# Why was Dawn's (the spacecraft) flight path circular?

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In comparison to the twin Voyagers, Dawn's flight path circled the sun in an anti-clockwise manner. The map of its path shows its targets, Ceres and Vesta were in a trajectory that was close to a straight path, if Dawn had taken a clockwise direction, with a close flyby of Mars. I can't find a reason to show it was studying the space around Lagrange point 3 (in relation to Earth), as according to this flight path map. So what was the reason for a longer direction?

Dawn was not exploring L3. But instead it explored both Vesta and Ceres. In order to do so in a cost-saving manner, Dawn was also a mission demonstrator for an interplanetary mission equipped with ion propulsion.

Because of the low specific impulse of ion propulsion, Dawn cannot fly in a straight line, as this would imply a high initial velocity and the spacecraft couldn't deaccelerate in time. Therefore, Dawn started with low $$v_0$$ at Earth, and raised its orbit slowly and continuously, creating the spiral you see.

The Earth orbits the Sun in a counter-clockwise direction. Covering $$2 pi imes 150 imes 10^6$$ million kilometers in $$365.25 imes 24 imes 3600$$ seconds, or 29.9 km/sec.

If you wanted to leave the Earth in a clockwise heliocentric orbit you'd have to accelerate Dawn by an extra 60 km/sec to do that, which is unfeasible with current rockets.

Likewise approaching Ceres you'd also be going the wrong direction to try to enter into orbit, you'd face the problem of Ceres moving counterclockwise at 17.9 km/sec while Dawn would be moving roughly the same speed clockwise, so another 35.8 kilometers per second relative motion that you would have to "dispose of" somehow.

As this answer points out, Dawn used electric propulsion which can only slowly change velocity. If you are in a heliocentric orbit and tried to spend many years decelerating to a dead stop and slowly re-accelerating the other direction, in the mean time you'd fall towards (but probably not into) the Sun, if it were possible at all (it would probably run out of propellant).

Dawn was not built to withstand a close pass with the Sun, and so it would not have survived.

Launches often try to use every last bit of "free" velocity from Earth's motion, not only by the deep-space launches orbiting the Sun in a prograde direction (the same direction the Earth and all other planets rotate) but by first launching into orbits around the Earth in a prograde direction, taking advantage of the Earths rotational velocity around its axis.

## Gemini 10

Gemini 10 was the eighth crewed Earth-orbiting spacecraft of the Gemini series, carrying astronauts John Young and Michael Collins. Its primary purpose was to conduct rendezvous and docking tests with the Agena target vehicle. The mission plan included a rendezvous with the Gemini 8 Agena target, two extravehicular activity (EVA) excursions, and the performance of 15 scientific, technological, and medical experiments. The scientific experiments were related to (1) zodiacal light, synoptic terrain, and synoptic weather photography, (2) micrometeorite collections, (3) UV astronomical camera, (4) ion wake measurements, and (5) meteoroid erosion.

#### Mission Profile

Gemini 10 was launched on 18 July from Complex 19 at 5:20:26 p.m. EST (22:20:26.648 UT) and inserted into a 159.9 x 268.9 km orbit. At orbit insertion Gemini 10 was about 1600 km behind the Gemini Agena Target Vehicle 10 (GATV-10) which had been launched into a near circular orbit about 100 minutes earlier. Rendezvous with GATV-10 was achieved on the 4th revolution at 10:43 p.m. and at 11:13:03 p.m. docking was achieved. A large out-of-plane error in the initial orbit required the Gemini to use 60% of its fuel for the rendezvous, over twice the planned amount. As a result most of the mission plan was revised. To conserve fuel, Gemini 10 remained docked to GATV-10 for the next 39 hours and used the GATV propulsion system for maneuvers. The planned docking practice runs were cancelled.

A 14-second burn of the GATV-10 primary propulsion system was used to raise the dual spacecraft apogee to 764 km. While the spacecraft were docked, a bending mode test was conducted to study spacecraft dynamics and other experiments were performed. Another burn of GATV-10 at 3:58 p.m. on 19 July brought the spacecraft into the same orbit as the GATV-8, which had been launched on 16 March for the Gemini 8 mission. At 4:44 p.m. the Gemini cabin pressure was reduced to zero and the hatch was opened. Collins stood up in his seat 3 minutes later and began photographing stellar UV radiation. Partway into the standup EVA Young and Collins began to experience severe eye irritation from an unidentified source and Young ordered termination of the EVA. Collins sat down and the hatch was closed at 5:33 p.m., and a high oxygen flow rate was used to purge the environmental control system.

Gemini 10 separated from GATV-10 at 2:00 p.m. EST on 20 July. A series of manuevers using its own thrusters brought Gemini 10 within about 15 meters of GATV-8. At 6:01 p.m. (48:41 ground elapsed time) the cabin was evacuated and the hatch opened for Collins to begin his second EVA. Collins left the spacecraft 6 minutes later attached to an umbilical cord and travelled to the GATV-8. Despite difficulties due to lack of handholds on the target vehicle Collins removed the fairing and retrieved the micrometeoroid detection equipment. During the EVA he lost his camera. He also retrieved the micrometeorite experiment mounted on the Gemini 10 spacecraft, but this apparently floated out of the hatch and was lost when Collins reentered the capsule. The EVA was limited to 25 minutes of outside activity due to lack of fuel. Collins reentered the capsule at 6:32 p.m. and the hatch was closed at 6:40. The hatch was reopened again at 7:53 p.m. to jettison 12 items before reentry. After about three hours of stationkeeping Gemini 10 moved away from GATV-8. At 8:59 p.m. the crew performed an anomaly adjust maneuver to minimize reentry dispersions resulting from the retrofire maneuver.

