Space Sunday: Venus’ transformation, SLS and an asteroid

An artist’s depiction of Venus evolving from a potentially habitable water world to the hot desert it is today.
Credit: NASA Goddard

Venus has been the subject of a number of recent studies, one of the most intriguing of which suggests it’s runaway greenhouse effect was started by what might at first seem an unlikely candidate: Jupiter.

Our solar system is a place of mysteries, both in itself and in comparison with many exoplanet systems. While of the latter have Jupiter-size worlds, unlike our solar system, these tend to be found fairly close in to their parent planet (although there are some exceptions). It’s believed that these planets actually formed further out from their parent stars, but then migrated inwards under gravity, until a point of equilibrium / resonance was reached.

This idea of planetary migration has led to theories on how our own solar system may have developed early it its life, with one of them in particular being of interest here. Called the Grand Tack Hypothesis, it suggests that Jupiter likely  formed some 3.5 AU from the Sun (1 AU = the average distance separating the Earth from the Sun) – or about 1.5 AU closer to the Sun that its present orbit. During the initial evolution of the solar system, it gradually migrated closer to the Sun, perhaps getting as close as 1.5 AU –  a little further out from the Sun than the present orbit of Mars, before the combined gravities of the other outer planets – most notably Saturn – gradually teased it back outwards again, until that point of equilibrium / resonance was reached, leaving them all in the orbits we see today.

What is particularly interesting about the Grand Tack Hypothesis is that it accounts for a number of inconsistencies visible in the solar system today if it is assumed the planets all formed more-or-less in their current orbit sand never shifted very far from them. These can be summarised as:

  • Why is Mars so small when compared to Earth and Venus? If the planets all formed within or close to their current orbits, then most models built around this idea result in Mars being of a comparable size to Earth.
  • Why don’t we see a “super Earth” (or “mega Mars”) between the orbits of Earth and Jupiter? Again, models based on all the planets  forming in their present day orbits around the Sun indicate that there would have been sufficient accretion disk material in the region of Mars for a solid planet 1.5 times the mass of Earth (or greater) to have formed.
  • Why is the asteroid belt so relatively uniform? Again, if Jupiter formed 5-5.5 AU from the Sun, then the material within the accretion disk should have resulted  not only in a Earth-size Mars, or a possible “mega Mars”, but should also have resulted in the formation of many more planetismals or “mini Marses” forming, smaller than Mars as weknow it today, but potentially somewhat larger than the likes of Vista and Ceres within the asteroid belt.
Many models of solar system suggest that, had it not been for Jupiter migrating from it’s point of initial formation in towards the Sun and then back out again, our solar system would look very different today, with an Earth-sized (or larger) “mega Mars” and an asteroid belt that includes multiple “mini Marses”. Credit: Sean Raymond

A migration of Jupiter towards the Sun accounts for the first two of these inconsistencies in much the same way: as it moved in towards the Sun, Jupiter both “ate” a lot of the material of the accretion disk sitting between the current orbits of Earth and Mars and also “pushed” some of it inwards, helping in the eventual formation of both Earth and Venus.

Then as it reversed course, it “ate” more of the debris, whilst pushing some of it away. Thus, the inward and outward movements of Jupiter left only sufficient material occupying the area of Mars’ orbit to accrete and form a relatively small rocky world. By the same measure, this pushing / absorbing of material within what would become the asteroid belt meant that material was much more widespread and unable to accrete sufficiently in order to create multiple “mini-Mars” planetismals.

But what does this have to do with starting the extreme greenhouse effect on Venus? The answer to this is quite complex.

In effect, Jupiter’s motion inwards not only pushed material towards the Sun that helped Earth and Venus to form, it also became sufficiently close to them both to encourage them into exaggerated elliptical orbits around the Sun, which at the time was somewhat cooler and dimmer than it has been for most of its adult life. Thus, Venus likely formed within what was then the Sun’s habitable zone, allowing an abundance of liquid water to form across the planet’s surface, whilst its elliptical orbit meant it experienced significant seasonal changes during the course of it’s “year”.

In particular, whilst “close” to the Sun, during its “summers”, the ocean-rich Venus would be subjected to greater amounts of evaporation of water from its oceans, which would in turn be subject to greater amounts of UV radiation. This radiation would split the water vapour into elemental oxygen and hydrogen, with the latter easily stripped away from the planet’s atmosphere by the solar wind, leaving the oxygen to combine with carbon to form carbon dioxide, generating what is called a “moist greenhouse” effect.

Jupiter’s migration would likely have helped Venus initially develop into a substantially wet world – which in turn likely started it on its way to having the runaway greenhouse effect we see today. Credit: NASA

At the same time, the mass of water in the Venusian seas gave rise to a process called tidal dissipation which over time, gradually “dampened” Venus’ exaggerated orbit around the Sun, allowing it to be pulled into the kind of circular orbit it has today, eliminating the seasons whilst holding the planet closer to the Sun than it had been, further increasing temperatures and accelerating the moist greenhouse effect. This in turn would be aided but the Sun increasing in it radiative output, further accelerating the greenhouse effect as more and more water evaporated from the surface oceans, until the point of no return was reached.

A further result of the Sun’s increasing outflow of heat meant that its habitable zone was pushed outwards to encompass the Earth – but being that much further away from the Sun and under a greater influence of the gravities of the outer planets, Earth didn’t suffer either form that initial kick into a moist greenhouse effect, allowing it to maintain its seasons, and remain a more comfortably warm, wet planet.

