JPL and NASA News

Bill Wheaton, IPAC

2000 June

He's Back!

After some high-tech adventures of my own (a virtuoso, state-of-the-art hip repair) caused me to go on vacation here for 6 months, it is a pleasure to be able to return to writing these columns once again. What with the disasters on Mars, and various other ups and downs, there is much more that needs attention even than usual; but as usual, "rather than try to cover everything I will only attempt to uncover a little", as a favorite physics professor in college said long ago.

NEAR Shoemaker at Eros

NEAR, the Near-Earth Asteroid Rendezvous mission, to asteroid 433 Eros, finally slipped into Eros orbit on 14 February 2000, nearly 14 months late. To celebrate, it was renamed NEAR Shoemaker on 14 March 2000, in honor of the pioneer planetary geologist Dr. Eugene M. Shoemaker, tragically killed in a 1997 car accident in the Australian outback. We discussed NEAR in some detail here in 1998 November, its scientific objectives and instrument complement. But complications arose immediately afterwards. Most of you probably remember the truly hair-raising save of the entire mission in late December 1998. Without the major rocket engine burn planned for 20 December, Eros would pass NEAR by, and the spacecraft would fall in its orbit back to the orbit of Earth from which it came months earlier. The maneuver began on schedule, but due to a software error in the on-board computer, it aborted catastrophically when it was only about 10% complete. The spacecraft was left out of control, in an orientation unknown either to controllers on Earth or to its own control computer. Because the parabolic high-gain antenna was not pointed towards the Earth, normal communication was impossible, and because the solar panels were not oriented towards the Sun, there was every danger that the batteries would run down before control could be recovered. Meanwhile, Eros was receding into the distance at nearly 1 km per sec, about 2,000 mph.

Heroic efforts by mission controllers and analysts, aided by an emergency mobilization of the world-wide resources of NASA's Deep Space Network, saved NEAR from the awful fate that threatened. Control was regained just before Christmas, a new command sequence was designed and beamed up, and on 3 January 1999 a revised large rocket burn brought NEAR almost to a stop (with respect to Eros) -- but trailing far behind, and with only a limited amount of fuel available to catch up. Catching up then took the remaining time, over a year, until last February 14; but only about 55 meters per sec worth of fuel had to be used, because the spacecraft veritably oozed back across the intervening million km to Eros, at an average approach speed of only about 30 m per sec, something like 65 mph.

Since that time NEAR Shoemaker has been busily carrying out the scientific program originally planned. Regular readers may recall that Eros is one of the largest near-Earth approaching asteroids; its size has by now been measured to be about 33 km by 13 km by 13 km, with a mass of some 7.3 trillion metric tons, giving a density of about 2.5. As described here in November 1998, NEAR is the first mission ever to orbit a grossly non-spherical body; one which is moreover tumbling, end-over-end, in a flat spin. This introduces huge complications into the orbit determination and navigation of NEAR, which become rapidly move severe as the spacecraft moves into closer and closer orbits.

Happy to say, all those problems seem to have been successfully solved by the wizards of astrodynamics at JPL and APL. (NEAR Shoemaker is managed and operated by the Applied Physics Laboratory, APL, at Johns Hopkins University; however, the orbit determination has been done by JPL, under a subcontract with APL.) NEAR has fixed solar panels which need to mostly point at the Sun. The science instruments all look out together, perpendicular to this axis. Thus it makes sense normally to orbit in a plane perpendicular to the solar direction; this is why the figure says "View from the Sun". This plan also gives good shadows along the terminator for excellent visibility of optical features. The spin axis of Eros is essentially fixed in space. Fortunately for the complexity of the navigation at least, it does not wobble about this axis, as in principle it might. When NEAR first reached orbit in February, it was summer at the north pole, and the spin axis pointed not far from the Sun. Since then in its apparent motion about the sky, the Sun has moved about 60 degrees, and NEAR is lately orbiting over the poles. As the Sun passes south of the equator, the north pole, as in September on Earth, will enter its winter darkness, and the south pole will emerge from the shadow that has heretofore hidden it.

