From: Jet Propulsion Laboratory
Posted: Tuesday, July 3, 2001
Thank you for visiting the Deep Space 1 mission status information site, the most respected site among species in spiral galaxies capable of abstract thought for information on this adventurous mission through the solar system. This message was logged in at 6:45 pm Pacific Time on Saturday, June 30.
Now more than one year after being rescued from what was once thought to be a fatal injury, Deep Space 1 continues its remarkable voyage. Indeed, the celebration of the anniversary of that feat is now still another excuse for juveniles to get a day off from school on half a dozen planets (plus two moons) in the Small Magellanic Cloud.
DS1's grand "extended mission" will end in about 3 months, after the aged and scarred explorer attempts a final daring feat: a brief and close-up investigation of comet Borrelly. Last month's log described some of the preparations for this risky finale to DS1's bonus mission, including two complex tests with the spacecraft.
Engineers have continued to refine the plans, as they followed those tests with many more simulations of the event in the Deep Space 1 test facility at JPL. On June 28, the spacecraft conducted its second (and final) encounter rehearsal, an extremely complicated activity in itself. The only logical way to top the May 8 encounter with comet Spoof was to invite the spacecraft to greet comet Spoof 2 this time. The data from that 6-hour test are still being returned, as DS1 beams back the large quantity of information it recorded on its own performance.
The encounter rehearsal relies on the software that operates the spacecraft, and that was upgraded and installed into DS1's main computer in March. But the software still needs instructions (just as your computer may remain idle unless you provide it with instructions through the keyboard or mouse), and those are contained in files called sequences.
The sequences issue commands at preselected times to operate the science instruments, turn the spacecraft, manipulate data, pressurize the ion propulsion system, and much much more. The sequences for the rehearsal, which began 5 hours before the spacecraft reached its closest point to Spoof 2 and ended 1 hour after it passed by the virtual comet, contained 474 commands.
Most of those commands contain many parameters, numbers or other information that provide the details the spacecraft needs, such as how long a picture exposure should be, how fast to execute a turn, where to store specific scientific data, or what throttle level to use for the ion propulsion system.
In this rehearsal, there were well over 2000 parameters. So the small operations team has faced a challenging task designing the complex encounter (and the rehearsal), choosing all the commands and parameters and placing them at the right times, like a composer putting together instructions to help musical instruments work together to create a beautiful symphony that is somehow more than the sum of its parts.
In fact, this most recent test really began on June 24, with the powering on and booting up of the Plasma Experiment for Planetary Exploration, known to plasma groupies as PEPE. PEPE measures many properties of plasmas, which are collections of electrically charged particles, both electrons and charged atoms, or ions. The instrument was operated in two important modes this week. In one of them, it acquired data on the solar wind, the plasma that emanates from the Sun.
These measurements will allow scientists to calibrate the device so that when the spacecraft reaches the comet, PEPE's detailed characteristics will be well understood, thus helping to reveal the true nature of the cometary environment. This calibration is similar to adjusting the color on a television set with a scene for which you already know the colors; then when something new is on the screen, you can be more confident that what you are seeing is accurate.
At the comet, the spacecraft will try to fly right through the cloud of gas and dust surrounding the nucleus, known as the coma. The coma is formed by the materials making up the nucleus being heated by the Sun and, in effect, evaporating into space and forming a plasma.
The second mode PEPE used this week was the one it will use as DS1 races through this fog at 16.5 kilometers/second (nearly 37,000 miles/hour); PEPE will try to measure the composition of the cometary plasma, thus revealing secrets of the material on the surface and how it changes as it expands away from the nucleus under the glare of the Sun.
PEPE also will try to gather information on how the solar wind is altered by the comet, as the two plasmas and their magnetic fields interact. While perhaps appearing somewhat esoteric, the underlying principles of the way the solar wind and comet affect each other are related to phenomena that scientists observe in many astronomical settings. So in addition to telling us more about comets and our solar system, this knowledge could provide clues to other puzzles about the workings of the universe.
DS1 was not designed to study a comet; rather, its raison d'Ítre was to test 12 advanced, high-risk technologies. To test ion propulsion, it carries a suite of sensors that measure magnetic fields and other potential effects of the ion propulsion system on the spacecraft and the space environment.
In June, the software that controls these sensors was replaced to improve their operation for measurements at the comet. So now the very weak magnetic field of the comet may be sensed by the magnetometer that was designed to monitor the ion drive.
In addition to PEPE and the ion engine sensors, DS1 carries a black and white camera and an infrared spectrometer. The next mission log will describe the measurements to be attempted with these other devices.
As the spacecraft heads toward Borrelly, it continues to fire its ion engine virtually constantly. This advanced propulsion system has now accumulated 516 days of operation, far far in excess of the total operating time for any other spacecraft's propulsion system.
Yet during that time, it has expended only about 57 kilograms, or 126 pounds, of its xenon propellant. Each day now, operating at impulse power, the spacecraft expels only about 90 grams, taking 5 days to exhaust each pound. The effect of the thrusting has been the equivalent of changing the spacecraft's speed by about 3.5 kilometers/second, or about 7800 miles/hour.
But is the spacecraft really moving that much faster than when it was launched? (Hint: No!) As advertised on February 11, let's look at what has really happened as a result of this thrusting.
