astronomy – Akshat Rathi

Are we alone in this universe?

April 18, 2015 Akshat Rathi1 Comment

The answer to the question can never be “yes”, for not every nook and cranny of the universe can ever be searched. Instead, the day we can say “no” for sure is nearing.

The chief scientist of the US space agency NASA, Ellen Stofan, recently said, “I believe we are going to have strong indications of life beyond Earth in the next decade, and definitive evidence in the next 10 to 20 years.”

This new optimism among astronomers is buoyed by many factors. Mainly made possible because of the increasing capabilities of astronomers to peer into the sky and collect lots of useful information.

Take for instance the explosion in the discovery of exoplanets—that is, planets found around stars other than our sun. We have nearly 2,000 confirmed possible alien worlds and another 2,000 waiting to be approved. These exoplanets, detected by the faint decrease in starlight as a planet crosses it, come in many shapes and sizes. But a recent estimate suggests that as many as one in five sun-like stars have planets that can host life.

The famous Drake equation—a thought experiment that estimates the number of detectable civilisation in our galaxy—has “average number of planets that can potentially support life per star that has planets” as one among its seven determining factors. The higher the average, the greater the number of detectable civilisations in the Milky Way. And the data is saying that the favourable conditions required to create life are more common than we previously thought.

Fiction no more

But there is a counter argument, too. Almost all the exoplanets discovered are in the Milky Way, and at least some of the habitable ones have existed for as long as 10 billion years. That is twice the length of time the Earth has had to create us.

Thus, if there are indeed some alien civilisations on any of these planets, at least one ought to have developed the capability of space travel. And, even if their spacecrafts travelled at a fraction of the speed of light, they should have colonised the entire galaxy in a few million years. But that hasn’t happened. So perhaps life is not all that common as it may seem.

So, if the Milky Way is barren, what is it that we can do to look farther to find life? Looking for life outside the Milky Way is much more difficult. So far, we have confirmed the existence of only two exoplanets outside our galaxy. But there are good reasons to believe that the average number of habitable exoplanets in other galaxies would be similar to that in the Milky Way.

A recent study, published in the Astrophysical Journal, suggests that if we are to believe in the hypothesis that any sufficiently advanced civilisation, once evolved, will take over an entire galaxy, then we may have a plausible way of looking for life at the intergalactic scale. To come to this conclusion, the study marries science fiction and cutting edge research.

In a 1937 novel, Olaf Stapledon proposed that an advanced civilisation will try to capture as much energy as is produced by its star so as to continue to grow. This it could do by building a sphere enclosing the home star, so as to capture every photon emitted. Thus, if such a civilisation spreads through the galaxy, we may have entire habited galaxies become dull to our eyes on Earth because they emit less light.

But, in a twist Stapledon would be pleased to hear, in his study Roger Griffith at Pennsylvania State University suggests that such “dark galaxies” would emit a peculiar infrared signal and could perhaps be found. He scoured the data of 100,000 galaxies collected by NASA’s Wide-field Infrared Survey Explorer and found about 50 galaxies that fit the criteria.

So, even if the Milky Way is barren, the answer to the age-old question of whether we are alone in the universe may not be negative. And, yet, it is quite possible that Griffith’s findings may also have a mundane explanation, such as large swathes of interstellar dust blocking the galaxy from Earth. We need to work harder to find out.

First published in Lokmat Times. Image by bflv. under a CC-BY-NC-SA license.

Asteroids are fascinating – not just because they can destroy humanity

February 4, 2015February 4, 2015 Akshat RathiLeave a comment

Look up in the night sky, if you are lucky you might be able to see Vesta, the only asteroid—among millions that lurk between Mars and Jupiter—bright enough to be visible to the naked eye from the Earth. Given their dull (non)appearance in the sky, it is no wonder that the first asteroid was only discovered in 1801 when relatively powerful telescopes started to be built.

However, in the two centuries since, we have learnt a lot about these celestial bodies. They could prove to be both a curse and a blessing. A curse because they could bring about the end of human existence, and a blessing because they could be the launchpad for building human colonies in outer space.

The definition of an asteroid has changed over the years, but today we know them as small bodies found in the inner solar system. Asteroids can also be called failed planetesimals—tiny fragments that had the potential to grow larger under the influence of gravity and become a planet when the solar system was being formed billions of years ago, but couldn’t.

Living on a pale blue dot

We now know that these “failures” pose a high risk to life on the Earth. Only last week a half-kilometre wide asteroid passed by the Earth. If it had hit us, it could have caused human extinction. Fortunately it was about 1 million km away, which is three times the distance between the Earth and the moon. But the risk is real. In the past one such asteroid impact was responsible for extinction of dinosaurs and an older one caused even more destruction and made all the oceans boil.

The good news is that, with enough warning, we already have the technology to deflect asteroids. But the bad news is that we may not get enough warning, because there are thousands of asteroids—some of which could be on a collision course with the Earth right now—that we have not yet discovered.

This threat has spurred some to take matters in their hands, rather than wait for government agencies to do the work. The B612 foundation, for instance, is planning to launch a private space mission in 2018 that will discover and track hundreds of thousands of such asteroids.

