The current mission design calls for a January 2015 launch (with a window from late 2014 to early 2015). IVO would be launched into a Venus-Earth-Earth gravity assist trajectory, similar to the interplanetary path taken by Galileo between 1989 and 1995. Calibration for IVO's various instruments would be performed during one of the Earth encounters, though McEwen does note that an asteroid encounter during one of the apohelions could be used as a dress rehersal for the Io flybys.
After a six-year cruise, IVO would enter orbit around Jupiter in 2021. Prior to Jupiter orbit insertion, IVO would perform its first flyby of Io. While other information I have found suggested that no science would be acquired during this first encounter, McEwen presentation points out that the reason for performing this encounter is to acquire unique equatorial science since the encounter doesn't actually reduce the delta-V, like the inital Io flyby did for Galileo. IVO would be inserted into an inclined, 200-day-long initial orbit following JOI.
During IVO's orbital tour, planned to last at least 1.5 years, the spacecraft would flyby Io at least six more times, with the goal of having more than ten flybys with the help of an extended mission. With the various sources I have, I am not certain if they plan to have six flybys during the primary mission with at least four more during an extended mission, or if the six-flyby number is only a baseline for mission success. The trajectory design calls for the Io flybys to also be used to "pump down" the spacecraft's apojove, eventually bringing IVO's orbital period down to 30 days. The first few flybys of Io would be further out, with altitudes between 500-1000 km, until estimates of Io's position are refined. Later encounters can then be brought down to altitudes approaching 100 km, supporting magnetometer and mass spectrometer science. Like Galileo, data would mostly be taken during the few days surrounding perijove and would be transmitted to Earth near apojove, though the much greater bandwidth with IVO would support more apojove science, such as imaging of Jupiter and eclipse observations of Io and Europa. Also, thanks to IVO's increased bandwidth, up to 20 Gb of data would be returned after every flyby, much greater than Galileo's entire Ionian dataset. I can't recall, but that maybe more data than Galileo returned during its entire Jovian tour.
For planetary protection purposes (God forbid we contaminate poor Amalthea), the spacecraft would be impacted with Io at the end of its life. However, several end of mission scenarios have been developed, including one that would put IVO into very large orbits, with periods up to one-year in length, in order to monitor Io for up to a decade and to perform a lifespan test for the Advanced Stirling Radioisotope Generator (ASRG), the next-generation power source to be used by IVO. In another scenario, IVO's perijove would be raised to pull it out of the worst of Jupiter's radiation belts, again to extend the spacecraft's life, though there maybe issues with the requirement to dump IVO on Io at the end of the mission.
Io Volcano Observer, being half a science mission and half a engineering test, has a very focused set of mission goals. As you can see in the slide at right, the primary objective of this study is to see if an Io mission can be accomplished with a Discovery budget, assuming NASA furnishes the ASRG and NEPA.
The baseline mission was designed as simply as possible while still accomplishing a focused set of science goals. From the presentation, these include:
- Understanding active Ionian volcanic processes
- Understanding the process of tidal heating
- Understanding loss of material from Io and following that material to the magnetosphere, plasma torus, and neutral clouds
There are also secondary goals to study the rest of Jovian system. Most of these observations would be acquired near apojove. These include: monitoring of Jovian clouds systems near the poles (complementing data from Juno), distant observations of the Galilean satellites including Europa's atmosphere, close-up observations of Jupiter's inner moons near perijove, and observations of Jupiter's rings and the Io Plasma Torus. Due to data volume restrictions on the later, shorter orbits, many of these observations would be acquired during the initial, 200-day orbit, when data volume would be more plentiful.
At the Io workshop, McEwen presented details on the planned baseline payload as well as other instruments that could be used on IVO. With this payload, McEwen and his team hope to accomplish the science goals discussed above.
The baseline payload for IVO includes the following instruments: a narrow-angle camera with an angular resolution similar to Galileo, a thermal mapper similar to THEMIS on Mars Odyssey, an Ion and Neutral Mass Spectrometer, and magnetometers. Other potential instruments outlined by McEwen include a wide-angle camera, a near-IR spectrometer, and an EUV/FUV spectrometer.
The narrow-angle camera, as outlined by McEwen, would provide medium-resolution surface monitoring and high-resolution surface imaging during flybys, while keeping noise to a minimum. Unlike most spacecraft imagers in use, the camera would use a CMOS focal-plane system with both framing and pushbroom modes. The fast readout times for the CMOS system would help keep radiation noise down by reducing the amount of time between the image exposure and its storage in memory. The CMOS detector would be split up so that the bottom half of the detector (1000 lines by 2000 columns), would be used for clear filter, framing-mode imaging, similar to Cassini's ISS. Up to half of the detector would be used for color filter, pushbroom imaging, similar to the Mars Reconnaisance Orbiter's HiRISE camera. This upper section would consist of up to 15 color filter strips of 64 lines each. These strips would use digital Time-delay Integration to produce nearly simultaneous color imaging (necessary for lava temperature measurements), super-resolution, and faint-target imaging. At minimum, McEwen's group would like to include broadband UV, blue-green, red, and near-IR dTDI lines, though other filters could be included, mostly narrowband filters designed for spectroscopy.
IVO's thermal mapper would be designed much like Mars Odyssey's THEMIS instrument. The instrument would include at least three bandpasses between 2 and 20 microns and would have a resolution about 12.5x coarser than the NAC (assuming new detectors are used, compared to THEMIS). With Thermal Mapper, IVO planners hope to map and monitor Io's thermal emission, perform thermal emission spectroscopy, and analyze Io's polar thermal emission and compare it with results from Galileo at low latitudes. For a baseline mission, the expected bandpasses would be at 2, 5, and 20 microns, though more bandpasses maybe possible. Some that interest IVO planners occur between 7 and 9.5 microns, wavelengths diagnostic for silicate mineralogy.
The Ion and Neutral Mass Spectromter (NMS) was covered recently in a blog post, so check that out for details of that instrument. IVO planners seem confident that they can detect most of the expected species in Io's atmosphere (like SO2, SO, O, and NaCl) thanks to its high signal-to-noise ratio and 1-300 amu mass range.
The final pieces of the baseline payload are a pair of fluxgate magnetometers. These very small magnetometers would be attached to 1-meter long brackets bolted to the spacecraft. These magnetometers would be similar in design to those carried on recent ESA planetary missions. Now whether they would actually find a magnetic field at Io...
These instruments would be bolted to the top deck of the spacecraft, below the High-gain antenna. This would force decisions between having a gravity pass and a remote-sensing pass, much like Cassini. Much of the electronics for these instruments would be stored below this top deck, in a radiation-shielded "vault", in hopes to reduce the chances for radiation affecting measurements.
A major limiting factor for the lifespan of IVO is the high charged particle environment near Io. Even with the high-inclination orbits, IVO planners expect betwen 115 and 230 krads, though the spacecraft electronics may have a much reduced exposure during the primary mission.
Another issue is the current projected price tag. A JPL Team X assessment in November came up with a cost $471 million. I'm not sure if that is for the baseline mission, but at least $21 million will need to be trimmed from the estimated budget to get IVO within the Discovery costcap.
This presentation lays out a pretty compelling case for a cost-effective mission to Io. As of earlier this month, IVO planners were preparing to finish their final report for the DSMCE program and were looking forward to submitting this proposal for the next Discovery Announcement of Opportunity coming (hopefully) sometime around mid-year.
Link: Io Volcano Observer (IVO) [pirlwww.lpl.arizona.edu]