Retrorocket ignition took place during the 43rd revolution on 21 July at 3:30:50 p.m. EST and splashdown occurred at 4:07:05 p.m. in the western Atlantic at 26.74 N, 71.95 W, 875 km east of Cape Kennedy and 6.3 km from the target point. The crew was picked up by helicopter and taken to the recovery ship U.S.S. Guadalcanal at 4:34 p.m. and the spacecraft was aboard at 5:01 p.m. Total mission elapsed time was 70:46:39. Of the primary objectives, only the docking practice was not accomplished due to lack of fuel, although the fuel budget also resulted in small revisions in some of the other objectives. The first rendezvous and docking maneuvers were successfully accomplished. All experiments obtained data except for the Gemini 10 micrometeorite collector, which was lost by floating out of the spacecraft. The landmark contrast measurement experiment was deleted due to lack of fuel. Gemini 10 demonstrated the ability of an astronaut to travel to another spacecraft and back and the use of powered, fueled satellite to provide propulsion for a docked spacecraft.

#### Spacecraft and Subsystems

The Gemini spacecraft was a cone-shaped capsule consisting of two components, a reentry module and an adaptor module. The adaptor module made up the base of the spacecraft. It was a truncated cone 228.6 cm high, 304.8 cm in diameter at the base and 228.6 cm at the upper end where it attached to the base of the reentry module. The re-entry module consisted of a truncated cone which decreased in diameter from 228.6 cm at the base to 98.2 cm, topped by a short cylinder of the same diameter and then another truncated cone decreasing to a diameter of 74.6 cm at the flat top. The reentry module was 345.0 cm high, giving a total height of 573.6 cm for the Gemini spacecraft.

The adaptor module was an externally skinned, stringer framed structure, with magnesium stringers and an aluminum alloy frame. The adaptor was composed of two parts, an equipment section at the base and a retrorocket section at the top. The equipment section held fuel and propulsion systems and was isolated from the retrorocket section by a fiber-glass sandwich honeycomb blast shield. The retrorocket section held the re-entry rockets for the capsule.

The reentry module consisted mainly of the pressurized cabin which held the two Gemini astronauts. Separating the reentry module from the retrorocket section of the adaptor at its base was a curved silicone elastomer ablative heat shield. The module was composed predominantly of titanium and nickle-alloy with beryllium shingles. At the narrow top of the module was the cylindrical reentry control system section and above this the rendezvous and recovery section which holds the reentry parachutes. The cabin held two seats equipped with emergency ejection devices, instrument panels, life support equipment, and equipment stowage compartments in a total pressurized volume of about 2.25 cubic meters. Two large hatches with small windows could be opened outward, one positioned above each seat.

#### Control, Propulsion, and Power

Attitude control was effected by two translation-maneuver hand controllers, an attitude controller, redundant horizon sensor sytems, and reentry control electronics, with guidance provided via an inertial measuring unit and radar system. The orbital attitude and maneuver system used a hypergolic propellant combination of monomethylhydrazine and nitrogen tetroxide supplied to the engines by a helium system pressurized at 2800 psi. Two 95 lb translation thrusters and eight 23 lb attitude thrusters were mounted along the bottom rim of the adaptor, and two 79 lb and 4 95 lb thrusters were mounted at the front of the adaptor. Power was supplied by a fuel cell power system to a 22- to 30-volt DC two-wire system. During reentry and post-landing power was supplied by four 45 amp-hr silver-zinc batteries.

#### Communications

Voice communications were performed at 296.9 MHz with an output power of 3 W. A backup transmitter-receiver at 15.016 MHz with an output power of 5 W was also available. Two antenna systems consisting of quarter-wave monopoles were used. Telemetry was transmitted via three systems, one for real time telemetry, one for recorder playback, and a spare. Each system was frequency-modulated with a minimum power of 2 W. Spacecraft tracking consisted of two C-band radar transponders and an acquisition-aid beacon. One transponder is mounted in the adaptor with a peak power output of 600 W to a slot antenna on the bottom of the adaptor. The other is in the reentry section, delivering 1000 W to three helical antennas mounted at 120 degree intervals just forward of the hatches. The acquisition-aid beacon was mounted on the adaptor and had a power of 250 mW.

#### Reentry

At the time of reentry, the spacecraft would be maneuvered to the appropriate orientation and equipment adaptor section would be detached and jettisoned, exposing the retrorocket module. The retrorockets consisted of four spherical-case polysulfide ammonium perchlorate solid-propellant motors mounted near the center of the reentry adaptor module, each with 11,070 N thrust. They would fire to initiate the spacecraft reentry into the atmosphere, with attitude being maintained by a reentry control system of 16 engines, each with 5.2 N thrust. The retrorocket module would then be jettisonned, exposing the heat shield at the base of the reentry module. Along with the ablative heat shield, thermal protection during reentry was provided by thin Rene 41 radiative shingles at the base of the module and beryllium shingles at the top. Beneath the shingles was a layer of MIN-K insulation and thermoflex blankets. At an altitude of roughly 15,000 meters the astronauts would deploy a 2.4 meter drogue chute from the rendezvous and recovery section. At 3230 meters altitude the crew releases the drogue which extracts the 5.5 meter pilot parachute. The rendezvous and recovery section is released 2.5 seconds later, deploying the 25.6 meter main ring-sail parachute which is stored in the bottom of the section. The spacecraft is then rotated from a nose-up to a 35 degree angle for water landing. At this point a recovery beacon is activated, transmitting via an HF whip antenna mounted near the front of the reentry module.

