Chapter 16. Encounter Phase     CONTINUED

Occultations

Occultations provide unique opportunities to conduct scientific experiments. Occultations of interest include the earth, the sun, or another star disappearing behind a planet, behind its rings, or behind its atmosphere, as viewed from the spacecraft. They are of interest when involving either the main planet or its satellites. During the one-time only opportunity for occultation by a planet during a flyby mission encounter, or repeatedly during an orbital mission, onboard instruments may make unique observations. Here are a few examples,:

• An ultraviolet spectrometer watches the sun as it disappears behind a planet's atmosphere (or a satellite's atmosphere), obtaining spectra deep into the atmosphere, that can be studied to determine its composition and structure.

• A photometer watches a bright distant star as it passes behind a ring system. Results yield high-resolution data on the sizes and structures of the ring system and its particles.

• The telecommunications system is used by Radio Science to measure the Doppler shift around closest approach to a satellite, to determine the object's mass.

• Radar Science commandeers the telecommunications system to perform a bistatic radar experiment: The Deep Space Network (DSN) observes the radio signal glancing off a satellite's surface as the spacecraft directs its downlink signal at just the right spots on the satellite to achieve this specular reflection. Results can yield information on the structure and composition of the surface. (While this kind of experiment doesn't actually need an occultation, it might be performed near occultation.)

• Radio Science observes the spacecraft's radio signal on Earth as the spacecraft passes behind a planet or a satellite, yielding data on the composition and structure of its atmosphere (see Chapter 8).

  STELLAR RING OCCULTATION INGRESS (Simulated star as viewed from spacecraft)

• Radio Science ring occultations provide data on the ring system's structure and composition. Of course this involves recording the downlink as the DSN sees it through the rings. The spacecraft's transmission is trained on Earth as the rings drift by.

Recall that the most precise Radio Science investigations require a two- or three-way coherent mode, receiving an uplink from the DSN as discussed in Chapter 10. However, with an atmospheric occultation, or an occultation of an opaque ring section, this coherent link is generally possible on ingress only; the spacecraft is likely to lose the uplink from DSN when it passes close behind the planet, and therefore would be incapable of producing a coherent downlink. For this reason, some spacecraft are equipped with an Ultra Stable Oscillator (USO) in a temperature-controlled "oven" on the spacecraft which is capable of providing a fairly stable downlink frequency for a short time when an uplink is not available. Calibrations, of course would, be accomplished shortly before and after the occultation.

Extrasolar Planetary Occultations

Somewhat related to this subject, many Earth-based telescopes, and some proposed orbiting instruments, are equipped with an occulting disk (coronograph) that can block out light from the central star. This facilitates examining the star's "planetary" disk of material orbiting it. Scattered light in the instrument and/or in the Earth's atmosphere limits its effectiveness, but studies exist for orbiting a separate large occulting disk to improve experiments' effectiveness from Earth-based or orbiting telescopes.

Gravity Field Surveying

There are two reasons for surveying the gravity field of a planet. First, highly accurate navigation in orbit at a planet requires a good model of variations in the gravity field, which can be obtained by such a survey. Second, gravity field measurements have the unique advantage of offering scientists a "view" of mass distribution both at and below the surface. They are extremely valuable in determining the nature and origin of features identifiable in imaging data. JPL has pioneered the field of mapping planetary mass concentrations. Application of these techniques to Earth helps geologists locate petroleum and mineral deposits, as well as provide insight to geological processes at work.

VENUS GRAVITY FIELD VS. TOPOGRAPHY Click image for info.

With an X-band (3.6 cm wavelength) uplink received at a spacecraft, and a coherent X-band downlink, spacecraft accelerations can be measured to tens of micrometers per second squared. This translates to a sensitivity of milligals in a planetary gravity field. (One gal represents a gravitational acceleration of 1 cm/sec²).

The most accurate and complete gravity field coverage is obtained from low circular orbit. Mars Global Surveyor is conducting a gravity field survey from circular orbit as one of its first-priority investigations. Magellan's orbit was elliptical during its primary mission, and meaningful gravity data could be taken only for the portion of the orbit that was plus and minus about 30 degrees true anomaly from periapsis, which occurred at about 10 degrees north latitude. After aerobraking (see below) to a low circular orbit, Magellan conducted a high-resolution gravity field survey of the entire planet.

