Galileo's Mission at Jupiter - Day 4 of the Callisto 30 Encounter: The Meat and Potatoes

Today's activities include the main focus of attention for this orbit, the close flyby of Jupiter's moon Callisto. This occurs at 5:14 a.m. PDT [See Note 1] at a distance of only 123 kilometers (76 miles) above the icy surface. At that time, Galileo is flying at a speed of 9.7 kilometers per second (6.0 miles per second or 21,700 miles per hour) relative to that satellite. Coincidentally, this flyby occurs on the fortieth anniversary of President John F. Kennedy's famous speech to the U.S. Congress, on May 25, 1961, in which he committed the nation to a manned moon landing by the end of the decade. At that time, the U.S. had only launched one spacecraft that had successfully escaped Earth orbit and fulfilled its mission -- the Pioneer 4 Lunar flyby in March of 1959. The U.S. and NASA were still over a year away from launching our first successful planetary mission, the Mariner 2 flyby of Venus. See how far we've come! In contrast, the Galileo spacecraft had five successful planetary encounters (one of Venus, two of Earth, and two of asteroids) even before reaching its primary goal of Jupiter!

But now the spacecraft is approaching Callisto's night side. This makes it difficult to observe Callisto, because to do so would also mean looking very near the Sun, which is hazardous to our remote sensing instruments. So we take this opportunity to continue our observations of Jupiter itself. The Near Infrared Mapping Spectrometer (NIMS) performs the last two of its three limb-to-limb, pole-to-pole global maps of the disk of Jupiter, looking for compositional variation in the atmosphere of the planet.

The science instruments then shift their attention to Callisto. Shortly after 3 a.m. PDT [See Note 1], Radio Science configures the spacecraft for an occultation. On Tuesday night, the body that came between Earth and Galileo was Jupiter. This time, Callisto itself blocks our communications with Galileo for about an hour. The radio signal to Earth is changed to a pure tone, with no telemetry modulation, and the science team will be looking for the effects on the radio signal of the extremely tenuous charged-particle atmosphere of the satellite. They will be measuring the vertical distribution of free electrons above the surface. While no telemetry is being sent to the ground, engineering and science measurements will be stored both in on-board computer memory, and on the tape recorder.

In particular, the Fields and Particles instruments [Energetic Particle Detector (EPD), Heavy Ion Counter (HIC), Magnetometer (MAG), Plasma instrument (PLS), and Plasma Wave Subsystem (PWS)] will be recording continuous high-rate, high-time-resolution data for about an hour centered on the closest approach. This recording will assist with studies of the Callisto's induced magnetic field and its interaction with the Jovian magnetosphere. The measurements from the various instruments will contribute to an understanding of particle pickup processes near Callisto, and thermal and non-thermal plasma interactions in the region.

About 45 minutes before closest approach, the Photopolarimeter Radiometer instrument (PPR) performs a brief calibration of its signal before embarking on a high-resolution scan from east to west across the night side of Callisto. This observation begins 14 minutes before closest approach and its purpose is to see how quickly different types of terrain cool off at night.

A scant three minutes before closest approach, the spacecraft once again appears from behind the satellite as seen from Earth. This will be the first signal we will have heard in an hour, but it will only be the pure tone of the Radio Science occultation experiment as it probes the tenuous atmosphere from the ground up. The first telemetry is still an hour away!

Just at closest approach, the Solid State Imaging camera (SSI) snaps the highest resolution near-terminator images ever taken of any icy satellite! From this vantage point, we may be able to see some boulders on the surface, and will be able to see small impact craters as well. Travelling at over 21,000 miles per hour at the time, the pictures would definitely be smeared by the motion while the shutter is open. However, the spacecraft employs a technique of moving the camera in the opposite direction of the spacecraft motion to try and keep the desired surface features steady.

Shortly after this, SSI takes the first of two sets of pictures of a domed crater. Eighteen minutes later, another set is taken from a different angle. This will allow scientists to reconstruct a stereo view of the surface, and to accurately measure the heights of the features they see.

NIMS next undertakes a high-resolution observation of a multi-ringed impact structure called Asgard, to study the composition of the surface materials. This is followed by a similar observation of a region near the crater Bran. SSI also views Bran crater with moderate resolution, to provide context for closer images taken during the 20th flyby in the Galileo series.

Context observations are important because they allow for more precise location of high resolution images and also reveal the kinds of terrain and the geologic features (such as large impact craters) that may have influenced the area seen at high resolution. SSI takes several opportunities as the spacecraft recedes from Callisto to look back and collect such context images.

SSI also takes pictures of an area called the Valhalla Antipode. Valhalla is a gigantic, continent-sized impact structure on Callisto, and the antipode is that area of the satellite which is exactly on the opposite side of the body. It is thought that the tremendous seismic energy generated by the impact would be focused on the point on the surface diametrically opposed, and may disrupt the surface in unusual ways. These images will determine if there is any 'weird terrain' caused by this seismic disruption at this location.

PPR performs two scans from east to west across the satellite to measure temperatures and determine how the surface materials warm up during the course of a Callisto day (one Callisto day lasts for over 16 Earth days). The final science observation of the day, at 8:30 a.m. PDT, is a PPR scan from pole to pole, to determine the temperatures at the poles. These areas are likely to be the coldest places on Callisto, and may therefore be a place where exotic volatile materials might collect, similar to the way ice may collect at the poles of Earth's Moon and Mercury.

Though this may seem like at least a full day's effort, the entire block of Callisto observations described here take place over a scant four hours -- five and a half hours, if you include Radio Science!

On the engineering side of the house, this evening sees a standard test of the spacecraft gyroscopes, in preparation for an Orbit Trim Maneuver which will be executed next Wednesday.

Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occurred is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 50 minutes to travel between the spacecraft and Earth. All times quoted above are in Earth Received Time.