I’ve written on numerous occasions about the various challenges facing any human mission to Mars. Perhaps chief among these challenges are the matters of radiation exposure and transit time. As I’ve noted in past articles (such as this one) on this topic, crews going to Mars face multiple risks, just two of which are radiation (both solar radiation and Galactic Cosmic Rays (GCRs)) and transit times.
The former is particularly deadly in that solar storms can deliver lethal doses of radiation exposure over a matter of a few hours (or less). However, they can be mitigated through the use of careful mission planning (avoiding, where possible, launch windows when solar activity is at or near its peak); and providing on-board radiation shelters which use a dozen centimetres or so of a suitable material (such as water) which can be used should a storm threaten.
By contrast, GCRs are less “immediate” in the risk they present, but they are constant and all-pervasive. They are also far more high energy than solar radiation, making shielding against them a more complex issue. requiring a lot more in the way of shielding. For example, gamma radiation from a typical solar storm requires around 13-15cm of water to mitigate much of its threat; GCRs require at least two metres of water (at one tonne per cubic centimetre) to reduce the threat by 50%. And while they might not be immediately deadly, GCRs can cumulatively have a major impact on health, such as reactivating cancer-giving strains of the herpes virus normally dormant in the human body, such as the highly contagious Epstein–Barr virus (EBV).
Ergo, crewed Mars vehicle require more wide-ranging and effective shielding in order to reduce the long-term impact of GCRs on Mars-bound (or Earth-returning) crews. Currently, two such shielding materials exist: Kevlar and high-density polyethylene (HDPE). Both are very effective in absorbing GCRs – just 5 cm of either will do the same job as 2 metres of water. However, while both could be incorporated into the structure of a crewed Mars vehicle, they would need to offer protection right across all crewed areas, not just a relatively complex shelter. As both have a mass in the same orbit as water (1 gram per cubic centimetre for the latter; 980 grams per cubic centimetre for HDPE and up to 1.44 grams for Kevlar), this means that both could come with a significant mass penalty.
As such, more lightweight – and preferably more efficient – shielding materials are required. One of the most promising is that of carbon nanotubes, some of which are very efficient in dealing with various forms of radiation. Single-walled carbon nanotubes (SWCNTs) can reflect up to 99.9% of solar electromagnetic radiation striking them, whilst boron nitride nanotubes (BNNTs) can absorb some 72% of neutrons (common to GCRs) in just a thin layer – more than can be achieved by using 5 cm of HDPE or Kevlar. In fact, NASA’s Langley research centre has in the past experimented with trying to “weave” BNNTs into structure that could be used within habitat units of spacecraft.
The problem here is that nanotubes are both expensive to manufacture and difficult to manipulate / use. Hence why, in the 35 years since serious nanotube production started, less than 10,000 tonnes have been produced world-wide. However, a team of researchers at the Korea Institute of Science and Technology (KIST), have been looking at the potential for nanotubes in a range of applications – including their use as a shielding material – and have developed a means of potentially overcoming the issue of using nanotubes to create materials the application of 3D printing.
In particular they have developed a means to combine both SWCNTs and BNNTs into “mats” of material which can be “woven” together as a part of the printing process to fulfil a number of roles. Most particularly, in terms of space applications, these “mats” remain all of the radiation shielding capabilities common to both SWCTs and BNNTs. Thus, single layers of a “mat” could be used to provide individual protection for circuitry and chips forming the electronics on robot spacecraft, or be layered to produce very lightweight, efficient and very flexible material for shielding all the habitable areas of a crewed spaceship. What’s more, the material can withstand massive temperature swings (from -196ºC to +250ºC), potentially allowing it to be used both internally and externally on space vehicles.
This material represents a completely new concept in shielding technology-it is as thin as tape and as flexible as rubber yet simultaneously blocks both electromagnetic waves and neutron radiation.
– Dr. Joo Youngho, principal investigator, Ultrathin, Stretchable, and 3D-Printable Complementary Nanotubes–Polymer Composites for Multimodal Radiation Shielding in Extreme Environments
The research still requires a lot more work before this approach can be thought of as truly viable, but the implications of such a shielding capability for something like crewed missions to Mars would be enormous.
Currently, it takes between 6 and 9 months to travel between Earth and Mars (or vice versa) when launching at the most energy-efficient times (approximately once every 26 months). This could put significant strain on a crew, limited as they would be to just a small circle of people with whom they could communicate in real-time and the limited amount of space available within their spacecraft in which they might fine solitude and peace keeping their own company.