Atmospheric Entry and Aerobraking

GALILEO JUPITER PROBE

The Magellan spacecraft was not designed for atmospheric entry. However, the periapsis altitude of Magellan's orbit was lowered, by the use of propulsive maneuvers, into the upper reaches of Venus's atmosphere near 140 km above the surface. This is still high above the cloudtops, which are at about 70 km. Flying at this altitude induced deceleration via atmospheric friction during the portion of the spacecraft's orbit near periapsis, thus reducing the height to which it could climb to apoapsis (recall the discussions in Chapter 3). The solar array, consisting of two large square panels, was kept flat-on to the velocity vector during each pass through the atmosphere, while the HGA trailed in the wind. The solar array reached a maximum of 160° C, and the HGA a maximum of 180° C. After approximately 70 earth days and one thousand orbits of encountering the free molecular flow, and decelerating a total of about 1250 m/sec, the apoapsis altitude was lowered to a desirable altitude. The periapsis altitude was then raised to achieve a nearly circular orbit. The objectives of this aerobraking experiment were to demonstrate the use of aerobraking for use on future missions, to characterize the upper atmosphere of Venus, and to be in position to conduct a full-planet gravity field survey from a nearly circular orbit. Click the cartoon at left for further information.

Descent and Landing

Even though Huygens enjoyed the immense fortune of surviving impact with the surface of Titan, and returning data for over 90 minutes, the mission was classified as an atmospheric probe, not a lander. Its prime mission was to collect data while descending for more than two hours through the atmosphere.

JPL's Surveyor missions landed on the Moon via propulsive descent, coming to rest on crushable foot pads at the lunar surface.

For a possible future mission, some members of the international science community desire to land a network of seismometer-equipped spacecraft on the surface of Venus to measure seismic activity over a period of months or years.

Balloon Tracking

The planned Mars Balloon is designed to descend to just above the surface. Carrying an instrument package, including a camera, within a long, snake- or rope-like structure, it will rise and float when heated by the daytime sunlight, and will sink and allow the "rope" to rest on the surface at night. In this way it is hoped that the balloon package will visit many different locations pseudo-randomly as the winds carry it. In doing so, it will also yield information on atmospheric circulation patterns. The Mars balloon is designed jointly by Russia, CNES, and The Planetary Society, a public non-profit space-interest group in Pasadena. It will depend upon an orbiting spacecraft to relay its data home.

The Mars Global Surveyor spacecraft carries radio relay equipment (see Chapter 11) designed to relay information from landers, surface penetrators, balloons, or Mars aircraft. The Mars Odyssey and Mars Express orbiters are similarly equipped. Future Mars orbiting spacecraft will also have relay capability, as did two Mars-bound spacecraft that were lost: Mars Observer in 1993 and Mars Climate Orbiter in 1999.

Sampling

This is all good, but the scientific community really would like to directly examine samples in the laboratory. A robotic sample return mission to Mars could be in the future. NASA has conducted long-range studies and technology development and may continue to do so for planning purposes. Several different scenarios are envisioned for accomplishing this, some of which would include a rover to go around and gather up rock and soil samples to deposit inside containers aboard the return vehicle. Interest in returning samples from Mars has heightened recently in the wake of discoveries including recently flowing springs, and ancient lake beds on Mars. Such samples would be examined for fossil evidence of life forms.

Sampling of cosmic dust in the vicinity of the Earth has also become an endeavor of great interest, since interplanetary dust particles can reveal some aspects of the history of solar system formation. Space shuttle experiments have so far been successful at capturing three 10 µm particles from Earth orbit, one intact.

Launched in 1999, Stardust collected samples of material from the coma of Comet Wild 2 in January 2004, for return to Earth January 15, 2006. The return capsule's entry, descent, and landing on Earth were flawless, and scientific analysis of its thousands of samples is underway, with help from members of the public being sought. Genesis succeeded in returning samples of the solar wind, despite its unexpectedly hard landing on Earth in September 2004, when its parachute failed to deploy.

A spacecraft isn't always needed if you want to collect interplanetary material. Dust from interplanetary space rains continuously into Earth's atmosphere, which slows it gently because of the particles' low mass. In 1998 the NASA Dryden Flight Research Center flew one of its ER-2 high-altitude research aircraft with an experiment for the Johnson Space Center that collected high-altitude particulate matter -- or "cosmic dust" -- on two collector instruments mounted on pods under the wings.