Comet or Asteroid: what is the difference?

Because looking up the difference between a comet or asteroid drives me nuts.

They are both:

  • Small chunks of “stuff” that hang around in the solar system. Most describe some sort of orbit, but some pass once and we never see them again.
  • The vast majority can’t be seen with the naked eye
  • They can be made from variety of materials, including rock, ice, carbon-rich materials or nitrogen-rich materials
  • They are much smaller than our moon, have very little gravity and therefore an irregular shape.

Comets are:

  • More likely to be made from water ice or have a thick coating of water ice.
  • The evaporating ice is what gives them an atmosphere that is not bound by the gravity of the comet (because the comet is too small). The water vapour gives them their tail.
  • They originate in the outer solar system.

Asteroids are:

  • Mostly rocky
  • They hang around in the inner solar system (anywhere up to Jupiter) which is also where they are thought to have been formed


  • Some asteroids have also been found to outgas water, while some comets have passed the sun so many times that whatever water ice they had on the surface has gone, leaving the rocky core.

So yeah, there isn’t a clear line to separate between the two and no clear definition. For the sake of convenience, some people use the word asteroid for bodies that move in the inner solar system. The are more likely to be rocky.

Comet or Asteroid: what is the difference? was originally published on Must Use Bigger Elephants


Jupiter’s moon Io: some facts

His Name In Lights is set on Jupiter’s moon Io, which has captured my interest for some time. I have at least one other Io story in the making. Expect an announcement about A Perfect Day Off The Farm fairly soon. In that story, I explain the concept of stick farms, which is something that would actually work.

If you came here from the novella His Name In Light’s product page on my author site, click this link to go back after you’ve read this

Io is a body slightly bigger than Earth’s Moon which orbits Jupiter at roughly the same distance as the Moon orbits Earth. It is the closest of the larger moons of that planet. Jupiter, of course, is much, much bigger than Earth, and this has all sorts of consequences for its neighbourhood.

Io was discovered in 1610 by Galileo when he was looking at Jupiter and noticed three little dots close to it. Moreover, the dots were placed in a straight line (that’s a dead giveaway for a planetary or any kind of system that involves things orbiting each other), and on top of that, the next day, the dots were in different positions. The discovery of the four Galilean moons (Io, Europa, Ganymede and Callisto – the god Jupiter’s lovers in ancient mythology), proved Galileo’s hypothesis that other planet-moon systems existed and that, by extrapolation, the Earth was not the centre of the universe, nor the centre of the solar system.

You can see Jupiter and its dancing dots with a decent set of binoculars in the night sky. Because Jupiter is so large and has so much gravity, the moons race around the planet like bats out of hell, and their positions are visibly different every day.

Being the closest moon, Io orbits Jupiter every 42 hours. Since the circle it describes is the same size as the Moon’s orbit around Earth, it means it’s moving at a speed of over 17 kilometres per second. By comparison, the Moon moves at a speed of one kilometre a second. This thing is racing!

The same applies for the other three moons, Europa (3.5 days), Ganymede (7.15 days) and Callisto (16 days). All move at comparatively high speed, which is why they’re so much fun to observe. You can see them move.

The moons’ gravities also interact with each other, since all of them are roughly the size of our Moon and quite close together, astronomically speaking. They have developed a pattern of orbital resonance, where Io will orbit exactly twice within one orbit of Europa, and Europa will orbit twice with each orbit described by Ganymede. Which also means that Io will orbit exactly four times for one orbit of Ganymede. Callisto is the odd one out and isn’t playing this game.

This orbital resonance means that the three moons will frequently line up in a straight path and will be at cross-angles to each other at other times. This means that there are huge tidal forces exerted on each of those moons, but especially on poor little Io.

When telescopes became good enough to observe very fuzzy details of the surface, scientists thought at first that there were two moons. Sections of the surface are highly reflective, and later pictures of the moon show a kaleidoscope of colours in yellows, greens and reds, leading to the nickname ‘pizzaface’.

We now know that Io is made primarily of sulphur and silicates (aka ‘rock’). The atmosphere is very thin and patchy and when present consists of sulphur dioxide. There is no water at all, in contrast to the other three Galilean moons, which have significant percentages of water on their surface. It is thought that radiation from Jupiter has evaporated any water present on the surface.