#### Gemini Program

The Gemini program was designed as a bridge between the Mercury and Apollo programs, primarily to test equipment and mission procedures in Earth orbit and to train astronauts and ground crews for future Apollo missions. The general objectives of the program included: long duration flights in excess of of the requirements of a lunar landing mission rendezvous and docking of two vehicles in Earth orbit the development of operational proficiency of both flight and ground crews the conduct of experiments in space extravehicular operations active control of reentry flight path to achieve a precise landing point and onboard orbital navigation. Each Gemini mission carried two astronauts into Earth orbit for periods ranging from 5 hours to 14 days. The program consisted of 10 crewed launches, 2 uncrewed launches, and 7 target vehicles, at a total cost of approximately 1,280 million dollars.

## Planetary Voyage

The twin spacecraft Voyager 1 and Voyager 2 were launched by NASA in separate months in the summer of 1977 from Cape Canaveral, Florida. As originally designed, the Voyagers were to conduct closeup studies of Jupiter and Saturn, Saturn's rings, and the larger moons of the two planets.

To accomplish their two-planet mission, the spacecraft were built to last five years. But as the mission went on, and with the successful achievement of all its objectives, the additional flybys of the two outermost giant planets, Uranus and Neptune, proved possible -- and irresistible to mission scientists and engineers at the Voyagers' home at the Jet Propulsion Laboratory in Pasadena, California.

As the spacecraft flew across the solar system, remote-control reprogramming was used to endow the Voyagers with greater capabilities than they possessed when they left the Earth. Their two-planet mission became four. Their five-year lifetimes stretched to 12 and is now near thirty-seven years.

Eventually, between them, Voyager 1 and 2 would explore all the giant outer planets of our solar system, 48 of their moons, and the unique systems of rings and magnetic fields those planets possess.

Had the Voyager mission ended after the Jupiter and Saturn flybys alone, it still would have provided the material to rewrite astronomy textbooks. But having doubled their already ambitious itineraries, the Voyagers returned to Earth information over the years that has revolutionized the science of planetary astronomy, helping to resolve key questions while raising intriguing new ones about the origin and evolution of the planets in our solar system.

History Of The Voyager Mission

The Voyager mission was designed to take advantage of a rare geometric arrangement of the outer planets in the late 1970s and the 1980s which allowed for a four-planet tour for a minimum of propellant and trip time. This layout of Jupiter, Saturn, Uranus and Neptune, which occurs about every 175 years, allows a spacecraft on a particular flight path to swing from one planet to the next without the need for large onboard propulsion systems. The flyby of each planet bends the spacecraft's flight path and increases its velocity enough to deliver it to the next destination. Using this "gravity assist" technique, first demonstrated with NASA's Mariner 10 Venus/Mercury mission in 1973-74, the flight time to Neptune was reduced from 30 years to 12.

While the four-planet mission was known to be possible, it was deemed to be too expensive to build a spacecraft that could go the distance, carry the instruments needed and last long enough to accomplish such a long mission. Thus, the Voyagers were funded to conduct intensive flyby studies of Jupiter and Saturn only. More than 10,000 trajectories were studied before choosing the two that would allow close flybys of Jupiter and its large moon Io, and Saturn and its large moon Titan the chosen flight path for Voyager 2 also preserved the option to continue on to Uranus and Neptune.

From the NASA Kennedy Space Center at Cape Canaveral, Florida, Voyager 2 was launched first, on August 20, 1977 Voyager 1 was launched on a faster, shorter trajectory on September 5, 1977. Both spacecraft were delivered to space aboard Titan-Centaur expendable rockets.

The prime Voyager mission to Jupiter and Saturn brought Voyager 1 to Jupiter on March 5, 1979, and Saturn on November 12, 1980, followed by Voyager 2 to Jupiter on July 9, 1979, and Saturn on August 25, 1981.

Voyager 1's trajectory, designed to send the spacecraft closely past the large moon Titan and behind Saturn's rings, bent the spacecraft's path inexorably northward out of the ecliptic plane -- the plane in which most of the planets orbit the Sun. Voyager 2 was aimed to fly by Saturn at a point that would automatically send the spacecraft in the direction of Uranus.

After Voyager 2's successful Saturn encounter, it was shown that Voyager 2 would likely be able to fly on to Uranus with all instruments operating. NASA provided additional funding to continue operating the two spacecraft and authorized JPL to conduct a Uranus flyby. Subsequently, NASA also authorized the Neptune leg of the mission, which was renamed the Voyager Neptune Interstellar Mission.

Voyager 2 encountered Uranus on January 24, 1986, returning detailed photos and other data on the planet, its moons, magnetic field and dark rings. Voyager 1, meanwhile, continues to press outward, conducting studies of interplanetary space. Eventually, its instruments may be the first of any spacecraft to sense the heliopause -- the boundary between the end of the Sun's magnetic influence and the beginning of interstellar space.

Following Voyager 2's closest approach to Neptune on August 25, 1989, the spacecraft flew southward, below the ecliptic plane and onto a course that will take it, too, to interstellar space. Reflecting the Voyagers' new transplanetary destinations, the project is now known as the Voyager Interstellar Mission.

Voyager 1 has crossed into the heliosheath and is leaving the solar system, rising above the ecliptic plane at an angle of about 35 degrees at a rate of about 520 million kilometers (about 320 million miles) a year. (Voyager 1 entered interstellar space on August 25, 2012.) Voyager 2 is also headed out of the solar system, diving below the ecliptic plane at an angle of about 48 degrees and a rate of about 470 million kilometers (about 290 million miles) a year.

Both spacecraft will continue to study ultraviolet sources among the stars, and the fields and particles instruments aboard the Voyagers will continue to explore the boundary between the Sun's influence and interstellar space. The Voyagers are expected to return valuable data for at least another decade. Communications will be maintained until the Voyagers' power sources can no longer supply enough electrical energy to power critical subsystems.

