Category Archives: Stars

Historic close-ups of Pluto and its moon Charon present puzzle for scientists

After a decade-long journey by the New Horizons spacecraft through our solar system, we can finally add Pluto and its main moon Charon to the roster of large icy bodies whose landscapes we have seen. And it was worth the wait. The first detailed images are surprising, showing a remarkable lack of impact craters on both Pluto and Charon.

NASA’s probe passed within 14,000 km of Pluto on July 14, and – after a nervous 12-hour wait for its call home – has begun to send back its trove of data, which includes images revealing details as small as 100 metres across.

The most detailed image of part of Pluto (see lead image) is truly staggering. Not a single impact crater is to be seen in this region, so the surface must be very young – reshaped by some sort of geological activity such as faulting or icy volcanism.

It is rather early to speculate, but maybe Pluto captured Charon only a few hundred million years ago (rather than billions), and we are seeing the effect of the very strong tidal interactions that would have ensued. Pluto could, in fact, even be geologically active today. I watched this image come in via a NASA press conference with a group of colleagues, and we were both amazed and mystified.

Pluto – which has a diameter of 2,370km – shares its orbital space with many comparable-sized bodies and crosses the orbit of the giant planet Neptune, which is why it does not qualify as a planet. Nevertheless, it is a fascinating world, as indeed is Charon (1,208km diameter).

Both are bodies whose rocky interiors are deeply buried by ice of various kinds. There is probably water-ice at depth on Pluto, but the surface ice is a mixture of frozen methane, ethane, carbon monoxide and nitrogen.

Latest image of Charon with its enigmatic dark polar cap, and a stupendous fracture system running near to its equator.

Charon, with its weaker gravity, has lost the substances that can turn to vapour and escape more easily, and is mostly water-ice tainted by ammonia. That much we already knew. But New Horizons has been gathering data that will show us how these ices are arranged across each surface – and may find traces of other constituents.

Pluto (left) and Charon (right) in slightly exaggerated colour.
adapted from multiple datasets via, Author provided

Already the images are throwing up new questions. The composite image above, compiled from various images during New Horizons’ approach, shows slightly enhanced colour views of Pluto and Charon. Pluto is notable for patches of both bright and dark material in a belt straddling its equator. What are these? Is the bright, heart-shaped patch some kind of nitrogen frost or snow deposit? Is the dark stuff carbon or tar of some kind? (We know that solar ultraviolet radiation turn methane into tar). Charon, unique among known worlds, has a dark polar cap. Is that old, radiation-damaged methane, whereas the greyer equatorial region is cleaner water-ice?

The fact that Pluto has a red tint seems to have been surprising to some, but it has actually been well-known for decades.

An Open University PhD student has already made a preliminary photogeologic map of the half of Pluto seen during approach (see below). It shows the “heart”, the dark patches and other units in different colours – representing different terrains. Several impact craters have also been marked in green, and a few wrinkles on the surface.

A preliminary map of terrain units on Pluto (right), based on the image on the left.
Peter Fawden, Open University, Author provided

Then there are the tectonic features, the faulting or other deformation of the outer layer of a planet, to consider. These are better seen in black-and-white images. For example in the next image, just inside the eastern (right-hand) edge of the disk, the surface is cut by a fracture deep enough to cast shadows.

Pluto seen about a quarter of a rotation before the top image. The fractures at the lower right had rotated out of view by the time of closest approach. The ‘heart’ was just beginning to rotate into view on the left.

Charon has similar fractures too. These are also known on several icy moons, such as Ariel and Titania at Uranus, and Tethys at Saturn, where they are described as chasmata (Latin for chasms). The image below shows
Charon and Uranus’s moon Titania at the correct relative scale. They both have fractures visible near their right-hand edges, which could be a remnant of a time when the surface became broken, perhaps by tidal forces.

Fractures can be seen both on Charon (left) and Titania (a moon of Uranus).

Scientists are eagerly awaiting more data from New Horizons’ onboard memory, which is now nearly full, that will take a total of 16 months to transmit to Earth. This is because at a range of more than 4.6 billion km from Earth the signal is so weak that it has to be transmitted at a slow rate of about 1 kilobit per second (you may be reading this via wi-fi operating at tens or hundreds of megabits per second). Although we may have to wait a while before we see the best of the pictures, what we already have already seen is enough to greatly intrigue planetary scientists such as myself.

The Conversation

David Rothery is Professor of Planetary Geosciences at The Open University.

This article was originally published on The Conversation.
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Beyond Pluto: New Horizons’ mission is not over yet

When New Horizons phoned home this morning (Australian time) after its close encounter with Pluto, there was jubilation and excitement.

Now, as Pluto retreats into the distance, the slow trickle of data can begin. Sent to us at a rate of just 1 kilobit a second, it will take months to receive it all, and astronomers around the world are waiting on tenterhooks to get their hands on the data.

