Saturday July 27th, New York City
The last 24 hours were probably the most challenging of the entire voyage from Southampton to New York. As we came onto the continental shelf a few hundred miles off Boston, and travelled on a rhumb line straight into New York harbour, a force 7 gale was blowing Queen Mary 2 from side to side, creating the roughest seas that we'd encountered on the whole journey. My final lecture, on the Cassini exploration of the Saturn system, was one of the most challenging I've ever given. I had to adopt a wide stance, clutching the podium as the lecture theatre pitched from side to side. This was followed by two live planetarium shows, giving a much smaller audience a tour of the night sky from the comfort of the auditorium, and making the most of the superb system onboard QM2. I received lots of warm feedback at an informal 'meet the speakers' event in the bar that afternoon, and I hope that people learned at least one thing from each of the lectures I gave!
From noon on Friday, when we heard our last navigational announcement from the Commodore at 40.3N, 67W, we travelled the final 280 miles to New York. The ship was relatively quiet that evening as people packed and prepared for the early morning. At 04:30 am we passed under the Verrazano-Narrows bridge with our narrow clearance, and by 5 am we were staring out at Manhattan at dawn, with the Statue of Liberty out to the port side and our final destination, Brooklyn cruise terminal, on the starboard. It was our first glimpse of land and civilisation in almost 7 days, and it was with a mix of emotions that we realised the relaxed and peaceful days at sea were over. Tugs moved us into place, and we were alongside shortly after 7am. I bade farewell to the Queen Mary 2, and saw her again later in the day from Battery Park, Lower Manhattan, as she departed for her 202nd crossing of the Atlantic Ocean.
Friday, 26 July 2013
Friday July 26th, 11am
It’s our final day at sea, and the vast bulk of the Atlantic Ocean is now behind us, with anticipation for our arrival in New York City tomorrow morning. The fourth and final lecture in this Royal Astronomical Society series concerns an enormously successful robotic explorer in the distant solar system, the Cassini-Huygens mission to Saturn. Many of the topics I’ll describe have been the subject of previous posts on this blog, so I’ll refer the interested reader to those for more details. For example, I begin with a brief history of observation of the Saturn system, from the strange appendages observed by Galileo, to the explanation of the rings and the discovery of enigmatic Titan by Christiaan Huygens; Jean Dominique Cassini’s discoveries of a division in the rings and a multitude of icy satellites; and William Herschel’s in depth observations of Saturn’s changing appearance. Saturn’s first human-made satellite was named for Cassini; and Titan’s first entry probe was named for Huygens, honouring their pioneering studies of the ringed planet.
The Cassini mission was born of the early 1980s, as one of the first fully international robotic space missions, a collaboration between NASA and the European Space Agency. Indeed, when budgets were being slashed it was the international aspect of this mission that probably saved it. It launched in 1997; spent seven years in the frigid depths of space and finally arrived in orbit around Saturn in 2004 just after the northern winter solstice. It has been working out there on its lonely orbit for almost a decade now – following completion of its four-year primary mission; it’s lifetime was extended and extended and we now hope it will survive through until 2017, northern summer solstice. That is, the spacecraft will have observed all four seasons at Saturn, from winter to spring, summer and autumn, providing arguably our most comprehensive picture of a giant planet ever obtained. Ignoring the wonderful science from this mission for just one moment, the Cassini mission is a triumph of engineering – a spacecraft almost 7-m long and 4-m wide, weighing 2150 kg and powered by the radioactive decay of Plutonium-238. Hydrazine rocket thrusters allow the spacecraft to be agile and turn to point its suite of instruments at the variety of targets, but it is this which limits the lifetime of the mission – once the hydrazine supply is exhausted, the mission will be over.