Because of the huge tidal forces, Io is the most volcanic body in our solar system. On Earth, we think of tides as something that affects water, but that is just because water can easily move, so we notice the effect. Rock can’t move, so it gets hot in response to tidal forces. This is why the interior of Io consists of molten rock.

Geologists on Earth define an active volcano as one that has been known to exhibit activity within the last 400 years. Under that definition, every single feature on the 400-long list of mountains, depressions and rifts on Io is an active volcano.

In fact, the moon is so volcanic that there are no impact craters, like we can see on the Moon or Mars. That’s not because nothing’s ever hit Io, but because the evidence gets covered up pretty quickly. Essentially, Io is continuously being turned inside-out by its own fart-holes.

The spread of volcanic ash across the surface of Io amounts to an average of 1cm per year. At a geological scale, that’s massive.

It also means that any robotic craft or permanent fixture you were to put on the surface would need some sort of mechanism to keep itself un-buried from this material. To make matters worse, it’s most sulphur as well as Sodium and other elements that are Not Conducive to effective mechanical operation of equipment.

Mountains on Io are up to 8 kilometres in height. Some are volcanoes, but many are not, at least not at the top. The volcanic outlets are more likely to be at the bottom of the mountain.

Why? Well, imagine a layer of ice floating on water. Now pile ice chips on, and more ice chips and more ice chips… and eventually the layer of ice will start to protest, to buckle and twist and break, sink in one place and tilt upwards at another. Ah! Now we have mountains. The logic then also dictates that the breaks (= places where magma can rise to the surface) are next to the mountains.

What else would be special about Io?

The day on Io is 42 hours, the same as its orbital period. Because the moon is tidally locked with Jupiter, like our Moon, it always shows the same side to the planet. Which in turn means that a day on the moon is the same length as one orbit around the planet. On Io, however, this has an interesting complication.

As it races around Jupiter, Io experiences an eclipse every day when it goes into the shadow of Jupiter. The eclipse lasts 2.5 hours and in that dark time it gets so cold that the pathetic cover of sulphur dioxide that passes for atmosphere condensates on the surface as snow, only to sublimate again when the sun returns.

In fact, it is highly likely that the time of the eclipse is the darkest time of day on Io. At night, the Jupiter-facing (subjovian) side would have Jupiter in the night sky. To be honest, Jupiter will take up a very significant portion of the night sky. Think of the light it would reflect back to Io.

This only applies for the side that always faces Jupiter, though. On the other side, you would never see the planet, but you probably get some interesting views of the other moons.

No discussion about Io is complete without mentioning radiation. Every celestial body that has a magnetic field has Van Allen radiation belts. A magnetic field traps particles, either from the solar wind or cosmic radiation, in a donut-shaped torus around the planet. Crossing those belts at between 1000 and 60,000km from the Earth was one of the main concerns for the Moon astronauts. At about 370km, the International Space Station stays well clear of them.

Io orbits–you guessed it–smack bang in the middle of Jupiter’s Van Allen belt. Jupiter is the second-most magnetic object in the solar system (after the Sun). Io helps the particle soup along a bit by constantly spewing volcanic stuff into space.

All this amounts to a massive radiation load. So much that if you were to step onto the surface unprotected, you’d be dead in minutes. Radiation would also affect the operation of any robotic equipment you’d send there.

So, as you can see, there are serious challenges to doing anything on Io, and a lot of places in the solar system that are easier to visit. But Io is also one of the most interesting places in the solar system.

Reference: Io After Galileo, A New View of Jupiter’s Volcanic Moon. Rosaly M.C. Lopes and John R. Spencer (2006). Springer. 366pp. ISBN 978-3540346814.

Be warned, this book is pretty darn expensive. For those with a more casual interest, Io’s Wikipedia page has an extensive list of scientific references, some of which can be read or downloaded for free online. If you are interested in buying this book, you can get a copy here: Io After Galileo: A New View of Jupiter’s Volcanic Moon (Springer Praxis Books / Geophysical Sciences)

Find out where to buy His Name in Lights

Patty writes hard Science Fiction, space opera and fantasy. Her latest book is Trader’s Honour, in the space opera series The Return of the Aghyrians. If you’d like to be kept up-to-date with new releases, remember to sign up for Patty’s new release newsletter.