It’s not 100% certain Jupiter’s migration was the kickerstarter for Venus’ greenhouse effect. There are, for example other mechanisms that may have dampened Venus’ orbital eccentricity without the influence of a massive planet like Jupiter – such as Milankovitch Cycles. But given the way the Grand Tack Hypothesis helps explain a good deal about the early solar system, it seem likely it may well have been responsible. And if it is correct, it has significant implications for any Venus analogues orbiting other stars and our understanding of the mechanisms at work in the development of exoplanets.

NASA Increases SLS Cost Estimates, Programme Still “On Course”

Kathy Lueders, NASA associate administrator for human exploration and operations, said the agency was moving ahead with SLS development with the goal of a first launch of the heavy-lift rocket no later than November 2021 – but also indicated that cost estimates for the programme would be increasing.

On August 24th, 2020, Lueders announced that while work had been suspended due to the approach of two tropical storms, Laura and Marco, the agency’s Stennis Space Centre, Mississippi, was still on course to complete the green run test of the first SLS rocket core stage in October.

The first core stage of the SLS booster – CS1 – is rolled out of its assembly building at Michoud Assembly Facility, ready to be transferred to the Pegasus transportation barge that would take it by river to the Stennis Space Centre, January 2020. Credit: NASA / Eric Bordelon

This is a crucial test that sees all four of the RS-25 engines of the core stage fired and operated for the full 8 minutes they will operate across during a launch, and throttled in accordance with a typical flight profile. This allows the entire structure to experience the full stresses of an actual launch, and allows engineers to ascertain how well it stood up to them, and what might be required by way of improvements / alterations to ensure the stage is ready for flight operations.

Assuming the stage passes the test – and those it must complete in the run-up to the firing – , and assuming not significant issues are revealed in post-test inspections, the way should be clear for the core stage to be transferred to Kennedy Space Centre at the start of 2021, where it will undergo further tests and then integration with the rest of the vehicle ready for the uncrewed Artemis-1 mission, around the Moon.

Followig this announcement, on August 27th, Lueders confirmed NASA has increased the development cost estimates for both the SLS launch system and the associated Exploration Ground Systems (EGS) required to support launch operations. The new estimates put initial SLS development costs at US $9.1 billion (up from US $$8.7 billion is the last available estimates from the Government Accountability Office (GAO), and an EGS estimate of US $2.44 billion, up from US $2.33 billion.

The CS-1 core stage on the test stand at Stennis Space Centre. Credit: NASA

While not huge increases in themselves, they do mean that both programmes are now well in excess of their threshold for triggering a formal budget review by Congress, although it is unlikely either the House or the Senate will insist on any major changes to the programme as a result of the development cost increases. If there is to be a fight over funding, this is more likely to be over the Human Landing System (HLS) programme for the development of the vehicles never to deliver crews to, and return them from, the surface of the Moon. As I recently reported, under the 2021 federal budget proposal from the House, HLS will only receive some 20% of its requested funding, while the Senate has yet to confirm its proposed budget for NASA, including HLS.

Asteroid 2018 VP₁ May Hit Earth in November, but its NOT the End of the World

The end of August saw a lot of people getting all a-Twitter (literally) over the possibility of an asteroid striking the Earth on November 2nd, 2020.

The asteroid in question is an Apollo asteroid called 2018 VP₁, first spotted on November 3rd, 2018. Apollo asteroids tend spend most of their time beyond Earth’s orbit. However, as they pass through perihelion  – their closest approach to the Sun – some can cross our orbit around the Sun, and thus present a risk of collision.  Such is the case with 2018 VP₁.

One of the more dramatic images used to illustrate 2018 VP₁’s arrival in November 2020. The reality is very different. Credit: unknown.

While astronomers have been able to track the asteroid on a number of times, allowing them to fix its orbital period to a little more than 2 years, they have done so with a margin of error of +/- 12 hours. This means that as the asteroid reached its closest point to Earth on November 2nd, it could either pass by us harmlessly at a distance of up to 3.7 million kilometres – or it could slam into the atmosphere (although the chances of this happening stand at just 0.41%).

But even if it does hit that margin of error and end up occupying the same part of space as Earth and at the same time, we’re hardly in for a cataclysmic event.

To explain: in 2013, an asteroid some 20 metres in diameter entered the Earth’s atmosphere to eventually break apart in an air burst some 25-30km above the city of Chelyabinsk, Russia. While the shockwave from the meteor’s break-up was sufficient enough to smash windows and cause some 1,500 reported injured (the majority of which were hearing related or the results of broken glass), there were no fatalities or actual lasting damage or environmental harm.

While 2018 VP might generate a visually stunning fireball should it enter Earth’s atmosphere, just as the 2013 Chelyabinsk meteor did, but is unlikely to cause any of the damage or injuries seen with that incident. Credit: unknown

2018 VP₁, however, isn’t a Chelyabinsk size of object. It’s actually some 10 times smaller in size – just 2 metres across, and has a mass of between 100 and 1,000 times less than the Chelyabinsk superbolide. This means two things should it actually strike the Earth’s atmosphere. The first is that it is mostly likely to almost entirely burn-up during entry, and level only dust and tiny fragments to eventually fall to Earth.

The second is that even if it should several initial entry into the atmosphere, such is its small size, it will break up at a much great altitude then the Chelyabinsk  object, and with far less dramatic effect. It will, however, provide a potentially spectacular fireball as it passes through the upper atmosphere, particularly if it occurs at night and is captured on camera, either intentionally through something like the Global Fireball network, or by chance by someone with a camera (or even by security cameras, as happened a couple of years ago with an asteroid over Australia). But other than that, there really is no cause for alarm.

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