From: Robert Herrick Date: July 14, 2009 2:21:07 PM PDT To: Rebecca Ghent , Martha Gilmore , Robert Grimm , Catherine Johnson , Pat McGovern , Peter Mouginis-Mark , Jeff Plaut , "Sandwell, David" , Buck Sharpton , Mark Simons , Sean Solomon , Rudi Gens , Franz Meyer Cc: "Rogers, Ford" Subject: status report Hi all, As I mentioned in my previous email, I think we are getting to a point where we have enough knowledge of potential mission details that we can start going through the process of developing the science plan. Next week Buck will lead an introductory meeting, give a current status report, and we'll discuss the overall framework of what we'll be doing over the next several months. I will be getting with you shortly on the date and time of that telecon. Here are some of the developments over the past few weeks: * Trajectory and orbit - Our chances for success for InSAR are much better with a circular orbit because this makes it much easier to get the spacecraft to fly close enough to the original orbit on repeat passes. A circular orbit will also provide a lot more flexibility in imaging. Circularizing the orbit will, however, add a significant delay (~300 days) before data collection can begin and an added cost of a few million. The current plan is for a circular orbit at 600 km altitude, which will separate the ground tracks by about 11 km at the equator. We are currently discussing using the NEXT thrusters (ion propulsion) to get into orbit. NASA is encouraging use of this system in the New Frontiers AO. Cruise, orbit insertion, and circularization will total up to about 2 years. * Imaging capability - The preliminary engineering studies indicate that atmospheric attenuation on Venus relative to Earth will require that narrower swaths be collected (the beam needs to be focused more at the same power), but we will be able to achieve the same image resolution. The resolution can be tuned from low to high resolution. We can do reconnaissance mapping at 30-100 m with swath widths of a few tens of km. We will be able to get down to 1-m resolution for small spots (few sq. km) and 3-m resolution for narrow (3.5-km wide) swaths. The spacecraft has a wide "accessible swath", so by slightly adjusting the look direction between orbits we will be able to do orbit-to-orbit mosaicking of swaths even if the swath width is less than the ground-track separation. * Volume of data collection - The limiting factor on the amount of data we can collect is our ability to send it back to earth through the DSN. The data rate could be as low as a few Mbps when Venus is far away, and tens of Mbps when Venus is close to Earth. Overall, if we assume a very conservative, constant downlink of 4 Mbps, then in one Venusian day we could cover these percentages of the planet at different resolutions: o ~1% at 3 m resolution o ~2% at 5 m resolution o ~18% at 50 m resolution * Sample goals and data volume - As part of materials that we are producing for Ames in our discussions with them, we constructed a preliminary set of mission goals and required data volumes, summarized in the attached document. Because of the particular information Ames wanted, this document separates things in a manner that minimizes overlap between the types of data to be collected, and it is probably not how things will end up organized in the proposal. Have a look at it, and perhaps it will provide a jumping off point for your own thinking about science goals, the type and volume of data to collect, and so on. * InSAR issues - While we can always use stereo radargrammetry to dramatically improve the topography, for many areas deriving topography from InSAR is a preferable method, and only InSAR would allow measurement of active deformation. The current assessment for InSAR at this point is positive. We are looking into the following issues regarding InSAR, and we'll let you know what we find out. o Navigation: One has to be able to position the spacecraft on repeat passes within a few hundred meters of previous passes. A problem is that Venus is farther away than Earth and the spacecraft is thus more difficult to track, we won't have GPS, and we only know the geoid well to about degree 60. On the plus side, the planet is spherical with fairly low, smooth geoid anomalies. At this point we feel reasonably confident that the necessary orbital precision can be obtained if we are in a circular or near-circular orbit of 600 km or so. o Long-wavelength distortions: If the atmospheric structure changes between passes in a manner that those changes are wavy at wavelengths of 10 m to 10 km, then topography generated from repeat pass interferometry will have distortions that cannot be distinguished from signal. This problem could be mitigated by having three or more passes over the same feature. o Short-wavelength distortions: If the atmosphere is turbulent at scales of 1 cm to 10 m, then pixels will decorrelate between passes and portions of the image pair may not be usable. Overall I think we are on track to putting in a great proposal, and we look forward to talking with you next week. Sincerely, Robbie Herrick -- Dr. Robert R. Herrick Geophysical Institute University of Alaska Fairbanks 903 Koyukuk Dr. Fairbanks, AK 99775-7320 Ph: (907) 474-6445 Fax: (907) 474-7290