Pluto: Once shattered, twice shy

Like our own Earth, Pluto has an oversized satellite, Charon. It was discovered back in 1978 and is more than half the diameter of its parent.

Over the past few years, intense observation of Pluto in preparation for New Horizons’ arrival has revealed four more tiny satellites, Hydra and Nix, and tiny Kerberos and Styx.

Prior to New Horizons, our best view of the Pluto system came from the Hubble Space Telescope.
NASA, ESA, and L Frattare (STScI)

But how did this satellite system come to be? And why the striking similarity to our double-planet?

If we look at the great majority of satellites in our solar system we find that they can be split into two groups. First, have those that we think formed around their host planet like miniature planetary systems, mimicking the process of planet formation itself.

These regular satellites most likely accreted from disks of material around the giant planets as those planets gobbled up material from the proto-planetary disk from which they formed. This explains the orbits of those satellites – perfectly aligned with the equator of their hosts and moving on circular orbits.

Then we have the irregular satellites. These are (with a couple of noteworthy exceptions) tiny objects, and move on a wide variety of orbits that are typically great distances from their host planets.

These, too, are easily explained – thought to be captured from the debris moving around the solar system late in its formation, relics of the swarm of minor bodies from which the planets formed.

NASA graphic using New Horizons’ early pictures of Pluto and Charon to compare their sizes to that of the Earth.

By contrast, our moon and Pluto’s Charon are far harder to explain. Their huge size, relative to their host, argues against their forming like the regular satellites. Likewise, their orbits are tilted both to the plane of the equator and to the plane of the host body’s orbit around the sun. It also seems very unlikely they were captured – that just doesn’t fit with our observations.

The answer to this conundrum, in both cases, is violent.

Like our moon, Charon (and by extension Pluto’s other satellites) are thought to have been born in a giant collision, so vast that it tore their host asunder. This model does a remarkable job of explaining the makeup of our own moon, and fits what we know (so far) about Pluto and its satellites.

Pluto and its moons will therefore be the second shattered satellite system we’ve seen up close, and the results from New Horizons will be key to interpreting their formation.

Schematic describing our best theory for the formation of Pluto’s satellite system.

Studying the similarities and differences between Pluto and Charon will teach us a huge amount about that ancient cataclysmic collision. We already know that Pluto and Charon are different colours, but the differences likely run deeper.

If Pluto was differentiated at the time of impact (in other words, if it had a core, mantle and crust, like the Earth) then Charon should be mostly comprised of material from the crust and mantle (like our moon). So it will be less dense and chemically different to Pluto. The same goes for Pluto’s other moons: Nix, Hydra, Styx and Kerberos.

Pluto, the unknown

The most exciting discoveries from New Horizons will likely be those we can’t predict. Every time we visit somewhere new, the unexpected discoveries are often the most scientifically valuable.

Jupiter and its volcanic moon Io, taken by New Horizons as it tore past the giant planet en-route to Pluto.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Goddard Space Flight Center

When we first visited Jupiter, 36 years ago, we found that its moon Io was a volcanic hell-scape. We also found that Europa hosts a salty ocean, buried beneath a thick ice cap. Both of these findings were utterly unexpected.

The Death Star terrorised peaceful planets before Voyager sent back images of Mimas.
Flickr/Paul T, CC BY

At Saturn, we found the satellite Mimas looked like the Death Star and another, Iapetus, like a two tone cricket ball, complete with a seam. Uranus had a satellite, Miranda, that looked like it had been shattered and reassembled many times over, while Neptune’s moon Triton turned out to be dotted with cryo-volcanoes that spew ice instead of lava.

The story continues for the solar system’s smaller bodies. The asteroid Ida, visited by Galileo on its way to Jupiter, has a tiny moon, Dactyl. Ceres, the dwarf planet in the asteroid belt, has astonishingly reflective bright spots upon its surface.

Pluto, too, will have many surprises in store. There have already been a few, including the heart visible in the latest images (see top) – possibly the most eye catching feature to date. The best is doubtless still to come.

To infinity, and beyond!

Despite the difficulties posed by being more than four and a half billion kilometres from home, New Horizons is certain to revolutionise our understanding of the Pluto system.

The data it obtains will shed new light on the puzzle of our solar system’s formation and evolution, and provide our first detailed images of one of the system’s most enigmatic objects.

But the story doesn’t end there. Once Pluto recedes into the distance, New Horizons will continue to do exciting research. The craft has a limited amount of fuel remaining, nowhere near enough to turn drastically, but enough to nudge it towards another one or two conveniently placed targets.

New Horizons’ will continue its mission after flying past Pluto, studying objects in the Edgeworth-Kuiper belt.

Since the launch of New Horizons, astronomers have been searching for suitable targets for it to visit as it hurtles outward through the Edgeworth-Kuiper belt, en-route to the stars.

In October 2014, as a result of that search, three potential targets were identified. Follow up observations of those objects narrowed the list of possible destinations to two, known as 2014 MU69 (the favoured target) and 2014 PN70.

