We’ve often considered the effect of interstellar dust on a spacecraft moving at a substantial percentage of the speed of light. The matter becomes even more acute when we consider an interstellar probe arriving at the destination solar system. A flyby mission moving at ten percent of the speed of light is going to encounter a far more dangerous environment just as it sets about its critical observations, which is why various shielding concepts have been in play to protect the vehicle. But even at today’s velocities, spacecraft can have unexpected surprises when they arrive at their target.
We’re now looking toward a 2015 encounter at Pluto/Charon. New Horizons is potentially at risk because of the fact that debris in the Pluto system may not be found in a plane but could take the form of a thick torus or even a spherical cloud around the system. We don’t yet know how much of a factor impactors from the Kuiper Belt may be, but strikes at 1-2 kilometers per second would kick up fragments moving at high velocity, generating debris rings or clouds we have yet to see. For that matter, are there undiscovered satellites in this system that could pose a threat?
Image: Pluto’s newest found moon, P4, orbits between Nix and Hydra, both of which orbit beyond Charon. Finding out whether there are other moons or potential hazards near Pluto/Charon was the subject of a recent workshop that is gauging the dangers involved in the encounter. Credit: Alan Stern/New Horizons.
Working on these questions is the job of a team that met at the Southwest Research Institute (Boulder, CO) in early November. The New Horizons Pluto Encounter Hazards Workshop had plenty on its agenda. As principal investigator Alan Stern notes in this report on the New Horizons mission, the group was composed of about 20 of the leading experts in ring systems, orbital dynamics and the astronomical methods used to observe objects at the edge of the Solar System.
The Hubble Space Telescope will play a role in the search for undiscovered moons and possible rings, aided by ground-based telescopes that will study the environment between Pluto and Charon, space through which New Horizons is slated to move. Stern also notes that the ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope will be able to make thermal observations of the system, all of which should give us a better idea of the situation ahead even as plans go forward to consider alternate routes in case the current trajectory starts to look too dangerous. From Stern’s report:
Studies presented at the Encounter Hazards Workshop indicate that a good ‘safe haven bailout trajectory’ (or SHBOT) could be designed to target a closest-approach aim point about 10,000 kilometers farther than our nominal mission trajectory. More specifically, a good candidate SHBOT aim point would be near Charon’s orbit, but about 180 degrees away from Charon on closest-approach day. Why this location? Because Charon’s gravity clears out the region close to it of debris, creating a safe zone.
New Horizons is now approaching 22 AU out and has been brought out of hibernation until November 15 for regular maintenance activities. Tracking a spacecraft on its way to a dwarf planet we have never visited is intriguing enough, but the recent workshop was inspired at least partially by discoveries made after launch, such as the existence of the moon P4, which was found this summer. Stern mentions some evidence for still fainter moons that have not yet been confirmed, but it’s clear that space ahead may have more surprises in store.
Stern adds “it is not lost on us that there is a certain irony that the very object of our long-held scientific interest and affection may, after so many years of work to reach her, turn out to be less hospitable than other planets have been.” Indeed. Then factor in how much work went into getting this mission funded, built and flown — the payload aboard New Horizons is a precious thing indeed. We can only hope — and assume — that the observing campaign to verify the path ahead will be successful.