Traditionally the difference between comets and asteroids is simple. Comets form tails of gas and dust when they are near the Sun, and asteroids do not. But as we’ve learned more about asteroids and comets we’ve found that things aren’t as clear cut. 

Comet tails are produced when “volatiles” such as water ice on the comet’s surface are sublimed by the Sun’s heat. This means comets must have surface ice, leading to the idea that they are “dirty snowballs.” Asteroids, on the other hand, were seen as dry rocky bodies, hence their lack of tails. On a basic level that’s true, and comets do tend to be more icy than asteroids. But comets can be quite rocky in composition, and asteroids can have plenty of icy volatiles. So sometimes a comet can appear much like an asteroid and an asteroid can have a comet-like tail.

A good example of this can be seen in the asteroid P/2016 J1. It was discovered in 2016 by Pan-STARRS, and was found to have two separate components (called A and B). These two objects have extremely similar orbits, meaning that they were once a single asteroid that was broken in two by a collision or gravitational interaction. Computer simulations of their orbits show that the asteroid likely split apart about six years ago. That would make it the youngest fragmented asteroid we know.

P/2016 J1 is in the asteroid belt, taking about 5.5 years to orbit the Sun. At its closest approach (perihelion), the asteroid is about twice as far from the Sun as Earth. Last year when it was near perihelion, the two parts of the asteroid started to become active and produced long dusty tails. In other words they looked like comets. This was probably caused by ice that was exposed by the fragmentation. Once heated by the Sun, they started to sublime and created the tails we could observe.

This doesn’t mean comets are simply broken asteroids. What it does demonstrate is that the interiors of asteroids can have lots of icy volatiles. They are just hidden by an asteroid’s the rocky exterior. So comets and asteroids can’t easily be divided into dirty snowballs and dry rocks. Instead it is more a matter of degree. Asteroids and comets are far more similar than we once thought.

Paper: Fernando Moreno, et al. The splitting of double-component active asteroid P/2016 J1 (PANSTARRS). arXiv:1702.03665 [astro-ph.EP] (2017)

The post More Than Meets The Eye appeared first on One Universe at a Time.

Today the object 2015 TB145 will make a “close” flyby of Earth. It’s been nicknamed Spooky due to its Halloween arrival date, and we’re already starting to learn a few things about it.

Spooky was presumed to be an asteroid when it was discovered, but was unusual because of its high relative speed (35 km/s). Its orbit is also unusual, being more inclined away from the plane of the solar system than most asteroids. Its trajectory is more in line with comets, though Spooky has no indication of a coma or tail. But we’re now starting to get infrared data from NASA’s Infrared Telescope Facility (IRTF) in Hawaii and radio observations from Arecibo, which supports the idea that Spooky is a dead comet.

In particular, the infrared observations allow us to determine the brightness (albedo) of the comet. It’s albedo is about 0.06, which, though about as dark as asphalt, is actually brighter than most comets.  So it is likely an old comet that has lost all its volatiles (surface ice and such) and now looks more asteroid-like than comet-like at first glance.

The post Spooky The Dead Comet appeared first on One Universe at a Time.

Comets are frozen remnants from the formation of our solar system. They live on the outer edges of our solar system until some gravitational perturbation causes them to sweep near the Sun. Because they have been relatively undisturbed for most the Sun’s history, they provide an excellent window into the chemistry and composition of the early solar system. Which is why it’s interesting that ethyl alcohol has been observed in the tail of comet Lovejoy.

Lovejoy is a long-period comet, which means it’s probably originates from the Oort cloud, and hasn’t had time to become contaminated by material of the inner solar system. Finding ethyl alcohol streaming off such a comet is somewhat surprising. We’ve long known that alcohol is plentiful in space, but much of it is in the form of methyl alcohol, which is a simpler molecule than ethyl alcohol. In this case, it seems the more complex alcohol formed through surface chemistry interacting with ultraviolet light. The team also found quantities of glycolaldehyde, which is a simple sugar-like molecule, which also supports the surface chemistry idea.

Compounds such as these are necessary ingredients for life on Earth, so its possible that comets similar to Lovejoy could have seeded Earth with these molecules through early impacts. That’s still a bit speculative, but it now seems clear that both comets and meteorites contain such molecules, so its within the realm of possibility that they seeded Earth with the building blocks of life.

At the very least the observations of Lovejoy show that these molecules formed in our solar system early in its history.

Paper: Nicolas Biver, et al. Ethyl alcohol and sugar in comet C/2014 Q2 (Lovejoy). Science Advances, 23 Oct 2015

The post Alcohol In Comet Lovejoy Points To Early Chemistry appeared first on One Universe at a Time.