The final decision on which target to aim for will be taken after New Horizons has left Pluto far behind, but we can expect to keep hearing about the spacecraft for years to come.

The Conversation

Jonti Horner is Vice Chancellor’s Senior Research Fellow at University of Southern Queensland.
Jonathan P. Marshall is Vice Chancellor’s Post-doctoral Research Fellow at UNSW Australia.

This article was originally published on The Conversation.
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NASA has closest view of Pluto

Nathanelle Brusk
Displayed with permission from @Fact

New images of Pluto have arrived from a NASA space probe, and they’re already allowing scientists to update what we know about the dwarf planet – such as its size. NASA’s New Horizons probe has traveled more than 3 billion miles to send photos and data about Pluto back to Earth.

NASA is set to release more images and data gleaned from New Horizons’ closest approach to Pluto, which was achieved just before 8 a.m. ET Tuesday, when it was about 7,750 miles from the planet. At the time of the flyby, the craft was traveling at more than 30,000 mph.

That launch took place in January of 2006. Today, NASA says the time it took New Horizons to get to the destination was “about one minute less than predicted when the craft was launched.” The agency added, “The spacecraft threaded the needle through a 36-by-57 mile (60 by 90 kilometers) window in space – the equivalent of a commercial airliner arriving no more off target than the width of a tennis ball.”

If you’re wondering why the planet has a reddish hue, NASA has an answer saying the color is caused by “the presence of methane at the surface. When the sun hits methane, it forms red-colored molecules.”

That launch took place in January of 2006. Today, NASA says the time it took New Horizons to get to the destination was “about one minute less than predicted when the craft was launched.” The agency added, “The spacecraft threaded the needle through a 36-by-57 mile (60 by 90 kilometers) window in space – the equivalent of a commercial airliner arriving no more off target than the width of a tennis ball.”

If you’re wondering why the planet has a reddish hue, NASA has an answer saying the color is caused by “the presence of methane at the surface. When the sun hits methane, it forms red-colored molecules.”

We’ll update this post with news from the space agency, which is still compiling data that were already sent back to Earth – and is awaiting new information from the flyby. It’ll take months to receive all the data.

In focus: New Horizons’ crisp images shed new light on the origins of Pluto and its moons

Eighty-five years after Clyde Tombaugh discovered Pluto, we’re finally getting an up-close and personal look, thanks to NASA’s New Horizons mission. As a planetary astronomer who has investigated the formation of Pluto and other planets for nearly two decades, I’ve been waiting for New Horizons’ approach since it was launched in 2006.

We’re finally receiving the first detailed pictures of the icy dwarf planet. Data from the spacecraft should tell us more about the composition of the materials that originally spawned our solar system’s planets, and the nature of geological processes on icy planets.

Over the past month or so, as New Horizons approached the end of its three billion mile journey from Earth to Pluto, NASA has released a stunning set of images from the mission.

New Horizons image taken July 13 2015 shows that Pluto has a ‘heart.’
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute, CC BY

We now know that from a distance, Pluto has peach-colored surface features that resemble a heart, a whale and a doughnut. Pluto’s gray moon Charon has a dark polar cap (unofficially named Mordor) and several bright craters.

The four smaller moons.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute, CC BY

We’ve seen four smaller (6–63 mile, or 10–100 km, diameter) moons orbiting the Pluto–Charon binary. Discovered by the Hubble Space Telescope, moons Styx, Nix and Hydra are bright and probably icy, but Kerberos looks like a lump of coal.

Despite the high quality of these images, what everyone was really waiting for was “closest approach,” when several instruments would finally be able to resolve kilometer-sized features on the surfaces of both planets. These instruments can spot cracks like those on Jupiter’s moon, Europa, and geysers like those on Saturn’s moon, Enceladus. On Tuesday, New Horizons successfully zoomed by Pluto. Here’s what the probe saw.

An artist’s conception of New Horizons passing Pluto.

Logistics of the approach

Traveling at about 31,000 mph (14 km/sec), New Horizons’ seven instruments had to work quickly during the fly-by. Pluto is two-thirds the size of our moon; its binary companion, Charon, is only half the size of Pluto. Following a path that passes only 8,000 miles from Pluto (and 18,000 from Charon), on-board cameras snapped pictures of the surfaces of both worlds. Other instruments collected data on surface compositions, the extent of Pluto’s atmosphere, and the amount of dust orbiting them. After only 10 minutes, closest approach was over and New Horizons sped away from Pluto and the solar system, never to return.

Over the next 16 months, the on-board computer will patiently transmit all of the newly collected data back to Earth. With a slow data rate of only 1 kb/sec, it takes 42 minutes to transmit just one image.

Pluto and Charon superimposed onto the Earth.

The first set of images shows spectacular vistas on Pluto and Charon. Pluto has magnificent 11,000-foot mountains! Relative to the size of Pluto, these mountains are comparable to a 65,000-foot mountain on Earth – more than twice the height of Mount Everest.

