Pluto-bound Spacecraft Gets Assist from Hubble

Here’s a story I’ve been following a while, partly through the eyes of my friend Alan Stern, who is the PI for NASA’s New Horizons mission to the outer solar system. The spacecraft is set to do the first-ever  flyby of Pluto in July 2015 before it heads out to explore a narrow region of the Kuiper Belt. Not surprisingly, the mission planners want to explore more than Pluto and Charon (and have a look at the other Plutonian moons, if possible), but also study any other Kuiper Belt Objects (KBOs) that might be along the path.

To do that, the mission needs some help from telescopes that could actually detect those objects that might lie along the path of New Horizons‘ trajectory, or could be visited relatively easily with a tweak of the spacecraft’s flight path. There are some ground-based telescopes (Subaru, Gemini (both on Mauna Kea in Hawai’i)) that can possibly see such objects (but which are quite faint and difficult to detect). The best choice is to use Hubble Space Telescope (HST) to search for KBOs in the region where the spacecraft would be next year. So, the team put in for HST time.

Getting time on HST is extraordinarily difficult, and you have to have a good science case for getting it. In particular, since HST is booked (and actually overbooked) well in advance, the best shot the team had for using the observatory was to apply for Director’s Discretionary Time. This is time allocated to the director to use as he or she sees fit, usually for particularly compelling targets of particular scientific importance. A good example of DDT allocation would be for a comet that suddenly appears and is of interest, or for another ephemeral object (such as a supernova). Or, DDT can be allotted for long-term surveys such as the Hubble Deep Field work that has been showing us the most distant reaches of the observable universe.

The case for a KBO hunt by Hubble was particularly strong. In talking with Alan, he confirmed that HST would be the best shot that the team had at finding something that the spacecraft could be programmed to reach. That’s why the team put in for DDT. And, last month, they got the word that the director had granted Hubble time to search for KBOs in support of the New Horizons mission. A preliminary search done using HST from June 16 to June 26 resulted in analysis of 200 images, and researchers found 2 KBOs, which is remarkable for such a short search time. The stage is now set for a longer-term search that begins the week of July 6th and winds up sometime in August. The hope is that HST will uncover other objects that New Horizons will be able to study.

Why study the Kuiper Belt Objects? This area of the solar system is a rich treasury of solar system materials that are quite primitive—that is, they date back to the formation of the solar system. Understanding (and charting) the characteristics of these distant worldlets (with their icy components) will give us a good window into what conditions were like in the early solar system. In addition, the Kuiper Belt is the source of many comets that find their way into our view. Planetary scientists have studied KBOs for at least a couple of decades now and have learned a lot about these most distant objects (despite that distance!). For example, many of them have moons of their own (c.f. Pluto).  Some are made mostly of ice while others seem to have more rock in their interiors. Their colors vary from red to gray. Some KBO surfaces are covered in water ice, while others have nitrogen or methane or other ices.

The “discovery” of the Kuiper Belt in the early 1990s (back when I was in graduate school, actually) immediately expanded our perception of the solar system. Those outer regions are now actually “places” with objects to explore. The Kuiper Belt itself is quite large and fills a volume of space much larger than we thought. But, one of the biggest changes in our perception of our solar system comes into play as we seek to understand how planets formed and where they formed. It’s now known that many objects in the Kuiper Belt formed closer to the Sun and were somehow moved out to these outer reaches. Vast migrations of objects took place, making the early epochs of the solar system’s history quite unsettled, indeed. And, what we’ve learned about our solar system could easily be applied to help us understand the formation of such systems around other stars.

So, for those reasons and many other scientific aims, it’s a great thing that Hubble will sweep its gaze out to the Kuiper Belt. Who knows what it might find? Certainly, based on the two new KBOs found in just a short time, it will most certainly find more of these distant worldlets. But, even if it doesn’t find ONE KBO along the New Horizons flight path (or one that the spacecraft can easily reach), what it does find will add immensely to our knowledge of the distant reaches of the solar system.