Space Sunday: reusability, habitability, survivability

SpX-13 lifts-off from Space Launch Complex 40 at Cape Canaveral Air Force Station, Florida, marking the first time SpaceX has launched a previously-flown Dragon 1 resupply capsule atop a previously flown Falcon 9 first stage, in SpaceX’s 17th launch for 2017. Credit: NASA

SpaceX Has completed its first mission to the International Space Station with a Falcon 9 first stage and a Dragon 1 resupply vehicle which have both previously flown.

The launch took place at 15:36 GMT (10:36 EST) on Friday, from Space Launch Complex 40 at Cape Canaveral Air Force Station. As well as being the first time a previously used Falcon 9 first stage and Dragon capsule have flown together, the launch also marked the first from SLC-40 since a pre-launch explosion of a Falcon 9 rocket in September 2016, which completely destroyed the rocket and its Israeli payload, and severely damaged the launch facilities.

Three minutes after the launch, the first and second stages of the Falcon 9 separated, the latter continuing towards orbit while the former performed its “boost-back” manoeuvre, and completed a safe return to Earth and a vertical landing at SpaceX’s Landing Complex 1 at Canaveral Air Force Station. The landing marked the 20th successful recovery of the Falcon 9 first stage – with 14 of those recoveries occurring in 2017.

The Dragon capsule, carrying some 2.2 tonnes of supplies for the ISS, was first used in a resupply mission in April 2015. In its current mission, it reached the station on Sunday, December 17th, where it was captured by the station’s robotic arm and moved to a safe docking at one of the ISS’s adaptors where unloading of supplies will take place. The capsule will remain at the station through January, allowing science experiments, waste and equipment to be loaded aboard, ready for a return to Earth and splashdown in the Pacific ocean, where a joint NASA / SpaceX operation will recover it.

The SpX-13 Dragon sits alongside the International Space Station on Sunday, December 17th, waiting to be grappled by one of the station’s robot arms and moved to its docking port. Credit: NASA/JSC

The mission is a significant milestone for SpaceX, bringing the company a step closer to it goal of developing a fully reusable booster launch system. Thus far the company has successfully demonstrated the routine launch, recovery and reuse of the Dragon 1 capsule and the Falcon 9 first stage. On March 30th, 2017, as part of the SES-10 mission, SpaceX performed the first controlled landing of the payload fairing, using thrusters to properly orient the fairing during atmospheric re-entry and a steerable parachute to achieve an intact splashdown. This fairing might be re-flown in 2018. That “just” leaves the Falcon 9 upper stage, the recovery of which would make the system 80% reusable.

However, recovering the second stage is a harder proposition for SpaceX – at one point the company had all but abandoned plans to develop a reusable stage, but in March 2017, CEO Elon Musk indicated they are once again working towards that goal – although primary focus is on getting the crew-carrying Dragon 2 ready to start operations ferrying crews to and from the ISS.

The major issues in recovering the system’s second stage are speed and re-entry. The second stage will be travelling much faster than the first stage, and will have to endure a harsher period of re-entry into the Earth’s denser atmosphere. This means the stage will require heat shielding and a means to protect the exposed rocket motor, as well as the propulsion, guidance and landing capabilities required for a full recovery.

SpaceX has proven the reusability of the Falcon 9 first stage (left) and the Dragon capsule system (right). All that remains is developing a reusable second stage, most likely for use with the Falcon Heavy – or as a part of the ITS / BFR. This image shows the discontinued proposal for a reusable Falcon 9 second stage. Credit: SpaceX

The problem here is that of mass. The nature of rocket staging means that – very approximately, every two kilos of rocket mass on the first stage reduces the payload capability by around half a kilogramme.  With a second stage unit, this can drop to a 1:1 ratio. So, all the extra mass of the re-entry / recovery systems can reduce the total payload mass, making the entire recovery aspect of a Falcon 9 second stage both complex and of questionable value, given the possible reduction in payload capability. However, with the Falcon Heavy due to enter service in 2018, a reusable second stage system does potentially have merit, as the combined first stages of the system can do more of the raw shunt work needed to get the upper stage and its payload up to orbit.

