Space Sunday: meteors and motors

The 2013 Chelyabinsk meteor

A meteor is the fiery phenomenon resulting from an asteroid or other celestial body entering the Earth’s atmosphere. Often called a shooting star, if it does not fully vaporise and a part of it hits the Earth’s surface, it is called a meteorite.

On December 18th, 2018, a meteor roughly the size of a school bus blew apart under the pressures of entry into the Earth’s atmosphere 26 km (16 miles) above the Bering Sea. The explosion released some 173 kilotons of energy – about ten times more that released by the bomb dropped on Hiroshima in 1945. It is the second largest meteor explosion recorded since NASA started officially tracking them 30 years ago, after the 2013 Chelyabinsk explosion in Russia.

And no-one actually saw the meteor until after it had blown itself apart. In fact, no-one was aware of what had happened until three months later.

It was on March 8th, 2019 that the meteor’s arrival was noted by human eyes. Peter Brown, a meteor scientist at the Physics and Astronomy department of the University of Western Ontario, was reviewing data from the system used by the Comprehensive Test Ban Treaty Organization to detect atmospheric explosions caused by nuclear tests. This system is comprised of seismic and acoustic sensors capable of picking up infrasound, inaudible to the human ear, at a distance of tens of thousands of miles.

Brown noticed that many of the system’s sensors detected the sound waves from an explosion originating over the Bering Sea, and he calculated that had anyone been below it, the sound would have been deafening. He reported his findings to the United States Air Force, and a review of logs from their spy satellites revealed the passage of the meteor had been noted. A further check with NASA revealed their database of atmospheric impacts has logged the event, which was then officially announced.

This prompted a race to verify, and Simon Proud, a meteorologist and specialist in satellite data at Oxford University in the UK, decided to check the archive of images collected by a Japanese weather satellite that sends data to his department. He found that the satellite, Himawari, had indeed visually recorded the event. And it was not alone.

NASA’s Earth-observing Terra satellite also spotted the meteor with two different instruments — the Multi-angle Imaging SpectroRadiometer (MISR) and the Moderate Resolution Imaging SpectroRadiometer (MODIS). MISR team members combined some of their imagery into an animated GIF, which NASA released Friday, March 22nd.

An animated GIF of views from the Multi-angle Imaging SpectroRadiometer instrument on NASA’s Terra satellite, taken a few minutes after a fireball exploded over the Bering Sea on December 18th, 2018. Credit: NASA/GSFC/JPL / MISR Team

It is estimated that the meteor was some 10 metres (33 ft) across, and has a mass of around 1,360 metric tons. It probably entered the denser atmosphere at a speed of 115,200 km/h (71,600 mph). By comparison, the 2013 Chelyabinsk asteroid was about 20 m (65 ft) across, massed about 10,000 tonnes, and generated 440 kilotons of energy when it exploded. Even so, that event is dwarfed by the 1908 Tunguska event, which generated a force of 10-15 megatons (roughly 85 times greater than the December 18th, 2018 explosion), flattening 2,000 square km (800 sq mi) of forest through its air blast.

So why wasn’t the December meteor seen? Well, firstly, because it entered the Earth’s atmosphere above a very remote place in the world; simply put, there weren’t that many people under its path to see it. But more to the point, there is an awful lot of rocky debris in space; as I noted in my previous Space Sunday report, Earth shares its orbit around the Sun with a great cloud of dust and rock, and more is constantly falling in towards the Sun from further out in the solar system. As such, meteors are actually a common event – not that it makes them any the less dangerous.

CENOS track of NEOs recorded over the last 30 years

Many of these lumps of rock and ice – as with the December 18th, 2018 rock – are simply too small to be easily located and tracked. Others, like the Chelyabinsk meteor, are occupying orbits that effectively mean they are hidden by the glare of the Sun, and remain unseen until the enter the atmosphere. Nevertheless, over the last 30 years, NASA’s Centre for Near Earth Object Studies CENOS has located and tracks some 20,000 near-Earth objects (NEOs) some of which may at some point come close enough to the Earth to enter the atmosphere, around 50% of them are between 140m and 1 km in size – large enough to pose a serious threat.

While none are as big as the one that struck Chicxulub, Mexico, 65 million years ago and brought about the extinction of the dinosaurs, those at the upper end of the scale could still result in serious loss of life were one to explode over a populated area. So tracking NEOs helps to reduce that risk by producing us with the advance warning needed to evacuate areas – or even to develop a plan to deflect the incoming object – something missions to asteroids like Ryugu and Bennu may also help us to achieve by teaching us more about the nature of asteroids.

Keeping with meteor impacts, roughly 12,800 years ago Earth went through a brief cold snap unrelated to any ice age. Geologists have, for decades, argued for and against the idea it was caused by a meteor airburst or impact, referred to as the Younger Dryas Impact Theory, which also caused the final demise of the Clovis culture in North America.

