Space Sunday: asteroids, telescopes and dust

Credit: Mopic/Shutterstock

Saturday, June 30th marked International Asteroid Day, a global event involving researchers, astronomy groups, space agencies and more talking about asteroids  – and the risk some of them present to Earth.

Since 2013, and the Chelyabinsk event which saw a meteor  roughly 20 metres across, caught on film as it broke up high over the Russian town, the tabloid media has seemingly been obsessed with reporting meteors about to collide Earth and wreak havoc.

Fortunately, the vast majority of the estimated 10 million objects which have orbits passing close to Earth – referred to as NEOs, for Near Earth Objects, are unlikely to actually strike our atmosphere or are of a small enough size not to pose a significant threat if they did, despite all the screaming of the tabloids.

A map showing the frequency of small asteroids entering Earth’s atmosphere between 1994 and 2013. The dot sizes are proportional to the optical radiated energy of impacts measured in billions of Joules (GJ) of energy. A total of 556 events are recorded on the map, representing objects ranging in size from 1m to 20m. Credit: NASA’s Near Earth Object (NEO) programme

Which is not to say NEOs don’t pose a potential threat. Not all of the 10 million objects with orbits passing close to, or intersecting, the orbit of Earth have been properly mapped. Take 2018 LA (ZLAF9B2), for example. As I reported at the start of June, this asteroid, some 2 metres across, was only identified a handful of hours before it slammed into Earth’s upper atmosphere over Botswana at approximately 17,000 kilometres per second, to be caught on film as it burnt up. The energetic force of the accompanying explosion has been estimated to have been in the region of 0.3 to 0.5 kilotons (300 to 500 tonnes of TNT).

To offer a couple of quick comparisons with this event:

  • The 2013 Chelyabinsk superbolide (roughly 10 times the size of 2018 LA (ZLAF9B2) disintegrated at an altitude of around at 29.7 km at a velocity between 60,000-69,000 km/h, producing an energy release equivalent to 400-500 kilotons (400,000-500,000 tonnes of TNT). This was enough to blow out windows and send 1,491 people to hospital with injuries, including several dozen temporarily blinded by the flash of the explosion. The first 32 seconds of the video below convey something of the force of that event.

  • In June 1908 a cometary fragment estimated between 60 and 190 metres cross disintegrated some 5 to 10 km above Tunguska, Siberia. This generated an estimated downward explosive force of between 3 to 5 megatons and an overall force of somewhere between 10 to 15 megatons (again for comparison, all the bombs dropped by allied forces in World War 2 amounted to around 3.4 megatons of combined explosive force). This is believed to have generated a shock wave measuring 5.0 on the Richter scale, flattening an estimated 80 million trees covering an area of 2,150 square kilometres. Were it to occur today, such an event would devastate a large city.

There are two sobering points with these two events. The first is that astronomers estimate only about one-third (1600) of objects the size of the Tunguska event meteoroid which might be among that 10 million NEOs have so far been mapped. The second is that many NEOs can remain “hidden” from our view. the Chelyabinsk superbolide, for example passed unseen as the Sun completely obscured its approach to Earth.

There have been several proposals for trying to deal with the potential risk of a PHA – Potentially Hazardous Asteroid – impact over the years. One currently in development is the NASA / Applied Physics Laboratory (APL) Double Asteroid Redirection Test (DART) mission intended to demonstrate the kinetic effects of crashing an impactor spacecraft into an asteroid for planetary defence purposes.

The target for this mission is rather interesting. DART will be launched on an intercept with 65803 Didymos, an asteroid around 750 metres across – but this will not be the vehicle’s target. That honour goes to a much smaller asteroid – around 170 metres across (so in the size range of the Tunguska object) – orbiting 65803 Didymos and informally referred to as “Didymoon”.

Originally, DART was to be a part of a joint NASA/APL and European Space Agency effort, with ESA supplying a vehicle called the Asteroid Intercept Mission (AIM). This would have been launched ahead of DART on a trajectory that would place it in orbit around the 65803 Didymos / “Didymoon” pairing, allowing it to track / guide DART to its target and record the entire impact and its aftermath.

AIM never received funding, leaving the NASA/APL mission, which is currently scheduled for launch in 2021 and will intercept “Didymoon” in 2022. However, in the last few weeks, ESA has announced a revised mission to 65803 Didymos called Hera. Like AIM, it is designed to orbit the asteroid and is moon, and a call has been made to combine it with DART under a new joint mission called Asteroid Impact and Deflection Assessment (AIDA).

This would require DART to be delayed for a number of years to give ESA time to obtain approval for Hera and design, build and launch the craft – so the intercept would not take place until 2026. While this is a delay, it would mean that scientists would be able to better characterise “Didymoon” ahead of DART’s arrival, and witness the impact and its aftermath in real-time.

The original DART / AIM mission – to study the use of kinetic vehicles to divert an asteroid – now potentially superseded by the DART / Hera mission. Credit: NASA / APL / ESA

It’s not clear whether or not DART will be delayed. If it isn’t, then it has been proposed DART carries a camera equipped cubesat similar to those AIM would have used in support of its mission. This could then be separated from DART ahead of the impact so it could image the event as it flies by “Didymoon”. The Hera mission would then arrive a few years after the impact and assess the outcome, including imaging the impact crater on the asteroid and changes to its orbit and its rotation, which can help scientists determine how efficient the impact was in transferring its energy into “Didymoon”.

In the meantime, a new Earth-based telescope – the Large Synoptic Survey Telescope (LSST) – is due to see “first light” in 2019. When operational, this 8.4 metre telescope will survey the visible sky every week down to a much fainter level than that reached by existing sky surveys, allowing it to catalogue 90% of the near-Earth objects larger than 300m and assess the threat they pose to life on Earth.

