As an asteroid affectionately known as "The Rock" soared past Earth recently, I thought it's time to tackle the differences between some very similar astronomical objects: asteroids, meteors and comets.​

Let's start with asteroids and comets.

Both asteroids and comets are chunks of space debris (usually rocks covered in ice) that orbit around the Sun.

But they come from different places.

Asteroids are rocky, inactive objects that are formed in the warm inner solar system region between the planets of Mars and Jupiter.

Comets are mostly made up of ice embedded with dust particles. They are formed further out, in the colder outer reaches of our Solar System. 

A comet's ices can vaporise in sunlight forming an atmosphere of dust and gas. This atmosphere (also known as a coma) sometimes forms the tail of dust and/or gas you often see trailing behind a comet.

If an asteroid or a comet is nudged out of its orbit by the gravitational pull of the other planets in our solar system, they can pass close to the Earth. 

OK, this is where we have to introduce three more terms: meteoroid, meteor and meteorite.

A meteoroid is, basically, a tiny piece of space debris. It could be a small asteroid, which forms when bigger asteroids collide, or the debris from a comet, which is formed when a comet passes near the Sun and releases debris that was once embedded in the (now melted) ice of the comet. 

When a meteoroid enters the Earth's atmosphere it burns up, causing a flash of light and is now known as a meteor.

A meteor also goes by another, more common, name - it's a shooting star.

If any of the meteor is not burnt up in the atmosphere and survives its descent to the Earth to land on its surface, then it is known as a meteorite. 

Here's a quick picture from tes.com to help differentiate these cosmological entities:

We know a lot more about comets thanks to ESA's Rosetta mission and there's plenty of information on the differences between comets, asteroids and meteoroids here.

You can find out more about comets here and asteroids here. Or, click here to find out how to catch a meteorite.

​And if you'd like to know what would really happen to you if an asteroid hit Earth, this article is quite shocking.

This week, "negative mass" hit the headlines. It's got a lot of scientists jolly excited - and I've received a lot of messages asking me to explain what negative mass is.

Here we go.

Let's start with Newton's second law of motion. Newton's second law can be summed up using the following equation: F = ma, where 'F' is the force (that you shove the object with), 'm' is the mass of the object and 'a' is the acceleration it moves with.

In other words, if you push an object, it will accelerate in the direction you pushed it.

That's common sense, right?

With negative mass, the opposite happens.

You push an object and it accelerates towards you.

​That's pretty weird, right?

Well, maybe not. An electrical charge can be positive or negative.

So, why can't a mass be positive or negative?

You can. Negative mass has cropped up in many other scientific theories, including those attempting to prove the existence of wormholes.

But no fundamental particles with negative mass have ever been discovered. This means we have no experimental insights into how a particle with negative mass would behave.

"What's a first here is the exquisite control we have over the nature of this negative mass, without any other complications," said co-author of this week's paper on negative mass Michael Forbes, assistant professor of physics at Washington State University.

That's why this latest breakthrough where a negative effective mass has been detected is so important - it gives us a way to understand negative mass and the theories that surround it. 

It could give us further insights into cosmological phenomena including neutron stars, dark matter, dark energy and black holes.

Despite the excitement surrounding this week's announcement, scientists have NOT actually created negative mass. They've created a negative effective mass. That one word makes the world of difference - you can find out more here.

And if you want to read the full scientific paper describing the phenomenon, click here.

Black holes are incredibly dense regions of space that exert such a massive gravitational force, even light cannot escape from them.

That’s a lot of science in one sentence.
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So, let’s break the science of black holes down.

​A black hole is a large amount of matter that is squashed to a tiny region of space.

​Imagine fitting every piece of Lego in the world (that's 400 billion bricks) into one brick.

But a black hole doesn't look like a Lego brick or a star, or any other astronomical object out there.

We actually don't have a clue what a black hole looks like because it doesn't have a surface like a star does.

What it does have is tremendous mass.

Let's go back to our Lego example. If we squashed every Lego brick in the world into one brick, that's going to be one immensely dense Lego brick. It'll fall through the desk and have a huge gravitational field because the more mass an object has, the greater its gravitational pull.

The gravitational field of a black hole is so large that even light cannot escape its gravitational pull.

The boundary of this region where nothing useful can escape the black hole's grasp* is called the event horizon. In other words, if light crosses the event horizon, it won't be seen again.

It also means you can't photograph or even see a black hole. 

Black holes are detected by observing how the material surrounding them behaves.

