On Monday, September 26th, after some teasing beforehand, NASA provided an update on the venting of water by Jupiter’s icy moon, Europa.
As I noted in my last Space Sunday report, Europa is covered by shell of water ice, much of it discoloured by mineral deposits and by deep cracks, beneath which it is believed to have a liquid water ocean about 100 km (62.5 miles) deep. The ocean is believed to be made possible by tidal flexing enacted by the massive gravity of Jupiter as well as from the other large Galilean moons. This generates heat within Europa, and this heat stops the water from freezing solid.
In 2012, The Hubble Space Telescope (HST) captured what appeared to be a huge plume of water erupting some 200 kilometres (125 mi) above the surface of Europa, using its Space Telescope Imaging Spectrograph (STIS) instrument. The update offered on September 26th provided information on further plumes, strengthening the case of water existing under the ice crust of Europa in the process – a crust which may be far thinner than thought.
Over a 15-month period, astronomers used Hubble’s STIS to observe Jupiter and Europa in the ultra-violet spectrum. During that time, Europa occulted (passed in front) of Jupiter on 10 separate occasions. The observations were an attempt to examine a possible extended atmosphere around the moon, which is slightly smaller than our own. However, on three of the passes, astronomers witnessed what appeared to be plumes of water erupting from the surface – and in pretty much the same location as seen in 2012. Analysis of the plumes revealed they were made up of hydrogen and oxygen consistent with water vapour being broken apart by Jupiter’s radiation in a process known as radiolysis.
The plumes are not constant, but rather flare up intermittently, possibly as a result of the surface ice on Europa flexing in response to the same gravitational influences that are keeping the ocean beneath the ice from freezing out. This suggests that the icy crust is, at least around the region where the plumes are occurring, thinner than had been thought. This is important, because it could mean that any automated mission sent to Europa could have a fair chance of cutting its way through the ice to deploy a submersible vehicle which could then search for any evidence of life in Europa’s salty ocean – which contains between two and three times as much water as all of Earth’s oceans combined.
The Gentle Crunch: Rosetta Mission Ends
The European Space Agency’s Rosetta spacecraft said farewell on Friday, September 30th, bringing the 12-year mission that bears its name to a close.
Launched in 2004, Rosetta was a daring attempt to rendezvous with a short-period comet, 67P/Churyumov-Gerasimenko, then orbit it and study it as it swept through the inner solar system and around the sun on its (roughly) 6-year obit. The aim was to give us unique insight into cometary behaviour and – more directly – to study one of these tiny lumps of mineral and chemical rich rock “left over” from the solar system’s formation, and thus gain greater understanding as to how things came to be, and perhaps how life itself might have begun.
Rosetta travelled almost 8 billion km (5 billion miles), including three flybys of Earth and one of Mars, and two asteroid encounters, before finally arriving at 67P/C-G in August 2014. In November of that year, The Philae lander was deployed in the hope of studying the comet from the surface and gathering samples of its material for analysis. Unfortunately, Philae’s anchoring mechanism failed, sending the little lander bouncing across the comet, until it came to rest in a location where it was receiving insufficient sunlight to recharge its batteries. Nevertheless, in the time it did have before its batteries were almost depleted, the washing machine sized lander some 80%+ of its science goals.
Meanwhile, Rosetta studied the comet in the long fall towards the Sun, and carried out an extensive mission of study, analysis and image capture, much of which has completely altered thinking around comets like 67P/C-G. For example, the mission discovered that water within the comet has a different ‘flavour’ to that of Earth’s oceans, suggesting that the impact of such comets with primordial Earth played far less of a role in helping start Earth’s oceans than had been thought.
As the comet became more active during its approach to the Sun, Rosetta found complex organic molecules – amino acid glycine, which is commonly found in proteins, and phosphorus, a key component of DNA and cell membranes – were present in the dust vented by 67P/C-G, reinforcing the idea that the basic building blocks for life may have been delivered to Earth from an early bombardment of such rocks. The mission also confirmed that the comet’s odd shape – two potato-like lobes of different sizes joined at a narrow waist – was the result of a very slow-speed collision very early in the comet’s 4.5 billion-year age.
