The so-called 'Pioneer Effect' continues to trigger study. Both Pioneer 10 and 11, as discussed in these pages back in November, have shown changes in their expected trajectories since they moved 20 AU beyond the Sun. In fact, since 1980 radio signals from the Pioneers have been slowly shifting to shorter wavelengths, which seems to imply a slight but interesting deceleration. This has led to at least one proposal for a mission to investigate the Pioneer effect. Both Galileo and Ulysses data have been examined for evidence of a similar effect; while Galileo's data were too limited for use, Ulysses did show a provocative, though extremely slight, change to its own acceleration (though at a much smaller distance from the Sun). Now a new paper notes the difficulties in measuring the Pioneer anomaly, and discusses a way of using asteroids and comets to measure gravitational effects in the outer Solar System. The paper is by computer scientists Gary Page and John Wallin (George Mason...

With the Jupiter Icy Moons Orbiter (JIMO) on hold, NASA is talking to the European Space Agency about a possible joint mission to Europa. A BBC story reports that a prime driver for ESA is the need to use radioisotope thermal generators (RTGs) on the mission, a power source with which the Europeans have little experience. RTGs are needed on missions to the outer planets because they increase the power available to the spacecraft, allowing for a wider range of experiments with more sophisticated instruments. Solar panels remain an option in Jupiter space, but aren't nearly as effective. The other driver, of course, is the recent success of the Cassini/Huygens combined mission, whose stunning images of the Saturnian system and data from the Titan descent and landing have many scientists now thinking of Europa. The moon's cracked ice seems to have been shaped by tidal forces from Jupiter, with reason to believe that an ocean of liquid water might be found beneath an ice crust tens of...

January 11 to February 14, 2006 marks the launch window for NASA's New Horizons mission to Pluto and the Kuiper Belt. At the moment, New Horizons is in pieces, or as principal investigator Alan Stern puts it in an update on the mission, it's in "...boards, boxes and a spacecraft bus on the cleanroom floor at the Johns Hopkins Applied Physics Laboratory..." The high-gain antenna is being checked, and the main computer system installed. According to Stern: The bird also received a guidance, navigation and control software load, and the first testing of the autonomy system (that provides for fault protection) has taken place. Coming soon to the spacecraft are the redundant flight computer, the gyros and the Ralph remote-sensing package. We are now approaching the time - only weeks away - when the last avionics box goes on the spacecraft and New Horizons is dressed in thermal blankets for environmental testing in a large vacuum chamber at NASA Goddard Space Flight Center. New Horizons...

Has Voyager 1 left the heliosphere? The question is a reminder that the Voyagers are our first interstellar probes; they'll still be returning data when they move into the interstellar medium. The heliosphere is a kind of bubble created by the solar wind from the Sun, that stream of high-speed charged particles constantly blowing into space at roughly 400 kilometers per second. Observing how Voyager 1 makes the transition across the boundary of the heliosphere will provide our first in situ study of interstellar space. Some scientists believe that at roughly 90 AU from the Sun, Voyager 1 has already pushed up against the 'termination shock,' that region where the speed of the solar wind drops to subsonic levels. Now new data studied by French and Finnish researchers indicate that the shape of the heliosphere may be distorted, further complicating the question of just where the true interstellar medium begins. Rosine Lallement and colleagues used data collected by the Solar and...

Our first interstellar mission won't be a long jump to Alpha Centauri or Barnard's Star. In fact, we've already launched not one but several interstellar missions -- the two Pioneer probes, and the two Voyagers that followed them, will all exit the Solar System; i.e., they will eventually cross the boundaries of the heliosphere to emerge into pure interstellar space. Some scientists believe that Voyager 1 is already pushing up against the so-called 'termination shock,' where the speed of the solar wind of gas and charged particles from the Sun drops to subsonic levels. But we need far more information than the Voyagers, with their rapidly fading signals, can tell us. The next mission designed to explore the outer limits of the Sun's influence will be the Interstellar Boundary Explorer (IBEX). Under development at Southwest Research Institute, IBEX is designed to explore how the solar wind interacts with the interstellar medium through which our entire Solar System moves. IBEX won't...

An asteroid called 2004 TP1 came within 13 LD of Earth on November 2 -- LD stands for 'lunar distance,' and is the average distance between the Earth and the Moon (238,855 miles, or 384,401 kilometers). Asteroid 2004 RZ164 will come even closer, at 7 LD on December 8. Both objects are considered Potentially Hazardous Asteroids (PHAs, as acronym-obsessed scientists like to call them). That means they are larger than 100 meters in diameter and come too close to Earth for comfort. 653 Potentially Hazardous Asteroids are now known. We've discussed such objects as perhaps the most significant reason for building up a space-based infrastructure that could ward off a potential strike. A good place to track them is the NASA-sponsored site Spaceweather.com, which bills itself as 'News and Information about the Sun-Earth Environment.' The site likewise tracks solar wind conditions (currently moving at 493.7 kilometers per second, based on data transmitted from the Advanced Composition Explorer...

When it comes to interstellar work, don't forget the Kuiper Belt. Although amateur astronomer Kenneth Edgeworth was the first to predict its existence, the Belt was named for Gerard Kuiper, who analyzed it in 1951. It is a region of thousands (and perhaps millions) of small, icy moons and cometary debris that exists from the orbit of Neptune well into deep space. Our first interstellar missions will be explorations of this area and the vast Oort cloud of comets that may extend as much as a light year out from the Sun. And yes, in a true sense, the Voyager probes could be considered interstellar missions, still reporting data as they move on toward the heliopause. But we may learn a good deal about Kuiper Belt objects by studying the findings of a spacecraft considerably closer, the Cassini Saturn orbiter. Cassini's Ultraviolet Imaging Spectrometer tells us that Phoebe, a tiny world about one-fifteenth the diameter of Earth's moon, is probably itself a Kuiper Belt object that was...

NASA has just announced that it has selected Northrop Grumman Space Technology as the contractor for co-designing its proposed Jupiter Icy Moons Orbiter. JIMO will be designed to orbit and explore three of the most interesting Jovian moons: Callisto, Ganymede and Europa. All three may possess water, organic material and a source of energy, leading to the possibility of some form of life evolving there. Image: The surface of Europa as seen by the Galileo orbiter. Note the crustal blocks on the left that seem to have once broken apart, and then 'rafted' into their current positions. They're evidence of what may be a sub-surface ocean. Credit: Planetary Image Research Laboratory, University of Arizona. Studying these moons closely will involve long periods in orbit around each before moving on to the next target. The propulsion system envisioned here is nuclear electric. NASA's Deep Space 1 spacecraft has already demonstrated the principle, in which electrically charged particles are...

Worth noting in relation to the JIMO story above (and for the broader issue of generating power for deep space probes): "A Power Conversion Concept for the Jupiter Icy Moons Orbiter," by Lee S. Mason (Journal of Propulsion and Power Vol. 20 No. 5, 1 September 2004, pp. 902-910). From the abstract: "An analytical study was performed to compare design options for a reactor power system that could be utilized on a Jupiter Icy Moons Orbiter mission employing nuclear electric propulsion."