The fact that a comet forms a coma and tail affects its composition. The content of the tail is lost. Loss of matter from a comet is estimated to be about 10 to 50 metric tons per second (t/s), for a comet approaching the sun for the first time. This surprisingly small amount of matter—a maximum of 0.03 to 0.2 percent of the comet’s mass each time it passes the sun—shows that tails can only have very low density. Their sometimes enormously intense brightness is explained, in the case of the dust tail, by the large surface area of the microscopically small dust particles. In a plasma tail, every atom or molecule contributes to its luminosity. This leads to an increase in luminosity by many orders of magnitude compared to the size of the comet’s nucleus.
The origin of comets
We know that comets are objects left over from the creation of the solar system (primordial objects)—and not recent fragments created by later collisions of other, larger celestial bodies. This makes them especially interesting for scientists, because they provide a glimpse into the past.
The high proportion of volatile substances in comets’ nuclei, such as water and carbon monoxide, means that they must have originated in extremely frigid environments, and therefore in the outer reaches of the solar system. Most planetesimals in the region of the outer planets were probably drawn toward the four giant planets when the solar system was young. Due to orbital disturbances affecting the remaining chunks, many of them were so scattered that they left the solar system. It’s assumed that around a tenth of these scattered bodies form the distant Oort cloud. Objects orbiting nearer but still outside of Neptune’s orbit were less subject to this scattering process and formed the Kuiper belt.
The Oort cloud and parts of the Kuiper belt are the reservoirs of the majority of comets, which could number in the billions. Because long-period comets are very much scattered by the large planets—especially Jupiter—when crossing inner regions of the solar system, they are only identifiable as former members of the Oort cloud for a few orbits. So there must be a mechanism that brings the comets still visible today back toward the sun from their orbits far out from the sun.
It’s assumed, for short-period comets from the Kuiper belt, that objects in the belt collide with one another, causing chunks to be flung into the interior region of the solar system. The process of scattering of long-period comets is still not precisely understood. Weak gravitational effects near stars or the attraction of larger, trans-Neptunian objects could gradually cause changes to their orbits and redirect the distant, cold, comet nucleus into an elongated orbit toward the sun, leading to the discovery of new comets year-round.
Some disappear, never to be seen again, while others maintain their periodic orbits. However, other possible causes have also been considered, such as the influence of passing stars or undiscovered planets (e.g., ‘Planet X’) or the now refuted idea that the sun might have a companion star (‘Nemesis’).
When comets flying through the interior of the solar system contain a significant amount of ice and pass very close to the sun, they can sometimes be visible to the naked eye—which was very much the case with Ikeya-Seki (1965) and Hale-Bopp (1997).
As explained above, comets lose a portion of their mass with every orbit around the sun, especially volatile components in the outer layer of the nucleus. The closer the perihelion of its orbit is to the sun, the faster this process occurs, because ice sublimates faster and larger particles are also lost due to the outgassing of rock. This means the comet’s nucleus is barely recognizable as such after a few thousand orbits around the sun. This timeframe is significantly shorter than the age of the solar system.
Due to the evaporation of ice, the rock of the nucleus loses its composition and the comet gradually dissolves. This can be caused by splitting (as with Biela’s Comet 1833), by Jupiter's influence (Shoemaker-Levy 9 1994), or by the gradual distribution of particles along their original orbit. The latter is the cause of most meteor showers.
Interesting comets
Comets can still be observed nightly with a simple telescope. However, ‘large comets,’ ones that a layperson can follow in the night sky with the naked eye, only appear about once every 10 years.
Halley’s Comet was the first comet recognized as a periodic comet (in 1705 by Edmond Halley) and the first comet whose nucleus was able to be photographed by space probes (1986). It has even been visible as a ‘large comet’ several times.
Encke’s Comet (discovered in 1818) has the shortest orbit of all known comets, at 3.31 years.
Comet Biela (1845-46) was the first comet to be observed disintegrating.
Comet Donati (1858) was the first comet on which outgassing in the coma was observed. It was voted by artists to be the most beautiful object of the century.
The Johannesburg Comet—almost at the same time as Halley’s—made 1910 the unusual year of two large comets.
Comet Ikeya-Seki is one of the brightest comets of the last millennium. In October 1965, it reached around 60 times the brightness of the full moon, and was clearly visible during the day beside the sun.
Comet Shoemaker-Levy 9 disintegrated near Jupiter. Its 21 fragments impacted the planet between July 16 and 22, 1994. Traces of it were visible for several weeks.
Comet Hale-Bopp was visible to the naked eye for more than 18 months in 1996 and 1997, a record for all known comets.
Comet Tempel 1 was the target of NASA’s Deep Impact mission, in which a 372 kg projectile, composed mainly of copper, impacted the comet on July 4, 2005, with a relative speed of 10 km/s. The resulting dust cloud was observed by the probe itself, numerous Earth-based telescopes, as well as the Hubble Space Telescope and the ESA space probe.
Comet Wild 2 was the first comet to have some of its coma particles collected by a probe (‘Stardust’). The samples were brought back to Earth in 2006.