For instance, black holes exert a powerful gravitational pull on nearby stars. If you look more closely at the stars' movements, you can calculate the mass of its invisible partner - and this could be a black hole if the inferred mass is large enough.

You can also investigate the gas and dust surrounding a black hole. This is funnelled by gravity into a disk and, as this material swirls around the black hole, it is moving so fast that it heats up and emits X-rays. We can detect these X-rays from Earth.

Another hint of a black hole is that light from the stars behind a black hole will be bent by its gravitational pull. You can measure the deflection of this light to derive the existence of a black hole - this bending is a process called gravitational lensing.

Black holes can also collide and create gravitational waves. Gravitational waves were detected last year and helped to prove one of Einstein's most fundamental theories - the General Theory of Relativity.

Yet, our understanding of black holes and their potential to help us understand our universe is still far from understood.

To quote theoretical physicist John Wheeler: “[The black hole] teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as ‘sacred,’ as immutable, are anything but.”

NASA's explanation on black holes is a great first port of call, Wired gives more details on how black holes form and this article from Space gives you more information on the different types of black hole in our universe.

And you can't go wrong with this brilliant Black Hole overview from New Scientist or this video explaining black holes:

* 30 years ago, Stephen Hawking suggested that black holes should release heat. He, more recently, argued that information could, technically, escape from a black hole.

​Hello. I’m the freelance writer who gets tech. I have two degrees in Physics and, during my studies, I became increasingly frustrated with the complicated language used to describe some outstanding scientific principles. Language should aid our understanding — in science, it often feels like a barrier.
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So, I want to simplify these science sayings and this blog series “Sunday Science” gives a quick, no-nonsense definition of the complex-sounding scientific terms you often hear, but may not completely understand. 

If there’s a scientific term or topic you’d like me to tackle in my next post, fire an email to [email protected] or leave a comment below. If you want to sign up to our weekly newsletter, click here.

So, last week I dealt with dark matter. This week we'll be talking about dark energy.

Let's just have a recap on dark matter and visible matter.

The visible universe is made up of ordinary matter - that includes our Earth, the Sun, stars and other galaxies. It makes up less than 5% of mass in the universe.

The rest of the universe is made up of dark matter (25% of the universe's mass) that's difficult (some say impossible) to detect.

And there's also a force present in our universe that repels gravity - this is known as dark energy (and accounts for 70% of the universe's mass).

And our universe is expanding - that's an important point too. But first...

Dark matter produces the attractive force of gravity, but dark energy produces a repulsive force - commonly referred to as anti-gravity. That's the difference. Dark matter attracts, but dark energy repels.

Remember I said that the universe is expanding? Well, two teams of scientists tried to measure the rate of that expansion in the 1990s. They were shocked to discover that the universe isn't just expanding - it's accelerating.

Scientists had previously assumed that gravity would eventually slow down the expansion of the universe. They were wrong. More baffling still, this repulsive force gets stronger as the universe expands.

It's a bit like Ironman throwing his helmet in the air and, instead of it falling back to Earth, it accelerates away from him.

No one really knows. The term "dark energy" was coined for this mysterious force and that's all dark energy really is at this stage - a name.

But theories do exist. One is that dark energy is a fifth fundamental and unknown force called quintessence that only exists when the universe is a certain size. Under quintessence, dark energy could eventually disappear and our universe will stop accelerating. 

Another idea is that dark energy is consistent with a cosmological constant. In other words, dark energy is constant and our universe will continue to accelerate in its expansion.

Alternatively, dark energy could result from the weird behaviour seen for the physics of the incredibly small - aka quantum mechanics. On such tiny scales, energy and matter can appear out of nothing for the tiniest of instants. The constant appearance and disappearance of matter could produce dark energy.

But every possible explanation still does not explain why dark energy exists in the first place.

If you Google "dark energy", there are a myriad of articles and videos out there. It's a big topic.

The most up-to-date and accessible article I could find is by Adrian Cho - Is dark energy an illusion? It discusses some of the challenges in the dark energy space right now (no pun intended).
 
And this video is a great round-up of dark energy:

This week, the University of Manchester unveiled a graphene sieve that can turn sea water into drinking water.

The implications are tremendous. Not just for the potentially life-saving devices that could result, but for the  material responsible for this breakthrough - graphene. 

Graphene is a form of the element carbon that is just a single atom thick. It is extracted from regular graphite (which is basically pencil lead) using a simple method where atom-thick layers  are lifted off graphite using a special form of sticky tape.