Alpha Centauri DOES have planets!

Ever since I wrote the two posts about the potential for planets at Alpha Centauri, those posts have been the most popular on this blog. With all the exoplanet action, this discovery was only a matter of time, because it seems that more than 90% of stars–maybe substantially more than that–have planets. I’ve written here about why it can be very hard to detect planets, even in stars that are “close”.

So it seems that Alpha Centauri B has a planet, which is now boringly named Alpha Centauri Bb. I think it needs a more interesting name.

The planet in question is about 13% more massive than Earth, which suggests that it is rocky, but orbits the Sun-like star far too closely to harbour life. The distance between it and the star is a mere 6 million kms (vs 150 million for Earth) and the surface temperature a mere 1200-odd degrees which makes it, as scientists wryly remark “unsuitable for life”. Or at least life as we know it. Science Fiction writers can go crazy here.

It is my prediction that if this planet exists this close to the star, others will be found. Already, some scientists say that the signal is too complex to suggest the planet’s existence with certaintly. A complex signal could well mean more planets. Trouble is, finding a Mars or Earth-sized planet possibly orbiting in a plane at an angle to the line of observation, as part of a triple-star system, and orbiting at a fair distance–as in 1 AU or more–is going to be like the proverbial needle in the proverbial haystack.

Meanwhile, get your space ships ready.

How many people are needed for a space colony?

Shifting Reality is set in a space station orbiting Epsilon Eridani b. The planet, which my characters call Sarasvati, is a gas giant which I have given rings, and the station’s main industry is the harvest of ice from these rings for the production of water, oxygen and fuel. The station is one of four human settlements in the solar system, three of them mining stations. These four settlements survive as independent communities, if not entirely from each other, then certainly from Earth, 10.5 lightyears away.

All of which raises the question: just how big does a community need to be to be truly independent, while maintaining the standard of living we’re accustomed to?

Let us take a few steps back and ask a question that will probably sound stupid, and variations of which make up many jokes: how many people does it take to change a lighbulb.

Duh, I hear you say. Changing a lightbulb is so easy, that’s why the jokes exist. I totally just googled “lightbulb jokes” and there is an entire site devoted to them, from which this beauty: How many porn actresses does it take to change a light bulb? A: Well, it looks like 2 of them are really doing it, but the real answer is actually none. They’re just faking it. Mwahahaha!

Anyway, the point is, changing a lightbulb is an extremely easy task.

Or is it?

Think of all the assumptions underlying the word lightbulb: that we know what it is, what it does and how it works. That there is a reliable supply of electricity. That there is a company that makes ladders. That we have a house that has a ceiling for a lightbulb to hang on.

So, how many people does it really take to change a lightbulb? How many people does it take to make sure that lightbulbs can exist and work so that the Irishmen, or Canadians, or porn actresses, can change them?

There’s the manufacturing: the glass, the metal, the factory that puts it together and puts it in a box. There is the mining. There is the power stations and the industry associated with making them work (coal mining, hydro-power and dams, wind farms, whatever). None of those industries will work without buildings or other places to house them. People make these buildings. Those places need to be cleaned and maintained and you need people to do this. None of those companies can survive for very long without an influx of new workers, so there are places for the new generation to be trained. Looking further into the future, each of those companies needs to make provisions to allow their workers to look after the very young. None of any of these people do well without food, so there are the people who cook, and they, or course will need something to cook, so you need agriculture, after all those people have used transport to get to their work, and would like to flush the toilet after they’re done, and would also very much like to buy all the items they need in a shop. And doing all of this naked could get rather embarrassing, not to mention cold. I have not yet mentioned the internet, or medical care.

By the time the insignificant light bulb is screwed into its insignificant fitting, many people have made contributions to it. Yes, many of those contributions are exceeding fleeting, but in terms of our question–how many people would a truly independent human colony need–by no means insignificant. All those things need to work and need to be in place before someone can climb up the ladder and change that lightbulb.