Saturn and its Rings
This lecture gives a whistle-stop tour of some of the discoveries of the Cassini-Huygens mission. Starting from the gas giant itself, we’ll talk about the different atmospheric layers, what we think is present deep down inside the planet, and what forces drive the weather we see. When you observe Saturn through a telescope, you’re seeing light being reflected from the clouds and hazes in the upper atmosphere; possibly fluffy clouds of ammonia ice rather than the water clouds we observe in our own atmosphere. Those clouds are blown around the planet by winds racing east and west. We’ll talk about small storms and lightning (including the southern storm alley), and the seasonal eruptions of globe-encircling storm systems that endure for many months. One such storm erupted in 2010, and its aftermath is still being felt today. Cassini is currently in a high inclination orbit, allowing it to gaze down at the poles of the giant planet to explore the mysterious hexagonal wave around the North Pole, and the twin hurricane-like cyclones churning like giant plugholes at both poles. It has also captured movies of the aurora dancing in Saturn’s high atmosphere in response to pressures from the solar wind and plasmas being injected within the planets magnetic field environment.
Saturn’s delicate and beautiful rings serve as a wonderful laboratory for gravitational interactions. We’ll describe the ring structure, their potential origin and the process of continual renewal and recycling. Discrete features like elusive spokes in the B ring; structures towering above the ring plane and casting shadows back across the main rings; shepherding moons generating wakes in the rings and beautiful gravitationally sculpted structures seen by Cassini’s high-resolution cameras. Beyond the rings, a vast array of unique satellites orbit the giant planet, including Phoebe (a remnant of solar system formation captured by Saturn’s immense gravity); Hyperion (with the appearance of a sponge); Mimas (with the huge Herschel crater making it look suspiciously like a science fiction icon); and Iapetus (with it’s asymmetric brightness and enormous equatorial ridge). Tiny Enceladus, with its four south polar fissures actively venting ice and gas into space, is intricately connected with Saturn’s diffuse E-ring and a truly remarkable discovery.
The jewel in Saturn’s crown is this enormous satellite, shrouded in a thick smoggy orange atmosphere and the second largest moon in our Solar System. Although Voyager had captured images of Titan thirty years ago, the thick hazes were impenetrable, preventing any glimpses of the surface of the moon. What would we discover there, and what might this unexplored terrain look like? The mystery of Titan was one of the driving goals of this mission to the Saturn system. Imaging systems on the Cassini orbiter provided access to wavelengths where the hazes are transparent, providing glimpses through the smoggy atmosphere. Radar swaths have shown the undulating terrain, from dunes to impact scars, mountain ranges and river valleys. And the Huygens probe, designed by the European Space Agency, became the first human-made object to touch down on Titan in January 2005. At that time no one knew what we might be landing on, from solid surfaces, to hydrocarbon sludge, or even into a vast ocean of liquid methane and ethane. The probe descended beneath a parachute, being buffeted by the winds until coming to rest in a dried up river bed; pebbles of water ice appearing rounded by the flow of fluids across the surface, and hints of moisture in the upper layers of the soil. It’s too cold on Titan for that fluid to have been water – instead, methane is a fluid at these temperatures, and a ‘methane-cycle’ on Titan mimics the ‘water-cycle’ on Earth, with methane clouds, methane rain, methane rivers and lakes. Evidence for this methane cycle can now be seen everywhere on Titan, and the river networks and lakes reveal a very Earth-like world.
One of Cassini’s most enticing discoveries is that of lakes and seas of hydrocarbons at the northern pole of the planet. For the first time in the history of our solar system exploration, we have seen standing bodies of liquid on another planet, and can envisage what it would be like to sail those Titanian seas. We have the technology to do so, and could one day see a long-live vessel bobbing on the hydrocarbon sea, floating in and out of different drainage deltas and looking back at the hills, valleys and mountains on the Titanian shore. It’s a lovely idea, and one I hope we’ll one day see with our robotic explorers.
By 2017, the Cassini mission will have completed an in-depth orbital reconnaissance of the whole Saturn system, and it will be time to draw this hugely successful mission to a close. It cannot simply be left in the Saturn system to potentially collide with, and contaminate, any of the pristine environments to be found on the satellites. So it must be disposed of by burning up in Saturn’s atmosphere, a dramatic fireball at the end of the mission. But before that happens, mission planners have designed a dramatic series of final orbits, taking more risks with this grand old spacecraft. It’ll be flying closer to Saturn than ever before, within the rings and skimming just above the cloud tops. It’ll be measuring the strength of the close-in magnetic field, and also the gravitational parameters of the planet itself, using these to probe down to the centre of the planet at depths we’ve never seen before. It’s been described as a new mission, a new lease of life for this ageing spacecraft, and contributing even more to our knowledge of the Saturn system.