## NASA's Stardust adjusts flight path for comet meetup

Just over two weeks before its flyby of comet Tempel 1, NASA's Stardust spacecraft fired its thrusters to help refine its flight path toward the comet. The Stardust-NExT mission will fly past comet Tempel 1 on Valentine's Day (Feb. 14, 2011).

The trajectory correction maneuver, which adjusts the spacecraft's flight path, began at about 4 p.m. EST (1:00 p.m. PST) on Monday, Jan. 31. The Stardust spacecraft's rockets fired for 130 seconds, consumed about 300 grams (10.6 ounces) of fuel and changed the spacecraft's speed by 2.6 meters per second (about 5.8 mph).

"An almost six-miles-per-hour change may seem insignificant when we're closing in on the comet at 24,236 miles per hour [39,000 kilometers per hour]," said Tim Larson, Stardust-NExT project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "But we're still two weeks and 8.37 million miles [13.5 million kilometers] away from the comet. At that distance, our burn will move our location at time of closest approach to the comet by almost 1,900 miles [3,058 kilometers]. By observing the results of these planned maneuvers and making further rocket burns, that's how we get a spacecraft to be where we want it, when it's on the other side of the solar system."

NASA's plan for the Stardust-NExT mission is to fly the spacecraft to target a point in space about 200 kilometers (124 miles) from comet Tempel 1 at the time of its closest approach -- about 8:56 p.m. PST on Feb. 14 (11:56 p.m. EST). This is a bonus mission for the comet chaser, which previously flew past comet Wild 2 and returned particles from its coma to Earth. During this bonus encounter, the spacecraft will take images of the comet's surface to observe what changes have occurred since a NASA spacecraft last visited. (NASA's Deep Impact executed an encounter with Tempel 1 in July 2005).

Along with the high-resolution images of the comet's surface, Stardust-NExT will also measure the composition, size distribution and flux of dust emitted into the coma, and provide important new information on how Jupiter-family comets evolve and how they formed 4.6 billion years ago. A Jupiter-family comet is a comet whose orbit has been modified by close passages to Jupiter. They have orbital periods less than 20 years.

Launched on Feb. 7, 1999, Stardust became the first spacecraft in history to collect samples from a comet (Wild 2), and return them to Earth for study. While its sample return capsule parachuted to Earth in January 2006, mission controllers were placing the still-viable spacecraft on a path that would allow NASA the opportunity to re-use the already-proven flight system if a target of opportunity presented itself. In January 2007, NASA re-christened the mission "Stardust-NExT" (New Exploration of Tempel), and the Stardust team began a four-and-a-half year journey for the spacecraft to comet Tempel 1. This will be the second exploration of Tempel 1 by a spacecraft (Deep Impact).

Its final orbit will only be 375 km above Ceres, but you have to give it time.

Dawn is powered by xenon ion engines, which are extremely efficient, but very weak. The usual comparison is that they are pushing the craft forwards about as much as a sheet of paper pushes down on your hand. Their advantage is that they can do this for a very long time. This is why Dawn has been able to visit Vesta and Ceres.

A probe with conventional chemical rocket engines can change its speed very quickly, but it only has fuel to fire a few times, briefly. What has been done in the past is that probes have been carefully aimed at their destinations, and then when they arrive, they swing in really close to the planet and fire their engines when grazing past at the closest point. This is the best way to slow down. By doing that over a few passes, it can be done with only brief firings and that saves on fuel. This is called an Oberth maneuver.

Dawn doesn't have to fire engines at the closest approach to Ceres. Its engines just keep firing, and firing until finally it slows down enough to orbit much closer. In fact the word 'firing' doesn't really apply. The engines have been running, continuously, for thousands of days. (Well, there's three, and they switch which is on periodically, and there were some periods of coasting.)

Braking this way takes weeks. It has already been braking for weeks as it got closer to Ceres. Then at first it was captured only into a highly elliptical orbit whose far point was very far away from Ceres. Still it runs its engines over a large portion of its orbit rather than applying much greater force briefly at the orbit's lowest point, and thus the Oberth effect is much less important in its maneuvers.

But because the engines are so very efficient, still less fuel is needed that if chemical rockets had been used. The luxury of having engines that run on and on may also have helped mitigate the problems of dealing with the failed reaction wheels, and also the (temporary) failure of an entire engine did not doom the mission.

(Note - Probes like Cassini and Voyager managed to visit several destinations by making extremely clever use of gravity assists as well, carefully entering into the gravity well of a planet or moon in just the right way so that they got swung around them and thrown out again in just the right direction to proceed on to the next destination. It is only because they were set up to do this that they managed to make it to several awesome destinations. But they wouldn't have been able to slingshot very much with these low gravity destinations.)

## Is Dawn's upcoming low periapsis orbit for XMO7 &ldquoresonant&rdquo?

In his Dawn Journal blog, mission director and chief engineer Marc Rayman discussed the challenges of bringing the spacecraft into its final orbit in detail.

During XMO7, Dawn should orbit Ceres once every 27 hours and 13 minutes, which equals exactly three times the dwarf planet’s rotational period. In what is known as a three-to-one resonant orbit, the probe should complete one orbit around Ceres for every three rotations the dwarf planet makes. (emphasis added)

An orbit who's period matches the sidereal rotation period of a body is called a synchronous orbit. Geosynchronous (but non-Geostationary) satellites around earth have an Analemma shaped ground track. According to my own answer to the quest Are there terms for Earth orbits with rational number multiples of 1 sidereal day? an orbit who's period is artificially synchronized to a rational fraction of the sidereal period of the central body is called a repeat ground-track orbit and not a resonant orbit.

These are non-resonant orbits where the orbital period is matched to a rational number times the sidereal rotation period of the orbited body. Spy satellites used to do this so that the time of day was about the same for each pass over a given area to be photographed, to reduce the variation in shadows and illumination.