However, if we had a more efficient propulsion system, one that could use a lot less fuel far more efficiently and for longer, then it would be possible to break out of the current 26-month, 6-9 month transit flight constraints to a greater or lesser degree. This would help reduce the stresses that might otherwise build-up in such a restricted environment, and also help reduce (to a degree) the crew’s deep-space radiation exposure risks.
One way to achieve this would be through the use of Nuclear Thermal Propulsion (NTP). However, such a system has yet to be developed and brings with it the need for shielding for the crew against the nuclear reaction, with all the added mass and complexity that brings.
Another alternative is that of electric propulsion. This is not as powerful as NTP and cannot even match the specific impulse that can be generated by chemical motors. However, it is a) highly efficient, b) already in use and c) unlike chemical rockets, it can maintain its thrust more-or-less continuously for comparatively little fuel mass. Take NASA’s mission to the asteroid 16 Pysche, for example. This uses Hall-effect thrusters which, while relatively low-power have maintained a steady thrust since the mission launched 2.5 years ago, accelerating the spacecraft from a few tens of thousand km/h as it departed Earth orbit to more than 135,000 km/h today – and it is still accelerating for the time being; all for just 1.6 tonnes of propellant.
However, the Psyche spacecraft masses just 2,6 tonnes overall. A crewed Mars vehicle, with its habitat units, control centre, solar arrays for electrical power, life support systems and so on, is going to mass tens of tonnes (a minimum of 45 tonnes has been estimated for just a basic habitat/lander craft). As such, if electric propulsion is to be used, then much more powerful thruster systems will be required.
This is exactly what NASA’s Jet Propulsion Laboratory (JPL) has been working on: a “next generation” nuclear-electric motor called the magnetoplasmadynamic (MPD) thruster. Rather then just relying on electric power to drive the thruster, the MPD introduces a magnetic field into the drive process, making the thruster far more efficient and with a greater output. It also utilises lithium as a the propellant rather than the more usual xenon or krypton, for an increased energy output. As a result, a test article of the MPD has already proven itself to be able to operate for relatively long periods (albeit days rather than months or years), producing a steady 120 kilowatts of thrust, more than 25 times that produced by the hall-effect thrusters on the 16 psyche mission.
This is an impressive start, but to power a crewed spaceship of the kind currently being considered for human Mars missions, the propulsion system would have to be capable of consistently generating up to four megawatts of energy, both to accelerate the vehicle during the first half of its voyage out from Earth (or Mars) and then as a braking system to reduce its velocity to a point where it can enter orbit around its destination. However, the JPL team are reasonably confident that with time and experimentation, they could likely iterate the MPD to a point were it is consistently generating around a megawatt of power, thus allowing multiple engines (4-6, allowing for reserve engines being carried to deal with any failures) to be used to propel a potential Earth-Mars-Earth vehicle, all of which would require far less fuel than any chemical propulsion system, and would not require refuelling at Mars.
There are a few wrinkles in this approach that need to be addressed, however. For example, to produce such a level of power output, the MHD would also produce a lot of heat – around a constant 2,800ºC. Thus, the materials used in the thruster system would have to be capable of running continuously in the face of this temperature for thousands of hours of use. As such, much more in the way of development and testing is required before the MPD thruster would be ready for practical use – which will take years or possibly decades. But once developed and tested, it could offer a means to either shorten the transit times between Earth and Mars by virtue of its constant thrust, or deliver heavier payload to Mars over roughly similar time-frames as the current Hohmann orbits, and with none of the angst people have around nuclear thermal systems.
3I/Atlas
On July 1st, 2025, 3I/ATLAS was confirmed as the third known interstellar object (ISO) to be passing through the solar system. It also became the third such object to ignite daft claims that such objects are of alien manufacture sent to spy on us, despite the evidence it is lactually a comet. By the end of October 2025, it was passing around the Sun at the start of its way out of the solar system, and by April 2026 it was once again passing beyond the orbit of Jupiter.
However, between July 2025 and April 2025, 3I/Atlas was the subject of intense study by observatories on the ground and in space, with some interesting discoveries being made along the way. The James Webb Space Telescope (JWST), for example, revealed the comet’s coma (the cloud of dust and material formed when a comet approaches the Sun and its ices sublimate, releasing material) to be composed primarily of carbon dioxide in an 8:1 ratio compared to water, much higher that with solar comets, which typically have a 4:1 ratio; indicating the comet likely formed in a very different environment compared to our own solar system.
This view was further enhanced following observations of the comet made by the Atacama Large Millimetre/sub-millimetre Array (ALMA) located high in the Chilean Andes. These revealed 3I/ATLAS is made of an astonishingly high ratio of semi-heavy water (HDO, also known as deuterated water, on account of one of the hydrogen atoms being replaced by a deuterium atom) relative to water.
