Space Sunday: to land on Europa

An artist’s impression of the Europa Lander. Credit: NASA

Of all the planets and moons in the solar system, the two that – next to Earth – are likely to be homes to oceans of liquid water are Jupiter’s moon Europa, and Saturn’s Moon Enceladus. The latter, as I’ve noted in this column, has visible evidence of geysers venting water vapour around its southern polar regions, while in November 2019, the the W.M. Keck Observatory indicated they had directly detected water vapour around Europa (see here for more) – evidence that has since been added to through further study of the data gathered by NASA’s Galileo mission that ended in 2003.

Given their distance from the Sun, both of these moons are covered in shell of icy material  that is believed to encase a liquid water ocean, likely heated from within by hydrothermal vents, themselves the result of both moons being “flexed” by the gravitational influence of their parent planets and the other large moons orbiting them. And where there is water, heat and a source of energy for sustenance, there is a possibility that life may also be present – which makes both Enceladus and Europa potential destinations in the search for life beyond our own world; and of the two, Europa is somewhat “easier” to reach.

A high resolution image of Europa’s chaotic surface taken by the Galileo mission. It shows terrain where blocks of material have shifted, rotated, tilted and refrozen. Credit: NASA/JPL

To this end, and again as has been written about in this column, in 2024 NASA intends to send the Europa Clipper to the Jovian system, placing it in a orbit around Jupiter that will allow it to make repeated fly-bys of Europa, joining the European Space Agency’s Jupiter Icy Moons Explorer allowing it so study the moon in detail, and characterise its surface and any ocean that might lay beneath.

However, to have a real chance of detecting any evidence of microbial life on Europa, scientists argue that a landing there is required, and as planetary scientist Conor A Nixon reminded me via Tweeter, a proposal to put a lander on the surface of Europa has been in development for over two years – although it has yet to reach the point of actually being funded. Were it to go ahead, it would – amongst other things – be the heaviest robot mission launched from Earth; so heavy, it would require either the Falcon Heavy or NASA’s massive Space Launch System (SLS) to throw it on its way to Jupiter – with the SLS being the preferred vehicle, as it would allow the mission to reach Jupiter after just a single gravity assist from Earth, shortening the flight time.

The proposed Europa Lander mission outline, as it stood in 2018, and reviewed in 2019. Credit: NASA

The primary objectives of the mission would be to search for subsurface biosignatures; to characterise the surface and subsurface properties at the scale of the lander to support future exploration of Europa and determine the proximity of liquid water and recently erupted material near the lander’s location; and assess the habitability of Europa via in situ techniques uniquely available to a landed mission. Under current plans, last revised in 2019, the mission  – outside of this launcher – will comprise five core elements:

  • The Europa Lander: a battery-powered vehicle intended to operate on the surface of Europa for 22 terrestrial days, and carrying a suite of around 14 scientific instruments / experiments.
  • The Descent Stage (DS): to reduce the risk of contaminating / damaging the lander’s touch-down point, it will be winched down to the surface by a “sky crane” vehicle similar to the one used to put the Curiosity lander on Mars and will be used with the Perseverance rover in February 2021. Once the sky crane has done its job, the sky crane will boost itself into an orbit where it will eventually burn-up in Jupiter’s upper atmosphere.
    • Together, the lander and the DS form what NASA call the Powered Descent Vehicle (PDV).
  • The De-Orbit Stage (DOS): a propulsion unit intended to slow the PDV into a decent to the surface of Europa.
    • When combined the DOS and PDV form the De-Orbit Vehicle (DOV).
  • This assembly is carried to Jupiter within the carrier stage, comprising two parts:
    • The carrier vehicle, which provides communications, power and flight management hardware and software.
    • A protective bio-barrier dome designed to protect PDV from the risk of contamination / damage during the 5-year trip to Jupiter.
The Europa Lander’s component element. Credit: NASA

Currently the only part of the mission that has been funded – via NASA’s 2018 budget – is the development of some of the science packages the mission might carry; but it is hoped further funding will be forthcoming in 2020/21, now that the Europa Clipper mission has received funding. This means that the proposed time scale for the mission – launch in 2025 atop a Block 1b SLS booster, followed by a 5-year trip to Jupiter (with a single fly-by of Earth) – is tight.

On arrival at Jupiter the carrier stage would initially enter a Jovian orbit to commence the “Jovian Tour” element of the mission that will last around two years. This will see the vehicle use Jupiter’s orbit to swing it into a position where it can enter Europa’s orbit. Once there, it will carry out a survey of the moon to locate / confirm potential landing points. Only after a landing zone has  been finally selected will the De-orbit. Descent and Landing (DDL) be initiated.

After the sky crane has placed the lander on the surface of Europa, operations will commence more-or-less immediately, starting with the deployment of the lander’s high-gain communications antenna. A key element of the lander will be a robot arm that will allow it to obtain a number of sub-surface samples from a depth of up to 10 cm below the surface. It is believed this will be of sufficient depth where sub-glacial microbial colonies similar to those found in the McMurdo dry valleys of Antarctica. These samples, gathered from various points around the lander, will be delivered to the lander’s on-board science labs for analysis.

