Dr. Marc Rayman's Mission Log

  August 8, 1999

Mission Update:

Thank you for visiting the Deep Space 1 mission status information site, known on the vast majority of solid surfaces in the solar system, as well as in over 95% of the atmospheres and more than a few of the major pools of liquids, as the most authoritative source information on this technology validation mission. This message was logged in at 7:30 pm Pacific Time on Sunday, August 8. With DS1 having been so busy and productive recently, there is a great deal to tell, so get comfortable for a long report.

After its remarkably successful mission of testing what were high-risk advanced technologies critical for future space science missions, Deep Space 1 blazed a new trail with its extraordinary encounter with asteroid Braille on July 28. As the hundreds of trillions of loyal readers to these logs know, reaching this tiny asteroid was a bonus. The visit to Braille was icing on the cake, and it proved to be exciting indeed.

DS1 managed to swoop to within about 26 kilometers, or just 16 miles, of the surface. This is far far closer than any spacecraft has ever come to an asteroid, and it's only about twice as high above the asteroid as a commercial jet flies above the Earth. Yet it was going at 15.5 kilometers/second or 35,000 miles/hr, well over 50 times as fast as a commercial jet or over twice as fast as the space shuttle. Getting to this close encounter with this asteroid was like kicking a soccer ball on Earth and scoring a goal on the moon.

DS1 collected all the data that was wanted to test the autonomous navigation system, to measure ions and electrons (or charged particles) in the vicinity of the asteroid, and to search for magnetic and electric fields near the asteroid. The spacecraft also acquired pictures and infrared spectra, except it did not get close-in images and it got only some of the spectra. To have gotten the kind of images DS1 did capture would have required a telescope 200 times more powerful than the Hubble Space Telescope if they had been taken from Earth orbit.

DS1 flew for over 9 months from Earth to Braille, testing its technologies most of the way, and yet the asteroid is so tiny that it did not show up in pictures taken by the spacecraft until about 2 days before DS1 arrived. In fact, this was the smallest celestial body ever to be targeted for an encounter by a spacecraft. It is so distant and faint that even its location was not well known. And when it finally showed up in the pictures, it was about 430 kilometers, or over 260 miles, from its expected location, requiring the spacecraft to change its course to reach it.

Last week's log told the chilling tale of the spacecraft's entry into standby followed by the dramatic and truly heroic recovery by the operations team. Following that, AutoNav tracked the asteroid down to 70 minutes before closest approach. At 28 minutes before the closest approach, it correctly switched to a different operating mode and as part of that used a different portion of the camera for its navigational sightings. The asteroid however did not register in the camera, so AutoNav had no new information with which to update its estimate of the actual location of the asteroid. All it had was its last estimate made at 70 minutes, or 65,000 kilometers (40,000 miles), away. And that was not accurate enough to keep the asteroid in the camera's view down to the time that the close-in images were to be collected.

Essentially, this is similar to the situation you would face if you were trying to drive down a dark country road and you had merely a glimpse of the surroundings at the beginning of your drive. You couldn't expect to get to your destination just on that basis. AutoNav however had accurately estimated the actual time that it would get to the asteroid, and so it ordered the various sensors to collect data at the correct times. Finally AutoNav switched back to its normal mode and just after the spacecraft zipped by the asteroid, it slowly turned to look back at it. The camera pointing was dead on and it got pictures and infrared spectra 15 minutes later.

Apparently the strangely shaped Braille, illuminated from the side by the Sun, caused it to be surprisingly dim. And the camera simply could not register that faint light as DS1 approached the asteroid. But on the other side, as DS1 receded, the asteroid did show up in the camera.

The pictures reveal that Braille is a very irregular, elongated object, only about 2.2 kilometers in one axis and 1 kilometer in another, or about 1.4 miles by 0.6 miles. This is smaller than many mountains in the United States. The real scientific prize however is in the infrared spectra. Infrared light is beyond what our humble eyes can detect, but DS1 can detect it, just as there are kinds of light invisible to humans but that bees can see, or kinds of sound imperceptible to people but within the hearing range of dogs. When a spectrum is taken, the light is broken into its individual components, much as looking through a prism, or like a rainbow in which white light is separated into its various colors. And such a spectrum is very valuable to scientists because many materials reflect infrared light differently. So an infrared spectrum contains the unique signature of the material whose light is being analyzed, like a fingerprint.