As we were preparing to embark on our passage to New York, on Friday July 19th at around 21:30, the Cassini spacecraft trained its powerful cameras back onto the inner solar system, to capture another stunning image of the Earth and moon system, as seen by a lonely robotic explorer a billion miles away in orbit around Saturn. While we’ve been sailing, that image was beamed to Earth, assembled by computers in California and processed to show the results to the world. All of humanity, once again sharing just a few pixels, and showing how small, how fragile, and how precious our home world truly is. It seems like the perfect place to end this series of lectures on board the Queen Mary 2.
Thursday, 25 July 2013
Thursday July 25th, 40.8N, 57.7W
The ocean around us today has changed from the green and rather forbidding cold, foggy ocean of the Labrador Current to the warmer, indigo blue colours of the Gulf Stream. A fresh wind is blowing, but we have clear blue skies and a beautiful blue sea. The depth under our keel is 5000 m, deeper than at any other point in this journey, as we pass within 200 nautical miles of Sable Island, Canada, a small, narrow crescent-shaped sand island around 13 square miles in size and home to only a handful of people. We’re now 2408 miles from Southampton, with only 745 miles to run to New York on a straight line. The Commodore describes this area as the region of strongest hydrographic contrast in the world, as the ocean temperature has changed by 10 degrees from the cold arctic currents of yesterday to the warm Gulf Stream today. But the Gulf Stream itself is complex and inconsistent, with whorls, eddies, gyres all changing the speed and temperatures as we make our passage, and showers and squalls are expected for this evening. We can also spot brown clumps of weed floating in the open ocean, Sargasso weed from the Bahamas being pushed north by the Gulf Stream and appearing like brown clumps of grapes in the seawater.
It is this changeable weather that has prevented us from hosting star parties from the upper deck on this particular crossing – the entertainment folks need time to advertise these events, and rely on weather forecasts. The full moon and long summer days also mean that conditions are best late at night, when they want to minimise disturbance to guests. That said, I’ve been doing my best to point out to interested guests Venus on the western horizon after sunset; the summer triangle (Deneb, Vega and Altair; in Cygnus, Lyra and Aquila); star hopping from the Plough to Polaris, Arcturus and Leo; as well as Cassiopeia and her entourage east of North at 10-11 pm (Cepheus, Andromeda, Perseus, etc.). The great square of Pegasus should be on the horizon to the northeast. Saturn is a good object for viewing in the early evening too towards the South West in the Constellation Virgo; and Jupiter is rising just before dawn. We’ll talk about all of these in Fridays’ live planetarium shows, but please do stop me to ask me if you’re out on the upper deck this evening!
Wednesday, 24 July 2013
Wednesday July 24th, 15:30
As I write this I’m sat on Deck 9 looking out over the thick fogs of the Grand Banks. This is the third of a series of public lectures for the guests on Queen Mary 2 as part of the ‘Cunard Enrichment Programme’ for this transatlantic crossing, and concerns a natural phenomenon with wide-ranging implications for the continued survival of our species.
On a bright, cold February 15th morning in the southern Ural region of Russia, an asteroid hurtled into the Earth’s atmosphere at speeds exceeding 41,000 mph. It was totally unexpected, and no one saw it coming. It became a brilliant meteor, outshining the light of the morning Sun to cast shadows across the town of Chelyabinsk. This airburst explosion had the energy of 440 kilotonnes TNT, and the resulting shockwave injured 1500 people and caused an estimated $33 million in property damage. It’s possible that fragments of the meteorite smashed a 6-m wide hole in Lake Chebarkul’s frozen surface. All this from an asteroid thought to have been around 17-20 m wide weighing around 10,000 tonnes. So why didn’t we have any warning? How frequent are these events? It left us all wondering – what would it have been like if this had been my town, my home, my place of work? And history tells us that the consequences could have been much, much worse.