When the Rosetta spacecraft reached comet 67P/Churyumov–Gerasimenko, one of the striking features was its “rubber duck” shape with two distinct lobes. One of the questions it raised was whether the comet had simply eroded in its middle region, or whether it had formed from a low-speed collision. We now know the ducky comet was once two comets.

Surface structures point to a two-comet origin. Credit: ESA

Computer simulations have shown that low-speed collisions between comets can create a single comet similar to 67P, but other models show how comets can erode irregularly to create similar shapes. So there has been debate over which process formed 67P. But new work has verified the collision model by looking at the surface gravity of the comet. As a surface of a comet settles within its gravitational field, the surface will tend to be oriented perpendicular to the direction of gravity in the region. Imagine a pile of sand on Earth. It can have high or low points in the sand, but if you jostle it, the sand will tend to level off. Not all surfaces will line up to gravity, but statistically most will.

So the team looked at several surface structures on 67P and compared them to the gravitational map to find a statistical correlation between the surface gravity. They then compared the results to two computer models. One where the comet eroded in the center, and one where two comets merged. What they found was that the real 67P agreed with the merged model.

So the one comet we’ve actually landed a probe upon was once two.

Paper: Jutzi and Asphaug. The shape and structure of cometary nuclei as a result of low-velocity accretion. Science 348 (6241): 1355-1358 (2015)

The post The Comet That Was Once Two appeared first on One Universe at a Time.

Comets are known for their tails. In the old days that’s how you distinguished a comet from an asteroid, though with our modern understanding the line between comet and asteroid as blurred a bit.  Still, one of the big areas of comet research has been on just how comet tails and coma form.

The traditional view has been that water and other volatiles are ejected from the comet through sunlight. The ejected molecules are then broken up through a process of photoionization, where ultraviolet light strikes the molecules, causing them to break apart. It is these ionized molecules that we observe as the coma and tail of a comet.

But we now know that reactions in space can be much more complex and subtle. In this case, we now know that free electrons play a role in the breakup of ejected molecules. The work was recently published in Astronomy & Astrophysics, and details the role of electrons in the chemistry of cometary atmospheres. The data for this research came from the Rosetta probe orbiting Comet 67P/Churyumov-Gerasimenko. Using ultraviolet spectroscopy, the team found that rather than ionizing various molecules directly, the ultraviolet light liberates electrons from water molecules in the coma. These electrons then interact with other molecules to break them apart.

It’s a rather surprising result, but it’s also a great example of how better data allows us to refine a simple model into a more subtle and accurate one.

Paper: P.D. Feldman, et al. Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta. A&A DOI: 10.1051/0004-6361/201525925 (2015)

The post Electric Avenue appeared first on One Universe at a Time.

This week the research journal Science published a special issue on the initial results of the Rosetta/Philae mission. Along with it came several high-resolution and color images of the comet 67P Churyumov-Gerasimenko. The images alone are pretty astounding. The surface is diverse, with evidence of fracturing, faint jets, and a mottling of colors. It is clearly a complex and dynamic body. While the images are appealing, other data is telling a fascinating story.  One of these stories concerns the formation of the comet’s magnetosphere. 

A color image of 67P’s surface. Credit: ESA/Rosetta/NAVCAM

Comets like 67P don’t have a strong intrinsic magnetic field, so by themselves they don’t have a magnetosphere to protect themselves against solar wind. It’s icy surface is therefore bombarded by ions and magnetic fields. But as the comet approaches the Sun, volatiles such as water sublime to produce a thin atmosphere. Some of the water vapor is then ionized by ultraviolet sunlight, and these ions interact with the magnetic fields of interplanetary space. As the atmosphere of the comet gets thicker, and more of it is ionized it becomes conductive. The solar wind accelerates some of the atmospheric ions at high speed, and the resulting current induces a magnetic field around the comet. Eventually the magnetic field is strong enough that it pushes back against the solar wind, creating a magnetosphere.

At least that’s what we’ve thought. Previous fly-bys of comets such as Halley have found that comets don’t have strong intrinsic magnetic fields, and we’ve seen more active comets affect the surrounding magnetic environment. But now results from Rosetta have confirmed this model by directly observing it happen.

The team observed both neutral and ionized water in the comets atmosphere, as well as the magnetic fields in the region. They collected data as the comet was more than 3.6 AU to when it made its closest approach to the Sun at about 1.25 AU. They found an initial faint atmosphere penetrated by the solar wind, then watched as the atmosphere thickened to the point where it started resisting the wind through an induced magnetosphere. They also found the atmosphere is unevenly distributed about the comet (as different surface regions produced more or less volatiles) and that even a faint atmosphere interacts more strongly with the solar wind than initially supposed.

This is why missions like Rosetta are important. Observing the complex interaction of solar wind and cometary atmosphere can only be done on site. It doesn’t show up in images. So enjoy the flood of new color comet pictures, but remember that we can’t see the colors of the wind.