At the end of October 2007, the 17P/Holmes comet appeared to increase in brightness in the space of 36 hours, from 17 to 2.5 m. The comet suddenly appeared 500,000 times brighter than usual and was visible in the sky with the naked eye.
67P/Churyumov-Gerasimenko is the first comet on which a probe gently landed in 2014 as part of the Rosetta mission.
Glossary of Acronyms
ABS – Anti-lock Braking System
AFC – Asteroid Framing Camera
ALMA – Atacama Large Millimeter/submillimeter Array
APXS – Alpha Proton X-ray Spectrometer
AU – Astronomical Unit (the distance from the Earth to the sun)
BEC – Bose-Einstein Condensate
CapCom – Capsule Communicator
CHON – Carbon, Hydrogen, Oxygen, and Nitrogen particles
CIVA – Comet nucleus Infrared and Visible Analyzer
COM - COMmunications
CONSERT - COmet Nucleus Sounding Experiment by Radiowave Transmission
COSAC – COmetary Sampling And Composition
DART – Double Asteroid Redirection Test
DLR – Deutsches Zentrum für Luft- und Raumfahrt (German Aerospace Center)
ELF – Enceladus Life Finder
ESA – European Space Agency
ESM – External Service Module
ESOC – European Space Operations Centre (Darmstadt, Germany)
ESTRACK – European Space TRACKing network
EVA – ExtraVehicular Activity
g – g-force (gravitational force)
GEND – Gateway Exploration Neutron Detector
GOLA – Gateway Orbiter Laser Altimeter
GTR – General Theory of Relativity (aka Theory of General Relativity)
HALO – HAbitation and Logistics Outpost
HLS – Human Landing System
HUT – Hard Upper Torso
ISS – International Space Station
JWST – James Webb Space Telescope
LAMP – Lunar Activity Monitoring Program
LOLA – Lunar Orbiter Laser Altimeter
MOM – Mission Operations Manager
MUPUS – MUlti-PUrp
ose Sensors for surface and subsurface science
NASA – National Aeronautics and Space Administration
NRAO – National Radio Astronomy Observatory
OCSS – Orion Crew Survival Suit
OGS – Optical Ground Station (telescope)
RF – Radio Frequency
ROLIS – ROsetta Lander Imaging System
ROMAP – ROsetta lander Magnetometer And Plasma monitor
SAFER – Simplified Aid For Extra-vehicular-activity Rescue
SEP – Solar Electric Propulsion system
SESAME – Surface Electrical Sounding and Acoustic Monitoring Experiments
SLS – Space Launch System
TIRA – Thermal InfRAred instrument
TU – Technische Universität (technical university)
UV – UltraViolet
VR – Virtual Reality
WHC – Waste Hygiene Compartment
Metric to English Conversions
It is assumed that by the time the events of this novel take place, the United States will have joined the rest of the world and will be using the International System of Units, the modern form of the metric system.
Length:
centimeter = 0.39 inches
meter = 1.09 yards, or 3.28 feet
kilometer = 1093.61 yards, or 0.62 miles
Area:
square centimeter = 0.16 square inches
square meter = 1.20 square yards
square kilometer = 0.39 square miles
Weight:
gram = 0.04 ounces
kilogram = 35.27 ounces, or 2.20 pounds
Volume:
liter = 1.06 quarts, or 0.26 gallons
cubic meter = 35.31 cubic feet, or 1.31 cubic yards
Temperature:
To convert Celsius to Fahrenheit, multiply by 1.8 and then add 32
To convert Kelvin to Celsius, subtract 273.15
Excerpt: The Triton Disaster
5/23/2080, VSS Freedom
“What is that?”
Startled, Nick turned towards the sound of the voice. One of the passengers, the skinny bald one, was watching the radar image over his shoulder.
“During the flight, you should...” Nick began, but then shook his head. Here we go again, he thought. There wasn’t really anything wrong with people asking him questions during the flight. They were, in the end, paying for him to entertain them.
“Let me see...” He looked for the bald man’s name tag and read it. “Mr. Wiseman. We’ll find out shortly.”
He shifted the radar image to center on the shadow the man had spotted. Usually the autopilot handled the radar and Nick didn’t have to check it. The pilot only had to step in if something was endangering the ship’s flight path and the autopilot hadn’t determined a detour. In other words, never. Whatever was casting the shadow must have been rotating, since the intensity changed at a rate of approximately once per minute. Nick retrieved the orbital data and nodded. It was probably one of the Spacelink satellites a crazy billionaire had paid to have fired into low orbit, way back when, only to just leave them up there when his company had gone bankrupt.
“Mr. Wiseman? This looks like an old Spacelink satellite. It’s a miracle it hasn’t burned up yet.”
It was strange, though. At such a low orbit, the atmosphere would have caused so much deceleration that the satellite should have fallen long ago. But during one of the launches the satellite deployment hadn’t gone as planned, so four of them had ended up in higher orbits. Nick remembered this only because it had delayed his own first launch into space by a month. NASA had wanted to be sure that the private firm had its technology under control.
“Spacelink?” the curious passenger asked.