A Hierarchy of Impact Events
First, some terminology. A meteoroid is a small grain, maybe up to 1-m in size, and possibly left behind by a passing asteroid or comet. We call these objects meteors when they enter the Earth’s atmosphere to burn up due to friction, resulting in ‘shooting stars’ and regular meteor showers throughout the year. If the object survives the heat of entry and makes it to the surface, we have a meteorite. The terms bolide and superbolide are sometimes used but have no precise astronomical definition, they refer to the brightness of the resulting fireball. An airburst, such as that at Chelyabinsk, is a mid-air explosion that makes no contact with the ground. Our best example of an airburst occurred in the remote Siberian region of Tunguska in 1908, when a small asteroid or comet (60-190 m wide) exploded 5-10 km above the Earth’s surface, knocking over 80 million trees over an area 830 square miles in size. The dust and particles from the explosion were reported to produce sky glows in the evenings over Europe for many days afterwards. Given the upheavals of the first decades of the 20th century, no expeditions were sent to the region for almost ten years, so our knowledge of this airburst comes from the devastated landscape and eyewitness accounts of the event. The Tunguskan airburst was the largest impact event in recent history.
When meteorites do reach the ground, the consequences can be enormous. Our solar system bears the remnant scars of this bombardment, and there is no finer example than the Barringer crater 43 miles east of Flagstaff, Arizona. Estimated to be 50,000 years old, this crater was created when a nickel-iron meteorite 60-m wide impacted with the Colorado plateau with an energy of 10 Megatonnes TNT. The meteorite itself was vapourised as it blasted a hole 1200 m wide and 170 m deep. It is named after Daniel Barringer, who first suggested that it had been caused by an impact, which was later verified by Eugene Shoemaker as the first evidence of an extraterrestrial impact on Earth.
Larger collisions can have consequences for the existence of life on our planet. Buried beneath the Yucatan peninsula of Mexico lies the Chicxulub meteorite crater. An oil survey had first detected an underwater ring beneath the Gulf of Mexico, and mapping of gravitational anomalies later confirmed the presence of an enormous crater 110 miles wide. The 65-million year age of the crater coincides with the end of the Cretaceous period, the start of the Paleogene (the K-Pg boundary) and the end of the age of the dinosaurs. This impact, and the environmental conditions that followed, triggered mass extinctions across our planet and ushered in the age of the mammals, and ultimately the human species. Impact processes have shaped our evolution and could one day threaten our survival, but thankfully extinction level events are rare. To understand the nature of the impactors and the frequency of such events, we shall now look outwards to the other bodies in our solar system.
Impactors in Our Solar System
The birth of our planet was complex and violent, as small rocky protoplanets grew larger and larger over time through collisions and mergers, gravity clumping material together until the planets as we know them today had formed. But this process was chaotic and remains incomplete, with vast fields of debris remaining throughout our solar system that was never incorporated into the forming protoplanets. Gravity serves as the great meddler, and as the planets moved and jostled to assume their present-day locations, they sent material flying in all directions, leading to a period of heavy cratering on all the planets known as the Late Heavy Bombardment around 3.9 billion years ago. We need only look at the full moon tonight to witness the scars left by this bombardment.
Some of that solar system debris can be found in the broad asteroid belt between the orbits of Mars and Jupiter. Jupiter’s immense gravity prevented this material from forming a planet, but large objects do exist – Ceres is known as a dwarf planet some 950 km in diameter, and soon to be visited by NASA’s Dawn spacecraft. Other large asteroids include Vesta, Pallas and Hygiea. We say that these objects are ‘volatile poor’, as many of their light molecules (water included) have been lost to leave dry and airless bodies. This contrasts them with the icy comets, rich in volatiles and icy material. For much of their existence, comets exist as dirty snowballs, only forming their characteristic tails when they wander into the inner solar system. As the cometary nucleus is warmed, they produce tails of dust and plasma that produce incredible spectacles in the night sky. We know of around 5000 cometary bodies in our solar system, but models suggest there could be as many as a trillion out there in the distant solar system. And the line between comets and asteroids can sometimes be blurred, as an extinct comet that has lost all its ice and volatiles would resemble an asteroid.