A resonant orbit on the other hand usually involves a gravitational interaction between two bodies. There are various situations where resonant orbits appear in the solar system involving three bodies (two moons and a planet, two bodies and the Sun) and there are at least two known examples of resonant tidal locking of the rotation rate of the smaller body with it's orbit: Mercury (3:2) orbiting around the Sun, and the Moon (1:1) orbiting around the Earth.

But I've never heard of the smaller orbiting body being in resonance with the rotation rate of the larger body. You could say that a geostationary non-station-keeping geosynchronous satellite that "falls into" a certain longitude range stationary points because of Earth's lumpy gravity is such a resonance, so maybe I have.

The problem though is that I am not sure why Dawn's orbit around Ceres is called "resonant" and not simply "synchronized 3:1". As far as I know, the term "resonance" always implies a locking or an interaction between two oscillators, so that they are "pulled into resonance" by the interaction.

QUESTION: I am wondering if Dawn's orbit is truly in a resonance with Ceres due to some multipole component of its gravity field, or if this is really just an artificially synchronized repeat ground track orbit and the term resonant doesn't actually apply.

I noticed that the Astronomy Now article Dawn spacecraft dropping to record low altitude at Ceres seems to be careful to never use the "r-word" when it describes the orbit:

The goal is to synchronise Dawn’s orbit with the nine-hour four-minute rotation of Ceres to ensure the spacecraft will repeatedly fly over a specific point on the surface – Occator Crater, where highly reflective salt deposits are visible – during the low point of each orbit.

“The flight team will synchronise the orbit so that each time Dawn swoops down to low altitude, it does so at just the right time so that Ceres’ rotation will place the Occator geological unit under the probe’s flight path,” Rayman writes.

## Dear Isaac Newdawn, Charles Dawnwin, Albert Einsdawn and all other science enthusiasts

For the first time in almost a year, the Dawn mission control room at JPL is aglow with blue.

The rope lights strung around the room bathe it in a gentle light reminiscent of the beam emitted by an ion engine on the faraway spacecraft as it maneuvers in orbit around Ceres. Dawn had not thrust since June, but it is now using ion engine #2 to fly to a new orbit around the dwarf planet. Thanks to its uniquely capable ion propulsion system, Dawn has accomplished far more powered flight than any other spacecraft, and more is ahead.

Dawn has spent most of the last year revolving around Ceres once every 30 days in extended mission orbit 5 (XMO5), a designation that illustrates the team's flair for the dramatic. (Your correspondent, as passionate as anyone about the exploration of the cosmos, can imagine only a few names more inspiring than that. Fortunately, one of them happens to be "XMO7." Read on!) As the probe followed that elliptical course, it reached down to a little less than 2,800 miles (4,400 kilometers) above the alien world and up to 24,300 miles (39,100 kilometers).

Dawn flew to high altitude late in 2016. Its work there is now complete, and defying expectations, the aged adventurer still has life left in it. As we saw in last month's overview of the two upcoming orbits, Dawn's next assignment is to go much, much lower.

XMO5 and the subsequent two orbits are elliptical, as shown in the illustrations last month and the new one below. Observing Ceres from a very low altitude is possible only in an elliptical orbit, not a circular one. Dawn was not designed to operate at low altitude, and its reaction wheels, which are so important for controlling its orientation, have failed, making the problem even more difficult. We have discussed this before and will address another aspect of it this month for the lowest orbit.

Although the elliptical orbits introduce many new technical challenges for the team, Dawn still takes a spiral route from each orbit to the next, just as it did earlier at Ceres and at Vesta when the orbits were circular. In essence, the ion engine smoothly shrinks the starting ellipse until the new ellipse is the size needed. These trajectories are very complicated to plan and to execute, but with the expert piloting of the experienced team, the maneuvering is going very well. (You can follow the progress with the mission status updates.)

The blue curve is Dawn's flight path from XMO5 (the outer green ellipse) to XMO6 (the inner one). Image credit: NASA/JPL-Caltech

Dawn began its descent on April 16. On May 15, with the blue lights turned off in mission control, the veteran explorer will begin its observations in XMO6. (As suggested last month, the targeted minimum and maximum altitudes for XMO6 are being updated slightly even as Dawn is on its way. In the next Dawn Journal, we will present the actual altitude range.) If all goes well, the control room will be lit up in blue again from May 31 to June 7, as the ship sails down to XMO7.

In XMO7, Dawn will swoop down to an incredibly low 22 miles (35 kilometers) above the exotic terrain of ice, rock and salt. The last time it was that close to a solar system body was when it rode a rocket from Cape Canaveral over the Atlantic Ocean more than a decade ago. (For readers unfamiliar with solar system geography, that was Earth.) The XMO7 ellipse will then take the spacecraft up to 2,500 miles (4,000 kilometers). Each revolution will last 27 hours and 13 minutes. In considerably less time than that (assuming you read at a typical speed), we will discuss why this orbital period is important.

Last month, we described some of Dawn's planned low-altitude measurements of nuclear radiation to reveal more about Ceres' composition. As a bonus objective, scientists would like to study the elements in one of their favorite places (and perhaps one of yours as well): Occator Crater, site of the highly reflective salt deposits, famous not only on Ceres but also on Earth and everywhere else that readers follow Dawn's discoveries. Studying this one crater and the area around it (together known as a geological unit) could reveal more about the complex geology there. But doing so is quite a challenge, as Dawn would need to pass over that region 20 times to allow the gamma ray and neutron detector (GRaND) to record enough of the faint nuclear radiation. This is the equivalent of taking a long exposure with a camera when photographing a very dim scene.