“Yes, that’s what the low orbit suggests. If it were an active satellite, the radar would issue a warning.”
“Then that thing is worth a lot!”
“Well, after so much time it’s become electronic waste.”
“Didn’t you hear that one of the company founder’s vehicles was auctioned off at fifty million the other day? A salvage company brought it back from its Mars orbit.”
The man was right. The Spacelink founder’s fans still adored him, and the fact that most of the other satellites in the series had burned up would increase the value of this specimen significantly.
“I think, Mr. Wiseman, that we should note what the exact path of this gem is. Then, later on we can—”
“But why later?” interrupted the passenger. He had become so loud that four of his five fellow travelers stopped photographing from their portholes to look up at him.
“We should discuss this in private,” said Nick, raising his arms.
“I’ve got an idea. I’ve booked an EVA with you, anyway. We’ll just use our time out there to bring this thing in.”
“We’d have to change course,” Nick replied. But he no longer protested, only searching for possible objections so the man could help clear them out of the way.
“You’re the pilot. But it looks to me like that thing isn’t that far off.”
“Up here, that’s relative. It’s above us. We’d need to decelerate to get into its orbit, then accelerate again to recover the lost time in a lower orbit. I have to get all of you back to New Mexico, preferably with a couple of gallons of methane left in the tank, or else my boss will give me the sack.”
Which might not be so bad, Nick thought. Then he’d never have to get back on this space bus and make stupid passengers with no training into astronauts.
“You’ll have to figure that out on your own. I can’t help you with that,” said Wiseman. “But I assume we have reserves on board. What if you lost me during the EVA?”
Nick sighed. “True, we have about twice as much methane in the tanks as we need. This is regulation in private space travel, and that’s what makes the tickets so expensive.”
“Just check it again. But let’s suppose we could bring in ten million for this thing there, then you’d get three million. Of course, since I discovered it, I get a little more.”
“No way. We’ll go fifty-fifty, or it’s not going to happen,” Nick replied.
“So you’re in?” asked Wiseman.
The bald guy had fallen for it. Nick gave a short laugh. “Are you a politician or something?”
“No, I’m a real estate agent,” answered the passenger. “Let’s just split the proceeds. It’ll bug me, but so be it. I’m Walter, by the way.”
Wiseman held out his hand and Nick gave him a palm slap.
Nick pressed a button on his control panel. An automated voice announced, “Your attention please. Orbit correction. Please ensure that your safety belts are securely fastened.”
The same text appeared on the displays adjacent to every passenger porthole. Everyone reached for their seatbelts except for a blond woman in her mid-40s, sitting on the right-hand side in the back, who ignored the announcement. She had headphones on and was bobbing her head back and forth with her eyes closed. When they’d crossed the Karman Line, she’d paid attention to outer space for just a moment. Nick figured her husband had given her this flight so he could screw his secretary in peace.
Nick sighed, lifted himself up, and floated over towards her. He knocked on the casing of her headset. When she opened her eyes in alarm, he pointed to the warning on her screen.
“Oh, excuse me,” she said loudly, buckling herself in.
Nick returned to his seat. Meanwhile, the computer had calculated the new course and all he had to do was press the start button. It was a good thing he’d had such thorough training in orbital navigation. But would he really still be able to calculate the necessary deceleration phases on his own? The thrust of the engines pushed him into his seat, and he closed his eyes and surrendered to it.
His seat was shaking. Nick winced. Had he fallen asleep? He checked the display. The ship had reached the pre-calculated position and was hovering, adrift in space.
Nick turned to the passengers. The blo
nd woman with the headphones appeared to be asleep. The others were glued to their portholes.
“Time to get out,” he said, nodding to Wiseman.
“Get out?” asked the Japanese woman.
“Mr. Wiseman here booked an EVA,” Nick replied. “He wants to take a look outside.”
“Oh, I’d like that too,” she said.
“I’m sorry, but you should have specified that when you made your reservation. It costs $5,000.”
“Can I pay now?” The woman grabbed her purse, presumably in search of her wallet.
“Unfortunately, no. We only have equipment for one guest on board. Planning, you know?”
She sank into her seat, disappointment written all over her face.
“Come with me to the back, Wiseman,” Nick said.
The realtor followed him to the rear of the ship towards the airlock. Nick activated the outer door, which opened laterally to reveal what looked like an oversized chocolate box. To save air, there was a pair of three-dimensional forms, vaguely human-shaped, made of soft but very durable material. The spacesuits were inside.
Nick took out Wiseman’s spacesuit first. “Slip it on,” he said. Then he donned his own.
The suits were so uncomplicated that even a layperson could put one on quickly. Most importantly, it was possible for someone with no previous training to fill one up with air. Nick still remembered the old NASA suits and having to cycle for half an hour to avoid getting the bends.
“It worked,” said Wiseman, smiling at him.
Nick checked the fit of the suit, tightened the buckles on the belt, and then put his hand on Wiseman’s shoulder approvingly. “Good job.” He pointed to the helmet. “When you close it, Wiseman, radio communication is activated automatically.”
“Roger that. And outside?”
The Dark Spring: Hard Science Fiction Page 28