2016_Z(Alternative words: zettahertz.)

Today’s question for Dr. Science is, “What’s the biggest optical telescope in the Empire? How far can it see?”

Over the years, a great many different telescopes have held that particular title: from the Great Eye at the Starspike (Eliéra’s oldest observatory, dating to the pre-Imperial era), through the first orbital telescopes, the large refractor at Farside Observatory, Seléné, and the Deep Orbit Oculus in far Súnáris orbit.

All, however, were rapidly outclassed by the discovery of very-long-baseline interferometry, which uses a technique referred to as aperture synthesis to correlate signals from a set of telescopes to produce images having the same angular resolution as an instrument the diameter of the entire set. Some limited use was made of these techniques with ground-based and orbital instruments, restricted by the difficulty in accurately quantifying optical-range photons for software processing, but once these difficulties were solved, construction began on much larger interferometric telescopes. Three particular examples of these held the title of largest optical telescope in turn, and while the others have been upgraded and remain in use, it is the last of these retains it today.

The first of these, the Barrascán Array, was constructed in the Meryn System, consisting of an array of millions of statites (produced by self-replicating, autoindustrial techniques) 48 light hours in diameter. Intended for general observation, the array possesses an angular resolution of 1.12 x 10-20 radians, enabling it to resolve objects 20 cm across at 2,250 light-years (i.e., the current fringe of the Associated Worlds, which was then unknown space).

The second, intended to carry out both exploration surveys and long-range observations of the galactic core, was the Very Long Baseline Observer, which made use of smaller arrays of deep-orbit telescopes located in systems across the width of the Empire, each reporting via the interstellar dataweave to the Exploratory Service’s headquarters in Almeä System. This gave it an effective diameter of 164 light-years, and thus an angular resolution of 3.74 x 10-25 radians, giving it the capability of resolving with micrometer resolution objects throughout the Starfall Arc, should its view be unobstructed. Indeed, if not for intervening objects, planetary rotation, local weather, and other such obstructions, it would be capable of reading a book over the shoulder of a sophont on any world in the galaxy — were one to pass within its view, since as you can imagine, an array of array of telescopes 164 light-years across is somewhat unwieldy to maneuver.

The apex of this technology is the Super-Size Synthetic Aperture, intended for in-depth studies of the deep universe. The SSSA takes the general concept of the VLBO even further by extending the array – by means of various treaty arrangements and leases – across much of the width of the Associated Worlds, reporting data back over tangle channels. Its effective diameter is no less than 1,825 light-years, giving it a theoretical angular resolution of 3.36 x 10-26 radians – which is to say, it can resolve a 33 m object at the rim of the observable universe.

The SSSA, however, is limited by the larger gaps between its elements, which are themselves limited to a single mobile telescope per system, and thus in turn by the amount of light collectable by each of these individual telescopes. It is also, unfortunately, constrained by the difficulty of maneuvering and recalibrating such a massive device, and by the political difficulties of passing through many different polities during reorientation, which tends to cause lengthy delays, increased costs, and where no permission can be obtained, gaps in array coverage. For most practical purposes, therefore, the VLBO can be considered the largest general-purpose optical telescope available to the Empire.

Dr. Science

– from Children’s Science Corner magazine


Ask Dr. Science: Starports

Today’s question for Dr. Science is, “What are starports for? Lots of starships call at my hab, and we don’t have one.”

Starports and starships have surprisingly little to do with one another.

If there were only starships and drifts, and perhaps the odd rock, we’d have no need of starports. The starships could simply pull up alongside their destinations and shift their cargo about with longshorebots and lighter OTVs and a few stout lads working out of docks and locks. Running a few insulated lines would take care of fueling, and in this scenario, no doubt the passengers – spacers all – would be happy enough to take a walk over. And outside local space, no-one cares where you heave to.

No, starports exist because the galaxy is full of planets, and because large numbers of people are perverse enough to want to live on them. (See my earlier column, Yes, They Store Their Air On The Outside (And Why We Can’t).)