Figure from APL, Johns Hopkins University

The initial trajectory led to a circular 200 km orbit (the bundle of adjacent orbits in the figure) with a 10 day period, which NEAR entered on March 3, and where it remained for about a month. During this period the shape, rotation, and mass distribution were determined to guide operations at closer distances. It then moved to a ±100 km, 3.4 day orbit where it stayed only from 11-22 April, before moving to the current 50 km, 1.2 day orbit, reached April 30. This orbit gives the Multispectral Imager (MSI) camera a current field of view of typically 1.4 km and a resolution of about 5 m. It will remain in such low orbits, dipping for a week to 35 km in July, until late August.

Fascinating as the pictures are, they are not the main part of the mission science results, only a little of which have yet been released. NEAR includes, beside the 7-color visible and near-infrared MSI camera, an infrared spectrometer, a laser altimeter, an X-ray and gamma-ray spectrometer, and a magnetometer, as well as an important radio science investigation. Early scientific results are to appear, as has been customary, in several papers published together in the American journal Science. Eros was classified as an S-type asteroid, the class that dominates the inner part of the Asteroid Belt. The spectra of such asteroids seems to imply that they consist of iron- and magnesium-bearing silicates (pyroxene and olivine) mixed with metallic nickel and iron.

On the other hand, the relationship between asteroids and meteorites is not clear. The "ordinary chondrites", the most common meteorites, are primitive, undifferentiated objects, apparently unchanged since the Solar System formed 4.6 billion years ago. The "stony-iron" meteorites seem to be remnants of a body that was large enough to have a melted-iron core. Spectroscopically, it appears that the S-type asteroids are unrelated to the ordinary chondrites. Yet if so, then we do not know where these most common meteorites come from. Before NEAR Shoemaker, Eros appeared to have some similarity to both the stony-irons and ordinary chondrites, but seemed more closely related to the stony-irons.

Now, somewhat surprisingly, it has become clear that Eros contains very little iron. In fact no magnetic field has been detected at all, probably a disappointment for the magnetometer team. Yet the implications are very interesting. Also, no sign of any significant concentrations of mass have yet been found. Technically, the gravity seems to be consistent with the shape, assuming uniform density, out to at least the sixth order in the standard mathematical expansion of the gravitational field. The absence of iron suggests that either the S-type asteroids are quite different than the stony-iron meteorites they seem to resemble; or maybe Eros is in some way atypical, perhaps not type S after all.

Information about NEAR and Eros may be found at  http://near.jhuapl.edu/ ,  including (of course) lots of pictures.

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Short Items

Despite the gloom that prevails about Mars, the Mars Global Surveyor keeps quietly returning wonderfully comprehensive information about the planet. Unless more pressing matters intervene, I hope to cover some of these in the near future.

BOOMERANG, a recent balloon-based experiment that observed the cosmic microwave background for 10 days with high precision from Antarctica, now gives compelling evidence that the large-scale geometry of the Universe differs from flatness -- which is to say ordinary Euclidean geometry -- by no more than 15% or less. Unsurprising as this might seem from the view of 19th century physics, it is in fact not at all required by more modern theories. Yet it is the value predicted by the most popular "cosmic inflation" models for the early Universe. The problem is now that flatness seems to require a somewhat delicate relationship between the total density of matter (both ordinary and "hidden" or dark), and "quintessence", that mysterious source of the recently resurrected Cosmological Constant; which Einstein and everyone else abandoned long ago when Hubble discovered the cosmic expansion.

Finally, Ed Stone, director of JPL during most of the 1990's, and a hero of the Voyager missions to Jupiter, Saturn, and beyond, has announced plans to retire next year. JPL is operated for NASA by the California Institute of Technology, and Caltech president David Baltimore has lately announced formation of a committee to search for a successor.

Bill Wheaton
Waw@ipac.caltech.edu