Aside from many craters, Charon has a large dark region, deep valleys, and prominent cliffs. One of the canyons is four to six miles deep. Relative to Charon’s radius, this canyon corresponds to a 37 mile (60 km) deep canyon on the Earth – over 30 times deeper than the Grand Canyon!

Learning more about the solar system’s start

These data will help to complete a “fossil record” of the early history of the solar system. Nearly all stars like the sun form with a “protosolar nebula,” a disk-shaped collection of gas – mostly hydrogen and helium – and micron-sized dust grains. Over time, these tiny dust grains agglomerate into small pebbles. Pebbles grow into boulders, then mountains and finally planets.

The inner parts of this protosolar disk are warm, too warm for water to condense out of the gas. Agglomeration here leads to rocky planets like the Earth and Venus.

The outer parts of this disk are very cold. Ices of ammonia, methane and water condense out of the gas onto the grains and pebbles, leading to icy worlds like Pluto and Charon.

When icy worlds get large enough, they collect hydrogen and helium from the disk and grow into gas giants like Jupiter and Saturn. From the extreme pressure of the overlying gas, the iceball that became Jupiter has been crushed into a core of metallic hydrogen. Pluto can tell us what that iceball was like 4.5 billion years ago.

The canyons, mountains, and valleys on Pluto and Charon help us learn how rapidly Pluto and Charon formed. When planets form in a few million years, heat generated from collisions and the decay of radioactive nuclei melt the core (and sometimes, like the Earth, the entire planet). Within this liquid, heavy atoms like iron sink to the center; lighter atoms like oxygen rise to the surface. The planet becomes differentiated, where its various constituents separate into different layers based on their densities.

Differentiated planets have geysers and oceans. On Pluto and Charon, any surface water freezes quickly. But internal heat from radioactive decays and tides can maintain a subsurface ocean. This kind of vast underground reservoir of water keeps a planet round. Once New Horizons measures the “roundness” of Pluto and Charon, we will know whether either has a subsurface ocean. If they do, they probably formed rapidly.

Today’s images suggest Pluto and Charon are geologically active, differentiated worlds. Perhaps they have underground oceans. Once all the data are analyzed, we’ll know for sure.

Charon’s newly clear topography in a New Horizons image taken July 13 2015 from a distance of 289,000 miles.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute, CC BY

Hypothesis testing

Personally, I’m looking forward to data that can test the “Giant Impact” hypothesis for the origin of the Pluto–Charon binary. The hypothesis states that Pluto and Charon formed in separate locations of the protosolar nebula. Much later, Charon skimmed the surface of Pluto and kicked up a lot of icy debris. Tides between Pluto and Charon stabilized their orbit, locking them into a structure where each has a “day” equal to their 6.4 Earth day orbit around a common center of mass. With this tidal-locking, one hemisphere of Pluto perpetually faces one hemisphere of Charon. The four small moons formed out of the debris.

For the next month or two, New Horizons will continue to collect images of the Pluto system. Once all these data are transmitted to Earth and analyzed in detail, we will know more about the shapes of Styx, Nix, Kerberos and Hydra. If these moons grew out of smaller particles in the debris from the Pluto–Charon collision, they should be round. If the moons are large fragments from the collision, they should have more jagged, irregular surfaces.

Hydra appears to be irregularly shaped.

From the image released today, Hydra looks like an irregular fragment with bright and dark features on its surface. Once the New Horizons team processes all of the Hydra images, we will have a better idea of its true shape.

Now that New Horizons has done its job, scientists around the world will begin to analyze the data. It may take several years, but I am sure we will have some amazing surprises. In the meantime, we can rejoice that we live in an age where our tools can visit worlds throughout the solar system. And we can all enjoy the beautiful images of Pluto, Charon and their family of small moons.

The Conversation

Scott Kenyon is Senior Astrophysicist at the Smithsonian Astrophysical Observatory at Harvard-Smithsonian Center for Astrophysics.

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From comets to planets near and far, space probes reveal the universe

If space is humankind’s ultimate challenge, then the first step starts close to home – we have still to explore much of our solar system that spans across enormous distances, never mind those galaxies and stars more distant still.

To learn more we must get closer, dispatching spacecraft such as New Horizons – which has just returned our first ever close-up images of Pluto after a nine year journey. Here are my top five missions that are chipping away at what we don’t know and building up a better sense of universe around us.

1. New Horizons

Time for your close-up, Pluto.

Rocketing past at 14km/s, the New Horizons spacecraft has provided our first close view of Pluto which will enormously improve our understanding of this distant body. Our facts regarding this icy dwarf are sketchy at best. Just knowing what Pluto looks like makes it much more real.

The probe’s brief two-hour visit captured images of parts of Pluto and its largest moon Charon at high resolution, improving our understanding of planet formation. This is an amazing feat, considering the signals take more than 4.5 hours to reach Earth and that the sun is so weak at this extreme distance that solar power is not an option.