The Habitability of Rocky Worlds Around a Red Dwarf Star

Red Dwarf stars are currently the most common class (M-type) of star to be found to have one or more planets orbiting them. Many of these worlds appear to lie within their parent’s habitable zone, and while that doesn’t guarantee they will support life, it does obviously raise a lot of questions around the potential habitability of such worlds.

There tend to be a couple of things which often run against such planets when it comes to their ability to support life. The first is that often, they are tidally locked with their parent star, always keeping the same face towards it. This creates extremes of temperature between the two side of the planet, which might as a result drive extreme atmospheric storm conditions. The second is – as I’ve noted in past Space Sunday articles – red dwarf stars tend to be extremely violent in nature. Their internal action is entirely convective, making them unstable and subject to powerful solar flares, generating high levels of radiation in the ultraviolet and infra-red wavelengths. Not only can these outbursts leave planets close to them subject to high levels of radiation, they can cause the star to have a violent solar wind which could, over time, literally rip any atmosphere which might otherwise form away from a planet. This latter point means that one of the most vexing questions for those studying exoplanets is how long might such worlds retain their atmospheres?

In an attempt to answer to that question, planetary astronomers have turned to a planet far closer to us than any exoplanet: Mars.

An artist’s impression of MAVEN in orbit around Mars. Data from the probe has been used by researchers in an attempt to calculate the possible habitability of rocky planets orbiting red dwarf stars. Credit: NASA/JPL

Operating in orbit around the red planet since November 2014, NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) has been in a unique position to comprehensively study Mars’ atmosphere. In particular MAVEN has been able to study the Martian upper atmosphere and ionosphere and its interactions with the Sun and solar wind through three years of fluctuating solar activity. This work has allowed a team of scientists led by professor David Brain from Colorado University’s Laboratory for Atmospheric and Space Physics (LASP) and a MAVEN co-investigator, to hypothesise how a Mars / Earth type planet with a tenuous atmosphere might be affected by the solar activity of a red dwarf star similar in nature to Proxima Centauri or TRAPPIST-1 (which has no fewer than seven rocky planets orbiting it).

For the purposes of their study, the researchers assumed their Mars-like planet would be orbiting its red dwarf parent close to the outer edge of the star’s habitable zone – a distance which, because of the star’s comparatively small size, would still place the planet well inside the orbit of Mercury around our own Sun. They also allowed more of the light emanating from the parent star, in keeping with most red dwarf stars observed. This is important, because ultraviolet radiation results in more charged particles being created within an atmospheric envelope, causing “sputtering” – energetic particles being accelerated into the atmosphere  to collide with other molecules, smashing some out into space while causing others to have further collisions, again potentially pushing additional molecules out into space, thus aiding atmospheric loss.

Extrapolating the MAVEN data and combining it with the likely behaviour of typical red dwarf stars, Brain and his team estimated that their example planet would lose between 3 and 5 times as many charged particles through “sputtering” as Mars experiences in its solar orbit, and some 5 to 10 times more neutral particles would be lost through photochemical escape (where UV radiation breaks apart molecules in the upper atmosphere). Finally, they factored-in the likely thermal escape from their planet’s atmosphere. This occurs when lighter molecules within an atmosphere – such as hydrogen – which are pushed to the “top” of an atmosphere as a result of thermal activity, and are shredded away by the local solar wind.

The MAVEN-based study suggests rocky planets orbiting a red dwarf star, such as K2-18c and K2-18b (foreground) many only be able to retain atmospheres conducive to life for extremely short periods Credit: Alex Boersma

From their work, the team conclude that, leaving aside the more violent outbursts from an M-type star which would accelerate atmospheric loss, most planets orbiting a planet orbiting in the star’s habitable zone  would have much shorter periods of time in which it might have the kind of atmosphere conducive to life.