If you can’t beat ’em, profit from ’em

Leaving the stories of doom behind, asteroids are fascinating for another reason: they hold clues about the formation of our solar system. For instance, according to theory, when the Earth formed into a planet, the process would have been too hot to have left any liquid water on the surface. So the current hypothesis is that water-rich celestial bodies, such as comets, brought the water when many crashed into the Earth during its early days.

A current space mission, called Rosetta, to the comet 67P recently tested this hypothesis. It found that the water on that comet was not of the same composition as that on the Earth. So if not comets, then the other celestial body that could achieve this feat are asteroids. This hypothesis will be tested by the Hayabusa-2 mission, launched in December by the Japanese space agency, when it will land three rovers on the asteroid 1999 JU3, and also collect and return a sample from the asteroid back to the Earth in 2020.

Even if they aren’t the source of terrestrial water, asteroids could be habitable, or at least resource-providing, stations for human colonies in outer space. And this isn’t just a crazy idea. Two private companies, Planetary Resources and Deep Space Industries, are currently working on projects to mine asteroids. They aim to harvest not just water, but also metals such as iron, nickel, platinum and palladium. Apart from the moon, few celestial bodies can boast of having such a close relationship with humanity.

First published in Lokmat Times. Lead image by ESA and ATG media lab. Asteroid image by Emily Lakdawalla (under CC-BY-NC-SA).

Cassini points to a hidden ocean on Saturn’s icy moon

April 7, 2014 Akshat RathiLeave a comment

Finding liquid water on a celestial body within the solar system is exciting. The only thing that is probably more exciting is finding an ocean full of it. Today such news comes via Cassini, which has made measurements that show that Saturn’s moon Enceladus has a hidden ocean beneath its icy surface.

While orbiting Saturn in 2005, Cassini found jets of salty water spewing from the south polar region of Enceladus. According to Luciano Iess of Sapienza University of Rome, lead author of the new study published in Science, “The discovery of the jets was unexpected.”

Geysers require liquid water, and we wouldn’t expect Enceladus to have any. It is too far from the Sun to absorb much energy and too small (just 500km in diameter) to have trapped enough internal energy to keep its core molten. The answer to how the water got there might lie in the details of the moon’s internal structure.

Water beneath an icy crust

The data to understand Enceladus’s internal structure came from by measuring changes in Cassini’s speed as it flew close to the moon. When passing the denser parts of the moon, it sped up by a few extra thousandths of a metre per second. That minute change was tracked through recordings of the radio signals Cassini was sending to NASA’s Deep Space Network station.

In making such tiny measurements, scientists had to filter out other factors that could influence Cassini’s speed. These include pressure on the spacecraft from sunlight, the nudge from heat radiating from its nuclear-powered electrical generator, and the drag of the particles it strikes as it passes through the south polar plumes.

Iess and his colleagues have produced a model of the internal structure of Enceladus using the measurements. They conclude that there is a core that is roughly 200km in diameter; above that lies a 10km-thick layer of liquid water, which is followed by 40km of ice crust. The water layer may extend all the way to the north pole, but its thickest part lies at the south pole.

It is possible that Saturn’s powerful gravity is responsible for the liquid water under Enceladus’s surface. Its pull could heat up the interior through a process called tidal kneading, which creates tides in the ocean causing internal friction and thus heat.

After the initial discovery of the plumes, Cassini’s minders put a lot of effort into determining Enceladus’s internal structure, but it still took nearly ten years to do so. This is because the time the spacecraft spends around Saturn is very valuable, and there are lots of other things worth studying.

Cassini can only make a handful of flybys near Enceladus while still paying attention to other moons, such as Titan. When approaching Enceladus, the controllers also had to make a choice about how to study the moon because of a limitation in how Cassini’s instruments are arranged. When making gravitational recordings it needs to point its antenna towards Earth, but in doing so all its other instruments face away from Enceladus. Of the 19 flybys, only three were used to make gravitational recordings.

“After spending eight years in the Saturnian system, one may think that the measurements are becoming repetitive and that Cassini has discovered everything in the reach of its instruments. This is far from being true,” Iess said.

Time is running out

“The evidence adds up to a large and active body of water under Enceladus’s southern polar region”, Helen Maynard-Casely of Australian Nuclear Science and Technology Organisation said. But she warned, “It is going to be a long time before we can verify if this ocean is there, if ever.”

The plutonium-powered spacecraft has enough energy to power itself till 2017. The trouble is that, in three years, it will only be able to make three more flybys of Enceladus, which is not enough to take more gravity data. Its end is slated to come when controllers drive it into Saturn’s atmosphere for incineration, because scientists are keen to avoid having it crash into Saturn’s pristine moons.

There is a push to send another mission to Saturn, but Jupiter’s moon Europa might be a better candidate to search for life. At 3,100km in diameter, it is much larger than Enceladus, and, in December, astronomers spotted water vapour coming from its south pole, as well.

The possibility of finding a large amount of liquid water is exciting because, for life to exist as we understand it, we need liquid water. Even on Earth, whenever untouched sources of liquid water, such as Lakes Vostok and Ellsworth under Antarctica, are studied, there is always the hope that we may discover new forms of life.

First published on The Conversation. Image credit: NASA/JPL/SSI/J Major