But the mission doesn’t end here: in 2019 New Horizons will visit a small object in the Kuiper belt, a region of space filled with small rocky planetoids, giving us a chance to examine the make-up of these remnants of the early solar system. And by 2026, it will reach the outer edges of the solar system.

2. Rosetta

Comet 67P/Churyumov-Gerasimenko, as seen from Rosetta 20km above.

Conceived decades ago, Rosetta flew alone through space for years before it reached the tiny comet that was its target and swung into orbit around it. Given the comet’s highly irregular shape this is an outstanding feat by itself.

The lander Philae managed to touch down and collect surface data of the comet, and while it was forced to shut down from lack of sunlight falling on its solar panels, it has now awoken and is transmitting data again. The Rosetta orbiter mission has also been extended to 2016 when it will also attempt to land on the comet.

The missions have improved our understanding of comets which contain frozen, icy rocks, and have measured the gas composition of jets streaming off the comet before they are altered by solar radiation.

But more than just hard numbers, this mission has been capturing images that speak for themselves, showing an ambitious mission conceived by many nations working together. Images such as Rosetta’s pictures of Philae descending resonates with us more than just hard facts and figures.

3. Dawn

Dawn is another mission expanding our knowledge of dwarf planets, in this case Ceres. It is now orbiting this interesting object having spent 2011 conducting similar work around nearby Vesta. Both Vesta and Ceres in the asteroid belt are protoplanets but of quite different composition.

Dawn has illustrated how powerful imagery can be. The most intriguing image is a crater that contains a handful of bright white spots on a surface otherwise darker than coal – unexpected, unexplored, challenging terrain.

What is the bright spot on the surface of Ceres? Dawn will find out.

4. Messenger

Incredibly detailed images from Messenger’s visit to Mercury.
NASA/JHU APL/Carnegie Institution of Washington

Messenger is still in my list of impressive space probes even though the mission ended with its controlled crash on Mercury’s surface this April. Sent to explore a planet of which we had barely any imagery of its surface, in four years Messenger managed to not only give us high-resolution maps of the innermost planet, it discovered water in its dark polar craters. On a planet baked by the sun this could only arrive from comets and water-rich asteroids – objects currently under investigation by Rosetta and New Horizons.

5. Curiosity

Curiosity Rover’s famous off-world self-portrait from Mars.
NASA/JPL-Caltech/Malin Space Science Systems

The last is the Curiosity rover. For me it sums up the efforts to explore our neighbour, Mars. These missions went in search of life and traces of water, carrying a complex laboratory, drills, laser and high resolution cameras.

Curiosity particularly illustrates the challenges we are capable of mastering to land a probe on Mars – described by NASA themselves as “seven minutes of terror”.

These rovers have achieved an outstanding feat, where now those exploring beyond Earth are not astronomers but geologists, the rovers’ equipment replacing the hammer and microscope used during fieldwork. The missions have added Mars to the “territory” that humans have access to. It’s even on Google Maps – imagery so good that we can see its surface as if we were there and can look at rocks in such detail as if we were picking up pebbles at the beach.

It’s out there waiting for us

Probes have witnessed solar eclipses and comet fly-bys that provide an entirely different view than is possible from Earth – something that adds a feel of awe and wonder, like looking back on Earth from the moon.

The sort of incredible images provided by these probes connects us with the solar system, bringing it closer to home. Famous images of Earth from space, such as the Blue Marble and the Pale Blue Dot catalysed our ecological conscience, reminding us of the fragility of our world in comparison to the vast, cold emptiness of outer space.

Such images lead us on to explore the universe and ourselves, and the findings of these remarkable spacecraft that have travelled millions, sometimes billions of miles through space over many years remind us that it’s out there to be discovered. The challenge and rewards await, as J F Kennedy said: we choose to go to space not because it is easy, but because it is hard.

The Conversation

Daniel Brown is Lecturer in Astronomy at Nottingham Trent University.

This article was originally published on The Conversation.
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Fly-by missions: what is the point when we have the technology to go into orbit?

New Horizons’ fly-by of Pluto and its moons is the latest in a historic string of missions to objects in the solar system. But given that a fly-by lasts for just a short time, how much can we really get out of it? There’s no doubt that the mission will yield a great deal of interesting data, but surely more would be gained if the spacecraft could go into orbit for a number of days or actually land on the surface and take physical samples.

New Horizons is hugely important because it is giving us a first glimpse into the unseen world of a third class of objects in the Kuiper belt – the building blocks of the outer solar system, located beyond the terrestrial and gas-giant planets. Fly-bys such as this are very exciting as they provide just one chance for unique measurements at the target.

While we are only at the very first stage of exploring Pluto and its moons, the fly-by will provide the foundations for future missions. Indeed, a fly-by is the first in the classical four stages of solar-system exploration and is followed – in this order – by an orbiter, a lander and the return of a sample from a body (marked 1-4 in the table below).

