Posted on by Steve Nerlich

It’s Not Just The Astronauts That Are Getting Older

Representing what may be the first long term lunar environmental impact study, recent laser ranging data from the Apache Point Observatory in New Mexico suggests the Lunar Ranging Retro Reflectors (LRRRs) left on the Moon by Apollo missions 11, 14 and 15 are beginning to shows signs of age.

Apache Point Observatory’s Lunar Laser-ranging Operation (the acronym says it all) has been collecting ranging data from the LRRRs since 2006, using a 3.5 metre telescope and a 532 nm laser.

A typical APOLLO observing session involves shooting the laser at the largest of the LRRRs (Apollo 15’s) over a ‘run’ of four to eight minutes. Each shot sends about 1017 photons to the Moon, from which only one returned photon per shot may be detected. This is why the laser is shot thousands of times at a 20 Hz repetition rate during each run.

If the return signal from the Apollo 15 LRRR is good, the laser is then directed to fire at the Apollo 11 and 14 reflectors. The laser can even be directed to the Russian Lunokhod 2 reflector, landed on the Moon in 1973, although this reflector does not return a reliable signal if it is in sunlight, probably because heating affects the reflectors’ refractive index and distorts the return signal.

Lunokhod 2 (moon walker in Russian), an 840 kg rover that landed on the Moon on January 15, 1973 and undertook scientific investigations on the lunar surface until May 1973.

The Apollo LRRRs were designed to remain isothermal, even in direct sunlight, to avoid the problem apparently suffered by Lunokhod 2. But a review of current and historical data has revealed a noticeable decline in their performance at each Full Moon. Since the reflectors are directed straight at Earth, they experience the most direct sunlight at a Full Moon.

Recent Apache Point Observatory data has been compared to historical data collected by earlier observatories involved in lunar laser ranging. For the period 1973 to 1976, no Full Moon deficit was apparent in data records, but it began to emerge clearly in a 1979 to 1984 data set. The research team estimate that return signal efficiency at Full Moon has degraded by a factor of 15 over the approximately forty years since the Apollo reflectors were placed on the Moon.

While heating effects may play a part in the performance degradation of the LRRRs, lunar dust is suggested to be the more likely candidate, as this would be consistent with the very gradual performance degradation – and where the most substantial performance loss occurs right on Full Moon. These findings may require careful consideration when designing future optical devices that are intended to remain on the lunar surface for long periods.

On the bright side – all the reflectors, including Lunokhod 2’s, are still functioning on some level. Hopefully, decades before their slow and steady decline progresses to complete failure, even more efficient replacement devices will be landed on the lunar surface – perhaps carefully positioned by a gloved hand or otherwise by robotic means.

This article was developed from this very readable scientific paper.

Posted on by Steve Nerlich

Astronomy Without A Telescope – The Hitchhikers Guide To The Solar System

Short on fuel, but good at astrophysics? It is possible to tour the solar system on less than 30 Altairian dollars a day by using the Interplanetary Transport Network (ITN).  

The ITN is based on gravity assist manoeuvres and low energy transfer orbits around and between Lagrange points. Using the ITN, it is theoretically possible to tour the solar system with an exceedingly economic use of fuel as long as you have an abundance of patience and don’t mind taking an often circuitous route to your destination.  

If you imagine the whole solar system as a rubber sheet which is warped by gravity wells, then the planets are really just small depressions of different depths pressed into the sides the Sun’s overarching gravity well.  

What’s important to this story is that the edges of those small depressions are nearly flat with respect to the otherwise steep slopes created by the Sun and the planets. It takes a lot less energy to move around on these flat edges, than it does trying to climb straight up the steep slopes.  

The flat edge that is present around the Earth’s gravity well is land marked by Lagrange point 1 (or L1) lying directly between the Sun and the Earth – and Lagrange point 2 (L2) on the opposite side of the Earth directly away from the Sun.  

It’s possible for a spacecraft to orbit a Lagrange point and be carried around the Sun with very little expenditure of energy. It’s because you are essentially riding the bow wave of the Earth as it orbits the Sun – so you are carried the Sun at the same orbital speed as the Earth (30 kilometres a second) without having to burn a lot of fuel in the process.