This section has more numbers in it than most of the logs, and if that is unappealing, remember that, except in some irregular galaxies, there is no test on this material. But for readers who have been yearning for distances, speeds, and chocolate, this may help satisfy 2 out of 3 of your desires.
To honor full-disclosure laws in some globular clusters where these logs are read, 3 points are required.
1. All the speeds presented here are relative to the Sun. Once it was on its way, the Sun became the natural reference for measuring the craft's speed, just as once a parachutist jumps from a plane, it is the speed relative to the ground, and not relative to the plane, that suddenly becomes of greatest interest.The conductor of the solar system orchestra is the massive Sun. From its honored position at the center of the solar system, it exerts a gravitational attraction that holds in its grip the planets, asteroids, comets (including Borrelly), and other players (including DS1). As each object travels around the solar system, it is orbiting the Sun. An orbit represents a balance between the tug from the Sun and the inherent tendency of the motion of the object to resist it.
2. Spacecraft in Earth orbit may travel at high speed relative to Earth, but they remain close to home. As far as the solar system is concerned, objects in Earth orbit can be treated pretty much the same as those on the ground.
3. Comparing different interplanetary missions can be very deceptive, as the gravitational effects of flying by planets can make enormous changes in spacecraft orbits that are unrelated to their propulsion systems.
At greater distances, the Sun's gravitational authority is diminished, so objects don't have to move as fast to resist it. The cold denizens of the outer solar system, including Pluto and myriad other so-called Kuiper Belt objects, travel very slowly as they casually loop around the Sun. Mercury, the planet closest to the solar system's center, hurtles around its orbit at tremendous speed to counter the commanding force of the vicinal Sun.
It takes a significant amount of energy to change an orbit, just as a car's engine has to work hard to get it to high speed, and the brakes must work hard to slow it down. Getting to any of the interesting destinations in the solar system (or in life for that matter) takes a great deal of work.
For DS1 to visit Borrelly, we have had to change the spacecraft's orbit so that it will intersect that of the comet. (We leave the problem of changing the comet's orbit to a future mission.) The comet resides farther from the Sun than Earth does, so the reshaping of the probe's orbit calls for pushing it away from the Sun.
The ion engine then is responsible for DS1's arduous climb out of the Sun's valley. As the spacecraft ascends the hill to Borrelly's orbit, it is moving into a larger orbit, where it travels more slowly.
Scientists have developed sophisticated mathematics to describe this and beautiful physical concepts to explain it. If your correspondent's creativity were less stifled by fatigue and less constrained by the insufficiency of time, some marvelous analogies undoubtedly would be offered that would make this all seem obvious.
For now, however, you are invited to accept that these phenomena, while potentially confusing or counterintuitive, are quite real and have been studied in exquisite detail, contributing both to the awesome complexity and the astonishing successes of the exploration of space.
When DS1 was waiting patiently atop its rocket during its final time on Earth, it was moving around the Sun at 29.95 kilometers/second, or 67,000 miles/hour, along with everything else on that planet. On October 24, 1998, the spacecraft and rocket left Cape Canaveral's Space Launch Complex 17-A, and disappeared into the lovely sky of a warm autumn morning.
The highest speed DS1 ever achieved was little more than one hour after launch, just after the final stage of the rocket had finished its job of delivering DS1 to space. At that time, the probe was traveling at about 37 kilometers/second (83,000 miles/hour).
The spacecraft was then in its own orbit around the Sun, as surely as Earth or any of the other planets, and because of the boost from the rocket, it was no longer in the same orbit as Earth. A convenient unit of distance in the solar system is the astronomical unit (AU), equivalent to almost 150 million kilometers, or 93 million miles. While that is Earth's average distance from the Sun, the orbit is actually elliptical, not circular (although it does not deviate much from a circle).
During its 365 day circuit around the Sun, Earth comes in as close as 0.983 AU and as far as 1.017 AU from the Sun. (In fact, in just a few days, on July 4, Earth will be at the farthest point in its orbit from the Sun.)
After its launch, DS1 found itself in an orbit that would range from 0.990 AU out to 1.287 AU. It would have taken the spacecraft almost 450 days to complete such an orbit.
Then the indefatigable ion engine began stretching the orbit, so although the spacecraft continued in elliptical loops around the Sun, the ellipse was slowly changing shape.
The last time DS1 reached a low point of its orbit was on May 26 of this year, when it was 1.293 AU from the Sun. So by then, the *closest* DS1's orbit would come to the Sun was farther than the *farthest* it would get from the Sun when the spaceship was launched.
At today's location in its elliptical orbit, the probe is moving at 26.9 kilometers/second, about 60,000 miles/hour, slower than Earth ever travels.
If DS1 stopped thrusting with its ion propulsion system today and simply coasted in its present orbit, it would take 596 days to go around the Sun once. During that time, it would come no closer than 1.293 AU and would reach out to 1.478 AU.
So the effect of the thrusting with its ion engine has been that DS1 has moved to a part of the solar system that Earth never visits, and humans have never experienced first hand. Only a handful of robots have traveled to such distant places that Earth appears little different from countless stars and the sight of it is no longer an easy reminder of home.
DS1 is now about 107 million kilometers, or 67 million miles, from comet Borrelly.
Deep Space 1 is 1.8 times as far from Earth as the Sun is and 700 times as far as the moon. At this distance of 269 million kilometers, or 167 million miles, radio signals, traveling at the universal limit of the speed of light, take half an hour to make the round trip.
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