The DOV configuration and the DDL phase of the mission

All this may sound simple, but the mission faces multiple issues / hazards. There are obviously those associated with the launch, journey to Jupiter, and landing – but Europa exists in a harsh radiation environment which could severely damage the lander’s electronics or affect the samples it digs up from under the surface, where they have protection.

To put the amount of radiation the surface of Europa receives into perspective, a typical dose of solar / background radiation received on the surface of Earth is about 0.14 rem/year; on Europa it is approximately 540 rem per day. This requires the lander’s more sensitive instruments to be sealed inside a protective “vault” with 8 cm thick walls deep inside the lander.

A further risk to the mission – one this is currently in part unknown – is the icy surface environment of Europa itself. A study published in October 2018 suggests that most of Europa’s surface could be covered with penitents – closely spaced ice spikes – possibly as tall as 15 metres. Whilst tall, these penitents are not large enough to register on even the highest resolution images gathered by Galileo, even after enhanced processing. However, radar and thermal data gathered by the mission are consistent with this interpretation. So any landing on Europa is going to require careful survey to find if there are suitable points where the lander can be winched down without being damaged by these spikes.

A final risk NASA has considered – although it would seem unlikely given the radiation environment – is that of the lander inadvertently carrying Earth microbes to Europa, which might then contaminate the environment around the lander. To prevent this, it has been suggested that at the end of the mission, the lander might self-destruct using an incendiary device, and that this system might also be triggered if the spacecraft loses contact with the Earth.

Despite these challenges, were it to go ahead, the Europa lander mission would be one of the most ambitious planetary science missions thus far undertaken.

In Brief

SLS First Launch Delayed to Late 2021

NASA now expects the first launch of the Space Launch System to take place in late 2021, with the SARS-CoV-2 pandemic at least partially contributing to the latest delays.

The formal announcement is still forthcoming, Prior to the pandemic, NASA had planned to perform a critical “green run” static-fire test of the rocket’s first core stage – a critical part of pre-launch preparations – in August 2020. Assuming this would have been successful, the core would have been shipped to Kennedy Space Centre in October, allowing the rocket to be assembled in readiness for its flight, carrying an uncrewed Orion capsule on an extended flight around the Moon, with the flight taking place in early 2021.

The SLS core stage on the test stand at the Stennis Space Centre, where it will undergo a “green run” 8-minutes test firing of its main engines. Credit: NASA

As a result of the SARS-CoV-2 situation, NASA’s preparations for the “green run” test have been frozen for two months. This means the static fire test is unlikely to take place before US Thanksgiving, 2020, so the core stage is unlikely to reach Kennedy until late 2020 / early 2021, with the launch potentially taking place around mid-2021.

China Criticised for Long March 5B Re-Entry

In my previous Space Sunday, I wrote about China’s test flight of their new crew capsule system (see: Space Sunday: rockets, landers, FRBs and the Moon). A noted in that piece, the launch left the 20-tonne core segment of the Long March 5B booster in low Earth orbit, with the expectation it would re-enter the Earth’s atmosphere on Monday, May 11th. It had been believed the stage would mostly burn-up, with any parts surviving the re-entry splashing-down over the southern Indian Ocean.

However, it appears the core stage, without any means to control it, re-entered the upper atmosphere off the coast of west Africa earlier than anticipated. Several elements of the stage survived the re-entry, including a 12 metre long pipe section, These fell to Earth some 2,000 kilometres down-range in the Côte d’Ivoire. Fortunately, no-one was apparently killed or injured in the impacts; however, the event has led to widespread criticism of the Chinese authorities, who gave no significant advanced warning of the stage’s re-entry, and have remained silent on the subject since.

NASA Advisory Committee Sceptical of 2024 Moon Landings

Members of a NASA advisory committee have expressed doubts that NASA can achieve the goal of safely landing humans on the Moon by the end of 2024, and raised concerns about the agency’s approach to developing lunar landers.

An artist’s impression of an Artemis lander and astronauts on the surface of the Moon

Among the most strident critics was Tommy Holloway, a former NASA space shuttle and International Space Station program manager. During committee discussion about potential findings and recommendations, he said he didn’t think the Human Landing System (HLS) programme could develop landers in time to take astronauts to the Moon in 2024, nothing that in the 1960s, it took Grumman seven years to design the Apollo Lunar Module, but NASA is expecting contractors for the Artemis programme to do so in half that time – but is itself hardly showing any sense of urgency in the matter. This lack of urgency was noted by other members of the committee.

They keep repeating things and doing some stuff that, if you make a decision and move, would allow the program to move on. I don’t think they have any chance of making 2024, and we can argue about that all day, but they won’t even make 2028 if we keep restudying everything.

– James Voss, former astronaut, and advisory committee member

Despite the criticism, and the fact the committee pointed to to past reports indicating the same concerns, and which called for NASA to cut back on trade studies and move to ensuring that there is adequate testing of the landers to avoid jeopardising safety, NASA managers remained optimistic that the 2024 target date could be achieved, pointing particularly to the use of commercial partnerships in the HLS development. However, even this drew further critique from other committee members, who noted that the partnerships NASA entered into for the development of vehicles to transport crews to and from the International Space Station extended well beyond their planned development cycle: originally intended to enter operations in 2015/16, both vehicles have yet to be classified as “operational”.