Attempting to repeatedly fly low over that geological unit presents daunting obstacles, as we will discuss. It may not work, but the team will try. That's part of what makes for a daring adventure! And accomplishing such a feat requires a special trick. Fortunately, the Dawn team has several at its disposal.

Recall that Dawn will loop around Ceres, going south to north at low altitude and back to the south again at high altitude. Meanwhile, Ceres will turn on its axis toward the east, completing one rotation in just over 9 hours, 4 minutes. (Note that Ceres turns quite a bit faster than Earth. A Cerean day is much closer in duration to a day on Jupiter, which is 9 hours, 56 minutes. All three turn east.) Therefore, the flight team will synchronize the orbit so that each time Dawn swoops down to low altitude, it does so at just the right time so that Ceres' rotation will place the Occator geological unit under the probe's flight path.

We mentioned above that Dawn's orbit will take 27 hours, 13 minutes. This period is chosen to be exactly three times Ceres' rotation period. Experts (now including you) describe this as a three-to-one resonant orbit, meaning that for every three times Ceres turns, Dawn turns around it once.

If this synchronization is clear, feel free to skip this paragraph. Perhaps get a snack until it's time for the next paragraph or, better yet, use this time to gaze at the mesmerizing beauty of the night sky and contemplate the magnificence of the cosmos. If the synchronization is not clear, find a globe of Earth. Now imagine a satellite circling it, flying from the south pole to the north pole over one hemisphere and back to the south pole over the opposite hemisphere. Suppose the first passage occurs over your location. If Earth didn't rotate, the second orbit would take it over the same place. (Of course, if Earth didn't rotate, you might run out of patience waiting for tomorrow.) Now rotate the globe a little bit while your imagined satellite goes through one revolution. If it flew over your location the first time, it will not the second time. And you can see that with Earth rotating at a constant speed, it requires a carefully chosen speed for the satellite to pass over the desired target on each revolution. The Dawn flight team will work very hard to help our distant explorer have the orbit needed to achieve the three-to-one resonance.

The accuracy necessary will be difficult to achieve, even for the Dawn flight team at JPL, where the best celestial navigators in the solar system get to work. The problems that must be overcome are manifold. One of them is that, lacking functioning reaction wheels, Dawn fires its small hydrazine-fueled thrusters to control its orientation in space. Whether to turn to keep its sensors trained on the ground, even with the constantly changing altitude and velocity in the elliptical orbit, or to point its main antenna at Earth, the reaction from a little burst of hydrazine not only rotates the spacecraft but also nudges it in its orbit. (We have described this several times in great detail before.) Each small push from the thrusters distorts the orbit a little bit, desynchronizing it from the three-to-one resonance.

Another difficulty is that, just like Earth, Mars, the Moon and other solar system residents (not to mention cookie dough ice cream), Ceres is not uniform inside. Its complex geology has produced some regions of higher density and some of lower density (although not with the same delectable composition as the ice cream). The total gravitational pull on the spacecraft depends on the dwarf planet's internal structure. We have described before how scientists take advantage of it to map the interior. But we have measured the gravity from 240 miles (385 kilometers) high. When Dawn swoops down much lower, our gravity map will not be accurate enough to predict all the subtle details of the mass distribution that may cause slightly larger or slightly smaller pulls at some locations. It will take quite a while to formulate the new gravity map. That new map may reveal more about what's underground, but until then, it will be harder to keep the orbit in sync.

On two occasions in mid-June Dawn will use its ion engine to tweak its orbit (in what we have described before as a trajectory correction maneuver) to help maintain the synchronization, but there will still be residual discrepancies.

We described and depicted last month how the low point of Dawn's orbit will gradually shift southward on each successive revolution. That means we will have only a limited number of opportunities to fly over Occator before the low point is too far south. Given the complexity of the operations, the planned measurements are not at all assured.

There are other aspects of this problem as well. While we will not delve into them here, engineers have been working hard on every one of them.

This view of Juling Crater was constructed from pictures Dawn took from its lowest orbit so far, 240 miles (385 kilometers) high. We have presented other views of this 12-mile (20-kilometer) crater, including last month, when we described the discovery that the amount of ice on the shadowed northern wall changed over six months in 2016. Ceres is not a static world. When Dawn dives down lower in June, it will obtain sharper images than this (at other locations). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

We have mentioned before that photography will be extremely challenging in XMO7, because of both the high speed so close to the ground and the difficulty pointing the camera accurately enough to capture a specific target. Let's take a more careful look at the nature of the orbit to understand more about the problem of trying to see any particular site.

You can think of the motion in an elliptical orbit as being somewhat like that of a swing. Imagine a girl named Dawn on a swing. Perhaps she is 10 and a half years old (like our spacecraft), usually (but not always) does what we instruct (like our spacecraft), feels energized by the light of the Sun (like our spacecraft), loves the idea of exploring uncharted worlds (like our spacecraft) and uses photomultiplier tubes coupled to a bismuth germanate crystal scintillator, lithiated glass and boron-loaded plastic to measure the spectra of nuclear radiation (okay, she is not like our spacecraft in every way).

When Dawn rides her swing, her speed is constantly changing. As she approaches the top of her arc, gravity slows her down and even brings her momentarily to a stop. She then begins to fall, accelerating as she gets lower. As soon as she passes the lowest point, her upward motion and the downward pull of gravity oppose each other, and once again she begins to slow. When her swing is pumped up (whether with her legs or by the push of her friend or her friendly ion engine), her arc will reach higher, and then she will speed through the low point even faster.

Of course, the swing does not trace out an ellipse, and the girl does not loop all the way around, but the fundamental principles of motion are the same, as methodically investigated by Galileo Galilei four centuries ago and explained by Isaac Newton in the second half of the 17th century. Dawn's elliptical orbit around Ceres will behave somewhat like the swing. At high altitude, far above the dwarf planet, the spacecraft will move at only about 120 mph (190 kph). Then, as gravity pulls it back down, the spacecraft will accelerate until it skims over the ground at 1,050 mph (1,690 kph) before starting to swing up again.

