JPL and NASA News

Bill Wheaton, IPAC

1999 March

Near Disaster on NEAR

As most of you will have heard, NEAR, the 433 Eros asteroid rendezvous mission, had a close call on 20 December, when it was expected to begin its final approach maneuvers. One of the common problems liquid rocket engines sometimes encounter is something called a "hard start". This just means an excess of propellant accumulated in the combustion chamber at ignition, before stable burning was established, leading to a pressure transient. In extreme cases hard starts can cause the chamber to be damaged or even to explode. For the case in point, a sensor shut down the engine when it detected an anomaly shortly after ignition.

At the time of the maneuver, NEAR was flying in solar orbit about 700,000 km ahead of Eros, moving approximately parallel to it and at nearly the same distance from the Sun, but traveling too slowly to be captured by Eros' feeble gravity. As a result Eros was steadily overtaking it, and would soon pass it by. The planned 20 December burn (others shortly following) would have increased NEAR's speed by about 0.96 km/sec, allowing a cautious approach to Eros orbit to be made during the following few weeks. But with the aborted burn, (a) the spacecraft entered safe mode, turning its high-gain antenna away from Earth, (b) communications were lost, so that NEAR's true velocity and position became increasingly uncertain, (c) as urgent as it was to regain control of the vehicle, it was even more essential not to send any commands that could compound the problems and possibly cause irrecoverable difficulties, (d) Eros continued to bear down on the spacecraft, eventually passing it on 23 December at a distance of 3951 km, (e) most serious of all, much (30 kg) of the available maneuvering propellant was expended during NEAR's automated attempts to recover from the aborted engine burn, considerably limiting the available options for future maneuvers.

Control of the spacecraft was finally regained after 27 tense hours, and a command sequence was quickly uploaded to observe Eros on the 23 December flyby. After this was successfully accomplished, controllers at the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland turned to the urgent problem of designing an orbit-capture maneuver for Eros, now rapidly dwindling into the distance. A burn executed on 3 January 1999 essentially completed the maneuver originally planned for 20 December 1998 and also one that had been scheduled for 28 December 1998. Unfortunately by this time NEAR was trailing Eros by over 1,000,000 km. With the limited available propellant, the quickest that little distance (hardly 3 times the Earth-Moon separation!) can be safely covered is about a year, so the rendezvous and orbit insertion must now be delayed until February 2000.

Despite this adventure, considerable useful information was obtained on the 23 December close pass. The dimensions of Eros have been measured to be 33 X 13 X 13 km, slightly smaller than originally estimated from ground-based radar observations, and, from its gravitational effect as NEAR flew by, its mass has been determined as well. The shape is something like a squashed, elongated groundnut. This information, together with the fact that no large moons were observed, will considerably facilitate the significant operational problem of designing a safe approach to orbit around an irregularly-shaped object which is rapidly spinning -- a problem never before encountered in Solar System exploration. Most of the surface was imaged, with a resolution ranging down to 400 m at closest approach, and spectral data show the surface composition to be variable. Many craters appear, ranging up to 8.5 km in size, but it appears that Eros may not be composed of unconsolidated rubble (as has been suggested for asteroid 253 Mathilde, based on data from NEAR's June 1997 flyby of that body), but rather may have a more rigid structure. The size and shape information give a preliminary density of 2.7 gm/cm³. This value agrees well with that previously determined for 243 Ida by Galileo in 1993, the only other S-type asteroid (believed to consist of a combination of silicate minerals and metals) to have its density measured to date. Mathilde, a C-type (carbonacious) body, had a much lower density of only about 1.5 gm/cm³.

Stardust Launched

On 7 February 1999 the Stardust comet sample-return mission departed for a rendezvous with comet Wild 2, scheduled for 2004, almost 5 years hence. Stardust will fly past Wild 2 at relatively slow speed, deploying "astrogel" collection panels to trap cometary dust grains. Astrogel is essentially a rigid foam of silica, with a density which may be made less than that of air. Because of its structure it is capable of non-destructively slowing and capturing comet grains for later examination. After a total of three loops around the Sun, Stardust's re-entry capsule will return to Earth and parachute to a landing in Utah in 2006. It is the first US mission dedicated to cometary science, the first sample-return mission launched since 1976, and also the first sample-return mission ever to venture beyond the Moon.

Other News

NASA recently announced the selection of five medium-class Explorer (MIDEX) missions for detailed study; later this year, two of these are expected to be finally chosen for flight. The five include the Swift Gamma Ray Burst Explorer, which will study bursts and their after-glows with gamma-ray, X-ray, and UV/optical telescopes; NGSS, a Next Generation Sky Survey in 4 infrared channels, 1000 times more sensitive than its predecessors; FRAME, the Full-sky Astrometric Mapping Explorer, able to determine stellar distances by parallax as far as the center of the Galaxy; AMM, the Auroral Multiscale Midex, a constellation of 4 identical spacecraft to study the spatial and temporal structure of auroras and the ionosphere; and ASCE, the Advanced Solar Coronal Explorer, a telescope with two advanced solar instruments that would be deployed from the Shuttle and later recovered.

Meanwhile, WIRE, the Wide-Field IR Explorer which we described last time, is now scheduled for launch on 2 March, while the launch of Chandra, the third Great Observatory (lately renamed from AXAF, the Advanced X-Ray Astrophysics Facility, for the great theoretical astrophysicist and Nobel Laureate Subrahmanyan Chandrasekhar), has been delayed until July. Finally SWAS, the Submillimeter Wave Astronomy Satellite, is already checked out in orbit and reports detecting large quantities of water in interstellar clouds.

Bill Wheaton
Waw@ipac.caltech.edu