On Earth, approximately 1 in 3,200 water molecules are HDO (with one in 41 million being heavy water (D2O), with which semi-heavy water should not be confused). On 3I/Atlas, the abundance of HDO is around 40 times higher than the abundance of semi-heavy water on Earth. Not only does this point to the comet being formed in a much colder – likely around -243ºC – environment than found within our solar system, it was also subject to very little in the way of stellar radiation, suggesting it formed at the very outer edge of its originating star system.
Further, as it passed around the Sun and was at its most active, the comet started outgassing more and more methane. This led to the theory that in its passage towards the Sun and the initial formation of its coma and tail, 3I/Atlas had shed the last of its cosmic ray irradiated outer shell, allowing its more “pristine” (i.e. preserved from the days of its formation) inner layers to be exposed to sublimation. In particular the abundance of methane being released further underlined the idea that the comet had formed in an extremely cold environment.
This is important because it directly impacts our understanding of the formation of stellar systems. These generally hold that star systems are born of relatively “hot”, compressed clouds of dust and gas, the majority of which collapses under gravity to form the central star, with any planets, asteroid, comets and such like forming in the immediate aftermath of the star igniting, when the “left over” material is still relatively dense – and warm – as it surrounds the newly-born star. Thus, 3I/Atlas potentially hints at an alternative path of stellar evolution we have yet to identify and understand.
Artemis Update
Following-on from my previous Space Sunday piece, the core stage of the Space Launch System (SLS) rocket that will be used in 2027’s Artemis 3 mission, completed its 1,450 kilometre journey by canal, river and sea from NASA’s Michoud Assembly Facility in New Orleans to the turn basin at Kennedy Space Centre’s (KSC) Complex 39 on April 27th, 2026.
The stage, lacking its four RS-25 engine units, which will installed as vehicle stacking starts within Kennedy’s Vehicle Assembly Building (VAB) reach the wharf in the basin safely aboard the Pegasus transport barge. Following its arrival, on April 28th, the stage was transferred by road from the basin to the VAB in readiness for vehicle stacking to commence.
At the same time as the stage was arriving at KSC, it was confirmed that Artemis 3 – planned as a crewed test of the available lunar lander vehicles required for missions to the surface of the Moon – has been pushed back from mid-2027 to an October-November 2027 time frame. This is apparently to allow both SpaceX and Blue Origin, the two contractors charged with supplying NASA with crew-capable lunar landers, with more time to have their first vehicles ready for testing in Earth orbit.
Whilst NASA is playing down the pushback, there is already mounting feeling in some circles that the mission will ultimately be pushed back until early to mid 2028, simply because there will not be any lander vehicle ready for Earth-orbit testing by a crew by late 2027.
On April 28th, 2026 NASA also released the first image of the heat shield used on the Artemis 2 mission to project the Orion capsule from the searing heat of re-entry into Earth’s atmosphere at the end of the mission.
As regular readers will know, there was considerable concern surrounding the heat shield after an identical unit used on the uncrewed Artemis 1 mission in December 2022 showed unexpectedly high levels of damage. Investigations revealed the worst of this damage – deep pits and holes within the ablative material of the heat shield were the results of gasses trapped in the layer being super-heated as the spacecraft “skipped” through the atmosphere before fully re-entering, resulting in them “blowing out” sections of the heat shield’s layers as they violently expanded.
As a result of this, the heat shields to be used from Artemis 3 mission onwards were put through a redesign prior to fabrication, but the shield for Artemis 2 had already been manufactured and installed – so the re-entry profile for the mission was changed in order to reduce the risk of outgassing and damage to the heat shield. Even so, fears remained as to the shield’s fitness for purpose.
Clearly it was up to the task as evidenced by the successful return to Earth by Artemis 2 crew, and within the image released by NASA on April 28th, it is clear that the heat shield more then withstood the stresses of the revised re-entry profile – even if the image is itself a most unusual one.
So keen were NASA engineers to see the state of the heat shield, that even as the Orion capsule floated in the Pacific Ocean off the Californian coast, and the crew were being recovered, a camera-equipped US Navy diver was tasked with swimming under the capsule and photographing the heat shield from below. The result is a somewhat eerie, almost sci-fi like underwater image of the heat shield, streaked with burn and ablation marks across its entire surface – as would be expected – but without any of the deep chadding and pitting seen on the Artemis 1 heat shield.
Obviously, the heat shield, recovered with the rest of the capsule and now back with NASA, will be examined more thoroughly, but this initial picture finally put to rest concerns that the Orion heat shield might be somehow, and potentially fatally, flawed.
Inara Pey: Living in a Modemworld 