They do have lots of facilities for starships associated with them – cageworks, chandlers, refueling depots, orbital warehouses, freight transshipment nodes, and suchlike – because it’s often convenient to keep them together in a central location, and because it helps pay the bills. But what starports are actually for is solving the interface problem.

One of the less believable realities of space travel is that – on most highly populated worlds, other than a few moons – the depth of the gravity well and the thickness of the atmosphere is such that it takes every bit as much delta-v to climb from the surface into orbit and as it does to make transit between a system’s worlds. The depth of the well and the passage through the atmosphere impose even more constraints on the structural strength and hull forms of starships, in ways that handicap them for operation in the space environment; most starships that are in operation today could neither support their own weight at the bottom of a planetary well, nor withstand the rigors of atmosphere entry. The need to transport freight and passengers between these two disparate environments is the essence of the aforementioned interface problem.

And so starports straddle this line, possessing both a dirtside half (the Down, or downport) and an orbital half (the Orbital, or highport), each composed of a variety of specialized facilities in close formation. The Orbital houses many starship service facilities, but the majority of its business is transferring freight and passengers to and from its counterpart. Except for relatively new colonies and those worlds with the wealth and traffic volume to support a space elevator (or more than one; Seranth has six elevators supporting its ring-city), this falls into a familiar pattern.

Freight is simple enough. Some worlds opt for pure mass-driver launch facilities, and some prefer laser-launchers, but wherever it can, the Imperial Starport Authority prefers to opt for the maximal efficiency of a hybrid system. Should you visit the freight terminal of any major downport, you’ll find it rather unimpressive in itself, despite the sheer size of the building, because it is merely the front end of an enormous mass driver – miles in length! – or array of mass drivers, ending at the peak of a mountain high enough to get the muzzle of the drivers above the thickest part of the planetary atmosphere – and if no mountain is conveniently located for the starport architects, an artificial one will be constructed for the purpose. Around the muzzle of the mass driver, a complex of gigawatt-range phased-array pulse lasers provides additional power and control.

Every few seconds, a freight container is taken from the outgoing queue, and locked into place within a reusable aeroshell, which provides both the streamlining necessary to penetrate the atmosphere, and ablative remass for the latter part of its flight. This aeroshell is then loaded into the mass driver and accelerated up to orbital velocities, with the mountaintop array selectively lasing the ablative remass (pulsed plasma propulsion) to provide guidance and additional delta-v as needed. (The degree to which it is needed varies by cargo: heavy hardbulk can withstand high accelerations, and as such most of the acceleration can be provided by the efficient mass driver, whereas more delicate cargoes require gentler acceleration for longer, and thus proportionately more of the total delta-v is provided by the lasers.) Upon its arrival in orbit, the aeroshell is caught by the muzzle of another, rather smaller, mass driver, this time operating in reverse, and converting the aeroshell’s residual kinetic energy back into electrical energy. Once it has been braked into the receiving station, the aeroshell is stripped off and sent for reconditioning and refueling, while the container is dispatched to the incoming queue, and thence to the appropriate orbital warehouse.

Ground-bound freight follows the reverse process, being accelerated by the small orbital mass driver onto a re-entry trajectory targeted upon the muzzle of its groundside partner; on its way down through the atmosphere (it is designed to be stable stern-down for reentry), the laser array and ablative remass are again called upon to provide guidance and, if necessary, additional deceleration. Plunging into the barrel of the mass driver, the reverse process is again used to brake it to a stop at the freight terminal, where the aeroshell is again stripped off and reconditioned, and the container routed onward to its final destination.

These systems are often operated in pairs, enabling the efficiency of using the captured gravitational potential energy of freight moving downwell – captured by the mass driver to the greatest extent that engineering and thermodynamics permits – to partially power the ascent of upwell freight. As you can imagine, a pair of these systems sending and receiving containers every few seconds, every hour of the day, every day of the week, can move an awful lot of freight!