Now an international team of scientists believe they have found geological evidence in South America that could settle the debate. Led by Chilean palaeontologist Mario Pino, the team has discovered a large, young impact crater in the Osorno province in southern Chile, close to the tip of the continent. Analysis of the impact site suggest it was created around 13,000-12,800 years ago – a time coincident to the Younger Dryas Boundary (YDB), which marks the time of the Younger Dryas Impact Theory, when there were numerous impact events across the northern hemisphere.

However, it is the size of the crater that suggests it may have played a significant role in the climate change that occurred in this period, causing widespread destruction, characterised by enormous biomass burning – around 10% of the Earth’s land surface, megafaunal extinctions and global cooling. Minerals found in the region are consistent with rapid temperature changes, further indicating the impact and the fires that followed it did indeed have a catastrophic impact on the global climate at the time.

SpaceX Starhopper Close to Starting Test Flights

Starhopper, the SpaceX vehicle designed to test capabilities that will be used in the company’s massive Space  Ship reusable passenger / cargo craft intended to take people to Mars, could be ready to start test flights in the next week or so.

The Starship Hopper under construction at the SpaceX facility at Boca Chica, South Texas; December 2018. Credit: Austin Barnard/Bloomberg

SpaceX hasn’t explicitly stated this will be the case, but they have notified local residents near their still-under-construction launch facility near Brownsville Texas that they’re establishing a “safety zone perimeter” in coordination with law enforcement agencies as a part of preparations for the flights.

As I reported at the time, the vehicle, which was being assembled outdoors at the SpaceX Boca Chica facilities, was initially completed in at the start of 2019. However, a storm in the area at the end of January 2019, resulted in the nose of the vehicle suffering damage.

A Raptor rocket motor being installed into the lower section of SpaceX’s Starhopper text craft. Credit: T.J. Cooney

That damage has yet to be repaired or replaced, but in the interim, the company has been installing the Raptor engine that will power the vehicle through its “hops” to altitude and back. For the first flights, only  one of the engines will be used, and SpaceX CEO Elon Musk has indicated these will see the vehicle tethered to the ground to reduce the risk of serious accident, and so will only be to a few metres height, could even take place without any replacement nose cone for the vehicle.

Britain’s SABRE Air-Breathing Engine Makes Progress

Reaching orbit is an expensive business. One way to reduce costs is by making launch systems reusable  – the approach being taken by SpaceX and Blue Origin in the United States. Another is (potentially) is through the use of single-stage to orbit (SSTO) vehicles that could effectively operate in a similar manner to aircraft: taking offer and returning to a runway.

However,  this brings with it problems of its own: rocket motors require a lot of fuel, and fuel requires space and creates weight. These tend to both make an SSTO unacceptably large and heavy, requiring even move fuel to lift them (hence why rockets are so big). But one company in the UK is making progress to address this.

The SABRE Engine

Reaction Engines has been working on SABRE – the Synergetic Air Breathing Rocket Engine. This is an elegant motor that operates in two modes: during take-off and ascent, it sucks in air from the surrounding atmosphere and uses it to generate thrust, allowing it to climb to an altitude of around 25 km (15.5 mi) and a speed of Mach 5.4. The system then switches over to using conventional on-board supplies of oxygen and hydrogen to power its way to orbit. Using the atmosphere means a vehicle using the engine requires far less on-board fuel, reducing it in both size and weight, making it a practical proposition.

SABRE has its roots in the UK’s HOTOL project of the 1980s. When the project folded, many of those behind that concept formed Reaction Engines Ltd., and started work on a concept called Skylon, which would use two SABRE engines for propulsion. The company worked on its ideas for some 20 years, with some interest from the UK government. Then in 2010, the project received a significant boost when the European Space Agency undertook an independent review of SABRE and found it viable.

This allowed the UK government and ESA to become directly involved in the engine’s funding, initially providing the money Reaction Engines needed to test of the engine precooler, an essential part of SABRE that ensures that the hot air stream entering at hypersonic speeds is kept at a consistent temperatures. If this could not be shown to work, the entire engine wouldn’t operate as designed.

As the tests were successful, Reaction Engines and ESA started work developing the engine core in 2016, and ESA recently completed a preliminary review of the overall design, with positive findings. This means the green light has now been given for Reaction Engines to build a core engine demonstrator for testing at a new facility they have been developing in Buckinghamshire, England.

This air-breathing demonstrator will be fully representative of the SABRE thermodynamic core cycle, fuelled by liquid hydrogen, and will contain heat exchangers plus combustion and turbomachinery modules. It will not, however, include the pre-cooler and rocket nozzle. However, providing the core tests are successful, this will be added in time as a part of the modular development of the engine.

So far, the UK government has provided some £50 million for SABRE development, and ESA a further €10 million, with some $37.5 million coming from Boeing and Rolls Royce, while BAE Systems has invested £20.6 million into Reeaction Engines in return for a 20% stake in the company. While SABRE was originally conceived as a part of the Skylon vehicle, the funding is purely for engine development. The cost of Skylon itself has been put at some £7 billion, and plans for any development of the vehicle await completion of the SABRE engine.

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