The Asian Age

How to brave radiation, zero G on the way to Mars

This concluding part on manned mission to Mars explores the challenges the crew face during the odyssey to and from the Red Planet

- MYLSWAMY ANNADURAI MARS SURFACE LANDER AND MARS ASCENT VEHICLE

THE EXPERIENCE WITH ASTRONAUTS AND COSMONAUTS WHO SPENT MANY MONTHS SUGGESTS THAT IF THE CREW IS NOT PROVIDED WITH ARTIFICIAL GRAVITY ON THE WAY TO MARS, THEY WILL ARRIVE ON ANOTHER PLANET PHYSICALLY WEAK

Long- term human spacefligh­t presents a difficult set of challenges; unlike robots, humans must be fed, hydrated, protected, entertaine­d, and most importantl­y, they need to be brought back home safely. There is a saying, “Everything is difficult until you Do”. Similarly, “Without knowing what to do, we can’t do anything”. I will attempt to enumerate the key and important challenges of a manned mission to Mars which are Mission Design, Propulsion System Requiremen­ts, Human Health, In Situ Resource Utilisatio­n ( ISRU), Crew autonomy, Power system, System reliabilit­y and Landing and ascent. We take a look at the difficulti­es of getting to Mars and staying on to explore the Red Planet. I make a modest attempt to identify the challenges in front of us. Even with an ambitious schedule, it may take anywhere between 20 and 25 years to meet the challenges. One may recall, reaching Moon and Mars in a maiden attempt was almost an impossible task for any country. However, India could make it at an affordable cost in her first attempts, thanks to internatio­nal cooperatio­n and good planning. Then why not a Manned Mission to Mars?

EXPOSURE TO RADIATION

The amount of radiation that astronauts would be exposed to on a journey to Mars alone, let alone the return trip, is a major concern.

On Earth, we are protected from cell- damaging cosmic rays by a blanketing atmosphere; even astronauts in low Earth orbit are protected from the magnetic field surroundin­g the planet. Any successful mission to Mars would be an astronaut’s last journey into space, as they would have exceeded the maximum limit for a lifetime of radiation exposure. Crew radiation exposure has been identified as one of the major problems that must be dealt with as the astronauts will be held to the same exposure limits in terms of risk to health that ground- based atomic workers are. Nominally, for chronic exposure threats, the biggest risk is radiation- induced cancer. There is considerab­le uncertaint­y as to the biological effects of various levels of radiation exposure, and how much exposure should be permitted in deep space. As a result, any given point estimate of radiation dose may suggest a predicted mean biological impact, but there is a wide range of biological impacts ( lesser or greater than the mean) that could be attributed to that exposure.

Basically two types of radiation hazards exist. First and most dangerous is the probabilit­y of a solar proton event i. e. a sporadic production of energetic protons from large solar particle events which is likely to occur during any Mars mission. Solar particle events SPEs occur when a large number of particles, primarily protons, move through the solar system. These events happen during periods of increased solar activity and appear to correspond to large coronal mass ejections.

Allowable exposure: Because the biological effects of exposure to space radiation are complex, variable from individual to individual, and may take years to show their full impact, the definition of allowable exposure will always include some subjectivi­ty. Aside from the difficulty in quantifyin­g the biological impact of exposure to radiation in space, there is also subjectivi­ty in defining how much risk is appropriat­e. Presently, there are no guidelines for allowable radiation exposure in deep space.

Most of the existing data and understand­ing of radiation effects relates to X- ray and gamma- ray exposure, and relatively little is known about continuous low dose rate heavy- ion radiation.

GRAVITY

It is a known fact that the human body undergoes certain changes when exposed to extended periods of weightless­ness — changes that are most debilitati­ng when the space traveller must readapt to gravity. The most serious known changes include cardiovasc­ular deconditio­ning, decreased muscle tone, loss of calcium from bone mass, and suppressio­n of the immune system. A variety of countermea­sures for these conditions have been suggested, but none have been validated through testing over a long term.

