Rush hour in Martian Orbit

We have two Mars orbiters arriving at Mars later this month (21 and 23 September) .  NASA's Mars Atmosphere and Volatile Evolution (Maven for short) and India's Mangalaan Orbital Mission (MOM for short.  NASA spent $671 million on Maven.  India's MOM is famous in technical circles for getting to Mars for about one tenth that.  Both orbiters still have one heavy duty maneuver before they can be considered to have "arrived"  That is the Mars Orbit Insertion (MOI)  a 34 minute burn of the rocket engines to slow the orbiter to Mars orbit velocity.  That's a long burn, especially for an engine that has been floating in vacuum, unused, for nearly a year.   To make the MOI dicier, the maneuver must be executed by the onboard microprocessor, since radio signals from Earth take 20 minutes to reach Mars.  15 years ago a gross software fault caused a Mars Orbiter to crash on Mars from a failure of the MOI maneuver. 
   Maven carries instruments to verify a Mars creation theory.  Now that we have good evidence of free surface water in the distant Martian past, the theory suggests that the water vapor escaped into interplanetary space due to Mar's weak gravity.  Maven's instruments will measure the flow of gases and ions in the upper Martian atmosphere, hoping to show that water is still escaping and measure the rate, as a way of figuring how long surface water lasted on Mars, before it escaped into space.
  To add to the fun, Siding Spring, a comet, will swing by Mars on 19 October.  It is believed that Siding Spring is a new comet, on it's first trip into the inner solar system.   It is thought that new comets are chunks of ice and gravel that have been floating in interstellar space ("the Ort Cloud") since the beginning of the solar system, and Siding Spring represents matter from the dawn of time, or at least the birth of the solar system which is a long time ago.  Scientists are eager for any information the Mars orbiters can gather from Siding Spring. 

Going to Mars on a budget

The unmanned Indian Mars mission, Mangalyaan, was launched for a mere $80 million according to Aviation Week.  Whereas the next US Mars mission, Mars Atmosphere and Volatile Evolution (MAVEN for short), due to launch in a week or two, will cost $671 million. eight times as much.  Granted, the Indian mission only carries 15 kilograms of scientific experiments, but still the difference in cost is striking.  "If India can make the world's cheapest car and the world's cheapest tablet, launching the cheapest Mars mission is no big deal," quipped one Indian space scientist. 
   Mangalyaan has a long way to go.  It will be 10 months coasting out to Mars, at which point it has to make a burn to establish itself in an orbit around Mars.  We all hope that after 10 months in interplanetary space, all the equipment will still be in working order.  Mars is a tough target.  Over half the missions to Mars have failed for one reason on another, including missions by Japan and China quite recently.
  Good luck and God Speed.

Flying a satellite in atmosphere

The Mars Atmospheric and Volatile Evolution  ("Maven")  mission, getting ready to launch for Mars, is planning to do just that.  To investigate the Martian upper atmosphere, the satellite will be placed in an elliptical orbit around Mars, which dips into the Martian upper atmosphere to take readings.  The orbit will go within 150 kilometers of the surface.
   That's about the altitude that the old Mercury capsules orbited at.   On Earth, there is enough air left above that altitude to form the ionosphere, important to HF radio transmission.  Clearly the experimenters believe there is a Martian ionosphere, thick enough for Maven's instruments to take a reading on.
   They didn't say how long this can go on before atmospheric drag pulls the satellite down. 
   They mentioned that Mars used to have a strong magnetic field which disappeared some time in the distant past.  That's a new one on me.  It is thought that the magnetic field used to shield Mars from the solar wind.  When the field went away, they think the solar wind stripped away most of the Martian atmosphere.  Just how the ancient Martian magnetic field was discovered, and what might make it go away wasn't discussed. 
   Anyhow they hope to use Maven to measure what in the upper atmosphere might be related to, or causes of, Mars' lack of air and water.