In the case of a planet orbiting a “quiet” red dwarf star – such as Ross 128, where an earth-sized planet was recently discovered – this “habitable period” would be shorted by a factor of 5 to 20 when compared to Mars. For a more common type of red dwarf star, such as TRAPPIST-1, the “habitable period” for the planets could be cut by up to 1,000 times. When the likely solar conditions common to most red dwarf stars are further factored in to the calculations, it appears increasingly unlikely worlds around them would remain habitable by any form of like we might recognise for very long. Which is a bit of a bummer.

“Oppy” Passes Another Winter on Mars

NASA Mars Exploration Rover (MER) Opportunity is a little over a month from celebrating the 14th anniversary of its arrival on Mars (or to put it another way, its mission has lasted just seven months less than we’ve all been enjoying Second Life). Having arrived on Mars on January 25th, 2004, “Oppy”, along with its sister MER rover, Spirit – which arrived on Mars on January 4th, 2004 but ceased all operations in 2010, had a primary mission period of just 90 days; however, at the time of writing these words, Opportunity has been operating for 4,780 days – a remarkable achievement by any standard. What’s more, the hardy little rover has just survived its 8th Martian winter.

NASA’s MER rover, Opportunity (MER-B) arrived on Mars in January 2004, and continues to explore the planet today. Credit: NASA/JPL

Eighth, because Mars orbits the Sun once every 686.971 Earth days (or 1.88 terrestrial years). While the planet’s axial tilt means Mars has seasons similar to Earth, each season is roughly twice as long as its earthly equivalent. Thus, while “Oppy” has been on Mars for almost 14 Earth years, it has only endured eight Martian winters – although they are winters far harsher, temperature-wise and length-wise, than anything we encounter here.

The rover has been studying the 22 km (14 mi) diameter Endeavour crater in the southern hemisphere of Mars since the end of 2011. It’s an area of particular interest as it shows much evidence for flowing water to have played a part in its past history. In particular, Opportunity entered the upper end of a fluid-carved region descending through the crater’s rim in July 2017, at the start of an extensive study of the area. Called Perseverance Valley, the sloping nature of the feature meant that the rover was well positioned to work through the first part of the Martian winter, the angle of decent maximising the amount of sunlight falling on Opportunity’s solar arrays. As winter closed on the rover, shutting down primary operations, this meant it was able to generate ample amounts of electricity to power the heaters essential to keeping it alive.

Now, with spring approaching, Opportunity has received a remote check over from Earth, and the mission team have found the rover to be in excellent condition overall. Not only has it had somewhat more power available over the winter period, it appears the rover’s position has meant less dust has accumulated across the solar arrays, resulting in less of a power generation drop-off than might have otherwise been the case.

Spring in the Martian hemispheres brings with it dust storms, both large and small – which the smaller ones, together with localised dust devils – being particularly good for the rover. This is because they act as scourers, passing over the rover’s solar panels and sweeping away dust deposits, further helping with electrical power generation. Dust was a particular concern for the MER team during this winter, as Jennifer Herman, the power subsystem operations team lead for Opportunity at NASA’s Jet Propulsion Laboratory, explained:

We were worried that the dust accumulation this winter would be similar to some of the worst winters we’ve had, and that we might come out of the winter with a very dusty array, but we’ve had some recent dust cleaning that was nice to see. Now I’m more optimistic. If Opportunity’s solar arrays keep getting cleaned as they have recently, she’ll be in a good position to survive a major dust storm. It’s been more than 10 Earth years since the last one and we need to be vigilant.

Two self portraits by Opportunity’s twin on Mars, Spirit, taken in 2007 and 2008, showing how local wind action – “dust devils” can “clean” the rover’s solar arrays of dust deposits. Credit: NASA/JPL

Since coming out of its winter hibernation, “Oppy” has resumed its study of Perseverance Valley, collecting colour, stereo, Panoramic Camera (Pancam) panoramas of the surrounding terrain. and resuming its drive further along the depression, stopping at identified “energy lily pads” – points the rover can “hop” to with short drives (8-10 metres (28-30ft) and then pause to top-up its batteries from the Sun. The latest of these “lily pad” points was reached on December 10th, and sits where the route the rover has been following forks. Since then, the MER imaging team has been using the vantage point to study which path forward the rover should take.