The spectrum of Braille tells a fascinating story. It is nearly identical to the spectrum of Vesta, one of the largest asteroids. That means their surfaces are made of the same material. And scientists already knew by comparing Vesta's spectrum with that of many minerals in laboratories that it is made of basalt. Now basalt is a rock that is formed when lava cools, and one question scientists have grappled with is: How did Vesta ever get hot enough to form lava? Vesta is too small to have the inventory of radioactive materials that a large planet like Earth has, in which the decay of those elements produces enough heat to keep the interior hot. There are several possible explanations, but one is that collisions with other asteroids caused enough heating to make the lava. The Hubble Space Telescope has revealed an enormous crater on Vesta that suggests a tremendous impact has occurred there. An exciting possibility is that such a collision sprayed many fragments into the solar system, and Braille is one of them.

Now the spectra of Vesta and Braille also match those of some meteorites. But it is not known how chips from Vesta, liberated in collisions with other asteroids, could reach Earth to fall as meteorites, as Vesta is in the main asteroid belt, between Mars and Jupiter. But now we have Braille as an example of an asteroid closer to Earth yet resembling Vesta, giving astronomers new clues to the trail followed by fragments of Vesta as they make their way to Earth. As these new data from Braille are analyzed more, they should add to the intriguing puzzle of how our solar system has evolved. In addition, as asteroids like Braille threaten Earth with catastrophic impacts in the future, a better understanding of their composition and structure will aid in determining how to protect our planet.

Braille is constantly bombarded by the solar wind, the stream of charged particles flowing from the Sun, and several experiments were conducted to search for any effects Braille might have on the solar wind or on the space environment. The experiments to measure ions and electrons near Braille were not expected to reveal any surprises, because, as one member of the DS1 team pointed out, it would be like trying to smell a bowling ball from 50 feet away. But there have been surprises before, such as Deimos, the small moon of Mars, giving off material that could be detected from much much farther away than DS1 flew from Braille. Always alert for surprises that nature might spring upon us, DS1 measured ions and electrons throughout its time near Braille. So far, the analysis has not revealed any unexpected results this time. Attempts to detect electric or magnetic fields associated with the asteroid also have not shown any positive indications yet. But, as is often the case in science, not finding a signal is a result in itself, as it allows scientists to eliminate some possibilities in attempting to fully characterize the asteroid.

DS1's mission, which has to its credit a wealth of technology testing and a bonus asteroid encounter, was scheduled to end on September 18. But this past week, NASA approved an extension to the mission. Now it is important to keep in mind that this continuation of the mission is threatened as long as NASA's budget remains in jeopardy, as it has been since a committee in the House of Representatives planned to cut NASA funding recently. The Congressional debate on the budget is not yet complete, but if the outcome is not favorable for NASA, the consequences to DS1, and many other important and exciting space projects, may be grave.

But if the funding is available, DS1 will combine its advanced technologies with the experience of the encounter with asteroid Braille to conduct two very exciting encounters with comets in 2001. And to prepare for this encore, less than a day and a half after passing the asteroid, DS1 resumed thrusting with its ion propulsion system, with AutoNav firmly at the helm again.

It is now on its way to Comet Wilson-Harrington, which it will reach in less than 1.5 years. When it was first seen, in 1949, it appeared to be a rather ordinary comet, but it was not observed again. In 1979, Eleanor Helin, who discovered asteroid Braille, discovered an asteroid known as 1979 VA. Several years later it was realized that the two objects were the same. So Comet Wilson-Harrington has stopped displaying its cometary activity and now looks like an asteroid. Gone are the typical tail and coma, which is the expanding cloud of gas and dust surrounding the nucleus. When DS1 reaches the comet in January 2001 it may provide clues to this mysterious body, now known as a dormant comet or a comet/asteroid transition object.

In September 2001, DS1 will sail past Comet Borrelly, one of the most active comets that regularly visit the inner solar system. It was discovered in 1904, but astronomers have calculated that in the 19th century it had several close encounters with Jupiter that changed its orbit. Many inhabitants of our solar system recall the thrill of Comet Shoemaker-Levy 9, which got so close to Jupiter that it was captured in the giant planet's firm gravitational grip, plunging into Jupiter's atmosphere in 1994. Borrelly escaped that fate and may reveal its cometary secrets when DS1 pays it a visit in just over 2 years.

Future logs will describe many more interesting details on these bodies now that DS1's mission is turning from testing technologies to investigating comets.

In the meantime, this past week the DS1 operations team conducted a few more tests with the spacecraft to prepare it for the many many months of thrusting with its ion propulsion system that will take it to these exotic destinations. Be sure to check in again in two weeks for all sorts of amazing statistics on how far DS1 will travel, how much its ion propulsion system will change its speed, and more.

Deep Space 1 is now over 30% farther away from Earth than the Sun is and 513 times as far as the moon. At this distance of almost 197 million kilometers, or 121 million miles, radio signals, traveling at the universal limit of the speed of light, take almost 22 minutes to make the round trip.

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