Below Mercury

by Anson, Mark

  Circuit breakers banged inside the cockpit as the power array short-circuited, overloaded, and blew apart in an explosion of violet flashes and glittering shards of glass.

  The shuttle broke free, rising up and away from the mountain peak. Behind it, the robot spun over the edge of the great outthrust peak in a shower of debris from the shattered power array, and fell. It tumbled as it fell, spinning down, down, towards the darkness of the crater that awaited it.

  The robot’s red eyes looked upwards one last time as it turned in its fall, and for a moment, they seemed to watch the shuttle as it climbed away, rising into the black sky. The robot’s limbs moved in a last, futile attempt to reach out and save itself, and then it plunged out of the sunlight into darkness, and was gone.

  Aboard the shuttle, Clare fought to control the craft’s attitude. One set of thrusters had been torn away, and the controls didn’t respond properly. She managed to steady the spinning shuttle by trial and error, until it settled out into its climb.

  She cancelled the alarms that warbled for her attention in the cockpit, and reset the circuit breakers. The shuttle’s systems gradually came back on line, and she reset the autopilot for the climb to orbit.

  They watched with relief as the white line of the shuttle’s path moved back towards the magenta curve of their required climb.

  ‘Can we make orbit?’ Matt asked, rubbing his neck where it had been wrenched sideways by the impact.

  ‘I think so. We’re lighter then planned now; the climb was planned for six people on board.’ She looked back at Matt, her expression stricken, as she realised what she had said.

  She looked frightened and vulnerable.

  Matt reached out, took her hand, and held it tight.

  ‘You did great,’ he said, smiling, and after a moment’s hesitation, she squeezed his hand back, hanging her head as the tears came. She wept for the loss of her crew, her ship, and everything else that she held close, for all the people who had died in the darkness of the forgotten mine, and for the horror of the silo.

  ‘I thought it had got us,’ she said through her tears, ‘I thought we were going down.’

  Matt said nothing, but unfastened his seat straps and leaned over to hug her, and it was quiet in the cabin; the engines hissed in the vacuum of space, propelling the little craft higher and faster, heading for the rendezvous point with the space tug.

  They could just make out the brilliant star of the tug’s reflective sunshade in the distance. It moved against the stars, growing larger with each passing minute.

  They didn’t say a word as they held each other during the long climb; each was too busy with their own thoughts, and the terror of the mine was too close behind them.

  The surface of Mercury dwindled to a distant landscape, and then to a curved horizon, and finally to a planet in space, turning beneath them.

  Clare reached out and triggered the automatic docking sequence, which would bring them to within manoeuvring distance of the tug. She did it automatically, her hands moving almost of their own accord, while her mind took shelter far away.

  ‘We’re going to have to send a report as soon as we’re on board,’ she said at last. ‘I’m looking forward to the reaction when we tell the FSAA what we found there.’

  ‘They’ll never believe us,’ Matt said flatly, looking into her eyes. In the subdued lighting of the flight deck, her pupils were very large.

  ‘Oh, I think they will.’ She spoke softly, but her voice was certain. ‘Everything that happened in the silo is in the flight recorder, and I’ve got the spaceplane’s last few minutes right here.’ She tapped the memory module hanging round her neck.

  ‘Oh, you beauty,’ Matt said, grinning. ‘Will you let me help compose the message?’ Their faces were very close.

  ‘I’ll think about it,’ she said, and pulled his face towards hers.

  BACKGROUND NOTES

  Mercury

  In 1610, the Italian astronomer Galileo Galilei (1564–1642), who would later be forced to recant his scientific beliefs in the face of a wrathful Church, observed Mercury for the first time with a telescope, and saw that it was a planet, a distant world in space.