Also the Lagrange points represent junction points to enable low energy transfer between different planetary orbits. As though the solar system’s space-time curvature makes for a giant skateboard park, it’s possible to step off L1 and follow a trajectory down to Venus – or you can coast across the flat edge of Earth’s gravity well for about 3 million kilometres to L2 and then step off on a long winding path to the L1 of Mars. Here you might rest again before perhaps shuffling across to Mars’ L2 and then on to Jupiter.  

Mathematical analysis of the gravitational interactions between three or four bodies (say, your spacecraft, the Earth and the Sun – and then add Mars too) – is complex and has some similarities with chaos theory. But such an analysis can identify interconnecting pathways right across the solar system, which ITN proponents refer to as ‘tubes’.  

The image on the left (Credit: American Scientist) shows an ITN ‘tube’ approaching Earth’s L2. At this point a cosmic hitchhiker can either double back on a trajectory towards Venus (red line), stay in orbit around L2 and tag along with Earth– or continue on through (blue line), perhaps entering another ITN tube on the way to Mars. The image on right shows a tongue-in-cheek depiction of the ITN tube network (Credit: NASA).

ITN principles have been adopted by a number of spacecraft missions to conserve fuel. Edward Belbruno proposed a low energy lunar transfer to get the Japanese probe Hiten into lunar orbit in 1991 despite it only having 10% of the fuel required for a traditional trans-lunar insertion trajectory. The manoeuvre was successful, although travel time to the Moon was five months instead of the traditional three days. NASA’s Genesis mission and the ESA’s SMART-1 are also considered to have used low energy ITN-like trajectories.  

So impoverished hitchhikers, maybe you can still have that grand tour of planets by using the ITN – but make sure you pack a towel, it will be a very long trip.

(Recommended reading: Ross, S.D. (2006) The interplanetary transport network. American Scientist 94(3), 230–237.)

Posted on by Steve Nerlich

What Can The (Dark) Matter Be?

What better place to look for dark matter than down a mine shaft? A research team from the University of Florida have spent nine years monitoring for any signs of the elusive stuff using germanium and silicon detectors cooled down to a fraction of a degree above absolute zero. And the result? A couple of maybes and a gritty determination to keep looking. 

The case for dark matter can be appreciated by considering the solar system where, to stay in orbit around the Sun, Mercury has to move at 48 kilometers a second, while distant Neptune can move at a leisurely 5 kilometers a second. Surprisingly, this principle doesn’t apply in the Milky Way or in other galaxies we have observed.  Broadly speaking, you can find stuff in the outer parts of a spiral galaxy that is moving just as fast as stuff that is close in to the galactic centre. This is puzzling, particularly since there doesn’t seem to be enough gravity in the system to hold onto the rapidly orbiting stuff in the outer parts – which should just fly off into space. 

So, we need more gravity to explain how galaxies rotate and stay together – which means we need more mass than we can observe – and this is why we invoke dark matter. Invoking dark matter also helps to explain why galaxy clusters stay together and explains large scale gravitational lensing effects, such as can be seen in the Bullet Cluster (pictured above). 

Computer modeling suggests that galaxies may have dark matter halos, but they also have dark matter distributed throughout their structure – and taken together, all this dark matter represents up to 90% of a galaxy’s total mass. 

An artist's impression of dark matter, showing the proportional distribution of baryonic and non-baryonic forms (this joke never gets old).

Current thinking is that a small component of dark matter is baryonic, meaning stuff that is composed of protons and neutrons – in the form of cold gas as well as dense, non-radiant objects such black holes, neutron stars, brown dwarfs and orphaned planets (traditionally known as Massive Astrophysical Compact Halo Objects – or MACHOs). 

But it doesn’t seem that there is nearly enough dark baryonic matter to account for the circumstantial effects of dark matter. Hence the conclusion that most dark matter must be non-baryonic, in the form of Weakly Interacting Massive Particles (or WIMPs). 

By inference, WIMPS are transparent and non-reflective at all wavelengths and probably don’t carry a charge. Neutrinos, which are produced in abundance from the fusion reactions of stars, would fit the bill nicely except they don’t have enough mass. The currently most favored WIMP candidate is a neutralino, a hypothetical particle predicted by supersymmetry theory. 

The second Cryogenic Dark Matter Search Experiment (or CDMS II) runs deep underground in the Soudan iron mine in Minnesota, situated there so it should only intercept particles that can penetrate that deeply underground. The CDMS II solid crystal detectors seek ionization and phonon events which can be used to distinguish between electron interactions – and nuclear interactions. It is assumed that a dark matter WIMP particle will ignore electrons, but potentially interact with (i.e. bounce off) a nucleus. 

Two possible events have been reported  by the University of Florida team, who acknowledge their findings cannot be considered statistically significant, but may at least give some scope and direction to further research.

By indicating just how difficult to directly detect (i.e. just how ‘dark’) WIMPs really are – the CDMS II findings indicate the sensitivity of the detectors needs to bumped up a notch.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Gravity, Schmavity

The axiom that what goes up, must come down doesn’t apply to most places in the universe, which are largely empty space. For most places in the universe, what goes up, just goes up. On Earth, the tendency of upwardly-mobile objects to reverse course in mid-flight and return to the surface is, to say the least, remarkable.

It’s even more remarkable if you go along for the ride.

If you launch in a rocket you will be pushed back into your seat as long as your rockets fire. But as soon as you cut the engines you will experience weightlessness as you arc around and fall back down again, following a similar path that a cannon ball fired up from the Earth’s surface would take. And remarkably, you will continue to experience weightlessness all the way down – even though an external observer will observe your rocket steadily accelerating as it falls.

Now consider a similar chain of events out in the microgravity of space. Fire your rocket engines and you’ll be pushed back into your seat – but as soon as you switch them off, the rocket ship will coast at a constant velocity and you’ll be floating in free fall within it – just like you do when plummeting to your accelerated doom back on Earth.

From your frame of reference – and let’s say you’re blind-folded – you would have some difficulty distinguishing between the experience of following a rocket-blast-initiated parabolic trajectory in a gravity field versus a rocket-blast-initiated straight line trajectory out in the microgravity of space. Well OK, you’ll notice something when you hit the ground in the former case – but you get the idea.

So there is good reason to be cautious about referring to the force of gravity. It’s not like an invisible elastic band that will pull you back down as soon as you shut off your engines. If you were blindfolded, with your engines shut off, it would seem as if you were just coasting along in a straight line – although an external observer in a different frame of reference would see your ship turn about and then accelerate down to the ground.

So how do we account for the acceleration that you the pilot can’t feel?

An improvement on the standard two dimensional rubber sheet analogy for curved space-time - although it still lacks the contribution of the all-important time dimension.

Without a blindfold, you the pilot might find the experience of falling in a gravity field a bit like progressing through a slow motion movie – where each frame you move through is running at a slightly slower rate than the last one and where the spatial dimensions of each frame progressively shrink. As you move frame by frame – each time taking with you the initial conditions of the previous frame, your initially constant velocity becomes faster and faster, relative to each successive frame you move through – even though from your perspective you are maintaining a constant velocity.

So – no force of gravity, it’s just geometry.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Don’t Make a Meal of It

You should always put out the old dinner set when you have astronomers around. It all starts innocently enough with imagine this wineglass is the Earth rotating on its axis… But then someone decides that large plate is just right to show the orientation of an orbital plane and more wine glasses are brought to bear to test a solution to the three body problem and…

My favorite dinner set demonstration is to use the whole table to represent the galactic plane – ideally with an upturned wide rimmed soup bowl in the middle to mimic the galactic hub. Then you get a plate to represent the solar system’s orbital plane and hold it roughly facing the galactic hub, but at a 63 degree angle from the horizontal. We know the equatorial plane of the Milky Way is tilted 63 degrees from the ecliptic – or vice versa since here we are arbitrarily making the galactic plane (table) the horizontal. This means galactic north is up towards the ceiling – and incidentally a line drawn north up from the galaxy’s centre (i.e. the galactic axis) passes fairly close to Arcturus.

Now for the Earth. Wine glasses make an excellent Earth model since the stem can represent the Earth’s axis of rotation. The glass is at least a bit round and you can see through it for a view of what someone would see from the surface of that glass.

Looking down on the solar system (plate) from its north, which is orientated away from the galactic hub (table), it actually rotates anti-clockwise. So if you hold the glass at the top of the plate – that’s Earth at about September, then move it to the left for December, down to the bottom for March, right side for June and back to September. 