In all the spacecraft operated in the harsh environment of the comet for 786 days, made a number of dramatic flybys close to its surface, survived several unexpected outgassings, and made two full recoveries for potentially serious “safe mode” situations. However, all things must inevitably come to an end, and with its manoeuvring propellants almost exhausted, on September 29th, Rosetta set course for a gentle crash landing on 67P/C-G.
The selected point for touch-down lay close to a place where much of the dust and internal material vented from the comet as it was heated by the Sun during its flyby in late 2015. The location was chosen in the hope Rosetta would reveal more of 67P/C-G’s secrets in it final few hours. The event couldn’t be seen in real-time due to the transmission delay between spacecraft and mission control in Germany, but a stream of images were returned for processing, while the science team watched the telemetry signal from Rosetta slowly fade and flatline as the vehicle gently crunched into the comet, 718 million km (446 million miles) from Earth, and 12:19 UT, on September 30th.
For those who had devoted the better part of their careers to the project – which has its roots in the 1980s – it was a bittersweet moment. The flatline indicated a friend had been lost, leaving mission managed Patrick Martin to sum things up.
Musk on Mars
On Tuesday, September 27th, a very nervous looking SpaceX CEO, Elon Musk, addressed the 67th International Astronautical Congress (IAC), in Guadalajara, Mexico, outlining his vision for the colonisation of Mars. In Making Humans a Multiplanetary Species, Musk offered an ambitious programme for the human colonisation of Mars and explorations deeper into the solar system which was a mix of visionary, borderline fanciful and sprinkled with some sharp pragmatism and humour.
The heart of the concept lies within a new Interplanetary Transportation System (ITS) SpaceX is in the process of designing and building, and which differs somewhat from previous indications. It will comprise two stages: a massive reusable booster stage, powered by 42 of SpaceX’s next generation Raptor engines (the first of which was successfully tested just before Musk’s presentation), topped by an integrated upper stage / reusable vehicle which can be either an interplanetary spacecraft or a dedicated fuel tanker.
This integrated upstage / vehicle will be 49.5m (162 ft)-long with a maximum diameter of 17 metres, as outlined in the diagram above.
One of these transporter variants of this vehicle would be launched, fully laden and with crew (/passengers eventually) but without fuel for its Raptor engines, into Earth orbit. Up to five of the tanker versions will then be launched (using the same reusable booster), to fuel the vehicle for its trip to Mars. Once fully fuelled, and with the launch window open, the massive craft wouldl then fire its engines to commence a 6-8 month trip to the Red Planet.
Once at Mars, the ship will make a landing and act as the habitat for initial crews, although Musk’s hope is that eventually a self-sustaining colony will develop on Mars as the years pass, with these big transports routinely flying people and materiel back and forth between the two planets, with each transport designed to be reused up to 12 times.
Musk’s reasoning for this approach is that if Mars is to be colonised, then the price per ticket must be brought down to the median cost of a house in the United States. Right now, the cost per person of a mission to Mars can be measured in the hundreds of millions; even the most cost-effective plan for sending exploratory teams to Mars, called Mars Direct, would require a start-up cost of US $3 billion a year over a decade just to get to the first launch, and then each subsequent launch would cost US $2 billion per launch – and they’d only be handling 6 crew at a time.
Thus, Musk figures on using the airline analogy: make the space vehicles reusable and you drive down the cost of operating them, provide a larger number of seats and you can reduce the overall cost per seat. It’s an interesting concept, and one which wowed the IAC audience, but whether it could actually be achieved or not is another matter entirely.
