Space Sunday: OSIRSIS-REx: sampling an asteroid

An artist’s impression of NASA’s OSIRIS-REx above the asteroid Bennu,with Earth added in the background for dramatic effect. Credit: NASA Goddard

After a period of delay, NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) is due to attempt the collection of a 60 gram (2.1 oz) sample from the surface of 101955 Bennu, a carbonaceous near-Earth asteroid, on Tuesday, October 20th.

Originally scheduled for August 2020, the attempt to gather the sample requires the space craft to slowly descend to within “touching” distance of the asteroid using a robotic arm. If successful, the sample gathering will open the door for OSISRIS-REx to complete the remainder of its mission before making its way back to Earth where the sample can be analysed.

Launched in September 2016, OSIRIS-REx is one of two such asteroid sample return missions currently in progress, the other being Japan’s Haybusha 2 mission (the original Hayabusha mission also returned samples from an asteroid – but they only amounted to around 1 milligram of material).

Having been launched well ahead of OSIRIS-REx, Hayabusha 2 is actually on its way back to Earth from asteroid 162173 Ryugu, with which it rendezvoused in June 2018. It spent 18 months surveying the asteroid, depositing four micro-rovers on its surface before gathering samples blown off of the asteroid by the force of a kinetic impactor (think bullet), allowing it to collect a mix of surface and sub-surface material. Currently, Hayabusha 2 will deliver its cargo back to Earth during a fly-by on December 6th, 2020, after which it may be tasked with a further sample return mission.

Image sequence showing the rotation of Bennu, captured by OSIRIS-REx at a distance of around 80 km. Credit: NASA Goddard

OSIRIS-REx reached it’s target, Bennu, at the very end of December 2018 and has spent most of the intervening time studying the asteroid in detail. Both Bennu and Ryugu are of interest to scientists for a number of reasons: they are both part of a class of asteroids that are believed to have been around since the formation of the solar system, and so they could help us learn more about that period.

Both are also in the Apollo asteroid group, meaning they routinely cross Earth’s orbit, and thus present a potential collision risk, and at 1 km diameter for Ryugu and just under 1/2 a km for Bennu, an impact from either would not be a Good Thing for Earth. So, another reason for sampling them is to determine their composition (and by extension, allow us to draw conclusions about the composition of other large Apollo asteroids) that may help make a determination of how to deal with them should that threat of impact become real (in fact, there is a chance that Bennu in particular might impact Earth between 2175 and 2199).

Finally, samples from both might offer clues as to how life-forming materials reached the surface of Earth.

Bennu has proven particularly intriguing for scientists. For one thing, it has proven to be entirely unlike anything that had been anticipated; rather than being relative smooth, with crater pits and sand-like regolith (surface material), Bennu revealed it is a boulder-strewn place with rocks in places comparable to mountains relative to its size, many of them placed so closely together, any attempt to gather samples near them would like result in a loss of the vehicle. This required a more extensive survey to determine potential sample sites, with five initially being identified, before these were narrowed to two, the primary, Nightingale, and a back-up.

This image, captured by OSIRIS-REx shows the Nightingale sample gathering crater surrounded by large boulders. The silhouette of the spacecraft (which is about the size of a van – excluding the solar panels and has a mass of just over 2 tonnes) added for comparison. Credit: NASA Goddard

The asteroid also demonstrated it can emit plumes of material from within itself when in the “warm” part of its 1.2 year orbit around the Sun. However, one of the most surprising discoveries was the identification of six bright boulders on the asteroid’s surface which, when subjected to spectroscopic analysis, revealed themselves to be of the same materials as boulders on Vesta, the second-largest asteroid in the solar system, surveyed by the NASA / ESA Dawn mission.

It’s believed that the presence of these rocks indicates that Bennu started life as part of a larger body – an asteroid or planetesimal – within the asteroid belt beyond Mars, where it was in collision with a fragment of Vesta, depositing material from the latter on its surface. That event, or another similar collision, led to a “catastrophic disruption” within Bennu’s parent, creating Bennu itself and sending it on its way into the inner solar system to be caught in an orbit much closer to the Sun.

The asteroid has also revealed itself to be particularly rich in carbon-bearing material, which can tell us how much water it may have contained (and how much might still be present as sub-surface ice). What is particularly interesting here is that many of the boulders on Bennu contain mineral veins composed of carbonate – which on Earth often precipitates from hydrothermal systems that contain both water and carbon dioxide. Some of these rocks are located around the Nightingale sample  recovery area. The presence of such carbonate strongly suggests that Bennu’s parent body, whether asteroid or small planetary body,was likely hydrothermally active. This has in turn given rise to the prospect that any sample returned by OSIRIS-REx might contain organic material.

The main reason Nightingale was selected for sample gathering however, is it that geologically speaking, it is quite young and pristine, and hasn’t been exposed to the harsh impact of solar radiation as much as other parts of the asteroid, and so may yield further secrets about Bennu – potentially further increasing the opportunity for organics (which might otherwise breakdown due to solar radiation) to be gathered in a sample.

The view through the sample camera (SAMCAM) on OSIRSIS-REx as it carries out a sample gathering rehearsal in April 2020. A further rehearsal was also carried out in August 2020. Credit: NASA/Goodard

For the sample gathering on October 20th, OSIRIS-REx will slowly descend from an altitude of some 770 metres above the asteroid, minimising the use of its thrusts once in motion to prevent contaminating the surface below it. For the descent, the satellite’s solar arrays will be raised into a Y-configuration to minimise the risk of them being affected by dust accumulation or even contact with the asteroid, should the vehicle be at risk of “toppling” at contact with the asteroid.