Passengers, though, are more fragile than most freight. (And rather less comfortable stepping into the breech of Heaven’s Own Sluggun, whatever the numbers might say.) They prefer to travel on shuttles, vehicles specifically designed to cope with the interface problem – with all that atmosphere in the way, you can’t just hop in a commutersphere or ride a candle!

But atmospheres aren’t all bad news. Given the depth of a planet’s well, you might expect that the shuttles would have to be huge lumbering ships to carry all the remass they needed to climb up to orbit; but since they spend so much time in atmosphere, they can use the atmosphere itself as remass, and only carry the little they need for the very end of their journey. Most shuttles have trimodal nuclear engines. They start out as simple tilt-turbine ducted fans when they leave the ground, until they can achieve the speed and altitude necessary to start using their reactors to heat the air directly, becoming nuclear-thermal scramjets, and this mode carries them up through hypersonic speeds to the very edge of space. At this point, before the air becomes too thin for them to function, they switch over to using their internal supply of remass, becoming true nuclear-thermal rockets until they dock with the highport and deliver their passengers. Refueling there, they land again using the same engine modes in the reverse order, and the cycle repeats.

It’s ironic, then, that the features most commonly associated with starports in the public mind – the enormous graphite-and-cerametal pads with their massive hidden cradles, the blast-deflecting berms, the “hot” shafts with their billowing wash-down sprays, and so forth – are those dating back to an earlier age of space, when planets truly were the center of civilization and mighty ships rose heavenwards on pillars of atomic fire, now sadly reduced to a minority of any starport’s business, handling a few special loads, private yachts, and those small tramp traders which service early colonies and outposts that cannot yet afford full starports of their own. But even they share this one commonality: a need to get to and from the planetary surface.

In the end, they’re all about the planets.

Dr. Science

– from Children’s Science Corner magazine

Ask Dr. Science

Today’s question for Dr. Science is, “Why do lighthuggers have to stay so far out? Can’t they use the same highports as normal starships?”

While it would certainly be more convenient to avoid the lengthy shuttle trip to meet a lighthugger, the risks attached to the amount of energy needed to propel a ship between the stars at near-light speed make them something best kept away from population centers.

The smallest lighthugger in production, close to the practical lower size limit, is the Evelantar-class staryacht, whose unfueled mass is 5,451 tons. It is propelled by a Nucleodyne Thrust Applications antimatter pion drive, with fusion supplementation for lower velocities, giving it a maximum cruising speed of 0.9 c.

The mass ratio, including operational safety margin, of the NTA pion drive – the ratio between its fueled mass and its unfueled mass – is 25; but since a lighthugger in many cases cannot guarantee that it can refuel at its destination, the Evelantar is equipped to carry fuel for a two-way trip.

Thus, such a staryacht can carry up to 136,275 tons of fuel one-way, or 272,550 tons fully fueled, of which just under half is antimatter in the form of metastable metallic antideuterium. And, of course, when fueled for a two-way trip, over half of its fuel – because of the additional fuel carried as a safety margin – remains in its cryocels when it arrives at its destination.

Such an amount of matter/antimatter fuel would, if detonated, produce an explosion of approximately 2.6 teratons. In orbit of a garden world, this would be sufficient to create massive earthquakes and volcanism, megatsunamis, global wildfires, major atmospheric damage, and a high-probability extinction-level event, in addition to the radiation effects. These radiation effects and indirect impulsive shock would also be lethal to any habitats or drifts within tens of thousands of miles of the explosion. And this is the fuel mass of the smallest production lighthugger.

While the probability of a cryocel containment-safety systems failure is infinitesimal, the magnitude of these consequences – along with the possibility of deliberate sabotage or the use of lighthugger fuel as a terror weapon – is sufficient for virtually all civilized systems to restrict lighthuggers to far outer-system ports of call.

Dr. Science

– from Children’s Science Corner magazine