Lessons from the past

The first fly-by was of our Moon, made in 1959 by the Russian Luna-1 spacecraft. And 50 years ago, nearly to the day (July 15), the US Mariner 4 made the first fly-by of Mars.

My generation was captivated by the historic fly-bys of the outer planets and some of their moons, and I’ve been lucky enough in my own career to have been involved in instrument teams for several historic fly-bys. These were the Giotto mission to comets Halley (1986) and Grigg-Skjellerup (1992), as well as several close “fly-by firsts” in the Saturn system with the Cassini mission (such as moons Titan, Enceladus, Rhea, Dione, Hyperion).

Flyby firsts.
Author provided

The Giotto fly-by of comet Halley only lasted a few days, but our knowledge of comets was revolutionised by this encounter. One of several probes to explore Halley in the mid-1980s, Giotto had the widest and most capable set of instruments and passed closer to its target than any of its companions.

It found cometary jet activity, a surprisingly dark surface, hydrocarbons in a crust and a complex bow shock and tail formation mechanism. These discoveries are now being followed up by the Rosetta mission and Philae lander at comet 67P.

Artist’s impression of Giotto approaching a comet.
Mirecki/wikimedia, CC BY-SA

But the fact that fly-bys happen so quickly can also make them very stressful and difficult to manage. When we were monitoring the Giotto spacecraft, flying past Halley at 68.4 km/s, it suddenly started spinning off its axis after encountering a dust particle near its closest approach. Fortunately it was possible to stop the wobble.

Comet Wild 2 as seen from Stardust on January 2, 2004

There are many other examples where data have been rescued – including with New Horizons during its worrying glitch (now fixed) on July 4.

New Horizons and beyond

After launch on an Atlas V in 2006, the 478kg spacecraft passed Jupiter only 13 months later, which was an express route. The main reason for the hurry was to reach Pluto before its tenuous atmosphere collapses by freezing as the planet moves further away from the Sun. The mission design of New Horizons gives a very fast fly-by at over 14 km/s (50,000 km/hour), with only a few hours and days for the highest resolution measurements.

Measured in “astronomical units” (one AU is about 149.6m kilometres), Pluto’s orbit takes it from its closest point to the Sun (29.7 AU, 1989), inside Neptune’s orbit (30.1 AU), through its current distance (nearly 33 AU) out to its furthest distance from the Sun (48.9 AU, 2113). Receding from the Sun, the surface temperature reduces from its current 40 Kelvin, leading to freezing of the atmosphere.

But why fly past, rather than going into orbit? The most simple answer is that a lot of energy, meaning a lot of fuel, would be needed to slow New Horizons enough to capture it into orbit. Instead, NASA opted to get to the Pluto-Charon system quickly with a relatively capable 30 kg payload, rather than taking a large amount of extra fuel using a different fly-by scheme, to get there before the atmosphere collapses.

New Horizons is already expanding the thin textbooks on the Pluto-Charon system with early images, and the data to be returned over the next 16 months from visible, infrared and ultraviolet spectrometers as well as plasma, dust and radio science instruments will broaden and rewrite them again.

Image of Pluto and Charon, taken with New Horizons.

But new questions will almost certainly arise, which can only be answered by a more detailed orbiter mission, following the usual exploration sequence. When that would happen is hard to say. The relative priority will need to be compared with missions to other objects, particularly those where the exploration stage is low, before possible implementation.

In the future, we can look forward to more detailed fly-by missions of objects where our knowledge is limited and later-stage missions such as Rosetta. Also we will visit new dimensions, such as ExoMars drilling underneath the Mars surface by up to 2 metres for the first time. The JUICE fly-bys of Jupiter’s moons Europa, Ganymede, and Callisto, before entering Ganymede orbit, will allow comparison of subsurface oceans, and Europa Clipper will fly past Europa 45 times to complete a detailed reconnaissance there.

These missions, and their follow-ons, will help us discover more about the humankind’s place in the universe – and whether we are alone. But it is clear that, while we have achieved a lot in solar-system exploration so far, there is still a large amount left to do.

The Conversation

Andrew Coates is Professor of Physics, Head of Planetary Science at the Mullard Space Science Laboratory at UCL.

This article was originally published on The Conversation.
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New Horizons finally gets up close with Pluto – for 15 minutes

As I began typing this column, NASA’s New Horizon mission was on its final approach to its primary target, Pluto. By the time I finished composing my deathless prose, the main mission was over. And I’m not a slow writer.

Launched in January 2006, the spacecraft has travelled for nine and a half years for a flyby lasting only about 15 minutes. It doesn’t sound much of a reward for all the effort of designing and building the spacecraft – but for planetary scientists, the data coming back from the mission is pure gold.

For now all we can do is wait. Early in the morning on July 15, New Horizons is expected to phone home and confirm that the fly-by went well. Later that day the first high-resolution images should start trickle back to Earth – revealing what Pluto and its moon Charon actually look like up close. However, it will take nearly a year for all the data from the instruments aboard the spacecraft to come back.