Contact will be made via a 3.35m long articulated robot arm called TAGSAM (Touch-And-Go Sample Acquisition Mechanism). On contact with the asteroid, a mechanism  in the arm will absorb the impact, initiating a 5-second period of contact. In this time, the cylindrical sampler head at the end of the arm will fire a jet of nitrogen gas directly at the ground beneath and hopefully capture any material up to 2cm across within itself.

At the end of 5 seconds, the energy absorbed by the TAGSAM arm will be released, gently pushing OSIRIS-REx away from Bennu. It will then back away for a short distance before initiating tests (via camera and a sensor) to confirm the amount of material captured.

If this proves to be too little, the capture process will be repeated – the capture mechanism carries enough nitrogen for three such attempts. If it is determined the issue is with the material at Nightingale being to bulky to capture, ORSIRIS-REx may be told to suspend operations and then relocate to Osprey, the selected back-up sample site. However, should the sample gathering be successful, TAGSAM will deposit the sampler head inside the Sample-Return Capsule (SRC) on OSIRIS-REx, where it will be stowed until the vehicle makes its return to Earth.

Overall, the entire sample gathering process should take around 4.5 hours, from the commencement of the slow, gentle to the 16m diameter sample site, and back to its 770m parking altitude.

Due to orbital mechanics, OSIRIS-REx will not be able to start back to Earth until  March 2021, and it  will pass Earth on Spetember 24th, 2023, when the SRC will be detached to parachute down for recovery at the US Air Force Test and Training Range in Utah. The samples gathered will then be shared with Japan and the mission’s core science partners from Canada, France, Germany, the United Kingdom, and Italy, and then with other organisations that request samples.

Three Raptors Nesting Under a Skirt

Tweeted by SpaceX CEO Elon Musk, this image reveals 3 atmospheric Raptor motors (the vacuum motors have much wider exhaust bells) mounted under Starship prototype SN8. The four square blocks with stubby cylinders attached visible around the vehicle’s skirt are the landing legs in their stowed position. Credit: Elon Musk / SpaceX

SpaceX is chasing down the first Starship hop to 15km altitude. Following the tank pressurisations tests for prototype SN8 I reported on in my previous Space Sunday article, the company has now mounted three atmospheric Raptor engines to at the base of the vehicle.

This led to speculation that SN8 could undergo one or more static firings of the engines during the past week, readying them for flight. Indeed, SpaceX filed road closure notices around the Boca Chica site for the 14th, 15th and 16th October,  actually performing what appeared to be fuelling operations on the nights of the 14th and 15th. However, no actually firings took place, and the notice for the 16th October was cancelled. Instead, new notices were posted for October 18th – so by the time you read this, a static fire test may have actually taken place.

SN8 apparently fuelled and venting excess pressure through the Raptor side vents in what appeared to be preparations for a static fire engine test on October 14th, although the motors were not actually used. Credit: Mary the BocaChicaGal

Betelgeuse: No Supernova, but Closer than Thought

A new study has further confirmed that – despite the hopes of some due to its recent behaviour – the super-giant variable star Betelgeuse isn’t close to going supernova, and in doing so, is causing a couple of facts about the star to be revised.

The speculation that the star may have exploded came as Betelgeuse went through a dramatic period of dimming in the latter part of 2019, losing between 25%-30% of its natural brightness. A variable star by nature that does periodically dim, the 2019 event was nevertheless unusual,  although most astronomers stated it would resume itself normal brightness in due course – as it did at the start of 2020.

The dimming of 2019 was eventually traced to two factors. the first was a convergence of the star’s two dimming cycles (one lasting 25 years, the other 425 days) that naturally made Betelgeuse look dimmer. The second was a large cloud of dust and material originally ejected by the star as hot plasma earlier in the year coming between Betelgeuse and Earth, making the star appear even dimmer in out skies and giving it an odd shape when viewed via telescopes.

The new study was an attempt to understand why, as well as dimming and brightening, Betelgeuse appears to pulsate. Using hydrodynamic and seismic modelling, the international team involved in the study found that for some reason the core of the star is emitting pressure waves analogous to sound waves, and these are cause the pulsations – although what is causing the pressure waves has yet to be determined.

In addition, the study appears to suggest two surprising things: that Betelgeuse could be up to 1/3 smaller than past estimates, and as a result, as much as 25% closer to us than previously believed (about 530 light years away). This is still not close enough to make it a threat were it to go supernova, but as a further assurance, the study suggests Betelgeuse is still burning helium, which means any  supernova won’t happen for another 100,000.

Orionids

Where to look for the Orionids meteors.  Credit: Starrynight.com

And staying with Betelgeuse – sort of – if you have clear skies near you on the 20th / 21st October, you might be able to catch sight of the Orionids meteor shower as it reaches its peak for 2020.

The best time to look is between 01:00-0:200 local time, and they can be seen in the direction of Orion (if above your horizon), which lies to the east-south-east in the northern hemisphere. Just look for the red eye of Betelgeuse and then a little more to the left and up to reach the radiant (the point at which meteors appear to originate).

Believed to be the result of the Earth passing through debris from the tail of Halley’s Comet, they tend not to be as numerous as the Perseids, but can reach around 20-25 an hour during their peak period.

 

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