Paper: Hans Nilsson, et al. Birth of a comet magnetosphere: A spring of water ions. Science, Vol. 347 no. 6220 (2015)

The post Colors of the Wind appeared first on One Universe at a Time.

Comet Lovejoy is still visible in the night sky, so if you get a chance you should check it out.  If you have seen it, you might have noticed a distinctly green color to it. That isn’t an optical illusion, because Lovejoy is really green.

Structure of a comet. Credit: NMM London

Usually when you observe a comet directly, it will appear white, or slightly bluish. The white is due to sunlight reflecting off the dusty tail of the comet, and the blue is from the gas tail of the comet. Photographs can typically reveal these colors in rich detail, compared to the limited color perception of our eyes. But sometimes the gas emitted can have a greenish hue. Many comets have an abundance of compounds such as cyanide (CN₂) and carbon (C₂). When ionized these compounds emit light in the green spectrum. The light they emit is much brighter than things like hydrogen (even though hydrogen is more common), and that combined with our sensitivity to green light means that a comet can appear green.

Green comets are fairly common, but make for a rare color in the night sky since there are no green stars.

The post Easy Being Green appeared first on One Universe at a Time.

This year starts off with a naked eye comet known as Lovejoy (or C/2014 Q2, for those who like to be specific). It’s a long period comet first observed by Terry Lovejoy back in August. The comet isn’t particularly bright, being just within naked-eye range at magnitude 5.  But it happens to be near the celestial equator, and even near the constellation Orion, so it is widely observable. Most people will require a small telescope or pair of binoculars to see it, but it isn’t difficult to find.

Path of Lovejoy in January. Credit Sky & Telescope

If you want to know where Lovejoy will be on a particular night, Sky & Telescope is a good resource. They have star maps showing the path of the comet, as well as tips on how to locate it relative to brighter stars. As with all long period comets, it is not certain just how bright the comet will become. At the moment it looks like Lovejoy will peak around magnitude 4 in mid-January.

So I encourage you to check it out. It’s relatively bright, easy to find, and it won’t be back for about 8,000 years.

The post Lovejoy appeared first on One Universe at a Time.

Results are starting to come in from the Rosetta mission, including a new article in Science on the composition of water on the comet 67P/C-G. The results support the idea that Earth’s water didn’t come from cometary bombardments.

The origin of Earth’s water has been a matter of some debate. The traditional view is that early Earth was too hot to retain its primordial water, so our planet was later wetted by cometary impacts during the late heavy bombardment period. It seems reasonable if you imagine comets as dusty snowballs. But as we now know, comets are more like snowy dustballs. And while asteroids are often thought of as dry rocks, they actually contain a great deal of water embedded in their minerals. So Earth could have gotten water from either meteor or comet impacts (or both).

We can actually determine the origin of Earth’s water by looking at trace isotopes within it. Typical water consists of two parts hydrogen to one part oxygen, hence H₂O. But there are other variations such as D₂O, which is two parts deuterium instead. The ratio of these two varieties of water (known as the D/H ratio) can tell us about the water’s origin. The D/H ratio found in water-rich meteorites is fairly consistent, and it is similar to the ratio of Earth’s water. That would indicate that our water came from asteroids, not comets.

In this new work, the authors measured the D/H ratio of water vented from 67P, and found it was nearly four times higher than Earth levels. This means that our water most definitely did not come from this kind of comet. Comet 67P is part of the Jupiter family of comets. Observations of cometary tails from other Jupiter family comets have given similar results, but this new result is much more accurate.

While this excludes one family of comet, it is possible that other comets might have contributed to Earth’s water. But since periodic comets such as 67P once originated from the same Oort cloud as other comets, that doesn’t seem likely.

Paper: K. Altwegg, et al. 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio. Science DOI: 10.1126/science.1261952 (2014)

The post A Comet’s Tale appeared first on One Universe at a Time.

[av_video src=’http://youtu.be/tMthy0RrORY’ format=’16-9′ width=’16’ height=’9′]

Yesterday Philae landed on a comet. It was the first successful soft landing on a comet, though it likely won’t be the last. This is something everyone should be excited about. With all the things to complain about in the world, here is a chance to celebrate a huge achievement of human ingenuity. We have a robotic probe on the surface of a comet.

Rosetta ground control at the moment Philae landed. Credit: ESA

There’s been a lot in the news about how this is the culmination of a 10 year mission, but that’s only the time since Rosetta’s launch. The idea for a comet lander dates at least as far back as the 1980s, when the Giotto spacecraft made a flyby of Halley’s comet. This led to plans for a mission to land on a comet and return with cometary material, but that was later scaled back to just a lander for budgetary reasons.