Comets are broken down into categories depending on how frequently they visit the inner solar system. Short period comets come from the realms of the giant planets and the Kuiper belt beyond Neptune. If a comet has it’s maximum orbital distance near Jupiter, then it would be known as a ‘Jupiter-family’ comet (of which there are around 450 with orbital periods less than 20 years). Comet Hartley 2 is a good example, and its nucleus was imaged in 2010 by NASA’s EPOXI mission. Halley-type comets have orbits that last between 20 and 200 years. At the other extreme, longer period comets all appear to come from an extremely distant cloud of cometary objects, taking hundreds of years between their visits. The hypothesised spherical Oort cloud (which has never been directly observed) could extend 50,000 AU from the Sun, and makes its presence known when gravitational perturbations kick an icy comet onto a trajectory into the inner solar system.
The size and brightness of a cometary tail depends on a multitude of factors, but those comets that put on a particularly magnificent display are known as the ‘Great Comets.’ Examples include Halley’s Comet, a frequent visitor to the inner solar system and known since ancient times (e.g., it was recorded in the Bayeux tapestry in 1066), last seen in February 1986 and returning again in July 2061; and more recently Hale Bopp in April 1997, which featured a tail 50 million km long and visible for 18 months to the naked eye. Some comets wander too close to the Sun and become ‘dirty snowballs in hell’ as they pass through the solar corona. One such comet, Lovejoy, was observed to pass through the corona in December 2011 and emerge on the other side intact. Finally, this year attention has been focused on two comets – PANSTARRS and ISON. The first arrived in the spring, but was difficult to identify in the sky. The second, ISON, should arrive in December, and has the potential to be a spectacular great comet. It’s a new arrival to the inner solar system, with pristine volatiles ready to vent, but although a tail is forming it’s hard to tell how bright it will become.
My own interest in impact events stems from a brief period of time in 2009 when my research took a very exciting and unexpected turn. But first rewind to 1994. You may remember the fate of comet Shoemaker-Levy 9, the 9th comet discovered by Gene and Carolyn Shoemaker with David Levy. In 1992 Jupiter’s immense gravity had disrupted this icy comet and broken it into fragments A-W, each 100-500 m in diameter and setting them on a course to collide with the giant planet in 1994. They slammed into Jupiter, creating plumes of material and dark bruises on the atmosphere that we’re visible for many weeks afterwards. These are classic examples of airbursts, even though they took place on a distant world, and allowed us to explore the detailed physics of the collisions. The superheated entry column left by the cometary fragments caused an ejection of the atmosphere (a mix of Jupiter’s atmospheric soup and vapourised comet), high above the clouds. The ejecta travelled ballistically, then slammed back down on the atmosphere to create the dark debris fans that characterised each impact site. This phenomenon was the chance of a generation to witness the aftermath of a cosmic collision.
In the summer of 2009, on a Sunday afternoon in Pasadena California, I was having a barbeque with some friends. I was due in the office that night to help my boss run a telescope on the summit of Mauna Kea, Hawaii, to observe the interactions of some giant storms on Jupiter. That afternoon, we received an email from a talented amateur observer, Anthony Wesley of Australia, who had spotted a new dark scar in Jupiter’s south polar region. He suggested that a fresh impact had occurred, for the second time in recorded history. So when we arrived at work that Sunday night, we immediately chose techniques to identify the smoking gun of an impact – the high altitude debris left by the collision – and it came into view in startling clarity. The discovery propelled us all into the world’s media for a few hours, as our image was reproduced around the globe – it was a terrifically exciting time!
Scientists scrambled for telescope time to diagnose what had happened. By studying the impact debris across a wide range of wavelengths, from the infrared to the ultraviolet, we could piece together the events like a detective story. Ultimately, the chemistry of the impact debris suggested that whatever had hit Jupiter that night had been depleted in ice and volatiles – no water-related chemistry was observed, and all the signs have since pointed to a rocky asteroid colliding with the giant in 2009. We even managed to persuade the Hubble Space Telescope to track the evolution of the impact debris – Hubble was still in testing and verification after the final servicing mission, so this was quite a feat and showed how the debris was blown around by Jupiter’s powerful winds. Evidence of the impact was still present 5 months on.