Dawn is much, much, much too far away for controllers to point its camera and other instruments as you might with a joystick or other controller in real time. Readers of the final paragraph of every Dawn Journal know that radio signals, traveling at the universal limit of the speed of light, usually take more than half an hour to complete the round trip. When Dawn is in XMO7 this summer, it will be about an hour. While the spacecraft is racing over the Cerean landscape, it can't wait for its radio signal to tell controllers what it sees and then, based on that, for a return radio signal to help it adjust the pointing of its camera. All the instructions from Earth have to be radioed in advance.

It is a very complicated process to go from measuring Dawn's orbit accurately to the probe actually aiming its camera and its spectrometers to collect new data, with many calculations and many steps in between, each of which has to be checked and double checked. The team has a special campaign planned for that purpose, and they will maneuver to XMO7 so that the best viewing will be in late June. But even when they work quickly for this dedicated attempt to get some bonus photographs of Occator, the entire process will take the better part of a week because of the spacecraft's orbital activities (e.g., while it observes Ceres, it cannot communicate with Earth), segments of its orbit where Ceres blocks its radio signal to Earth and so it is not possible to communicate, and the schedule for the large Deep Space Network antennas to shout so Dawn can then listen for what fades to become a long-distance radio whisper. Time needs to be allocated for computers and people to analyze data, to formulate and verify the new plans, to beam the instructions to Dawn and then Dawn finally to execute them. Meanwhile, even after the initial measurement of its orbit, while all this work is occurring on Earth, the ship will continue to be buffeted by the hydrazine winds and the gravitational currents, so its course will continue to change.

The consequence of all this is that by the time Dawn actually conducts its observations, its orbit will be different from what was measured days earlier. The carefully devised prediction that formed the basis of the plans could well be off one way or the other by four minutes or even more. (By the way, calculating now the credible magnitude of the error for this June campaign is a sophisticated science that, in itself, involves thousands and thousands of hours of computer calculations, performed on hundreds of computers working simultaneously. Epistemic knowledge does not come easily.)

From Dawn's perspective, descending and speeding north at 1,050 mph (1,690 kph) to the vicinity of Occator, faithfully pointing its sensors according to the plan worked out days before on a distant planet and stored in its computer, Ceres' rotation will carry the crater to the right at more than 190 mph (310 kph). Dawn's camera will take in a scene about 2.1 miles (3.4 kilometers) across, and at the spacecraft's high velocity, there won't be time to turn right and left to cover a broader swath. Even if the probe arrived at Occator's latitude a mere 20 seconds off schedule, a spot on the ground that was expected to be in the center of the camera would have moved entirely out of view and so would not even be glimpsed. If Dawn were four minutes too early or too late, the ground beneath the spacecraft (known as the ground track) would shift west or east by 13 miles (21 kilometers), and the terrain that's photographed could be entirely different from what was expected.

Occator Crater is 57 miles (92 kilometers) across, so all this work should allow GRaND, with its very wide field of view, to measure the composition in the geological unit that contains the crater. But the narrower view of the camera means we cannot be certain what features we will see. Fortunately, we already know that there is fascinating geology just about everywhere in and near Occator. Indeed, the dwarf planet is vast and varied, with a great many intriguing features. We are going to behold some amazing sights!

Before then, we will gain new perspectives from XMO6 in May. And as Dawn was getting closer to Ceres, together the pair were getting closer to the Sun until yesterday. Dawn isn't the only object in an elliptical orbit. Ceres, Earth, and all the other planets (whether dwarf or not) travel in elliptical orbits too, although they orbit the Sun. Ceres' orbit is more elliptical than Earth's but not as much as some of the other planets. The shape of Ceres' orbit is between that of Saturn's (which is more circular) and Mars' (which is more elliptical). (Of course, Ceres' orbit is larger than Mars' orbit -- it revolves farther from the Sun than the Red Planet does -- and smaller than Saturn's, but our focus here is on how much the orbit deviates from a perfect circle, regardless of the size.)

The location of Ceres and Dawn in the solar system is shown on April 28, 2018, when they were at perihelion, the minimum distance to the Sun. We have charted Dawn's progress on this figure many times before, most recently in September. Image credit: NASA/JPL-Caltech

In its 4.6-year-long Cerean year, Ceres, with Dawn in tow, reached the minimum solar distance of just under 2.56 AU (238 million miles, or 383 million kilometers) on April 28. Dawn also was in residence at Ceres when they were at their maximum distance from the Sun in January 2016. Although the dwarf planet's orbit is not elliptical enough that the additional solar heating is expected to have much effect, the upcoming observations in XMO6 will provide scientists with the opportunity to look for any changes just in case. (The change Dawn detected at Juling Crater is more likely related to the seasonal change of the angle of the Sun rather than the distance to the Sun.)

The solar system constantly performs a complex and beautiful choreography, with everything in motion. Dawn will complete its current elegant spiral in another two weeks, and then it will be time for the next act, XMO6 and, after that, the finale, XMO7. A great many challenges are ahead but the allure of the rich rewards of new knowledge, new insight, and a new adventure is irresistible as Dawn delves further into the unknown.

Dawn is 1,400 miles (2,300 kilometers) from Ceres. It is also 2.34 AU (218 million miles, or 350 million kilometers) from Earth, or 900 times as far as the Moon and 2.32 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 39 minutes to make the round trip.

