exactly happened, and how long will this process go on? In answering the first question, astronomers believe that as the neutron star is relatively young, it rate of rotation may have been held in check by the solar wind from the red giant. Over time, the interaction between the red giant’s solar wind and the neutron star’s magnetic field resulted in ongoing high-energy emissions from the dead stellar core.

As to whether this it a short-lived phenomenon or the beginning of a long-term relationship, Erik Kuulkers, ESA’s INTEGRAL project scientist, notes:

We haven’t seen this object before in the past 15 years of our observations with INTEGRAL, so we believe we saw the X-rays turning on for the first time. We’ll continue to watch how it behaves in case it is just a long ‘burp’ of winds, but so far we haven’t seen any significant changes.

So for now, we’ll just have to wait and see.

Air-Breathing Electric Thruster Tested

While it is true the that densest part of the Earth’s atmosphere extends to the edge of the mesopause, just 85 km (53 mi), and the Kármán line –  representing the boundary between Earth’s atmosphere and “outer space” sits at 100 km (62 mi) altitude above the surface of the planet – the fact is that Earth’s atmosphere extends much further from Earth – out as far as 10,000 km (6,200 mi) from the planet’s surface.

This means, for example, that the space station, which operates at an altitude of 400-410 km  (250-256 mi) is operating within the thermosphere, and despite the tenuous nature of the atmosphere at that altitude it is subject to drag which requires it periodically boosts its orbit. This atmospheric drag also extends to low-Earth orbit satellites (which operate up to 2,000 km (1,200 mi), requiring they also periodically need to adjust their orbits. The problem here is that while the ISS can be refuelled – satellites in low-Earth orbit have finite supplies of fuel they can use, which can limit their operating lives.

Now – in a world’s first – the European Space Agency has tested an electric thruster was can ingest scarce air molecules from the thermosphere as fuel, potentially allowing satellites in very low orbits around Earth to have greatly extended operating lives.

Ram-Electric Propulsion is a potential means of providing propulsion for low-orbiting satellites uses extremely rare air molecules in the upper reaches of the Earth’s atmosphere as a means to generate electric thrust. Credit: ESA

A test version of the air-breathing thruster (technically referred as Ram-Electric Propulsion) was recently tested in a vacuum chamber simulating the environment at 200 km altitude. In the test, the thruster was initial fired using xenon gas – a common fuel supply for electric thruster systems – generating a distinctive blue-green plume. A “particle flow generator” was then used to simulate the influx of rarefied air molecules into the thruster system as if it were moving in orbit around Earth, causing the exhaust plume to turn a milky-grey – a clear sign the thruster was burning air as propellant, rather than xenon.

Once the initial thruster burn was completed, the thruster was shut down, purged and than restarted a number of times only using the air molecules provided by the “particle flow generator”, proving the engine can be successful fired – and fuel – by upper atmosphere trace gases.

Placed in a vacuum chamber simulating the mix of atmospheric gases at 200 km altitude, the thruster was initially fired using xenon gas as a fuel, causing a distinctive blue-green exhaust plume (l). It was then fired – with the aid of a “particle flow generator” to simulate its movement through the upper atmosphere – purely using the available air molecules as a fuel supply (r). Credit: ESA

The test firing is the culmination of almost a decade’s worth of research into electric thruster systems. While there is still a way to go before it is ready for practical use, the approach has the potential to benefit more than just low-Earth orbit satellites.

With minimal adjustment the system could in theory be adapted for use on satellites intended to operate in orbit around Mars or even Titan, both reducing the amounts of on-board propellants such a vehicle would require and increasing the mass allowance for science systems.

Earthly Bacteria’s Survival Raises Hope for Life on Icy Moons

For many years, scientists have suspected that Earth’s hydrothermal vents played a vital role in the emergence of life, and that similar vents could exist within the interior of moons like Jupiter’s Europa and Ganymede, and Saturn’s Titan and Enceladus, potentially giving rise to  life within the deep water oceans believed to exist under the crusts of at least some of these worlds.

These theories gained pace when the NASA / ESA Cassini mission observed plumes of water vapour escape Enceladus’ southern polar region. not only did these plumes strongly suggest hydrothermal activity within the moon, but in passing through them, Cassini detected the presence of organic molecules and hydrated minerals – both of which are potential indications of life.

