JOIN USComets are long-haired stars that fascinate and enthrall scientists all over the world. The journey of two of such stars is under focus right now, writes C Sivaram
The Rosetta spacecraft launched in 2004 finally arrived at its destination, the Comet 67P, Churyumov-Gerasimenko, on August 6. To rendezvous with the comet, Rosetta travelled six billion km through the solar system making four flybys of Mars and Earth, using their gravity to pick up speed.
It also had a nearly three year hibernation period when the solar flux on its panels was at its weakest. The spacecraft finally woke up from its slumber in January, 2014.
The spacecraft became the first to orbit a comet and is currently only hundred km away from it. Due to the very low mass of the comet, the orbital velocity is only half a meter per second.
So, it takes three days to complete one orbit of the comet. Ten manoeuvres were required to gradually adjust the spacecraft’s speed and trajectory to match with that of the comet.
In November this year, Rosetta will actually lower a lander named Philae onto the comet surface. To ensure that it does not bounce back into space, the lander weighing hundred kg would be attached to the comet crust by a harpoon.
Philae will use a small drill to remove samples from the comet’s crust and this would be analysed in the attached automated lab. The mission is expected to last one year, till the end of 2015.
Origin of life
Many astronomers believe that during the early days of our solar system, the innumerable comets colliding with the earth, four billion years ago, could have kick-started life on Earth by bringing in large stocks of organic matter. Philae is going to search for the presence of such organic compounds.
It is also believed that comet strikes on Earth could have brought in much of the earth’s water apart from the complex molecules perhaps aiding rapid emergence of life on Earth.
Comets are believed to consist of ancient particles of ice and dust, left over from the formation of the solar system around four and a half billion years ago. The present cosmic target, 67P is supposed to be among the oldest materials in our neighbourhood.
The second comet of current interest is the one expected to come within a hundred thousand km of the planet Mars, on October 19 of this year. It was first spotted by Robert McNaught, at the Siding Springs Observatory, in Australia.
Thought to be an accidental discovery, it was believed that it might crash into planet Mars, but the recent indication is that it might just come very close to it. It is also known as Siding Spring.
What would be the consequences for Mars if the comet actually crashed into it? As a comet approaches the Sun, the volatile materials constituting it evaporate and it spews dust, water vapour and several other gases for a hundred thousand km or more.
So, comet Siding Spring could shower the outer parts of the Martian atmosphere with dust and gases, at least for a few days. The dust particles moving at velocities of fifty km a second could pierce the surface like bullets.
A greater danger is that the comet could break up (like comet Shoemaker-Levy did in 1994) hurling the remnants onto the Martian surface. If comet Shoemaker-Levy had broken up above the Earth’s atmosphere rather than Jupiter, humanity would not have survived.
A single fragment of the Shoemaker-Levy impacting Jupiter at sixty km per second had more energy than the impact of a Hiroshima bomb released every second for ten years.
Astronomers are now becoming more aware of possible impact of asteroids and comets on Earth. An impact of a ten km asteroid (that wiped out the dinosaurs) is expected only once in fifty million years. Some dedicated telescopes have made an inventory of over a thousand asteroids (a km or more in size) which could pose potential threats in the future.
Comets are more of a problem as their emergence from the outer reaches of the solar system cannot be predicted well. Far beyond Neptune’s orbit, our solar system is surrounded by a shell consisting of several billion of comets, known as the Oort cloud.
Perturbing gravitational forces acting on the Oort cloud can send several comets onto paths that carry them into the inner solar system. Comet orbits are hard to calculate as surface eruptions of gas and dust can change their path.
Debunking myths
Comets are made up of a loose mixture of rocks and ices and are often called dirty snowballs. Every time a comet approaches the Sun, it loses some of its mass as frozen ices evaporate and gases and dust are ejected forming a long tail. As large comets approach the Sun, volatile constituents vapourise forming a dusty gas tail that can exceed a hundred million km making them clearly visible from Earth.
The dirty snowball of dust and ice (unchanged for billions of years) warms and emits a huge cloud of material called the coma, followed by the tail. Most of the dense rock constitutes the core. After several thousand years, a comet can lose all its volatile matter (ices and dust) and ultimately lose its tail.
Appearance of a large comet in the sky was considered a harbinger of disasters like plagues and warfare. Even the appearance of Haley’s Comet in 1910 led to a fear that the world was about to end especially when astronomers detected sulphur compound and cyanide in its tail.
However, matter in a comet’s tail is too rarefied to cause any damage. It is interesting to note that in the distant past, comets were thought to be phenomena occurring within the earth’s atmosphere. Vapours within the atmosphere were considered to have soared up towards the heaven. It was Tycho Brahe, in the sixteenth century, who showed that comets are independent celestial objects well beyond the parameters of our own planet.
As all cameras orbiting above Mars are designed to focus on the planet, they are not likely to produce good images. But the Hubble Space Telescope and other observatories on Earth have the ability to capture clear images. Also, the Curiosity Spacecraft on Mars could get lucky and snap an image of the comet flying past it.