  In 1629, Johannes Kepler (1571–1630) predicted a transit of Mercury across the Sun for 7 November 1631, observable from Europe. Kepler died a year before he could witness the event, but three scientists in Europe were able to observe the transit, including Pierre Gassendi (1592–1655), who used a projection of the Sun through his telescope to see the event on a white screen. In a report of the transit, Gassendi wrote:

  ‘Crafty Mercury wanted to pass by unnoticed; he had come in earlier than expected, but could not escape without being discovered; I found him and I have seen him; something never experienced by anyone before me; on November 7th, 1631, in the morning.’

  Edmond (Edmund) Halley (1656–1742), who later had a comet named after him, was the first to realise that Mercury transits could be used to measure the distance of the Sun, thereby establishing the absolute scale of the Solar System from Kepler’s third law.

  Johann Schröter (1745–1816) produced the first crude maps of Mercury, but it is hardly surprising that they were not of a high quality, given the extreme difficulty of observing the planet. Mercury rarely strays more than a few degrees from the Sun’s disc, so that it can only be seen for a few minutes after sunset, or before sunrise, at certain times of the year. Even to prepare these simple maps must have taken years of patient observing using the telescopes of the time, waiting for the few moments of clarity in which faint surface features could be distinguished.

  Centuries later, we know much more about this tiny world that fleets by, so close to the Sun. The robot explorer Mariner 10 was launched towards Mercury in November 1973, making the first use of the ingenious ‘gravity slingshot’ manoeuvre to place it into an orbit that passed the planet repeatedly. Mariner 10 made three flybys over Mercury between March 1975 and March 1976, and mapped more than forty percent of the planet’s surface. Much of our knowledge of Mercury was gained from this hugely successful mission.

  The difficulties of getting to Mercury cannot be over-emphasised. Any object orbiting the Sun, for instance a planet or a spacecraft, has a certain amount of kinetic energy (from its orbital motion) and potential energy (from its distance from the Sun). A spacecraft ‘falling in’ towards the Sun changes some of its potential energy into more kinetic energy, so that by the time it arrives at Mercury it is travelling very fast indeed. Huge amounts of rocket power have to be expended to slow the craft down sufficiently to be captured into orbit around the planet.

  Mariner 10 was not designed to enter orbit round Mercury; it travelled too fast and lacked a rocket motor powerful enough to perform an orbit insertion. The Messenger probe, however, launched in August 2004, successfully entered orbit round Mercury in March 2011. After a complex and ingenious journey involving multiple flybys round the Earth, Venus and Mercury to slow itself down, Messenger was able to use a comparatively small rocket engine to insert itself into an orbit round Mercury. This was a truly astonishing achievement, a journey of nearly eight billion kilometres, lasting over six-and-a-half years.

  Messenger has already returned a wealth of scientific information from its three Mercury flybys, but now that it is in orbit the amount of data gathered will increase enormously. The high-inclination orbit will allow detailed examination of the cold polar craters, and the tantalising possibility of finding ice in the deep crater floors.

  Mercury’s density, as deduced from its orbital motion, is extremely high. If it were not for the Earth’s density being higher due to compression effects, Mercury would be the densest planet in the Solar System. Mercury is made almost entirely of iron; it has a huge iron core that takes up most of the planet, with a relatively thin silicate mantle and crust. Because of this, even though Mercury is just one-eighteenth of the Earth’s volume, its surface gravity is over a third that on Earth, and is equal to that of Mars, a much larger planet.


  In a strange parallel to the early history of the Earth, Mercury was once much larger. Long ago when the Solar System first formed, more planets turned round the young Sun than do so today. It is thought that, in a catastrophic impact with another early planet, Mercury’s outer layers were blown off into space. In the molten ruin of the planet, most of the liquid iron sank into the giant core, leaving the silicate minerals behind to form the mantle and crust. The primordial surface of Mercury was further altered by eruptions of basaltic rock from deep below the surface, and during subsequent crater-forming epochs.