Medieval humans spent a hideous amount of time simply surviving: chopping the wood, hunting the game, toiling in the fields. They might have been smart, but only the very rich ones had the time to do something with it, by buying labour to do their domestic work. When technology lifted, the growing of food was one of the first things to be large-scale outsourced to other parts of the population. Ditto with the building of houses, the very basic level of health care, the making of clothes, the raising of farm stock. Then: transport. Many people who first bought cars still did a lot of their maintenance themselves. The cars, of course, were manufactured, but these days very few people maintain their own cars. Your car might even need a computer technician. The making of clothes has gone the same way. Of course none of us, barring people in remote areas, ever produced our own electricity or looked after our own water and sewerage.

Each time living standards jump, we add an extra level of service, with associated necessary people to maintain it. If you are astonished by how many people’s efforts have touched the humble light bulb, you will be blown away by the number of people who have breathed over the space program.

I wanted to know what the web says about minimum population requirements, and if you google “how many people are needed for a space colony” the number that’s most quoted is ten thousand. I haven’t discovered the source of this number. Will quote it here when I find it.

Ten thousand is a large-ish village. My husband’s family lives in a town roughly that size. It’s very agricultural, so food isn’t a problem. There is a council that maintains roads and sewerage. But no one makes washing machines. And if someone is sick, you have to go to the hospital in the city. There is also no high school, and I’m not even speaking about a university.

Picturing that village with the people in it, it is my guess that if the rest of the world disappeared, they’d survive fine, but it would be accompanied by a very sharp drop in living standards.

Yeah, yeah, I can hear the protests. You train the people. And they multi-skill. Yep. Sure. And the people starting the colony could decide to only include smart people, and not take loafers, and expect everyone to put in their very best effort. But you know what? That will only work for so long. Loafers, disagreers and political stirrers are born. We’re in space 10 lightyears from Earth. What are you going to do? Execute them? And the last time a society as a whole tried to give everyone jobs and be hyper efficient, we called it communism. Ain’t gonna work.

What is more, society needs a fairly large proportion of people who are happy to just consume stuff. A lot of the above services will only work well at large scale. Ten thousand might be a nice number for a seeder population or might be a genetically justified number (it gets quoted in this respect), but it’s not a long-term viable population.

Approaching the problem from the other side, and forgive the macabre-ness of the following: if a quarter of the world population disappeared overnight, would our standard of living drop?

Yeah, yeah, I hear you, it depends on which ones. You could argue bluntly that the world could well do without the poorest people, but don’t go telling me that the rest of the people could just go on living as before. For one, who made that shirt you’re wearing?

A quarter isn’t that much, so what about if half the world population vanished, and let’s be fair and distribute the vanishing evenly over the entire population. Three and a half billion people, and you’d miss half the people who normally share your table at dinner. Don’t tell me worldwide standard of living wouldn’t be affected.

OK, still, it’s a matter of choosing the right people, and training. But, in order to have the same standard of living, those supposed ten thousand colonists will need the same access to extremely specialist medical care that we have. We have this care because people are allowed to specialise–because they outsource everything else. Forcing fewer people to do the same work means unspecialising them. A jack-of-all-trades will not do the same job as a specialist. End of story.

But, you can automate tasks.

Yes, and each level of technology you jump will require more dedicated specialists to maintain it. Dog, meet tail.

You need more than ten thousand. Substantially more. You need something the size of a decent city, just to maintain our current western standard of living.

Oh, I forgot, we expect these folk to maintain an interstellar space service as well…

There be telescopes!

A few pretty pictures. More here

Space tweeps and the large 72m telescope. Photo by Canberra photographer Martin Ollman

The large telecope at night.

2.55am and we’re receiving a signal from MSL in this windy, cold and rainy field. Photo by Martin Ollman.

Oops, I dropped the umbrella!

More space tweeps. I’m even in this one. Photot by Martin Ollman.

Pointed at the Moon…

Once more on the search for exoplanets, and Alpha Centauri

Excuse me for the absence of regular science posts, but my brain has turned to mush from writing fantasy. Isn’t it easy when you’re allowed to make everything up and as long as it makes sense, no one cares about accuracy? But, yes, I will return to where I left my characters in the Jupiter system, or in a space station orbiting the fictional gas giant Sarasvati, in the not-too-distant future.

