Introduction: transits of Venus are rare events for people bound to Earth. James Burke tipped me to the story about plans to observe transits of Venus as seen from other planets, which, of course, happen on different dates. Jay Pasachoff kindly explained his proposals for such observations to me in an email, which he has permitted me to reproduce below. -- Emily Lakdawalla
I am pleased to supply more information about our hopes and plans for observing transits beyond the one of Venus as seen from Earth on June 5/6, for which I have a grant from the Committee for Research and Exploration of the National Geographic Society.
As you know, David Ehrenreich and Alfred Vidal-Madjar in France, and others, [had] Hubble Space Telescope time to observe the June 5/6 transit with three instruments by looking for 0.1% dips of the brightness of the three areas of the Moon at which the instruments will stare. There will be calibration time before and after the transit. All times are about a half hour later than those for the Earth. Glenn Schneider and a colleague tried to use a Steward Observatory telescope to do something similar for the 2004 transit, but had a ground-based telescope and inadequate time on the transit from their location.
When I learned about the Hubble experiment, I arranged to meet with Ehrenreich and Vidal-Madjar (with whom I had written a paper about interstellar deuterium about 30 years ago) in Paris right after the Division of Planetary Sciences meeting in Nantes, where Widemann and I had coordinated our mountain-based 2012 observations. I had brought him into the analysis of the 2004 TRACE spacecraft observations that Schneider and I had analyzed and published [PDF], with the additional author Leon Golub, who had built the camera for TRACE. Widemann provided links to the data from Venus Express. Later, in a subsequent article, Paolo Tanga from an institute in Nice elaborated on the 2004 transit as observed from Pic du Midi, other ground-based telescopes, and Venus Express, for which I was again a coauthor along with several others.
After Widemann and I met with Ehrenreich and Vidal-Madjar at l'Institut d'Astrophysique in Paris, I was jealous of their successful proposal, so I thought about what other transit opportunities there might be. It turns out that a transit of Venus was visible from Jupiter earlier in May, which was too soon to propose for Hubble time, with another coming on September 20. I put together a team that involved Schneider, the French colleagues, and my former student Kevin Reardon, who is now at the Arectri Observatory. We also included one the two main scientists who have instruments to study the Total Solar Irradiance: Richard Willson of ACRIMsat, with whom we had put together NSF and NASA proposals for the current transit. Co-investigators also include the French group with the addition of Alain Lecavelier des Etangs from Paris, and John Clarke of Boston University.
Since Schneider was instrument scientist on Hubble's NICMOS and was very experienced with making Hubble proposals, he took my first draft and supervised the major proposal--which is always a very big effort for a Hubble proposal. There were a semi-infinite number of emails considering various possibilities, including using Jupiter's disk as a reflector and using one of Jupiter's moons. We decided that Jupiter's disk came nearest to filling the field of view from Hubble so observing its intensity over the 9 hours of transit plus some time before and after became the major thrust of our proposal, looking for a 1/100 per cent drop in the intensity. Io and Europa add to the orbits immediately flanking the transit, and we will use their added contribution to help in our calibration.
The proposal was submitted this spring; we expect to be notified on June 15. It would be nice to know before I give an invited, plenary talk about the transit of Venus at the AAS meeting in Anchorage on June 11, though we probably won't.
In any case, we then looked for a transit of Venus from Saturn, and found one on December 21. We brought Phil Nicholson of Cornell into the group because he is on the Cassini team. At a group meeting last month, he convinced everyone (we had sent him some PowerPoints and other ammunition) to provide Cassini time for that transit, also for an 0.01 per cent effect, on December 21.