Since that time, the amateur community has become adept at impact monitoring on Jupiter, using webcam movies to detect the telltale flashes of meteors exploding in the jovian atmosphere. We’ve now witnessed three such events since 2010, but none have left the dramatic atmospheric scarring of either the cometary impacts in 1994 or the asteroidal impacts in 2009. In essence, these are no different to what happened over Chelyabinsk in 2013, and the objects are even of a similar size. The frequency of impacts can be used to understand the rates of impacts in our solar system at the present time.
Preparing for the Future
So impact processes are commonplace in our solar system, having shaped planetary evolution in the past and still causing dramatic events today. So what is being done, and what can be done, to minimise the risks associated with these collisions?
The first task is to understand the frequency of impacts, which can be done via a chart showing the object size and energy versus the number of impacts over the years, centuries and millennia. As a benchmark, a 4-m stony asteroid is predicted to hit the Earth once a year or so; a Chelyabinsk-sized object every few hundred years; a football field sized asteroid every 2000 years; and a 1-km sized object every half million years or so, on average. Such a collision would have both regional and globally devastating consequences. But a 10-km impactor, like that which triggered the extinction of the dinosaurs, might be expected once every 100 million years or so. These are all averages and present an idea of probability, and should not be seen as a timeline for such events!
The second task is to assess the nature of the impact hazards today, by monitoring all ‘Near Earth Objects’ (NEOs) and assessing their potential to collide with Earth. For example, NASA has a congressional mandate to characterise all NEOs over 1-km in size, and this supports the international ‘SpaceGuard’ project scanning the skies for these objects. Almost 10,000 NEOs are known, the majority of them asteroids, and our models suggest that this represents over 90% of the population out there. But recall that Chelyabinsk was far smaller (<20 m), and would never have been picked up by such surveys. All objects are assigned a number on the Torino scale, which balances the probability of impact versus the amount of damage it would cause. When an object is first spotted it is assigned a high number, but as we refine our knowledge of the orbital parameters it is downgraded to zero. Today, only one NEO still has a nonzero probability of impact, 2007VK184, which has a 1/1820 chance of hitting us in June 2048. But as our knowledge of the orbit of this body improves with time, we fully expect this hazard to be downgraded too.
Finally, if we do spot a hazard heading our way, what safeguards are in place to prevent collision? It certainly depends on many factors, including the size of the threat and the amount of time we’d have to prepare. That’s precisely what projects like SpaceGuard is designed for. The method ultimately chosen to prevent a collision will depend on cost, performance, operations and risk, but can be loosely broken into two categories – destruction or delay. Destructive methods would break the impactor into smaller, safer fragments to remove the threat. Delay tactics would tweak the orbit of the impactor so that it passes us safely. Nuclear detonations; using ballistic impactors; or mass drivers and gravity tractors; focused solar energy or ion beams; or even attaching conventional rockets to the impactor, could all be viable options for destruction and delay. Our readiness will improve with our technology, but its our duty to be ready for the challenge when, not if, it finally arrives. I finish with a final quote from Sagan: “Extinction is the rule. Survival is the exception.” Something worth bearing in mind when we see the next Chelyabinsk.
Wednesday July 24th, 43.5N, 48.8W
We’re now off the southernmost point of the Grand Banks of Newfoundland, 294 miles southeast of St. Johns and about 124 miles to the northeast of the final resting place of the Titanic. History boards all around the ship tell the story of the Titanic’s fateful voyage, and the rescuers aboard the Carpathia. The Grand Banks rise above the seafloor 3 km below, sometimes as shallow as 200 m or so, but we expect it to deepen again as we make our way towards the eastern seaboard. There are 1981 miles of sea astern, and 1171 miles still to run to New York City. The warm moist air outside is interacting with the cold Labrador Current coming down from Greenland, and creating a thick impenetrable fog all around Queen Mary 2 today, making a walk outside rather eerie. The Commodore reminds us that sailors don’t like the ‘F’ word (fog). Sticking my head over the starboard side (i.e., north), I could see a ‘glory’ and a ‘fog bow’, two examples of the optical effects as the Sun penetrates through the tiny fog particles, not too dissimilar from a rainbow seen through rain droplets. The sea itself is relatively calm, and you can barely feel the motion of the ship through the waters.