An artist’s rendering of the LSST in its observational dome. Credit: Todd Mason, Mason Productions Inc. / LSST Corporation

Located on the El Peñón peak of the 2,682m high Cerro Pachón mountain in northern Chile, LSST was first proposed in 2001, with construction of the 3-part primary mirror commencing in 2007, initially funded through private investment, including software billionaires Charles Simonyi and Bill Gates providing US $20 million and US $10 million respectively for the project. Funding has since been taken over by the US National Science Foundation (NSF) after the 2010 decadal survey, ranked the LSST as the highest-priority ground-based science instrument in development.

Unlike almost all previous large astronomical observatories, data and images obtained by LSST will be made public as soon as it is taken. This amounts to 1,000 pairs of images and 15 terabytes of information from each night’s observation period.

By providing immediate public access to all the data it obtains, it will provide everyone, the professional and the “just curious” alike, a deep and frequent window on the entire sky.

The LSST tour“, part of the LSST website

An artist’s rendering of the completed LSST observatory superimposed over its construction site on the El Peñón peak, northern Chile. Credit: LSST Project/NSF/AURA

More Organics On a Minor Planet

In my previous Space Sunday update, I covered further studies into organic molecules found on the minor planet Ceres. Now a new study led by researchers from of the University of Heidelberg, Germany, reveals that Saturn’s moon Enceladus is spewing fairly complex organic molecules into space.

The study focuses on the icy geysers in Enceladus’ southern polar regions first imaged by the Cassini probe in 2005 and which give rise to an exceptionally tenuous atmosphere around the moon. Data indicated the material being vented was predominantly water vapour ice crystals containing some fairly simple organic compounds. The vapour most likely comes from a 10km deep ocean beneath the moon’s icy surface and which is kept in a liquid state in part by hydrothermal activity along the ocean floor.

Previous studies of the data on these plumes indicated the organic compounds within them had molecular masses mostly below 50 atomic mass units. However, using a revised approach, the new study reveals evidence of complex macromolecular organic material with masses above 200 atomic mass units. On Earth, such compounds, circulated in the waters of primordial seas by hydrothermal activity similar to that thought to exist on Enceladus, are thought to have played a vital role in the origins of life. While no-one is suggesting the compounds on Enceladus are direct evidence of primordial life in the waters there, the study does mark the first evidence of complex organic compounds existing on one of the solar system’s “icy water worlds”.

JWST Woes

On June 27th, 2018, NASA announced a further launch delay for the James Webb Space Telescope (JWST) – the third in its development. The flagship observatory will now not launch any earlier than March / April 2021. In addition, the project will break a pre-launch cost cap set by Congress.

The James Webb Space Telescope – launch delayed until at least March / April 2021. Credit: NASA

This delay comes just three months after it was announced JWST’s launch was to be delayed until May 2020. In 2017,  NASA had stated the telescope’s initial October 2018 launch would slip to June 2019.

The delays are said to have five root causes: human error, lack of experience in new technologies like the sun shield, embedded project problems, systems complexity and  excessive optimism on the part of the project managers. Human error alone (the use of the wrong solvent to clean the spacecraft’s valves, for example) have so far caused an 18-month delay to the project and added US $600 million to the total pre-flight bill.

The new time frame reflects a more cautious approach to completing the telescope’s testing and preparations for launch. It will come at a cost of a 10% over-run of the projects US $8 billion pre-flight budget, which will require Congressional authorisation; however, it is unlikely the project will be cancelled at this stage of its development.

Martian Dust Storm Now a Global Event

I recently wrote about the Martian dust storm affecting the operation of NASA’s Mars Exploration Rover Opportunity and which was being felt half a world away by the Mars Science Laboratory Curiosity rover.  Since then, the storm has continued to grow and is now considered to have a global reach – it is now, in official NASA parlance a PEDE, Planet-Encircling Dust Event, almost completely obscuring the planet’s surface features from observation from orbit and Earth.

Takes 15 days apart through the same telescope and viewing the same face of Mars. On the left, taken on June 8th and the storm started to rise, features such as Syrtis Major ( the dark India-shaped marking centre) are visible. On the right, taken on June 23rd, they are almost totally obscured by dust, as is the south polar cap (at the top of each image). Credits: Damian Peach (left) / Christopher Go (right)

It is the first PEDE to occur on Mars since 2007, but is considered much more intense in nature than that event. Due to the reduced amount of sunlight reaching the surface of the planet, the solar-powered Opportunity remains in a “sleep” mode to conserve power. However, there are slow-growing concerns that the intensity of this storm could prolong its duration and potentially impact Opportunity’s chances of recovery.

The storm is also likely to spoil the upcoming period of opposition – when Mars and Earth are at their closest to one another in their respective orbits – which occurs this July. Such opposition events usually affords excellent views of the planet’s surface features for many astronomers on Earth.

Two images from Curiosity’s Mast Camera (Mastcam) showing the impact of the dust storm on the sunlight reaching the surface of Gale Crater. The left image shows the “Duluth” drill site on Sol 2058 (May 21st); the right image is from Sol 2084 (June 17th). Both were imaged under the prevailing local daylight conditions and at the same time of day. Credit: NASA/JPL / MSSS

However, the storm is good news for those studying the complex weather on Mars. it has long been believed that these massive dust storms contribute to atmospheric losses and the intensity and likely duration of this storm, coupled with the availability of vehicles orbiting Mars is presenting scientists with an excellent opportunity to examine such events and their aftermath over an extended period of time.

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