Mars is a crowded place these days and is soon to get another, high-speed, transient visitor.
Comet Siding Spring (C/2013 A1) is approaching Mars and will pass within about 87,000 miles (139,500 kilometers) on Sunday 19th October.
Siding Spring’s nucleus will come closest to Mars around 11:27 a.m. PDT (2:27 p.m. EDT), hurtling at about 126,000 mph (56 kilometers per second). This proximity will provide an unprecedented opportunity for researchers to gather data on both the comet and its effect on the Martian atmosphere.
Siding Spring came from the Oort Cloud, a spherical region of space surrounding our sun and occupying space at a distance between 5,000 and 100,000 astronomical units. It is a giant swarm of icy objects believed to be material left over from the formation of the solar system.
Siding Spring will be the first comet from the Oort Cloud to be studied up close by spacecraft, giving scientists an invaluable opportunity to learn more about the materials, including water and carbon compounds, that existed during the formation of the solar system 4.6 billion years ago.
Currently NASA has three craft in orbit around Mars (Odyssey, MRO and MAVEN), the European Space Agency has MEX and the Indian Space Research Organisation has MOM. In addition there are two active rovers on the surface of Mars; Opportunity and Curiosity. All the orbiters face a small risk of damage – not so much from Comet Siding Spring itself but from its long dust tail. The rovers are not considered to be at significant risk since they will be protected by the Martian – albeit very thin – atmosphere. They have been moved to positions to observe.
SkyandTelescope: Such a close encounter means the dust tail left in C/2013 A1’s wake might graze Mars’s upper atmosphere. The smallest particles are only about a half millimeter across, but even these could severely damage a spacecraft when striking at 35 miles per second. Scientists predict that the time of greatest danger for the orbiters will occur about 90 minutes after Comet Siding Spring’s closest approach and last about 20 minutes.
The three NASA Orbiters and ESA’s MEX have re-positioned themselves and will take shelter on the far side of Mars as the comet flies past. The Indian MOM has very little fuel to expend for any major changes to its orbital path and will just try to get as far away from the dust tail as possible and keep its antennae crossed.
To avoid the threat of Siding Spring’s debris, NASA engineers will manipulate the orbiters’ trajectories so that all three will end up on the opposite side of the planet during the flyby. The MRO team executed one maneuver at the beginning of July, with another planned for the end of August. The Mars Odyssey team took similar steps on August 5th, and the MAVEN team will perform a precautionary maneuver shortly after the spacecraft enters orbit around Mars.
MEX is following the same strategy
The European Space Agency is taking similar precautions to protect its Mars Express (MEX) orbiter. MEX has a highly elliptical orbit that would leave it exposed to Siding Spring’s debris longer than MRO or Odyssey. On June 22nd the MEX team altered the orbiter’s track around the planet so that it will be hidden behind Mars for 27 minutes during the comet’s closest approach.
ISRO’s MOM will not be behind Mars when the comet makes its closest approach to the planet. They do not have the fuel to expend and so will just try and be as far away as possible.
Hindustan Times: “We have repositioned the Mars Orbiter, as the comet Siding Spring is expected to be close to the Mars on October 19. We have taken the Orbiter to a position farthest from the tail of the comet so that it doesn’t affect the satellite,” AS Kiran Kumar, director, Space Application Centre, Ahmedabad, said.
Fortunately the latest estimates have reduced the risk of collision somewhat:
ESA: Initial estimates gave the possibility that Mars Express might have to contend with a large particle flux – and that several (2? 3?) very high-speed (~56 km/sec!) particles might bash into the spacecraft. Happily, additional observations by ground and space telescopes (including the ESA/NASA Hubble Space Telescope) have allowed initial estimates to be refined and the risk is now understood to be much lower – and perhaps even as low as zero.