This morning I came across this very interesting article on the Centauri Dreams website. By the way, Centauri Dreams, the website of the Tau Zero Foundation, is a very rich source for writers of realistic SF, especially in relation to planetary exploration and interstellar travel.

The article summarises results and speculation arising from new planets discovered by the European Southern Observatory’s HARPS spectrograph, which provides the most accurate Radial Velocity measurements we currently have (see an earlier post on how planets are discovered). Because of its increased sensitivity, HARPS can detect smaller planets. The smallest planet found at this point in time is a mere 1.5 times Earth’s mass. One of the, perhaps expected, outcomes of the spectrograph’s bevy of newly found low-mass planets (super-Earths or near Earth-mass) is that there are many of these smaller planets, a lot more than there are very large planets, and that the previous bias was merely a product of larger planets being easier to detect. The galaxy is swarming with smallish rocky planets. It is quite likely that some will be found inside the habitable zone.

We may already have found some of these planets. Much was made last year of the ‘discovery’ of Gliese 581g, supposedly in the habitable zone of an M class star. However, further analysis has so far failed to confirm the existence of this planet. But the star has two other planets which orbit at the edge of the habitable zone, and out of these, Gliese581d looks the most promising. The width of the habitable zone is not absolute, but varies with the planet’s albedo (basically, how much light it reflects) and composition and (if any) atmosphere composition (see another post on that here). So a newly discovered planet, HD85512b, at 3.6 Earth masses, may also fit the bill. It is a little close to its K class parent star, but could harbour liquid water on its surface if certain conditions of composition and atmosphere are met (see original paper by Kaltenegger et al. here).

Using the HARPS spectrograph, another group of researchers report on the search for planets orbiting sun-like stars within 40 light years from our solar system. (original paper by Pepe et al here). This work has resulted in a the discovery of a number of planets, again, most in the smaller size category. One of the main targets for the hunt is Alpha Centauri B, but there are some problems, one of the main ones being that it is part of a triple star system, and that any model the describes the wobble of the star that is caused by an orbiting planet must take into account that there are two other stars in the system, and as you could understand that is tricky business.

Image depicting an exoplanet system snarfed from NASA JPL

Seriously Amazing Science 2 (links)

A post at Centauri Dreams suggest that instead of building very large interstellar probes (and their inherent logistic problems), we could look into building a swarm of very small ones. There are probably a whole bunch of story indeas incorporated in this very interesting post.

This coming Friday, 5 August, the Juno spacecraft takes off for a mission to Jupiter. Believe it or not, there are Lego figures aboard. One can only wonder at what the aliens (or future humans) are going to think when they find it. As aside here is a picture of the Lego figures in question.

New dwarf planets and other objects continue to be discovered in the vicinity of Pluto.

Astrobiology Magazine examines the influence of the absence or presence of a moon on a planet’s stability.

Scientists announce they have found signs of liquid water on Mars. Watch this space. There have already been models that suggest that in small micro-environments liquid water could exist on Mars. We may well find it sooner rather than later.

And this is just too silly to be true, but it seems it is: a man builds a nuclear reactor in his kitchen. Somewhere on my hard drive, I have a story about some Ethiopian thugs cobbling together a fusion reactor that ends up working a little bit better than they expect. I shelved it a while ago as being ‘too stupid’. Maybe it’s time to pull it out again…

Seriously Amazing Science (links)

Here is a new series which I’ll use to post fascinating science-related links I come across on the internet.

First of all, consider this image of a giant space squid

Secondly, this article about the Moon is rather long, but not overly scientific. About two-thirds down, it says “the Moon may still have a molten outer core”. Wow, just wow. I don’t know if there is a geologist in the house who can tell us about how, failing drilling in the surface, we can determine how warm a planet is at what depth, but it would be amazing to discover that the Moon is not as cold and dead as we thought.

Also an article on the future of manned (peopled?) space flight

Added to the above link, history is being made sooner, rather than later, with private space flight edging close.