  Chao Meng-fu crater

  The craters of Mercury are named after famous artists, painters, authors, and musicians. Chao Meng-fu crater is a very large crater, lying almost exactly on the South Pole of Mercury, and is named after Chao Meng-fu (1254–1322), a Chinese scholar, painter and calligrapher who lived during the Yüan Dynasty. His most famous work of art is Autumn Colours on the Ch’iao and Hua Mountains. On the handscroll of the painting, now preserved in the National Palace Museum in Taipei, he wrote:

  ‘The venerable Kung-chin is a man of Ch’i [Shantung]. After I had completed my service as T’ung-shou [lieutenant governor] of Ch’i-chou and returned home, I told him about the mountains and rivers of Ch’i. Among them Mount Hua-fu-chu is the most famous, having been mentioned in the Tso [a book by Tso Ch’iu-ming]. Its shape is lofty and precipitous, rising isolated in a most unusual way. So I painted this picture for him, setting Mount Ch’iao on the east; which is why I call it Autumn Colours on the Ch’iao and Hua Mountains. In the twelfth month of the first year of the Yüan-cheng era [1295]. Done by Chao Meng-fu of Wu-hsing.’

  Chao Meng-fu would have been surprised indeed to learn that, after his death, his name would live on to grace the lofty and precipitous mountains of another world.

  On the map of the crater, the position of the crater relative to the geographic South Pole is shown as suggested in J.K. Harmon’s 1994 paper Radar mapping of Mercury’s polar anomalies (for this and other references, see Bibliography), so that the crater rim lies on the South Pole, and not in the position shown in NASA’s 1978 Atlas of Mercury. The latitude and longitude of the crater quoted in the story reflect this as well. It will be interesting to see if this is borne out by the Messenger science results.

  The names of the central peaks in the crater are fictitious, and are taken from Chao Meng-fu’s inscription on his famous painting. Chu-Tsing Li’s book The Autumn Colours was invaluable in appreciating the work of this extraordinary and gifted artist.

  The map of the crater shows longitudes increasing to the west, in accordance with the original NASA maps, and International Astronomical Union conventions for planetary rotation. For navigation at the South Pole, however, where every direction is ‘north’, a grid north system is used, where north is aligned with the direction of 0° longitude, and east is 90° clockwise from this. This results in grid heading 90° lying on the 270° line of longitude, and vice-versa. This is intentional, and correct for this system of polar navigation.

  The detail of the floor of Chao Meng-fu crater is fictional, as this area is in permanent darkness and has not yet been mapped. The map is based loosely on the Mariner 10 airbrush maps as published in NASA’s Atlas of Mercury, with considerable license taken over the structure and position of the central peaks.

  If Mercury’s primordial surface solidified from a magma ocean, as seems possible, then it may be composed of anorthosite, like the Moon, with a subsequent covering of low-iron flood basalts. The impact event that formed Chao Meng-fu crater will have punched deep into this surface structure, and the described geology of the crater follows this supposition, with deposits of metals and ores concentrated around the rim of the impact melt, similar to the Sudbury Basin on Earth.

  The mine

  If there were indeed ice on Mercury, it would be a tantalising prospect to mine it and use Mercury as a refuelling base. Ice on its own, however, would not be enough to drive the commercial development of such an inhospitable world – only some scarce and incredibly valuable commodity could do this, and helium-3 fills this role in the story.

  Due to the likely large size and complexity of practical fusion reactors, small reactors would still need to utilise fission fuels, and this is reflected in the description of the mine’s main power source.

  Erebus Mine is intended to reflect a working mine, based on principles of terrestrial mining engineering and practice, extrapolated to a planetary mine in a hostile environment. Although much of the ice mining can take place using open-pit mining techniques, the high radiation levels on Mercury’s surface would require all the living accommodation and critical equipment to be located underground.

  While an open-pit mine would appear to be the simplest option for extraction of the surface ice deposits, there would be practical advantages in mining the ice underground using room-and-pillar workings, notably not having to work in the hostile environment of the crater floor. The design of the mine in the story is a compromise; the surface layers are mined using an open-pit operation, while underground room-and-pillar workings are used for the deeper deposits.