These layers are graphene, which is the world’s first 2D material with a range of exceptional properties. It is incredibly lightweight, flexible, stretchable, fire resistant, a brilliant electrical conductor and very tough - at about 200 times as strong as steel. It is also about a million times thinner than a human hair and is a low cost alternative to other competing commercial materials. This sheer range of killer properties in one material has piqued the interest of a wide range of industrial sectors.

But, while graphene has captured imaginations, it has struggled to capture the financial purse strings of many big businesses. As such, the supply chain is still embryonic and graphene remains the darling of the world's startups and research houses - and not of the world's leading corporations as commercial production struggles to gather pace.

So, could this latest breakthrough finally give graphene the momentum it needs to reach commercial maturity? If it does, a range of fascinating products could be developed in a range of industries. 

This is because graphene has a high conductivity - which makes it perfect for a range of applications, particularly when coupled with its other physical properties.

Chips
Graphene has been touted as a potential silicon replacement thanks to its low cost and resilient features. IBM also created a graphene chip in 2014 that was 10,000 times faster than standard silicon-based chips.

Electronic displays
Graphene is optically transparent and electrically conductive, making it a good candidate for future touchscreens, LCDs and LEDs. It's also flexible so "roll-up laptops" and other flexible displays could be a possibility.

Printable circuits
Graphene inks could print conductive patterns to unlock high-speed and low-cost printing techniques for electronic circuits. Commercial applications include disposable sensors, RFID tags and intelligent packaging and there have been interesting developments in this area.

Bioelectric sensors
Bioelectric sensory devices could be a natural successor to the graphene sieve. Thanks to the large surface area, high electrical conductivity, thinness and strength of graphene, this could allow for the fast and efficient monitoring of entities such as haemoglobin or glucose levels, as well as toxin detection.

One suggestion takes this concept one step further - it is the development of a "toxic" form of graphene to target and destroy cancer cells or even promote tissue regeneration.

Nuclear decontamination and bomb detection
Nuclear decontamination is one other application that could exploit graphene's ability to filter different atomic isotopes of radioactive hydrogen from ordinary hydrogen, which could clean up nuclear waste. ​
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Graphene foam could also outperform leading gas sensors to detect potentially dangerous chemicals and aid bomb detection.

​These are just a handful of possible uses that could hit the mainstream if the graphene sieve helps researchers enter the commercial world to gain investment for and interest into this novel and highly versatile material.

​You can find out more about graphene and its practical applications here:

Before we talk about dark matter, we need to talk about gravity.

Gravity pulls things together. It keeps the Earth orbiting around the Sun. It keeps you stuck to the Earth. The universe is full of matter and the attractive force of gravity pulls all matter together. 

The more massive an object is, the greater it's gravitational pull. In other words, you need to have a big mass to exert a big gravitational pull.

Now, let's looks at the above picture of our Milky Way galaxy. You can see the area where our Sun lives, nestled on the Orion Spur.

A galaxy is just a collection of stars, dust and other space matter that's usually circling around a massive black hole.

Everything is held together by gravity into one spinning mass. The more matter a body has, the more massive its gravitational field. And the Milky Way looks pretty massive, doesn't it?

But, there isn't enough visible matter in our galaxy for it to exist.

Our galaxy is rotating with such speed that the gravity generated by the all the stars, dust and other space matter you can see could not possibly hold it together. Our galaxy should have torn itself apart long ago.

That's right. Our galaxy isn't fat enough.

And it's not just our galaxy. There isn't enough matter to hold together other galaxies in our universe. 

This is where dark matter comes in.

Dark matter does not emit light or energy - so it's impossible for scientists to directly observe it. But we can detect dark matter through its gravitational effects.

And here's a sobering thought - roughly 80% of the matter in our universe is made up for dark matter. Only 20% is the stuff we can actually see. 

But what is dark matter? Well, no one really knows because we can't directly detect it. That's right. We have no idea what 80% of the matter in our universe actually is.

One idea is that it could contain "super-symmetric particles" - which is where every particle has a more elusive partner. There are hopes that CERN's Large Hadron Collider could detect super-symmetric particles. Another idea is that dark matter may be a special version of a particle called a pion. And most proposals assume dark matter is made up of a special type of matter that doesn't interact with itself very much - these particles are called WIMPs (Weakly Interacting Massive Particles).

There is another theory - that the laws of gravity need revising. Or there could be a mysterious "dark energy" that overcomes gravity.

We'll look at dark energy next week!

If you want to find out more about CERN and its efforts to detect dark matter - click here. And this video gives a great overview of what we actually know about dark matter (with a little more info on dark energy):

If you've got an hour to spare, this video from the World Science Festival gives a really in-depth explanation of dark matter: