Episode 7: Neighbourhood Watch Pt1! - Travel in our Solar System

This blog post is a transcript of Series 1, Episode 7 of my science podcast, Don't touch my Radium! All sources, references and recommended material can be found here.

In last week's episode we finally talked about what biosignatures we’re looking for!! We established that there are three categories, gaseous, structural and temporal. Gaseous are the most commonly searched for signatures as they are the most established ones with researchers using computer models and data from various missions to allow them to make determinations about the potential climates and habitability of exoplanets. This is mostly helped along by researchers ability to pick out mimic behaviours, circumstances where the gases are made by abiotic processes instead of biological ones, so that they can rule out false positives. Structural and temporal signatures could tell us about water and vegetation on the surface or if the planet has seasons like ours, however these will both depend on advancements in future technology as we don’t currently have the ability to gather enough light, and so enough information, to learn about these in a way that will eliminate false positives. So for now, we focus on searching for biosignatures in the atmospheres of other planets to see if there are signs out there that could only come from a biological process.

But we seem to be getting a bit ahead of ourselves! All this talk of exoplanets and habitable zones in extrasolar systems! As with most scientific discoveries, the discovery of a biosignature will require evidence building up until no other explanation exists. We can only search for life as we know it, atmospheres as we understand ours, planets that are not too hot, not too cold, that are just right. Just right to be Earth-like! But what if we had more information? What if we could learn from other types of planets, from other atmospheres? Earth is in the middle of our star's habitable zone, but Mars is also in this boundary! And who's to say that life can’t exist elsewhere, maybe on the moons of Saturn or Jupiter? We can’t possibly know what the limitations for life in our Universe are, we can of course focus on Earth-life, Terran-like potentials, but if we want to expand our knowledge base and concept of the conditions in which life can live then we have to learn more. We have a solar system of planets, dwarf planets, moons and asteroids all on our doorstep! So in this episode we’re going to bring the question back home, to our own neighbourhood.

We may not have the technology to travel the 6,000 years to get to the nearest Earth-like planet, Proxima Centauri b, but the clever clever researchers and engineers who work on the concept of space travel have developed ways of travelling in our solar system. Up to now we’ve only talked about putting a telescope into orbit around our own planet. Well we can send a probe, which is basically a travelling telescope/instrument/satellite depending on it’s payload, out into orbit around the other objects in our neighbourhood. So before we get into detail about the missions and search for life in our solar system, let’s talk about how we are able to travel around.

Space Navigation

Improvements in space flight and manipulation of the physics of our planets allow for space probes to be sent out, leaving earth's atmosphere, getting closer to their targets and letting us learn more than ever before about the composition and nature of the planets, moons and objects in our solar system.

Space Probes

“We’re sending a probe!”, is often heard from science to science-fiction, well probes are just satellites that have been launched into deep space. Technology and structure wise, they are very similar to orbital telescopes as the environment is very similar. What is new here is the navigation of space. With our current technology, probes never return! But the next generation are being designed to have the ability to bring back samples from comets, asteroids and planets. We’ve talked previously about how we get into orbit around our own planet, but how do we get across the vast distances between the planets in our solar system to get a probe into orbit around another planet, or just find a resting spot where it can chill out and observe the universe on a wider scale?

In terms of space probes, there are three types: interplanetary that fly by celestial bodies, orbiters placed in orbit, and landers: probes that land on the surface of a planet. From what we learned in episode three where we talked about rocket fuel and sending a satellite into orbit, we can probably all make some assumptions about how to find our way to another planet. We’ll need a rocket to carry the probe to a certain height in space, the probe itself will need an engine, fuel and thrusters of some sort to continue on it’s trajectory beyond the gravitational pull of the Earth so that it doesn’t get pulled into an orbit or back to Earth and it can use the thrusters to guide it towards it’s target. Well this is where I tap out of my guesstimates. Because once you are in the vacuum of space, with the vast distances, how do you get to where you want to go, and how do you send the information back to Earth?!

We learned in episode three that orbiting telescopes or scientific satellites will send their information to a communications satellite, most likely in geostationary orbit around our planet, and that satellite will communicate with the ground to relay the collected information. We can do something similar for a travelling probe. Firstly we’ll need to consider two things that are very intertwined, the navigation and the trajectory.

The ability of a space probe to navigate its surroundings depends on:

• Computer models of the motion of the probe

• An accurate model of the solar system

• Measurement system for determining the position and speed of a probe

• Location from where the measurements are taken

To break this down a bit more, you need to be able to lay in a flight path, to do this you have to have a pretty good idea of the environment the probe is going to fly through, you need a way to measure position and speed to keep track of its location and make sure it’s on the right path but to do this, you also need a way to communicate this information to controllers on the ground back on Earth.

At NASA, they use the antennas of the Deep Space Network (DSN) as the measurement system. This is an international array of ground based giant radio antenna as well as orbital antenna that acts as the largest and most sensitive scientific telecommunications system in the world. They transmit radio signals to the probe which returns them to earth with a slightly shifted frequency. Distance and speed of the probe is determined by looking at the difference between the transmitted and received information. They can actually get an accuracy of 0.05mm per second for the line of sight velocity in a 3m range, this is done using a high frequency of signals and an accurate atomic clock. There are three facilities around the world that NASA’s JPL operates and they are spaced equidistant at around 120 degrees longitude from each other, they are in California, Spain and Australia. What’s really cool about this is that it means that there is constant communication available for spacecraft and no communication is lost as the planet rotates.

The trajectory of travel then, is largely dependent on the requirement for propulsion fuel. Fuel is heavy and expensive (Remember episode three about satellites? $10,000 per 500g of payload?) so determining a method of travelling to another planet by minimising the fuel and mass of the probe is a necessity. There are a few methods of travel that can be manipulated to limit the fuel costs for space probes. We have, Hohmann transfer, gravitational slingshots and the interplanetary transport network. Let’s have a look at each of these.

Hohmann transfer

The Hohmann transfer begins with a satellite that wants to change its orbit around the same body. So we have a satellite in a low Earth orbit wanting to move to a high Earth orbit. To do this we assume that there is no other gravitational influence nearby that would affect the transfer, then we increase the velocity and guide the satellite to a new, higher orbit. This is achieved using a two impulse elliptical transfer between two coplanar orbits. Do you remember from episode three that we talked about rocket fuel and how it’s efficiency is the specific impulse? Well this is related to how fast the propellant is ejected, so we can think of using an impulse, as ejecting some fuel to propel the satellite forward with a velocity. The first impulse is to send the satellite to the desired height, while the second impulse will be to direct the satellite onto the new orbital path once it’s at the correct height. This method uses the least possible amount of propellant and can be applied to interplanetary transfers.

Let’s take sending a probe to Mars as an example. Each planet has what we call a sphere of influence where the gravitational attraction of the planet is larger than that of the sun. First we need to escape the potential well of the original planet using a hyperbolic escape orbit. The method uses a series of two body approaches, first it needs to escape the Earth's sphere of influence so it needs to use an impulse to attain an escape velocity, once it reaches the edge of this region the second stage will again use an impulse to alter the probes trajectory and accelerate it in the direction of Earths’ revolution of the Sun so that it ends up in an elliptical trajectory around the Sun. Then the third stage takes effect once the probe reaches the edge of the sphere of influence of Mars where it deccelerates to a hyperbolic approach capture trajectory using the gravitational field of mars as the attracting force. It’s kind of like throwing a dart at a moving object.

In order to do this, the positions between the planetary bodies involved must align in such a way that the probe sent out will reach a specific point at the same time as the planet it is attempting to orbit. Earth and Mars align to allow for a Hohmann transfer once every 26 months.

Using this technique to visit several planets is a bit more complex. When we do this, we are taking advantage of the gravitational fields of other bodies by entering their SOI in order to change direction or gain additional impulse. This is normally referred to as a gravitational assist, a flyby or a gravitational slingshot.

Gravitational slingshots

When our solar system formed the Sun took 99.8% of the nebular mass and in those early days, intense ejections of coronal mass would have transferred angular momentum from the Sun to the objects that made up the final 0.2% of the proto-planetary disk. Planets still retain most of the solar system's angular momentum, and this momentum can be tapped to accelerate spacecraft on gravity assist trajectories.

This is a bit hard to explain without visual representation so I’d recommend checking out the Planetary Society article or the YouTube video by the Canadian Space Agency that are listed in the references. But let's give it a try. Say Jupiter is travelling along its orbit around the Sun, and then we come along and we want to use it’s gravity to change our direction and gain some speed. We aim ourselves towards Jupiter at an angle (The angle depends on the direction change we’re looking for), and we head in from behind the planet, kind of towards the Sun. As we begin to feel some of the gravitational force from Jupiter it pulls us towards it and increases our speed, and as all objects exert a gravitational pull on each other, no matter how small, some of the angular momentum is transferred from Jupiter to our probe. This gives us enough speed to escape the gravitational field of Jupiter as we pass by, but we leave with a new trajectory and having gained some velocity and angular momentum.

So gravitational assists don’t just reduce the fuel costs, it also adds velocity to the spacecraft allowing for shorter mission times. And since we want to head out across the vast expanses of our solar system, the faster and easier it is to get there, then the quicker we can get answers.

Our solar system

In episode 1 we talked about the creation of our sun and the dispersion of material across the proto-planetary disk. We learned that the melting temperatures of the four main types of material determined its position in our solar system and condensation coupled with accretion led to the formation of our planets. This is the reason why only rocky material survives closer to the sun while ice, liquid and gas exist in more outer regions.

As we discussed last week in looking at biosignatures, we know from our own planet that when searching for life we look for rocky terrain, the presence of liquid water along with a protective atmosphere. This atmosphere is expected to contain hydrogen, oxygen, nitrogen and carbon in order to make up the composition of an Earth-like atmosphere, remember that we have CO2, H2O, CH4 and N2O. All the planets in our solar system have their own atmospheres, even some of the moons have an atmosphere! Not ours though, the craters on our moon have existed for hundreds and thousands of years because there is no wind or water to change them. In general, atmospheres throughout the solar system are different to Earths, but they do contain a lot of the same elements so we can analyze how these different compositions affect habitability. This presence of an atmosphere along with how it is structured and what its chemical composition is allows us to speculate about what kind of life it could support.

Another interesting topic in a planet's habitability is the presence of a magnetic field, we already know that Earth has one and that it protects our atmosphere from the solar winds reducing the stripping away of important materials. Interestingly Mars and Venus both do not have magnetic fields, so researchers are looking at what this tells us about habitability. You might remember that we mentioned before about how there’s indications that Mars used to have a magnetic field and it’s disappearance may be the cause of the radiation of the surface of the planet. Maybe if Mars still had a magnetic field there would be clear signs of life!

Each time a probe or a satellite is sent out into space it is sent on a mission, a mission to gather evidence, to go where no man has gone before (except for the moon, the men have been there). Let’s take a look at what we have learned about the objects in our solar system thanks to these brave little probes.

In our search for life as we know on extrasolar systems, the most likely place to find it is on a terrestrial planet in the stars habitable zone. We’ve talked about Earth and our formation, we’ve described the atmosphere and the magnetic field, so before we take a trip around our solar system let’s start with a recap about our own planet.

Earth

We mentioned in Episode 3 that there have been about 9000 satellites sent in orbit around the Earth. According to the Union of Concerned Scientists who have compiled a complete and publicly accessible database of all of Earth's satellites, as of December 2019 there are 2,218 operational satellites orbiting our planet. I couldn’t get an accurate idea of how many Earth Observation satellites there are currently in operation, there have been up to 700 launched since Sputnik 1 was first sent up, and of course there is the International Space Station, the biggest satellite up there! American Scientist reported in 2008 that there were 150 operational at that time and that was over a decade ago! I think what’s important for me to recognise here, is that although NASA is the biggest resource for me for this topic, there are about 45 other countries and agencies that are currently exploring the wonderful planet we live on and I totally recommend following the link in the references to the database to get an idea of just how international our scientific exploration of the world is.

Some facts about Earth:

• Earth is the largest terrestrial planet in our solar system, with a radius of 6,371 km.

• The planet orbits at 150 million km from the Sun, we refer to this as 1AU in order to make an easy comparison with the other objects in our solar system. Sunlight reaches us in 8 minutes.

• Our day is 23. 9 hours long and our year is 365.25 days, that 0.25 is why we have a leap year every four years.

• Earth has no rings and we have one moon, the Moon!

• Earth has a geomagnetic field that deflects solar winds protecting the ozone layer from being stripped away.

• The solid surface has a variety of mountains, canyons and plains of land but most of the surface is covered in liquid water.

• Earth's axis of rotation is 23.4 degrees giving us seasons that help our plant life to thrive.

Habitability:

Earth is the only inhabited planet and existence of life as we know it! We have a hospitable surface temperature with liquid water. Our atmosphere has a number of layers but closest to the surface it consists of about 78% nitrogen, 21% oxygen and the final 1% is made up of other gases; argon, carbon dioxide and neon. This ratio of gases is the right balance for us to have breathable air. But this doesn’t mean that life as we know it is limited to humans and animals walking around sucking up oxygen searching for a nutrient filled snack, or maybe a pastry.

Mostly we are looking for evidence of microbial life, small organisms that can thrive in an Earth-like environment or extremophiles, microbes that hang out in some inhospitable places where humanity could not survive. As we’ve said, Earth is just right for us, nice temperatures, plenty of oxygen, not much radiation thanks to the atmosphere. But there are places on our planet that have some more extreme living conditions and some organisms that are happy hanging out there. There are many types of extremophiles but by way of an example let’s have a quick look at five types laid out in a National Geographic article back in 2013;

• Tardigrades : you’ve probably seen a picture of these, they were coming up everywhere for a while. They are tiny millimeter long extremophiles that look kind of like a vacuum bag, to me anyway. These guys can withstand temperatures from -200 degrees up to 151 degrees, in Celsius people, can survive without oxygen and water and can take about a thousand times the amount of radiation we can. They can be found all over our planet.

• Brine Shrimp : these guys are down for salt, they have gills and glands that can filter out the salt to keep a balance but they can survive in waters that are 10 times more salty than the ocean such as the Great Salt Lake of Utah.

• Methane Ice Worm : these guys are up to two inches long and live on the seafloor of the Gulf of Mexico where they burrow into methane mounds and live on the bacteria that grows there. While out in some of our glaciers you’ll find the glacial ice worms that survive on algae with their body functions operating at freezing, 0 degrees Celsius. They apparently melt at about 4 degrees Celsius. Ew.

• Rushing Fireball Microbe : this is a thermophile who likes to hang out in 100 degree Celsius temps, boiling point so it’s pretty happy around Italy’s Vulcano Islands where it was discovered.

• Radiation-Resistant Bacteria : this guy can take 3000 times as much radiation as we can, the single-cell organism is referred to as the Lazarus microboe and it can be found on the inside walls of nuclear reactors!

So when it comes to microbial life, we can expand our limits, we can look further and deeper. We do not need to be limited to our own survival. The first four planets in our solar system, in terms of distance from the sun, are terrestrial (rocky) planets, Venus, Mercury, Earth and Mars. Rocky terrain helps to reduce the amount of solar radiation. Further afield we have the gas giants - Jupiter and Saturn, and the ice giants - Uranus and Neptune. But these aren’t the only objects in our solar system, some of these plants have moons, and I mean moons, we’re talking nearly 70 in total around Jupiter alone. And some of these moons are as big as ours, one is as big as the planet Mercury! So while the gas and ice giants themselves may not have surfaces that bear any similarity to ours, their moons might be a different story.

We’ll continue the search for life in our solar system next week and while we won’t go into detail about every object there is, that is a crazy big list, we will look over the habitability of our planets and a few choice moons.

Next week, Neighbourhood Watch Part 2 where we’ll explore the habitability of the worlds in our solar system. Thanks for listening. Come back next week if you’d like to learn more. If you want to get in contact you can email dtmyrad@gmail.com or drop by our instagram @dtmyrad.

References / Further Reading

Articles

• Gravity assist, David Shortt, 2013, The Planetary Society

• How do space probes navigate large distances with such accuracy and how do the mission controllers know when they've reached their target?, Jeremy Jones, 2006, Scientific American

• Signs of Life on Europa May Be Just Beneath the Surface, Mike Wall, 2018, Space.com

• 5 Extreme Life-Forms That Live on the Edge, Liz Langley, 2013, National Geographic

Education Sites / Resources

• About the Deep Space Network, Jet Propulsion Laboratory, NASA

• What is a Lagrange Point?, Solar System Exploration, NASA

• Basics of Space Flight, Solar System Exploration, NASA

• Orbit Transfers and Interplanetary Trajectories, S. Widnall and J. Peraire, Aeronautics and Astronautics, MIT

• What is L2?, Orbit/Navigation, esa

• Our Solar System, Solar System Exploration, NASA

• UCS Satellite Database, Union of Concerned Scientists

• ESA’s fleet of Solar System Explorers, Science & Technology, ESA

Missions

• Europa Clipper, Jet Propulsion Laboratory, NASA

• Juno, NASA Space Science Data Coordinated Archive

• Uranus Approach, Jet Propulsion Laboratory, NASA

• Dragonfly, Press Release, NASA

• Cassini, Solar System Exploration, NASA

• Mars 2020 Mission, NASA

• ExoMars, ESA-Roscosmos

• Trace Gas Orbiter, ExoMars, ESA-Roscosmos

YouTube Videos / Documentaries

• What is a gravity assist?, Canadian Space Agency, YouTube

• Exploring Our Solar System, Stuart Eves, The Royal Institution Lectures, YouTube

• One Strange Rock, Darren Aronofsky, National Geographic Documentary, 2018

• What a planet needs to sustain life, Dr David Brain TED talk, YouTube.

Mariner 10 concept image of gravitational assist.