So, holding your plate at 63 degrees to the table, now hold the wine glass tilted at 23.5 degrees to the plate. Assuming you left your protractor at home – this will mean the wine glass stem is now almost parallel to the table – since 63 + 23.5 is close to 90 degrees. In other words, the Earth’s axis is almost perpendicular to the galactic axis.

The range of different orientations available to you. The axis of Earth's rotation (represented by the 'celestial equator') is almost perpendicular to the orbital plane of the galaxy.

You should really imagine the plate being embedded within the table, since you will always see some part of the Milky Way at night throughout the year. But, in any case, the wine glass gives a good demonstration of why we southerners get such a splendid view of the galactic hub in Sagittarius. It’s hidden in the daytime around March – but come September about 7pm you get the Milky Way running almost north-south across the sky with Sagittarius almost directly overhead. Arcturus is visible just above the western horizon, being about where the galaxy’s northern axis points (that is, the ceiling above the middle of the table).

And if you look to the north you can see Vega just above the horizon – which is more or less the direction the solar system (plate) is heading in its clockwise orbit around the galaxy (table).

Now, what’s really interesting is if I add the Moon in by just, oh… Er, sorry – that wasn’t new was it?

Posted on by Steve Nerlich

Can a Really, Really Fast Spacecraft Turn Into A Black Hole?

This question was posed in an Astronomy Cast episode a while back. It offers an interesting thought experiment, although a reasonably definitive answer to the question can be arrived at. 

Imagine a scenario where a spacecraft gains relativistic mass as it approaches the speed of light, while at the same time its volume is reduced via relativistic length contraction. If these changes can continue towards infinite values (which they can) – it seems you have the perfect recipe for a black hole. 

Of course, the key word here is relativistic. Back on Earth, it can appear that a spacecraft which is approaching the speed of light, is indeed both gaining mass and shrinking in volume. Also, light from the spacecraft will become increasingly red-shifted – potentially into almost-blackness. This can be partly Doppler effect for a receding spacecraft, but is also partly a time dilation effect where the sub-atomic particles of the spacecraft seem to oscillate slower and hence emit light at lower frequencies. 

So, back on Earth, ongoing measurements may indicate the spacecraft is becoming more massive, more dense and much darker as its velocity increases. 

But of course, that’s just back on Earth. If we sent out two such spacecraft flying in formation – they could look across at each other and see that everything was quite normal. The captain might call a red alert when they look back towards Earth and see that it is starting to turn into a black hole – but hopefully the future captains of our starships will have enough knowledge of relativistic physics not to be too concerned. 

So, one answer to the Astronomy Cast question is that yes, a very fast spacecraft can appear to be almost indistinguishable from a black hole – from a particular frame (or frames) of reference. 

But it’s never really a black hole. 

Centaurus A with jets powered by a supermassive black hole within - the orange jets are as seen in submillimetre by the Atacama Pathfinder and the blue lobes are as seen by the Chandra X-ray space telescope.

Special relativity allows you to calculate transformations from your proper mass (as well as proper length, proper volume, proper density etc) as your relative velocity changes. So, it is certainly possible to find a point of reference from which your relativistic mass (length, volume, density etc) will seem to mimic the parameters of a black hole. 

But a real black hole is a different story. Its proper mass and other parameters are already those of a black hole – indeed you won’t be able to find a point of reference where they aren’t. 

A real black hole is a real black hole – from any frame of reference. 

(I must acknowledge my Dad – Professor Graham Nerlich, Emeritus Professor of Philosophy, University of Adelaide and author of The Shape of Space, for assistance in putting this together).

Posted on by Steve Nerlich

Astronomy Without A Telescope – The Only Way Is Up

Escaping a gravity well is never an easy proposition. Unlike other kinds of wells there are no walls, so your options are ilimited. Over the years we humans have experimented with a variety of ways of getting out – with varying levels of success.

Trying to build your way out was first attempted – at least allegorically – with the Tower of Babel which (again allegorically) did not go well. Even with today’s engineering, it remains a dubious prospect. The relatively new Burj Khalifa in Dubai has managed to scale only 830 metres. The official defintition of where ‘space’ starts is 100 kilometres (or 60 miles).

Firing yourself out of a cannon or strapping explosives to your throne in the case of Wàn Hù, generally described as a minor official of the Ming Dynasty circa 1500, is similarly fraught with problems. See the Mythbusters episode Ming Dynasty Astronaut to see how that worked out.