But what is so exciting about Pluto? It isn’t even a planet anymore! When the New Horizons mission was conceived, Pluto stood (or rather, orbited) proudly as the ninth, and newest, planet in the Solar System. But eight months after New Horizons left Earth on its journey to Pluto, the International Astronomical Union downgraded Pluto from “planet” to “minor planet”.

Pluto’s change in status has, however, definitely not diminished the importance of the mission. Indeed, it has probably enhanced the scientific significance of the findings. Back when we thought Pluto was a planet, it was merely the last member of a series which represented a progression from the inner rocky and metallic bodies such as Mercury, through the gas and ice giants like Jupiter and Neptune, to Pluto – a small body of ice and rock.

But we now know that Pluto is not an isolated entity – it is the largest body in a huge family of primitive objects, many of which have their own moons. According to current models of how the solar system formed, there were once several hundred thousands of objects beyond Neptune, but Jupiter’s motion scattered most of them much further out from the Sun.

There are, however, still likely to be more such remaining bodies, known as Trans-Neptunian Objects, than the asteroids in the main belt between Mars and Jupiter. These objects are probably even more primitive in nature than some comets, which have been modified as they approach the Sun.

We already know that methane and ammonia ices are present on Pluto – but are there any higher hydrocarbons, or biologically interesting compounds such as amino acids? It will be interesting to see how analysis of the surface ices compare with results from the Rosetta mission or from the Dawn mission at Ceres. Can we draw any comparisons with the photo-chemistry on Saturn’s giant moon, Titan? Will Pluto demonstrate that trans-Neptunian objects are the most unchanged and unprocessed objects in the Solar System?

First look

Previous images of Pluto have been poorly resolved – the best view by the Hubble Space Telescope is of a fuzzy grey blob (see image below). But over the past few weeks, we have been able to enjoy increasingly more detailed images taken by the New Horizons spacecraft on its approach to Pluto. For example, we’ve learned that the planet is slightly bigger than we thought. We have also seen features on the surface, including what are probably ice-caps.

Images of Pluto generated with the Hubble Space Telescope and advanced computers.
NASA, ESA, and Marc W. Buie/wikimedia

Although the closest approach to Pluto will be over in a matter of minutes, the amount of data captured will be immense. It could help us answer a number of questions about Pluto – such as the distribution of different ices (water, ammonia, methane), the relationship between rock and ice and the presence of a thin atmosphere. The fly-by could also shed light on whether there are indeed craters on the body and whether there is any evidence of resurfacing.

Image of Pluto and its moon Charon, taken with New Horizons.

There is no expectation that cryovolcanism or ice geysers will be observed on Pluto, it doesn’t have the same gravitational heat source derived from a giant companion such as the case for Jupiter’s moon Europa. But it is in a binary system with its almost equally-sized moon, Charon – so it may surprise us yet.

For me, one of the highlights of the coming months will be synthesis of three sets of data – from New Horizons on Pluto, Dawn on Ceres and Rosetta-Philae on comet 67P/Churyumov-Gerasimenko. This will give us real insight to comet-asteroid interrelationships, and the primitive material from which the Solar System was built.

Whatever comes from the fly-by, we already have enough fresh information about our far-distant neighbour to ensure it is no longer seen as an underworld, the underdog of our planetary system. It is not the last planet to be visited but it is the first trans-Neptunian object to be seen – and so becomes a member of a very exclusive club.

15 July: Success confirmed

Following a long day of waiting, mission scientists got their reward just before 01:00 (UT), when the signal transmitted from the New Horizons spacecraft about four hours earlier was received at mission control. Cheers were heard from the team as Alice Bowman, the Mission Control Manager announced “we have telemetry lock”. This translates as “the spacecraft has called up, we have answered the phone, and we can hear what New Horizons is saying”. More cheers as each sub-system reported in: “thermal systems nominal”; “propulsion systems nominal”.

During the wait, we were tantalised with a fresh image of Pluto and its largest moon, Charon. New images will be released soon. For now, the waiting continues as New Horizons moves away from Pluto and its moons, and looks for its next target, a second TNO, which will be encountered in around 4 years time…

The Conversation

Monica Grady is Professor of Planetary and Space Sciences at The Open University.

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The search of life beneath the ice: why we’re going back to Europa

Last month NASA gave the “all systems go” for a new mission to Europa. But why go back? After all, we’re still sifting through the data from the Galileo probes fly-bys from more than a decade ago.

The short answer: it’s all about life.

The Jovian moons – named after Jupiter’s lovers by Simon Marius – have been a source of scientific speculation since Galileo trained his telescope on Jupiter in 1610, announcing his discovery in the Sidereal Messenger.

But the idea that Europa and other moons of Jupiter might harbour life is relatively new, as is the notion they might have hidden oceans beneath their icy surfaces. Indeed, these speculations demonstrate just how fast our conceptions of the solar system, and life, can change.