The keen-eyed may have noticed that I stated the transporter will only carry the fuel needed to get to Mars. This is because Musk intends to use a page from the Mars Direct concept: ISRU – in-situ resource utilisation. The fact is that Mars does have almost everything humans need to establish a permanent presence there (if we can just figure out how to extract and refine a lot of it), and one of the most abundantly useful things it has is its carbon dioxide atmosphere.
While we may not be able to breathe it, we can – using a process called Sabatier Reaction, which dates back to the first decade of the twentieth century – take Mars’ atmosphere and create water and / or rocket fuel. All that is needed is some hydrogen, a catalyst such as nickel, some heat and time.
In 1996, the team behind Mars Direct showed that a vehicle flying to Mars with just 6 tonnes of hydrogen in its tanks could, over 18 months, leverage that hydrogen into 112 tonnes of oxygen / methane fuel – enough for a crew of six to use the vehicle to return to Earth at the end of their mission (they would fly to Mars in a different vehicle, which would be left behind as a functioning outpost / possible hub for a developing base).
Musk proposes the same approach – his Raptor engine is being designed to operate on a liquid oxygen (Lox) / methane mix. The major difference is that of scale. Mars Direct’s Earth Return vehicle would have a “dry” mass of some 35-40 tonnes, only requiring 112 tonnes of fuel to boost it back to Earth. With a 4x greater mass, Musk’s transporter vehicle would require considerably more fuel to be produced on Mars. Nevertheless, the idea is sound.
A lot of the finer points of getting humans to and from Mars and actually building a colony were glossed over during the presentation – doubtless the result of time constraints. For example, while Musk explained how a crew would be shielded against solar radiation, he gave no mention as to how SpaceX would deal with the fair greater risk posed by high energy galactic cosmic rays (GCRs), which could see an unshielded astronaut en route to Mars receive a daily radiation dose of around 1.8 milliSieverts – effectively a fully body CAT scan every 5-6 days for 6 months – which really isn’t recommended.
While materials such as hydrogenated boron nitride nanotubes (BNNTs), which are an effectiv and versatile GCR blocker, it is unlikely there will be sufficient quantities of them available for use in the fabrication of Musk’s transporter vehicle for a good few years to come, putting them outside his target time frame – if he sticks to it.
And it is that time frame which is still the biggest question mark over the idea. Musk hopes to start automated missions to Mars using an uncrewed variant of the Dragon 2 capsule and the Falcon Heavy booster in 2018, with up to 3 missions following in 2020 and 2022, before launching a modest crewed mission on his big booster / transporter in 2024.
While the Dragon missions will doubtless yield valuable data relating to propulsively landing a craft on Mars, the vehicle will only mass around 10 tonnes on landing. The SpaceX transporter will be 20 times greater than that, so whether half-a-dozen Dragon landings can supply sufficient information to assist in lading a 200-tonne craft on Mars is debatable. And, of course the rocket booster and transporter + their launch facilities (which will be placed at Kennedy Space Centre’s Pad 39A after all) have to be fabricated, tested, refined and – in the booster and transporter’s case – certified for human flight. All of which makes 2024 a very ambitious goal – which is why, in fairness, SpaceX only cautiously point towards it – they’re aware the actual launch could be several years beyond 2024.
But is the transporter actually needed for our initial attempts to explore Mars with a human crew? There is an argument to say that really, it’s not. There is much to be gained by sending smaller missions to Mars in the early years – teams of 6 to 8, with missions carried out along the lines of Mars Direct: straight from the launch pad to a landing on Mars, no faffing around with refuelling in Earth orbit, etc. Such missions would be extremely cost-effective, with Musk’s super booster adding even greater value by potentially sending two cargoes to Mars per launch, whilst allowing the company a longer lead time on developing and testing and refining the big transporter, and then introducing it when there is a genuine need for it.
I’ll have more to say on human mission to Mars – the options, the risks, the considerations, and so on, in a future Space Sunday. For now I’ll leave you with SpaceX’s ITS promotional video.