  The spaceplane

  The Olympus 240 spaceplane in the story is an example of a reusable, single-stage-to-orbit (SSTO) launch vehicle. To date, such a vehicle has never been realised due to the difficulties in meeting the two requirements in one vehicle. The Space Shuttle was a reusable launch vehicle, but was not single stage.

  The spaceplane in the story begins its orbital ascent from a fully loaded state, flying high in the atmosphere, so that it does not have to move its entire loaded mass from a standing start from a runway or launch pad. This saves a considerable amount of propellant, but the biggest savings (compared to a pure rocket) come from accelerating the vehicle to hypersonic speeds while still in the atmosphere, using air-breathing engines.

  This portion of the orbital ascent presents extraordinary engineering challenges. Considerable advances in materials science and propulsion technology would be required before a vehicle such as this would be practicable. The nose, leading edges and other surfaces of the spaceplane that are exposed to the highest temperatures would all require active cooling in order to survive the heat generated by atmospheric friction. The air entering the engines would also need cooling, as it would be red-hot from the compression caused by slowing it down to subsonic speeds.

  Supersonic combustion (‘scramjet’) technology aims to get round some of these problems, but this technology brings problems of its own, including low net thrust levels and the likely need for hydrogen fuel. Although liquid hydrogen has a higher specific impulse (bang per kilo) than any other practical chemical fuel, its very low density means that fuel tanks have to be very large, with severe penalties in drag at hypersonic speeds, likely outweighing any advantage.

  For these reasons, scramjets are not described in the spaceplane design, and liquid propane is used as a spacecraft fuel instead, due to its much higher density, and relative ease of storage and handling.

  The navigation and flight controls of the spaceplane are based loosely on those found on large civil aircraft, with some additions and changes necessary for spaceflight.

  The space tug

  The concept of a ‘space tug’ like the Baltimore dates back to the 1970s, to a vehicle that remained in low Earth orbit and could be used to move crew and cargo about in orbit. In the story, this concept is extended into the enormous vehicles that ply between the planets, true leviathans of deep space.

  The gaseous core nuclear engine that powers the Baltimore is based on concepts first explored in the 1950s. The engine utilises a nuclear reaction taking place in an incandescent, fissile gas (uranium tetrafluoride) to vaporise and expand the liquid propellant, providing thrust. Lower power settings provide heat energy to drive Brayton-cycle electrical power generators.

  The engineering challenges of a practical gas core nuclear engine are immense, particularly how to prevent loss of the fissile gas through the exhaust n
ozzle. The engine as described uses a closed-cycle system, keeping the fissioning gas inside a transparent container, and heating the fuel through intense thermal radiation.

  Again, liquid hydrogen is avoided as fuel for the nuclear engine (despite its higher specific impulse), and ammonia used instead, for its higher density and ease of storage. Ammonia also has many useful properties as a working fluid for the power generation and refrigeration circuits, and is plentiful in the outer Solar System.

  Alternative propulsion systems for deep space include various kinds of electric and electromagnetic thrusters. Although these are remarkably efficient compared to nuclear thermal engines, they do not offer the high thrust levels that are necessary for orbit insertion manoeuvres and tolerable journey times in a manned spaceflight context.

  The Baltimore also carries large stores of cryogenic propellants, to refuel the vessels that dock with it and to power its own manoeuvring engines, located at the end of the ship, near the reactor. These engines are conventional, i.e. non-nuclear, and are used for routine manoeuvres such as minor orbit changes.

  As described in the story, the entire crew module can detach from the rest of the tug in an emergency, such as a main engine failure. In this case, the conventional rocket engines on the crew module can utilise the cryogenic propellant stores to get the crew back home, or to a place of safety.

  The crew shuttle and fuel tankers

  The crew shuttle vehicle is designed as a utility crew transfer vehicle, and can work equally well on any airless planets and moons where the surface gravity does not exceed one third of a gee. As the story suggests, it is not designed for operating off rough surfaces, but from a prefabricated silo, which provides all the necessary ground support services during maintenance and turnaround.