Even if you do survive the initial blast, the huge acceleration required to achieve a projectile escape velocity of 11.2 kilometers a second from sea level will kill you anyway. And there’s also an issue of atmospheric drag – since the air in front of you will be superheated, your already Gforce-demised self will get cremated on the way up.

It would all be so much easier if someone could just throw down a rope. Various people have been attributed with first thinking up the space elevator – but it was probably Konstantin Tsiolkovsky – involving getting a base station into geostationary orbit and then lowering down from it kilometre-lengths of a carbon nanotube cable that we’ll be inventing any day now.

The last Saturn V launch (with Skylab 1) in May 1973

So for the moment at least, we are stuck with good old-fashioned rockets – for which we can also thank Mr Tsiolkovsky, amongst others. Although achieving a zero to 11.2 kilometers a second velocity at sea level will kill you – if you can get a bit of altitude at a lower acceleration rate, the escape velocity from that altitude will be lower. So as long as you can launch with enough fuel to keep gaining altitude, you can keep on applying this logic until you eventually escape the gravity well. We’ve done it with robotic spacecraft, but we’ve never done it with people.

Before I start sounding like a Moon landing denier, remember the Moon is still orbiting within Earth’s gravity well. Lagrange points 1 and 2, about 1.5 million kilometres away mark the edges of the Earth’s gravity well. L2 is perhaps the better target since you could use the Earth’s shadow to reduce your exposure to solar radiation. At 1.5 million kilometers, it’s about four times the distance to the Moon, so a one month round trip maybe. It’s still challenging and you’ll still collect a hit from cosmic rays – but nothing like the potentially suicidal two year round trip to Mars. So, if we can get past this obsession with landing on things, wouldn’t it be a wothwhile goal to try and finally get someone out of the well?

Posted on by Steve Nerlich

Astronomy Without A Telescope – Let’s Go Around The Room

My dog keeps me in touch with the universe. There are important reasons why he has to go out into the backyard about 9pm and then again around 5.00am – and at both times there are stars out. He has this very particular sound, the canine equivalent of ahem, to let me know it’s time.

Around February is great because on the night shift you see Orion and on the morning shift you see Scorpio. These are two of three constellations, I can easily identify without a book – the other one being the Southern Cross. And this is the only time of the year when I get to see all three, since by the time Scorpio is up in the evenings around August, Orion is already lost to the glare of daytime.

This reminds me of a plan I have to once and for all explain to people how the night sky works. You wallpaper a room with your equatorial and/or ecliptic constellations and on the roof put your circumpolar constellations, which would include the Southern Cross down here or Ursa Minor for your northern folk. Then in the middle of the room you put a big and glaringly bright light.

So around February, you are in that part of the room where when you face away from the light you can see Orion. Then spinning on the spot, you’ll be able to spy Scorpio just before you come around to face the bright light, which prevents you from seeing what’s on the other side of the room. Keep spinning and you come back to night time and admire Orion again – and so on.

Cool wallpaper - the Pleiades, Hyades and Orion as seen from the southern hemisphere

To progress through the year you have to start walking around the room, that is orbiting the bright light – and you can keep spinning on the spot for the day night effect if you like. Once you are around on the other side of the room – you get a much better night-long view of Scorpio, while Orion is lost behind the bright light. Your circumpolar constellations are still visible on the ceiling – but kind of upside down now.

It’s taken a few nights out with the dog to figure out which way you are supposed to spin – not to mention which way to put the wallpaper since if your at my latitude in the north, you’ll need to hang it upside down. For me, if I’m standing in front of Orion, Scorpio is going to be around to my right (but left for you) – and I’m going to orbit to my right (but left for you) – and I’m going to spin clockwise (but anti-clockwise for you).

I almost have it all visualized when there’s certain ahem as dog realizes master is staring vacantly at the sky again. Oh yeah sorry, good dog – and we go back inside.

Posted on by Steve Nerlich

More Evidence That Size Doesn’t Matter

Astronomers have managed to peer past obscuring dust clouds to gain their first peek at the gestation of a massive proto-star W33A which is about 12,000 light years away within the Sagittarius constellation. A spokesperson for the research team, who you may not be surprised to learn is British, described the sight as ‘reassuringly familiar, like a nice cup of tea’.