Speculative science, speculative fiction

A generation of space scientists and enthusiasts who grew up on Robert A. Heinlein’s “juveniles” will fondly remember Farmer in the Sky, written in 1950, when the Jovian moons were believed to be rocky, like our own Moon.

But in the late 1950s and continuing through the early 1970s, a growing body of telescopic data suggested that some of these moons, in particular Callisto, Ganymede and Europa, were covered in water ice. This speculation came from their high albedo, a measure of how much they light they reflect. With an albedo of 0.64, Europa is one of the most reflective bodies in the solar system.

In 1971, Carl Sagan suggested that the Jovian moons, including Europa, were of “major…exobiological significance”. In other words: they might harbour life.

Europa as seen by Voyager 2 during its close encounter in 1979.

The early 1970s also saw the first speculation that some outer moons of the solar system, including Europa, might hide an ocean beneath their surfaces. It was initially suggested this might be due to radiative heating, although it was later proposed that the heat might come from tidal forces induced by Jupiter, especially because of the synchronous orbits of the three innermost Galilean moons: Io, Europa and Ganymede.

The 1979 Voyager fly-bys confirmed that Callisto, Europa and Ganymede moons were covered in ice and that Io was extremely volcanic. The best images of Europa were taken by Voyager 2 from a range of 204,400 kilometres, showing Europa to be “billiard ball” smooth.

Not too hot, not too cold…

Things took a turn following the discovery by Robert Ballard’s 1977 expedition of entire ecosystems thriving near hydrothermal vents in the deep ocean. These vents existed in the “midnight zone”, without sunlight and photosynthesis, and changed the way we thought about life.

The discovery of life around deep ocean vents, like this one, raised the exciting prospect of life existing under the ocean on Europa.
P. Rona/NOAA

In 1980, scientists Gerald Feinberg and Robert Shapiro hypothesised that deep sea volcanism might support life on the Jovian moons. The Feinberg-Shapiro hypothesis is one of the major reasons for the current interest in Europa by astrobiologists.

In essence, it was proposed there might be a tidally heated habitable zone around giant planets, similar to the habitable, or “Goldilocks” zone around a star: where it’s not to hot, not to cold, and where liquid water and life can exist.

The idea of life on the Jovian moons was quickly picked up by science fiction writers. In Arthur C. Clarke’s 2010: Odyssey two (1982) and 2061: Odyssey three (1988), aliens transform Jupiter into a star kick-starting the evolution of life on Europa, transforming it into a tropical ocean world forbidden to humans.

In Bruce Sterling’s 1985 Nebula Award nominee, Schismatrix, Europa’s ocean is colonised by a group of genetically transformed post-human species.

Fire and ice

Europa and life were thus well and truly established in the minds of science fiction writers, planetary scientists, exobiologists and the public by the time NASA’s extraordinary Galileo mission began taking images of Europa in 1996.

This is the colour view of Europa from Galileo, taken in the 1990s, that shows the largest portion of the moon’s surface at the highest resolution.

By the completion of its primary mission on December 7 1997, Galileo had made eleven encounters with Europa. Galileo’s extended mission became one of “fire and ice”: its twin foci were Io’s vulcanism and Europa’s icy oceans. The Europa fly-bys took the probe to within a few hundred kilometres of the moon’s surface.

These extensive observations of Europa by the Galileo mission were compelling evidence for a liquid water ocean some 100 to 200 kilometres thick on which “floats” an outer shell of ice. Magnetometer measurements indicate the ocean is free flowing and salty.

Galileo also provided spectacular views of the icy terrain: ridges, slip faults and “ice-bergs”, all adding to the picture of a surface only 10-100 million years old, which is young by the four to five billion year age of the solar system.

The spacecraft, nearly out of fuel after an extended mission, was deliberately crashed into Jupiter on 21 September 2003 to protect Europa from possible contamination.

Europa Report

The data Galileo collected are still revealing new important finds. There evidence of clay-like minerals on the surface, possibly from asteroid or meteorite collision, and signs of sea salt, discoloured by radiation, making up some of the dark patches observed by both Voyager and Galileo.

A whole new generation of scientists is eagerly awaiting the data from the new mission. Astrobiology has become, since the early 2000s, a whole new science discipline. This “alien ocean” mission is clumsily named, at present, Europa Multiple Flyby Mission.

So the new mission, slated for a rendezvous with Europa in 2030, won’t involve a lander. And until we can send a probe into the icy depths of Europa’s sea, speculation about what might be lurking there, à la Sebastián Cordero’s Europa Report, will remain the domain of science fiction and scientists’ fantasy. Maybe one day, it will be science fact. Europa, here we come.

The Conversation

Morgan Saletta is Doctoral Candidate History and Philosophy of Science at University of Melbourne.
Kevin Orrman-Rossiter is Graduate Student, History & Philosophy of Science at University of Melbourne.

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