JOIN USComets have always been fascinating objects for laymen as well as professional astronomers. The October sky has four of them staring at us – C2013 V5 (Oukaimeden), C2012 K1 (Panstarrs), C 2014 E2 (Jacques) and the faint C2013 A1 (Siding Spring). All of these are being studied for their cosmic activities.
Although Comet Siding Spring is the faintest among these, it is making news because it is heading for a close encounter with Mars on 19th-20th October 2014. It has an orbit that brings it up from southern part of the celestial sphere towards the Sun. The comets can have all possible orientations for their orbits and very rarely, they move along the plane of ecliptic. Moreover, the orbit will bring it closest to the Sun on 25th October within a few days of its encounter with Mars. This point referred to as the perihelion can vary from fraction of AU to its fractions or multiples. It may be recalled that last October everyone was looking at comet ISON with great enthusiasm. It did not survive the closest approach at all. This year Siding Spring is posing the other extreme – its closest approach to the Sun is a good 1.4 AU away.
The comet was discovered in January 2013, by Robert McNaught. He used the 0.5 m Uppsala Southern Schmidt Telescope at the Siding Spring Observatory, Australia. Initial orbital calculations indicated a collision with Mars, but subsequent refined ones showed a close encounter. Since then it is being monitored by many ground-based telescopes and the space missions.
By January 2014, it seemed to increase in brightness much above the predicted values. The rate of production of dust, gas and chiefly water has been estimated from these observations. By the end of September it was producing 10^28 molecules of water per second.
The orbit has yet another peculiarity – it is retrograde. As mentioned earlier, the comets can approach the Sun from all possible directions. But it is approaching Mars from an opposite direction. As a consequence, the relative velocity of approach is very high 50-60km/s. Its orbit is inclined by 129 degrees.
The comet is leaving the OortCloud and approaching the Sun for the first time; thus it is a treasure house for scientists digging out the history of our solar system. It is bringing along material which was available for the Sun (to form the solar system) 4.5 billion years ago. The closest approach distance to a planet is another record. The last we had heard of a similar encounter was in 1983, when comet IRAS-Aracki-Alcock visited the Earth from a distance slightly more than that of the Moon. The fatal encounters happened in 1994 when Comet Shoemaker Levy crashed on Jupiter and the unknown objects (asteroid or comet?) hit the earth in 1908 and in last February.
A dust bath
The coma (the head) of a comet is generally 10000 to 100000km wide. Therefore Mars is anticipated to pass through the coma. Since the estimated production of dust is about 430 tonnes, Mars will have to endure a huge dust bath. Model calculations put the diameters of particles between 1 µm and 12.4 mm. The phenomenon may be compared to volcanic dust pumped into the Earth’s atmosphere. However the situation on Mars will be different; the organic carbon of the dust may enrich the methane content or may not, depending on whether the comet enhances the activity.
Considering the large extent of the coma, it is certain that the upper atmosphere will be strongly perturbed since large amount of material will be deposited. The associated tidal effects and green house effects have also been calculated. The comparison of properties of ice with H2O + CO2 prior to encounter and after the encounter should be interesting, especially keeping in mind the comets as possible source of water for planets.
Likewise the effect on the production rate of these molecules by the comet, if any, also will be interesting. With the known water molecule production rate, the temperature can increase by almost 30 degrees. This effect will continue for a good number of hours after the close encounter. The increase in density of hydrogen is expected to be almost double. How Mars is going to digest these sudden changes is going to provide some key points for understanding the evolution of atmosphere in our own planet.
The event has come as a rare opportunity to study the comet from close quarters for Mars Reconnaissance Orbiter which is capable of providing high resolution images of the comet. The other two orbiters are MOM (from ISRO) and Maven, both of which have special instruments to measure methane and D/H ratio.
The measurement of deuterium to hydrogen ratio is done by separately measuring the energetic UV photon (called Lyman Alpha) emitted by hydrogen and deuterium. This is important in understanding the process of mass loss from the atmosphere. Mars Odyssey is equipped with infrared measuring tools which will throw light on the thermal variations in the atmosphere.
Rover analysis
The two rovers moving on Mars can obtain images and try to get images of meteors too. As can be expected there is going to be a very high rate of small particles of size 0.5 to 1 micron or greater falling. This shower may start a few days prior to the close encounter. The estimated ZHR (zenith hourly rate) exceeds 600.
The danger in the form of energetic particles hitting the spacecrafts has been theoretically calculated and an assurance on the safety is hinted. This is based on the observations from Earth so far indicating a normal activity and expected continuation of the trend. The potential candidates for causing severe damage should be larger than 50 microns; these must have been already released (as on mid-September) so as to impact on October 19-20.
So far the observations have not indicated a positive trend. Even the velocities seem to be less than the critical value. Still, it is planned to manoeuvre the orbiters to move to the opposite hemisphere of Mars at the instant of closest approach. The rovers may also need to run for cover to protect themselves from the incoming comets and dust.
(The writer is Director, Jawaharlal Nehru Planetarium, Bangalore)