Space Sunday: moles, asteroids and a high speed planet

InSight’s scoop hovers over the HP3 mole. Credit: NASA / JPL

It’s now close to 15 months since NASA’s InSight lander arrived on Mars (see Space Sunday: insight on InSight for an overview of the mission and Space Sunday: InSight, MarCO and privately to the Moon for more on the mission and InSight’s Mars arrival). In that time the lander has completed a lot of science, but one thing has remained an issue: the HP³ experiment.

This is one of two surface experiments InSight placed on Mars, and comprises a base module and a long, slender self-propelled probe called the “mole”, designed to “burrow” its way down into the the sub-surface to a depth of up to 5 metres, towing a sensor-laden tether behind it designed to measure the heat flow from the planet’s interior. The mole has an internal hammering mechanism that is designed to drive it deeper into the ground, but this relies on friction against the material forming the walls of the hole it is creating – and this hasn’t been happening.

After a good initial start, the probe came to a halt with around 50% of its length embedded in the soil. At first it was thought it had hit solid bedrock preventing further motion; then it was thought that the mole was gaining insufficient traction from the hole walls, on account of the fine grain nature of the material it was trying to move through.

The HP3 “mole” showing the spring mechanism and “hammer” it drives into the ground. Credit: DLR

By mid-2019, engineers thought they had a solution: use the scoop at the end of the lander’s robot arm to compact the soil around the lip of hole in the hope of forcing sufficient material into the hole it would provide the traction the probe needed to drive itself forward. When this failed, the decision was made push the scoop directly against the side of the probe, pinning it between scoop and hole wall to again give the probe the traction it needed.

Initially, this approach worked, as I noted in Space Sunday: a mini-shuttle, Pluto’s far side & mole woes, but then the mole “bounced back”. Since then, the probe’s progress has been a case of “three steps forward, two steps back”, making some progress into the ground and then bouncing back – a source of much frustration among the science team.

After a year with the mole more-or-less “stalled”, mission engineers have decided to take more direct action. The decision has been made to try to “push” the probe using the robot arm’s scoop. This means placing the scoop on the top end of the mole – an approach that has so far been avoided out of concerns to might damage the sensor tether as it emerges from the same end of the probe. However, in manipulating the lander’s robot arm and its scoop over the course of a year, engineers are confident they can avoid harming the tether.

This latest effort to get the mole into the surface will take place in late February / early March. If it is successful, the team may revert to using the scoop to once again compress the sides of the probe’s hole to try to provide it with further traction as it continues to dig down into the subsurface material. Should the attempts fail, it’s unclear what might be tried to get the mole moving again; the mission team admitting they have “few alternatives” left to try.

How to Deflect an Asteroid

On April 13, 2029, an asteroid in the region of 370 metres in length and 45 metres across will pass by Earth at 30 km/s no further away from the planet’s surface than some of our geostationary satellites.

Called 99942 Apophis, an object I’ve written about in past Space Sunday articles, it is one of a large number of potentially hazardous objects – asteroids larger than 140 m in length that in crossing the Earth’s orbit as both they and the planet go around the Sun, pose a potential risk of one day colliding with us, with potentially devastating consequences. When it was   discovered in 2004, initial tracking of Apophis suggested it could collide with Earth in 2029. Further observations of the object showed this would not happen, not would it do so the next times it passes close to Earth in 2036, 2068 and 2082.

Extinction level event: a very large asteroid impact. Credit: Anselmo La Manna/YouTube screenshot

Which is not to say Apophis or 101955 Bennu, or one of the many other PHOs – Potentially Hazardous Objects – that are being tracked might one day strike Earth. The tipping point for such a collision comes down to such an object passing through, or close to, it’s gravitational keyhole. This is a tiny region of space – perhaps only 800 metres across – where gravitational influences – notably that of Earth – are sufficient to actual “pull” an objects course onto a collision with Earth.

Currently, plans to try to prevent such an impact revolve around identifying when an object has passed its particular keyhole, making an collision inevitable. There’s a reason for this: identifying where an objects keyhole might lie isn’t a precise science, and relies on scientists known an awful lot, including things like the size, mass, velocity and composition of these objects, what forces might be at work to influence their orbit, and so on. However, by leaving things until after an object has passed its keyhole means the time available to try to divert it is relatively short, perhaps months or just a couple of years or so, leaving very little time to plan and execute a mission to prevent any such collision.

Better then, to identify when an object is liable to pass close enough to its keyhole that it it will be drawn into a collision path. This would provide a far greater lead time for planning how to deal with it. This is exactly what a team of MIT researchers are suggesting in a part that also defines a framework for deciding which type of mission would be most successful in deflecting an incoming asteroid.

A keyhole is like a door—once it’s open, the asteroid will impact Earth soon after, with high probability

People have mostly considered strategies of last-minute deflection, when the asteroid has already passed through a keyhole and is heading toward a collision with Earth. I’m interested in preventing keyhole passage well before Earth impact. It’s like a pre-emptive strike, with less mess.

