How hard is it to hit a moving planet? Beware of the most often used analogy, that of firing a bullet at a target the size of a dime placed an unbelievable distance away. Not a good comparison unless you’ve aimed your rifle using Newton’s gravitational laws of motion and your bullet—on command—adjusts its path in-flight. And you have radar tracking and two-way communication as well. The truth is that technology from four decades ago proved fully capable: Mariner and Viking missions ventured to and successfully pioneered exploration of both Venus and Mars.

So why is Mars so “hard”? Mars Observer went silent just before arrival in 1993. Mars Climate Orbiter did not survive the insertion maneuver in 1999 due to a mix-up of English and metric units. That same year,  Mars Polar Lander was also lost on arrival. On the other hand, NASA has had remarkable successes too: Pathfinder / Sojourner set the way for today's duo of rovers scouting Martian geology. Ninety percent of that data comes to Earth via Mars Odyssey, and the “Energizer Bunny,” Mars Global Surveyor—since late 1996—has returned more data and mapped more of the surface than any other mission. So the odds are actually in JPL’s favor. Maybe Mars is just unlucky, and it’s the space travel, pushing the envelope, that’s hard.

Testing MRO’s cameras begins this week, and—fingers crossed—the HiRISE high resolution camera should deliver the best images ever, resolving objects as small as a kitchen table (about 1 meter across) on the surface: the Rovers should be easy targets. In fact, the scientific payload aboard MRO is the largest and most capable ever sent: its six instruments will study Mars in unprecedented detail, from the top of the atmosphere for weather and climate, to surface relief and mineral composition, to the subsurface for liquid water or ice. Over its mission, MRO will send ten times as much data as ever sent before: data rates are quoted in units of CD-ROMs per hour. And if NASA follows their custom of putting raw and processed images quickly on the web (at right: science mission images will be taken from a much lower in orbit and have seven times greater resolution), we shouldn’t have to wait long for remarkably detailed and unparalleled views.

After the excitement of arrival, the MRO team is now settling into six months of aerobraking. The arrival orbit is highly elongated, varying from 44,000 to just 400 km above the surface. As MRO reaches its closes point to the surface, it will skim through just enough to thin Martian atmosphere to experience some drag and loose speed. The result, over hundreds of aerobraking passes, will gradually alter the orbit to be nearly circular, polar and low-altitude. JPL flight engineers will control these dips with brief thruster burns at the top-ends of the orbit. A “walk-out” phase will then raise the low point back out of the atmosphere and into the intended, 200 by 150 mile, 2-hour, science-phase orbit. Remarkably, tiny variations in the drag experienced by the MRO, sensed by sensitive accelerometers aboard, will be used to probe the density, temperature and pressure patterns in the Martian atmosphere. The trade-off is time for science: aerobraking saves 50% on the mass of fuel needed to attain a circular orbit directly on arrival. JPL put that weight in instruments, not fuel.

It is quite possible that the site for man’s first steps on another planet will be first identified by MRO. As long as the gremlins can be kept away, we can expect exciting and spectacular discoveries to be posted at mpfwww.jpl.nasa.gov/mro/. 

At right: Detail of the detail image from the first returned from MRO's HiRISE camera. Image Credit: NASA/JPL-Caltech/ University of Arizona. Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 26 March 2006. Text and uncredited images copyright © 2006 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.