By Dr. Marc Rayman
JPL - June 22, 1999 - There is a great deal occurring as the mission of Deep Space 1 continues making great strides, so get comfortable for a long log entry! Of DS1's payload of 12 advanced technologies, 11 have now received 100% or more of their needed testing.
Nevertheless, additional tests are being conducted on many of them to assess how they fare as they continue operating in space. The twelfth technology is the autonomous navigation system, known as AutoNav. As of this past week, it is on schedule with 95% of its required testing complete.
With most of the technology testing behind it, the team's attention has turned to preparations for the July 29 encounter with an asteroid with the fanciful yet somehow fitting name 1992 KD. The primary objective of the event will be to provide that final 5% of the testing of AutoNav.
As a bonus, now that we have already squeezed more out of this mission than was required -- or expected by many people -- we want to do still more. With precious resources entrusted to NASA by taxpayers, the operations team wants to accomplish the most that it can.
So the two science instruments that DS1 has tested will be used to gather exciting scientific data (including pictures) during the encounter with this intriguing body. But keep in mind that this is primarily an exceedingly challenging test for the small portion of AutoNav that can only be exercised by visiting a solar system target. Last week, AutoNav passed other tests with flying colors.
On Monday, June 14, AutoNav made its first complete course correction. In the past it has corrected its course by modifying the direction and duration of thrusting that was planned for the ion propulsion system. But in this case, there was no reference plan for it to change; it had to start from scratch and decide the direction and duration of thrusting.
Based on its own determination of where it was in the solar system, where 1992 KD was, and what both of their motions were, AutoNav calculated what changes to make to keep it on track for the July 29 appointment with the asteroid.
But it turned out that in order to point the ion engine in the desired direction, the orientation of the spacecraft would have allowed the Sun to come too close to the camera and to the device that tracks stars, imaginatively known as the star tracker.
So the on-board system did what the operations team refers to as "vectorizing the burn": it computed two different orientations that were acceptable for ion engine firings that combined to produce the desired effect.
So on Monday AutoNav commanded the turn to the first orientation and fired the ion propulsion system for just over 4 hours. Later that day it turned to the second orientation and again used the ion engine for slightly over 4 hours of gentle thrusting.
The combined result of the two burns was to change the spacecraft's speed by 1.6 meters/second, or about 3.5 miles/hour, in just the direction needed to assure that Deep Space 1 continues to close in on the asteroid.
In effect, to keep the Sun away from the sensitive instruments, vectorizing the burn allowed the spacecraft to zig then zag, ultimately achieving exactly the needed maneuver.
As Deep Space 1 approaches 1992 KD, AutoNav will continue to fine tune the spacecraft's path occasionally. Until 2 days before the closest approach to the asteroid, AutoNav will use the extremely efficient ion propulsion system for these course corrections.
But in the final 2 days, when time is of the essence, it will use the faster but less fuel-efficient reaction control system (RCS). This system burns conventional rocket propellant, known as hydrazine, through combinations of 8 small thrusters.
Normally used to control only the orientation of the spacecraft, the RCS can also change the flight path using 4 of the thrusters that together will provide about 60 times greater thrust, but use about 15 times more propellant, than the ion engine.
For small maneuvers, that propellant cost is worth it to achieve the greater responsiveness, whereas for large maneuvers, the mass of the hydrazine the RCS would consume would exceed what the rocket that launched DS1 could have carried into space. The gentle and efficient ion engine easily wins when we can be patient.
On Friday, AutoNav's ability to carry out a course correction with the RCS was tested. (In the previous week, the ability of the RCS to change the spacecraft's speed was verified; but in this more recent test, the RCS burn was under AutoNav's control for the first time.)
Again needing to vectorize the burn, AutoNav flawlessly took the spacecraft to the first orientation, commanded the hydrazine thruster firing, turned to the second direction, commanded the second firing, then turned to point the main antenna back to Earth. The two burns together pushed the spacecraft along by about 1.1 meters/second, or 2.5 miles/hour.
In addition to AutoNav tests, a new test of beacon monitor operations was conducted this past week. When beacon monitor is used on future missions, it will summarize the overall health of the spacecraft.
Then it will select one of 4 radio tones to send to Earth to indicate how urgently it needs contact with the large antennas of the Deep Space Network, NASA's worldwide network of stations used to communicate with probes in deep space.
These tones are easily detected with low cost receivers and small antennas, so monitoring a spacecraft that uses this technology will free up the precious resources of the Deep Space Network, thus allowing more spacecraft to explore the solar system without having to expand the network.
Each tone is like a single note on a musical instrument. One tone might mean that the spacecraft is fine, and it does not need contact with human operators.
Another might mean that contact is needed sometime with a month, while a third could mean that contact should be established within a week. The last is a virtual red alert, indicating the spacecraft and, therefore, the mission are in jeopardy.
Earlier in DS1's mission, extensive tests were conducted of the tone reception, verifying the predictions of how much easier they are to detect than normal signals.
This week, a new test was begun of the sophisticated system on board that determines how healthy the spacecraft is. Rather than sending to engineers on Earth temperatures, pressures, voltages, currents, and other measurements, beacon monitor has within it a set of acceptable ranges for all of these.
The ranges depend upon which activity the spacecraft is conducting, and in some cases, the software combines different measurements in certain ways to arrive at its conclusion.
To establish this sophisticated capability, DS1 experts from each individual subsystem (such as power, telecommunications, and attitude control) provided programmers with the kinds of observations they consider important and helped define how on-board measurements should be used to assess the spacecraft's health.
Last week, the system began paying diligent attention on board, and it is continuing to do so. In parallel, human experts are still monitoring the spacecraft, and if any unusual readings occur, they will provide an opportunity to compare the on-board evaluation with that of the operations team.
Late this past week, Deep Space 1 was exactly as far away from Earth as the Sun was or some 8.5 light minutes away. It's thought-provoking to look at the Sun and think of the diminutive DS1, which has accomplished so many impressive feats and is still going on its journey through the vast solar system, now farther than the Sun.
Deep Space 1 Reports From Spacer.Com
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