The Rosetta spacecraft under construction. Philae is circled. Credit: Cumbrian Sky

Design and construction of the Rosetta and Philae spacecraft date back to the 1990s, with major construction beginning around the turn of the century. It was originally scheduled to land on a comet known as 46P/Wirtanen, but delays led to the change to 67P/Churyumov–Gerasimenko.

The reason comets are such an important target is because they contain the most primal materials of the solar system. Unlike most of the bodies in the inner solar system, which have been mixed and bombarded over the ages, comets have an origin in the outer edges of the solar system known as the Oort cloud.

Philae has a planned mission of about 6 weeks, so there is going to be a lot of data gathered, and over time results will start being published. But we’ve already learned a few surprising things. One is that the comet itself sings. Not in the traditional way we think of, but rather in electromagnetic waves. The comet’s weak magnetic field interacts with the coma and solar wind to create a warbling effect. You can hear the sound in the video above, where it’s been sped up from its actual millihertz range.

So we’ve sent a spacecraft into space, put it in orbit near a comet, landed on it with a smaller probe, and listened to the comet’s song. What an amazingly human thing to do.

The post Song for a Comet appeared first on One Universe at a Time.

In October 2007, comet Holmes experienced rapid brightening. By itself  this isn’t a huge deal. As comets move closer to the Sun, volatiles (gases and such) can warm and expand, and these can vent out of the comet rapidly, causing the comet to brighten suddenly. Sometimes this out-gassing can cause a comet to fragment, but often it is simply evidence of an active comet.

Comet Holmes was discovered in 1892 by Edwin Holmes (hence the name), and orbits the Sun at a distance between Mars and Jupiter (though tilted a bit from the planetary plane). It is normally rather dim, with a typical apparent magnitude of 17. Holmes wouldn’t have noticed it if it weren’t for the fact that it had brightened to magnitude 5.

Credit: John Pane

In 2007 Comet Holmes gained notoriety when it suddenly brightened from magnitude 17 to magnitude 3 in about two days. This made it visible to the naked eye under clear dark skies. The coma (the cloud-like feature surrounding the icy/rocky nucleus) expanded by a factor of four by the end of October, and this once faint object began to look like a traditional comet. You can see this change over time in a composite series of photos showing Comet Holmes from October 2007 to March 2008.

The exact cause of this particular brightening is not entirely clear. The brightening was unusual in just how intense it was. It was so bright that some suggested the comet was struck by a meteoroid, though that would be unlikely. A paper in Astronomy and Astrophysics calculated that the comet lost about 3% of its mass in the event. Since the comet didn’t fragment during such a tremendous release of material (Comet Holmes remains intact to this day) the authors suggested a covering of dust coated the comet, under which was a layer of ice. When the ice layer sublimed (when from ice directly to gas) the resulting expansion of gas and dust caused the rapid brightening.

Spectral analysis of Comet Holmes during the outburst, as presented in another A & A paper found not only water, but ethane, methanol and acetylene, which are all rather common hydrocarbons found in space. The distribution of various volatile materials is consistent with either the dust layer process or a pocket outgassing. So we aren’t entirely sure what caused the sudden brightening.

The post A Holmes Mystery appeared first on One Universe at a Time.

This week comet Siding Spring (also known as C/2013 A1) made a close approach to Mars. It’s not a particularly bright comet, but it was close enough for the Opportunity rover to take an image of it from the martian surface. Think on that one just a bit. We have robots on Mars doing comet photography now. At closest approach, Siding Spring was about 140,000 km from the center of Mars. If it had made a similar pass by Earth it would have been about half the distance to the Moon.

In recorded history, we’ve never had a comet pass so close to Earth. We’ve had bright comets, and Earth even passed through the tail of Halley’s comet in 1910, but never a really close approach. The closest recorded comet flyby occurred in 1770 by Comet Lexell. It was discovered by Charles Messier just a month before close approach, and came within 2.2 million kilometers of our planet. At it’s closest, it was noted that the nucleus had an apparent size equal to Jupiter, and its coma was larger than the Moon.

Comet with an orbit similar to Lexell’s calculated orbit. Credit: Popular Science Monthly (1881)

The comet was named after Anders Johan Lexell, who determined its orbit and demonstrated that it was a periodic comet with a period of about 5.5 years.  This meant its aphelion (greatest distance from the Sun) was in the region of Jupiter’s orbit. Although it was expected to return in 1776, it was never seen again. In the 1840s Urbain Le Verrier (famous for predicting the existence of Neptune) calculated that the comet made a couple close approaches to Jupiter that radically changed its orbit, possibly even throwing it out of the solar system.

Comet Lexell is now designated a lost comet. Siding Spring will soon head to the outer edge of the solar system, not to return to the inner solar system for a million years.

The post Lost Comet appeared first on One Universe at a Time.