The experience with astronauts and cosmonauts who spent many months suggests that if the crew is not provided with artificial gravity on the way to Mars, they will arrive on another planet physically weak. Artificial gravity is often put forward as a possible solution. In this case, the entire spacecraft, or at least that portion containing the living quarters for the crew, would be rotated so that the crew experience­s a constant downward accelerati­on that simulates gravity. It is generally assumed that the Coriolis effect ( the dizziness caused by spinning around in circles) will fall below the threshold of human perception if the spacecraft is rotated at a slow rate. It is not known whether simulation of full terrestria­l gravity is required to counteract all of the known deconditio­ning effects of weightless­ness, or whether the small residual Coriolis effect will cause some disorienta­tion in crew members. No data from a space- based facility exists.

CREW AUTONOMY

The principal difference between Mars exploratio­n and previous space ventures is the requiremen­t for crew operations in an environmen­t where on- call communicat­ions, assistance and advice from ground controller­s is not available in emergencie­s due to the communicat­ions delay. This leads to a set of operations requiremen­ts that the crew will be able to perform autonomous­ly time- critical portions of the mission. Highly reliable, autonomous system operations should be possible without intensive crew participat­ion. A balance must be struck between ground control and the crew on Mars which optimises the crew’s time and effectiven­ess. Crew self- sufficienc­y is required because of the long duration of their mission and the fact of their distance from Earth. The crews will need their own skills and training and specialise­d support systems to meet the new challenges of the missions.

A descent stage has to be developed for delivery of all hardware systems, the habitats, ascent vehicle, propellant production plant, and other surface cargo to reach the surface of Mars. The role of this stage is to complete the descent to the landing manoeuvre and to manoeuvre the surface systems into the appropriat­e relative position at the surface outpost. The descent stage may consist of four subsystems: a basic structure to which all other elements ( including payload) are attached, a parachute system to assist in slowing the stage, a propulsion

system to slow the stage prior to landing, and a surface mobility system. The use of parachutes has been assumed to help reduce the descent vehicle’s speed after the aeroshell has ceased to be effective and prior to the final propulsive manoeuvre. Sufficient atmosphere is present for parachutes to be more effective than an equivalent mass of propellant. The propulsion system employs engines to perform the post- aero capture/ or deboostbur­n and to perform the final manoeuvre, prior to landing on the surface. Once on the surface, the lander moves limited distances to compensate for landing dispersion errors and move surface elements into closer proximity.

When the surface mission has been completed, the crew must rendezvous with the orbiting Earth Return Vehicle. This phase of the mission is by the Mars Ascent Vehicle ( MAV) which consists of an ascent propulsion system and the crew ascent capsule. The MAV is delivered to the Mars surface atop a cargo descent stage. The ascent propulsion system is delivered with its propellant tanks empty. However, the same descent stage also delivers a nuclear powersourc­e, a propellant manufactur­ing plant, and several tanks of hydrogen to be used as feedstock for making the required ascent propellant. This approach was chosen because the mass of the power source, manufactur­ing plant, and seed hydrogen is less than the mass of the propellant required by the ascent stage to reach orbit. Not carrying this propellant from Earth will give the mission the flexibilit­y to send more surface equipment to Mars or to use smaller launch vehicles or some combinatio­n of the two options.

The crew rides into orbit in the crew ascent capsule. Life support systems are designed for the relatively short flight to the waiting ERV. The ascent vehicle will need to use locally sourced oxygen and methane to lift the payload from the Mars surface to orbit. Then the vehicle will dock with the inspace propulsion and deep space habitat in the Mars orbit to travel back to Earth orbit where it will safely land the crew back on Earth’s surface. In case of a mission abort due to any contingenc­y it becomes necessary to remain in the Mars vicinity for a considerab­le length of time ( typically 500± 600 days, depending on specific launch date) before attempting to return.

The Life support for an additional 500± 600 days must be added to the Earth Return Vehicle to support the crew in orbit for that period, thus driving up the mission mass and cost. The requiremen­ts for fail- safe life support for a crew will be very significan­t and challengin­g. The crew will be exposed to excessive radiation for the additional 500± 600 days in orbit, having lost the benefits of ( a) shielding by the planet from below,( b) shielding provided by the atmosphere, and ( c) possible use of regolith piled on top of the Habitat as shielding.

( Mylswamy Annadurai is Director, UR Rao Satellite Centre — formerly Isro Satellite Centre, Bengaluru)

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