– MIT study co-author, Sung Wook Paek

Apophis compared to the Empire State Building. It is not of a size to cause an extinction level event were it ever to collide with Earth, but it would result in a disaster on a massive scale. Credit: unknown

This does require identifying an object’s particular keyhole – but scientists are getting better at doing this through the medium of repeated observations of PHOs and studying them. Apophis and Bennu have had their keyholes recorded.

the MIT study notes that identifying where an object’s keyhole lies and when the object might come close to it to be pushed into a collision course with Earth serves three purposes.

Firstly, and as noted, it could greatly extend the time in which any potential deflection mission can be planned and executed. For example, waiting until after an object has passed its keyhole might only offer a year or so to try to deflect it; knowing that the object will encounter it keyhole half-a-dozen years hence, presents all of that time in which to execute a deflection mission.

Secondly, knowing where an object’s keyhole sit, and when the object might encounter it also allows scientists to potentially determine “safe harbours”  – locations within the object’s object around the Sun where a deflection mission would be more effective than in others.

These “safe harbours” are important, as the further away from Earth an object can be intercepted , then potentially the less force is required to deflect it. Thus, things like the Hollywood favourite for this type of mission  – the use of nuclear weapons – can be avoided. In this, the study points towards the use of kinetic deflection: firing a payload at the object so it impacts at a certain point and at certain velocity, its momentum pushes away from any path of impact.

In this, the study suggests three kinetic impactor mission types.

  • A “type 0” mission, using a single, heavy payload is launched at the object, using the best available data gathered on it, to try and deflect it with a single hit. However, as data specific to the object: its composition, density, etc., would likely only be best guesses, there is a not insignificant risk of failure.
  • A “type 1” mission, that would be in two stages: first, a scout mission would be launched to the object to gather precise information about the object’s composition, density, etc. This data would allow type and size of interceptor to be determined, together with the identification of its best point of impact and the velocity it would need when striking the object.
  • A “type 2” mission, that would be in three parts: a scout vehicle and interceptor launched lose together. The scout would perform the same function as for the “type 1” mission, and help guide the interceptor into the object. Data from both the scout and the initial impact would then be used to fine-tune a further impactor mission to fully deflect the object.

The team’s research offers a potential roadmap for more effectively dealing with the threat of known Earth-crossing NEOs, although they acknowledged there are likely to be many more small objects which, although not a global, national or regional threat should they strike Earth, could still do severe damage on a localised – city or smaller – level.

Currently, the MIT team are now working to further refine their work on further options for deflection – such as instead of relying on just one or two impactors, using a series of smaller, faster vehicles that would strike an object like a stream of bullets, each one only having a small effect on the object course, but which are cumulatively enough to push the object off of its Earth threatening course.

Exoplanet with An 18-hour Year

The Next Generation Transit Survey (NGTS) and the UK’s Warwick University are continuing to build up an impressive track record of unusual exoplanet discoveries. I last wrote about both in 2019 (see Space Sunday: ExoMars, a magic movie and a “forbidden planet”), reporting on the discovery of NGTS-4b, one of a number of strange, new worlds the university’s astronomers have discovered using the automated observatory.

Since then, the team have found several more of these massive planets orbiting in extreme proximity to their parent star. However, none come close to NGTS-10b, regarded as the most extreme large exoplanet yet discovered, and which has now been confirmed to be sitting on the edge of its destruction.

The 12 20cm telescopes of the NGTS array and members of the operating staff of the European Southern Observatory (ESO), Chile. The ESO Very Large Telescope can be seen on the hill in the distance. Credit: G. Lambert

The planet is some 20% bigger than Jupiter and has roughly twice the mass. It is orbiting a K5 V main sequence star called NGTS-10, a star around 70% the radius of our Sun and 1000 degrees cooler. NGTS-10b is actually so close to its parent – some 27 times closer than Mercury is to our own Sun – that it completes an orbit once every 18 hours. That makes it the closest “hot Jupiter” class of exoplanet to its parent, and the fastest orbiting exoplanet over all, yet discovered.

“Hot Jupiters” are massive, gaseous bodies sitting very close to their parent star that appear to be very rare: of the 4,000+ exoplanets so far confirmed, only 337 are classified “hot Jupiters”, and only 6 of these  have orbits of less than 24 terrestrial terrestrial hours.

At around 1,060 light years from Earth, NGTS-10b is a prime candidate for study by the James Webb Space Telescope, when launched, as it offers an excellent opportunity for the study of extreme atmospheric conditions in an gaseous exoplanet. It’s liable to be a place of extreme atmospheric conditions, as it is most likely tidally locked to its star. This means it always keeps the same face towards the star, resulting in it being super-heated, which the far side of the planet is darkness, and subjected to extremely low temperatures.

However, what is particularly interesting for astronomers is that NGTS-10b sits right on the edge of its Roche limit – the point at which the tidal forces of the star exceed the planet’s own force of gravity, marking it as the point at which the planet starts to break apart.

An artist’s impression of  “hot Jupiter” planet orbiting in close proximity to its orange main sequence parent. Credit: unknown

This isn’t going to happen ant time soon – the Warwick team calculate the planet’s orbital period is deteriorating by 7 seconds a year, so it’s unlikely to start breaking up for around another 36 million years.

However, studying its interaction over the next several decades will reveal if the planet is starting its final spiral in towards its parent, and allow direct measuring of the rate of any infall, allowing sstronomers to place tighter constraints on the efficiency of tidal interactions between stars and planets.

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