Another new article I wrote for The Conversation website on comets and asteroids

Looks like a comet but feels like an asteroid? That’s wild!

By Natalie Starkey, The Open University

Comet ISON’s fate has left many sad. For the public, the comet could have made for a spectacular view in December. For scientists, it would have been a chance to learn more about these mysterious bodies. But why are comets mysterious? And what really are they? Some recent missions are beginning to answer that question

In 2006 the NASA Stardust mission returned to Earth with the first samples collected directly from a comet. What scientists discovered when they analysed the samples was not what they were expecting. The results challenged many commonly-held assumptions about the composition of comets and asteroids, as well as the formation of the solar system as we know it today.

Before this mission, scientists had always characterised comets and asteroids as completely different. Asteroids make frequent deposits on Earth in the form of meteorites and are thought to have formed in the inner solar system, close to the sun. They contain rock minerals that formed in high temperatures early on in solar system history – more than 4.5 billion years ago. Their orbits are generally circular.

Comets, on the other hand, are less-frequent visitors from the outer solar system. They are often called dirty snowballs and are believed to act as time capsules from the deep freeze of the solar system, containing the primary dust and gases from which everything we can touch and smell began. Comets glow and produce a tail of gas and dust when they pass through the inner solar system because the sun’s heat causes frozen gases within them to sublime. Their orbit is different to that of asteroids, being generally more elliptical.

The Stardust mission collected grains from the tail of comet 81P/Wild2. Analysis of these grains revealed them to be high-temperature rock minerals, indistinguishable from those commonly found in asteroids, their presence indicated an inner solar system origin. But 81P/Wild2 looks, smells and acts just like a comet. And, having been wrenched from its more distant orbit to one in the inner solar system by the gravitational pull of Jupiter in 1974, its orbit classified it as a comet.

The big question is: how can a comet contain minerals formed in the inner solar system when it is supposed to have been formed some 30AU away from the sun, where AU is the distance from the Earth to the sun, and its surroundings are as cold as -200°C?

This question has thrown space scientists into disarray, showing that classic solar system formation models that have been relied on for many years do not square with the evidence in these samples. Unfortunately only one comet has been sampled directly of the trillion or so that are estimated to exist.

Even without real rock samples returned to Earth, astronomers have recently used an infrared telescope to study an asteroid remotely. It is called 24 Themis and is located in the outer part of the main asteroid belt, between Mars and Jupiter. It was found to contain water-ice, an unexpected component in an asteroid and something that would normally help to classify it as a comet. Also, asteroid P/2013 P5 was observed by the Hubble Space Telescope in September this year to have six cometary-like tails, even though it has an asteroid-like orbit. These results obtained remotely would not be understood so well without the Stardust mission.

So where does this leave us and why does it matter? The current thinking is that the two extremes of composition do exist. Some asteroids are true asteroids – they contain only material formed in the inner solar system and no water ice. Conversely some comets are true comets – they contain only outer Solar System material. But the samples obtained from 81P/Wild2 and remote sensing of other solar system objects highlights that solar system models must account for a continuum of compositions from asteroid to comet.

Many elements of the universe remain a mystery. For instance, it is still unclear if water and organic material were delivered to Earth by comets or asteroids. But perhaps more Earthly desires will drive research in this area. One that no longer seems science fiction is the mining of space objects for their precious metals.

Before anyone goes to the expense of sending a mining mission to an asteroid, they will want to check first that it contains the metals they require. If comets have such metals too, then perhaps asteroids won’t be the only potential mines we have.

Natalie Starkey receives funding from the Science and Technology Facilities Counci. She is affiliated with Planetary and Space Sciences at The Open University.

The Conversation

This article was originally published at The Conversation.
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My recent article about comet ISON from The Conversation website.

Comet ISON fizzles … but there’s a sting in the tail

By Natalie Starkey, The Open University

If there is one thing we know about comets, it is that their behaviour is really hard to predict and that they will always surprise us – and sometimes disappoint.

Unfortunately it looks like comet ISON, or most of it, did not survive its terrifying encounter with the sun yesterday when it made a close approach at just 1.2 million kms from our closest star’s fiery surface. This distance may seem large but it is close enough to have subjected the comet to temperatures of around 2,700°C. To survive such a close shave with the sun may sound unlikely. But a few other sungrazing comets have been known to survive closer passes so there was immense hope it would perform a death-defying stunt and emerge intact.

ISON did not leave us without a final serving of mystery though. Soon after reaching its nearest point to the sun (known as perihelion), Twitter and news agencies were alight, lamenting its loss following no signs of it emerging. It was assumed to have disintegrated – RIP ISON.

But then, moments later, new images emerged showing a hint of something appearing on the other side of the sun. Was this still a diminished comet ISON or a ghostly version of its former self? Well, comet experts are not sure.

Whatever appeared after perihelion had enough matter in it to produce a tail, which started fading as it got farther from the sun. But to be able to say anymore, scientists need more data, which they don’t have.

This is because ISON is still too close to the sun and remains too faint for equipment to record what data will be needed. What can be said for certain is that whatever the size of the nucleus of the remnant, if it has one, it will be much smaller than a few weeks ago when it was visible to the naked eye from Earth.

Comet of the century

Comet ISON had been dubbed the Comet of the Century. Ever since it was first spotted in September 2012 scientists believed it had the potential to shine brighter than the Moon. Although ISON did not quite live up to these predictions, it has certainly put on a good show on its perilous journey from the Oort cloud, a region of icy comets lying nearly a light year from the sun, where it lived for 4.6 billion years.

In recent weeks ISON has been entertaining us with a show of varying levels of dust and ice streaming from its nucleus. As it got closer to the sun, the dust production of ISON has, at times, seemed to completely shut down. This led some worried scientists to suspect the comet had died very early, its shining light distinguished. But then ISON surprised us all by turning on again and the last sightings before perihelion suggested that it had not yet run out of juice.

Bright comets from the Oort cloud passing the Earth are extremely rare. Astronomers study these icy objects to learn more about the mysterious visitors from the outer solar system. ISON holds secrets 4.6 billion years old, including potential information about the earliest formed gases and dust in our solar system. But despite what has become of comet ISON, scientists have acquired lots of new data about the comet on its journey to the sun, which will help them understand, and hopefully better predict, the behaviour of future cometary visitors.

The rarity of this event explains why scientists have been unsure how to predict what the comet was going to do as they do not have much to compare it to. Passing the sun is certainly no easy manoeuvre: the extremes of gravity and heat acting to rip apart and explode the comet nucleus, possibly vaporising it in a split second.

We have probably been denied the opportunity to observe a beautiful comet blazing through our night skies in December. But it is not not all bad news. ISON’s curtailed life allowed us to learn more about the behaviour of these ancient icy dustballs. And, while scientists pore over new comet data, we can all anticipate the next one to be spotted hurtling toward the sun. It could be a good few years, but there’s one thing we can be assured – comets will always take us on a thrilling ride.

This article was updated on December 1 to include the latest information about ISON.

Natalie Starkey receives funding from The Science and Technology Facilities Council. She is affiliated with Planetary and Space Sciences at The Open University.

The Conversation

This article was originally published at The Conversation.
Read the original article.