A dramatic plume sprays water ice and vapour from the south polar region of Saturn’s moon Enceladus. Cassini’s first hint of this plume came during the spacecraft’s first close flyby of the icy moon on February 17, 2005. Credit: NASA/JPL / Space Science Institute

To see if life could thrive in a possible ocean beneath the surface of Enceladus, a team of scientists conducted a test where strains of Earth bacteria were subjected to conditions similar to those believed to exist under the moon’s icy crust. For the sake of the tests, the team worked with three strains of Methanogen known as methanothermococcus okinawensis (M. Okinawensis), which can be found in deep-sea fissures and hydrothermal vents on Earth.

These strains were selected because of their ability to grow in a temperature range that is characteristic of the vicinity around hydrothermal vents and at low partial pressures of molecular hydrogen – both of which are consistent with analysis of Enceladus’ plumes, and what is believed to exist within the moon’s interior. They consume chemical products also found within the plumes, such as methane (CH4), carbon dioxide (CO2 ) and molecular hydrogen (H2), and emit methane as a metabolic by-product.

After subjecting the strains to Enceladus-like temperature, pressure and chemical conditions in a laboratory environment, the researchers found that one of the three strains was able to flourish and produce methane. The strain even managed to survive after the team introduced harsh chemicals that are present on Enceladus, and which are known to inhibit the growth of microbes.

Cross-section of Saturn’s moon Enceladus, showing how hydrothermal vents in the seabed could give rise to hotspots with sufficient heat and mineral release to support life – as well as heat the ocean under the ice and generate the plumes images by the Cassini mission. Credit: NASA/JPL / SwRI

From this, they determined that some of the methane found in Enceladus’ plumes were likely produced by the presence of methanogenic microbes. As Simon Rittmann, a microbiologist at the University of Vienna and lead author of the study, explained in an interview with The Verge. “It’s likely this organism could be living on other planetary bodies,” he said. “And it could be really interesting to investigate in future missions.”

In the coming decades, NASA and other space agencies plan to send multiple mission to the Jupiter and Saturn systems to investigate their “ocean worlds” for potential signs of life. The current focus for the first of this missions is liable to be to Jupiter’s Europa (Europa Clipper, which is proposed for launch some time between 2022-2025). However, it is hoped that at some point in the coming decades, there will be a lander mission to Enceladus.

James Webb: Further Delays Likely

The James Webb Space Telescope (JWST)  – once launched – is set to become Earth’s most powerful telescope to enter operations. In gestation since 1996, JWST is part of NASA’s Flagship programme, and is named after James E. Webb, the second administrator of NASA, who played an integral role in the Apollo programme. It is regarded as the scientific successor to the Hubble Space Telescope, but not a replacement, because the capabilities are not identical.

However, JWST’s development has not been without problems. In September 2017, NASA announced it was delaying the launch  – originally scheduled for October 2018 – to between March and June 2019, after concluding spacecraft development work was taking longer than expected. At the time, it was intended that the delay would give NASA an extra four months of schedule reserves – periods of time put in place so that unexpected issues could be dealt with without threatening the launch date.

The James Webb Space Telescope. Credit: NASA

Unfortunately, much of this reserve time was taken by Northrop Grumman – a prime contractor for JWST –  requiring an additional three months “due to lessons learned from conducting deployment exercises of the spacecraft element and sun shield.”  Specifically, while the sun shield – five layers of Kapton material that will deploy to the size of a tennis court to keep the telescope cold – successfully deployed over a planned 2-week test period, folding it back into its stowed state proved much harder – eventually taking two months to complete, with a number of tears occurring in some of the layers in the process. At the same time, it was discovered that the platforms attitude control thrusters needed to have faulty valves replaced.

JWST has now entered the phase of the project where problems are most likely to be found and schedules revised. As a result,and given just 6 week of reserve time remain in the current schedule, the U.S. Government Accountability Office has indicated that JWST’s launch should be delayed to allow for any additional issues or problems which still may arise during spacecraft integration and testing to be dealt with.

So far, NASA has not provided an updated schedule for JWST’s launch beyond the window of March to June 2019. The GAO said NASA senior management will formally identify a new launch window after being briefed on the independent review’s results in “early 2019.” However a further problem is that any rescheduling of the launch, will likely see JWST exceed its US $8 billion cap, requiring Congress approve additional funding for the mission.