Recently, astronomers have discovered trojan asteroids in Earth orbit. Trojans are objects that share the same orbit as their ‘host’ body, usually at a gravitationally stable point, such as the L4 and L5 Lagrange points. Because they share the same orbit as Earth without even being in danger of colliding with it, they are extremely hard to detect, because they’re near the Sun when seen from Earth, and you only get a bit of time after sunset or pre-dawn to observe them. It’s also thought that the Lagrange points are collection points for various items of natural space junk, and people have long been talking about sending missions there. That may happen since we now know there is something hanging around there for us to investigate.

So… does Alpha Centauri have any planets?

A short follow-up on this post I made a while ago (note the tantalising difference in wording of the title).

Just today, I stumbled across a scientific paper, thanks to a tweet by @b0yle, that outlines the priority of stars to which to send an as-yet-hypothetical interstellar mission (link here; the paper is not as jargon-laden as a lot of other scientific publications, so go read if this subject interests you).

Short answer: we still don’t know. And with our current techniques, we can’t be certain. We’re getting very close, though.

Some figures: there are 56 stars within 15 light years of us (give or take a few that may or may not be within this range and possibly very dim ones we haven’t yet discovered).

Most of those are M-class stars, ‘red dwarfs’. There are also 2 G-class stars, like the sun (Alpha Centauri A at 4.4 ly and Tau Ceti, at 11.9 ly). According to current evidence, up to 30% of all stars may have planets. At this point in time, most discovered planets have been very large.

The fact that we haven’t detected any in the Alpha Centauri system doesn’t mean that there are none.

Planet size is expressed in number of times the planet is heavier than Jupiter. It is still fairly hard to detect anything significantly smaller than that. Yet evidence suggests that smaller planets could be more common than larger ones.

Alpha Centauri is a star system of three stars (one K-class, one G-class–like the sun–and one M class). What is the statistical chance that none of them will have planets?

What if the Earth had no moon (part 2)

A while ago, in this blog post, I mused about what the earth would be like with no moon. You may hear the Moon blamed for things as diverse as reproductive cycles and people’s moods, but in that post, I argued that if all of a sudden, we’d find ourselves without a moon, not a great deal would change. The Sun causes tides, albeit smaller ones, and any stability issues of the Earth’s axis would not change overnight.

But this is not a realistic scenario. The Moon isn’t going to just disappear (the subject of Death Star-like blasting a planet apart is a subject for another blog post, but let’s just say it’s physically nigh impossible). The real question I should have asked is: what would life on Earth be like if Earth had never had a moon? If the Pluto-sized object that may or may not have collided with us to form the Earth and Moon as we know them had missed the Earth, and sailed straight on, to eventually burn up in the Sun.

The situation would look vastly different. The Earth-Moon system is vastly different from other known planets in terms of relative size. Only the largest moons of Saturn and Jupiter are similar in size to our Moon, but those planets are of course much more massive. The Earth-Moon system could well be referred to as a double planet. Of course Earth and Moon affect each other, and this influence comes in the form of gravity. While the gravity of the Moon affects all surfaces of the Earth equally, it’s only the oceans that can react to this gravity. Yeah, that’s how the tides are formed.

Imagine the Earth as an oblong bubble of water, with bulges both on the side where the Moon is at and the opposite side. This bulge orbits the Earth at the rate the Moon does. But the Earth itself rotates inside this envelope of water, and this creates friction at the place where water and solid surface meet. In effect, the water is forever trying to keep up with the planet. Friction creates warmth, and yes, loss of speed.

So it is that over the four-plus billion years of the Earth’s lifetime, the rotation speed of the Earth has slowed from an eight-hour day to our current twenty-four hours. Is still slowing, in fact.

Of course life on a planet with days of eight hours would be very different. We’d experience vastly more powerful weather and especially wind systems. With a rotation speed like that, there would probably not be much opportunity for much North-South weather movement, but we’d have strong bands of air movement, much like Jupiter (which has a daylength of ten hours). We’d have similar ever-lasting cyclonic systems.

And what would it do to the seashores, having the tides jump up twice in eight hours?

Or to the biology of animals evolving with that kind of daylength? Would we all have nervous tics from seeing the Sun whizz by?