There has been a standing debate in astronomical circles about whether or not massive stars form in the same way as smaller stars. The issue has been hampered by a lack of observational data on just how massive stars form – as they develop so quickly they are generally only seen in an already fully formed state when they pop out of the obscuring dust clouds of their stellar nursery.

Known as a Massive Young Stellar Object (MYSO), W33A is estimated to be at least 10 solar masses and still growing. Shrouded in dust clouds it cannot be observed in visible light, but much of its infrared radiation passes through those ‘natal’ dust clouds. A research team led by Ben Davies from the University of Leeds collected this light using a combination of adaptive optics and the Near Infrared Integral Field Spectrograph (NIFS), on the Gemini North telescope in Hawaii.

The research team was able to piece together an image of a growing star within an accretion disk – surrounded by a wider torus (like a donut) of gas and dust. There were also clear indications of jets of material being blasted away from the poles of W33A at speeds of 300 kilometres a second. These are all common features that can be observed in the formation of smaller stars.

Massive Young Stellar Object HD200775 within the reflection nebula NGC7023.

This adds to other recent findings about the formation of massive stars – including the Subaru Observatory’s direct imaging of a circumstellar disk around the MYSO called HD200775 reported in November 2009 and evidence of rapid formation of planets around massive stars in the W5 stellar nursery, reported by other researchers to the American Astronomical Society in January 2010.

These findings support the view that massive star formation occurs in much the same way as we see in smaller stars, where a centre of mass sucks up material from a surrounding gas cloud and the falling material collects into a spinning, circumstellar accretion disk – often accompanied by polar jets of material flung out by powerful electromagnetic forces within the growing star.

However, at least one clear distinction is apparent between small and massive star formation. The shorter wavelength, high energy radiation of newborn massive stars seems to dissipate the remains of their circumstellar disk more quickly than in smaller stars. This suggests that planet formation is less likely to occur around massive stars, although evidently some of them still manage it.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Wake Me When We Get There

Living on a planet comes with certain advantages. Gravity, for instance. The only reason hot air rises on Earth is that colder, denser air will always fall to displace it upwards. On the International Space Station everyone has to sleep next to a fan or they end up being enveloped in a bubble of their exhaled carbon dioxide (or worse). 

Perhaps this won’t be a problem for life forms that evolve in microgravity, since they might evolve to keep moving around all the time, even when sleeping – kind of like how sharks do in Earth’s oceans. 

But then, life forms that might evolve in microgravity are unlikely to be metabolically dependent upon either atmospheric gases or oceans, since if you are going to have an atmosphere or an ocean of any appreciable density – you kind of need something like a planet to start with. Hmm… 

So, can something interesting evolve in microgravity, in the the absence of a dense medium? Well, there’s Sir Fred Hoyle’s fictional Black Cloud – where something approximately 1 AU in diameter, with the mass of Jupiter and considerably less density than water still managed to be hyper-intelligent, despite the substantial impost placed on its speed of thought. I mean, that’s eight minutes travel time just to go hmm… 

Our one and only data point about how intelligence might arise organically suggests an electrochemical basis, while the hypothesized Black Cloud required an electromagnetic basis. The latter is feasible in a brain with a density considerably lower than water, but you would need to transmit thoughts at the wavelength of X-rays to effectively move them through the dense organic tissues we are familiar with on Earth. On that basis, too much thinking really could give you cancer. 

A black cloud - but probably not a smart one.

So, it seems plausible that intelligent, electrochemical thinkers generally evolve on planets – but we can still keep it open for some much bigger, though perhaps slower, electromagnetic thinkers to evolve in microgravity. 

And there are reasons to envy an entity that can survive long term in microgravity and can manage a slow and steady journey between stars under its own propulsion system. For us high density thinkers, there’s a time limit on just how long you can enjoy the particular gravity well you happen to have evolved in. Habitable zones don’t stay habitable forever. Firstly, gravity wells have a habit of attracting devastating meteor or comet impacts – and for the longer term, your star is eventually going to die. 

Probably, the smart thing to do first is to build a planetary defense system – noting the current population of dinosaurs on Earth is exactly zero.  In the longer term, you would need to make a run for it – ideally taking as much of the surviving ecosystem with you as you can. You never know when some giant energy-sucking alien artifact is going to show up, wanting to talk to a whale. 

Anyhow, it’s great that we are now in orbit on a regular basis. It’s a really good start.