Emily Lakdawalla wrote, "Although PT1 is the most likely of the Hubble-discovered objects to be targeted, it's possible that followup observations may make PT2 or PT3 more desirable." Assuming reachability, what characteristics would make a particular target more (or less) desirable? And how will you make those follow-up observations? More Hubble time? Additional telescopes? Thanks!
Hi ddbeck,
PT2 and PT3 are more desirable than PT1 simply because they’re brighter. Brighter means bigger, which means more science. Brighter means we can see the object sooner with LORRI. Brighter means the residuals on orbit fits will be smaller, and we will waste less fuel targeting the object and correcting our course. However, we do not have enough information yet to know if either PT2 or PT3 will be reachable. PT3 looks promising and PT2 is still possible, but not promising, and that’s the main purpose of our follow-up observations at present. We discovered PT2 and PT3 over a month after finding PT1, and the time between the first and last observations (“arc length”) is the major factor that dictates how well we can predict an object’s orbit.
The encounter with PT1 would also happen during the winter. The sun will interfere with communications only for a few days in January (not an encounter killer at all, but still sub-optimal). We could avoid that problem by going to the other objects. PT3 would have a summer encounter.
We will be following up with all our objects with Hubble, but planning for next year hasn’t happened in earnest. This week, we just started to get our last set of observations on these objects until spring. Next summer, when the conditions are most favorable, we plan to try to detect these objects from the ground, but it will take a lot of image stacking and we don’t know if we can do it. We’ll probably try with some of the big telescopes used for ground-based searching, like Subaru, Gemini and Magellan.
As for physical characteristics, another binary would be great (we can use Kepler’s laws to find the mass of the primary), but none of our potential targets appear to be binary. That’s not a surprise—these objects are at the limit of detection. A smaller body orbiting body would be beyond that limit. I’m told a fast rotator might make imaging more difficult, so we’d like to try to get light curves of these guys. We’re going to try to get colors on the object, but I’m not sure if we’ve decided if we would prefer to see a KBO with an unusual color or a typical one (red, for this type of object). The team still has more to talk about here.
Even if one of the brighter potential targets turns out to be reachable, we might just go with PT1 anyway. We’re interested in the idea of looking at some of the KBOs the search team discovered (but we can’t reach) at their closest approach to the spacecraft, and conserving fuel will allow that to happen.
But honestly, mission feasibility and reachability are our biggest concern.