## NASA spacecraft nears historic arrival at dwarf planet Ceres

NASA’s Dawn spacecraft took these images of dwarf planet Ceres from about 25,000 miles (40,000 kilometres) away on February 25th, 2015. Ceres appears half in shadow because of the current position of the spacecraft relative to the dwarf planet and the Sun. The resolution is about 2.3 miles (3.7 kilometres) per pixel. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

NASA’s Dawn spacecraft has returned new images captured on approach to its historic orbit insertion at the dwarf planet Ceres. Dawn will be the first mission to successfully visit a dwarf planet when it enters orbit around Ceres on Friday, March 6th.

“Dawn is about to make history,” said Robert Mase, project manager for the Dawn mission at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Our team is ready and eager to find out what Ceres has in store for us.”

Recent images show numerous craters and unusual bright spots that scientists believe tell how Ceres, the first object discovered in our Solar System’s asteroid belt, formed and whether its surface is changing. As the spacecraft spirals into closer and closer orbits around the dwarf planet, researchers will be looking for signs that these strange features are changing, which would suggest current geological activity.

“Studying Ceres allows us to do historical research in space, opening a window into the earliest chapter in the history of our Solar System,” said Jim Green, director of NASA’s Planetary Science Division at the agency’s Headquarters in Washington. “Data returned from Dawn could contribute significant breakthroughs in our understanding of how the Solar System formed.”

Ceres rotates in this sped-up movie comprised of images taken by NASA’s Dawn mission during its approach to the dwarf planet. The images were taken on February 19th, 2015, from a distance of nearly 29,000 miles (46,000 kilometres). Dawn observed Ceres for a full rotation, which lasts about nine hours. The images have a resolution of 2.5 miles (4 kilometres) per pixel. Click to view full-size. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn began its final approach phase toward Ceres in December. The spacecraft has taken several optical navigation images and made two rotation characterisations, allowing Ceres to be observed through its full nine-hour rotation. Since January 25th, Dawn has been delivering the highest-resolution images of Ceres ever captured, and they will continue to improve in quality as the spacecraft approaches.

Sicilian astronomer Father Giuseppe Piazzi spotted Ceres in 1801. As more such objects were found in the same region, they became known as asteroids, or minor planets. Ceres was initially classified as a planet and later called an asteroid. In recognition of its planet-like qualities, Ceres was designated a dwarf planet in 2006, along with Pluto and Eris.

Ceres is named for the Roman goddess of agriculture and harvests. Craters on Ceres will similarly be named for gods and goddesses of agriculture and vegetation from world mythology. Other features will be named for agricultural festivals.

Launched in September 2007, Dawn explored the giant asteroid Vesta for 14 months in 2011 and 2012, capturing detailed images and data about that body. Both Vesta and Ceres orbit the Sun between Mars and Jupiter, in the main asteroid belt. This two-stop tour of our solar system is made possible by Dawn’s ion propulsion system, its three ion engines being much more efficient than chemical propulsion.

“Both Vesta and Ceres were on their way to becoming planets, but their development was interrupted by the gravity of Jupiter,” said Carol Raymond, deputy project scientist at JPL. “These two bodies are like fossils from the dawn of the Solar System, and they shed light on its origins.”

Ceres and Vesta have several important differences. Ceres is the most massive body in the asteroid belt, with an average diameter of 590 miles (950 kilometres). Ceres’ surface covers about 38 percent of the area of the continental United States. Vesta has an average diameter of 326 miles (525 kilometres), and is the second most massive body in the belt. The asteroid formed earlier than Ceres and is a very dry body. Ceres, in contrast, is estimated to be 25 percent water by mass.

“By studying Vesta and Ceres, we will gain a better understanding of the formation of our Solar System, especially the terrestrial planets and most importantly the Earth,” said Raymond. “These bodies are samples of the building blocks that have formed Venus, Earth and Mars. Vesta-like bodies are believed to have contributed heavily to the core of our planet, and Ceres-like bodies may have provided our water.”

“We would not be able to orbit and explore these two worlds without ion propulsion,” Mase said. “Dawn capitalises on this innovative technology to deliver big science on a small budget.”

In addition to the Dawn mission, NASA will launch in 2016 its Origins-Spectral Interpretation-Resource Identification-Security-Regolith Explorer (OSIRIS-REx) spacecraft. This mission will study a large asteroid in unprecedented detail and return samples to Earth.

NASA also places a high priority on tracking and protecting Earth from asteroids. NASA’s Near-Earth Object (NEO) Program at the agency’s headquarters manages and funds the search, study and monitoring of asteroids and comets whose orbits periodically bring them close to Earth. NASA is pursuing an Asteroid Redirect Mission (ARM), which will identify, redirect and send astronauts to explore an asteroid. Among its many exploration goals, the mission could demonstrate basic planetary defence techniques for asteroid deflection.

## Serendipitous Juno Spacecraft Detections Shatter Ideas About Origin of Zodiacal Light

Look up to the night sky just before dawn, or after dusk, and you might see a faint column of light extending up from the horizon. That luminous glow is the zodiacal light, or sunlight reflected toward Earth by a cloud of tiny dust particles orbiting the Sun. Astronomers have long thought that the dust is brought into the inner solar system by a few of the asteroid and comet families that venture in from afar.

But now, a team of Juno scientists argues that Mars may be the culprit. They published their finding in a March 9 paper in the Journal of Geophysical Research: Planets. An instrument aboard the Juno spacecraft serendipitously detected dust particles slamming into the spacecraft during its journey from Earth to Jupiter. The impacts provided important clues to the origin and orbital evolution of the dust, resolving some mysterious variations of the zodiacal light.

Though their discovery has big implications, the scientists who spent years studying cosmic debris did not set out to do so. “I never thought we’d be looking for interplanetary dust,” said John Leif Jørgensen, a professor at the Technical University of Denmark.

Jørgensen designed the four star trackers that are part of Juno’s magnetometer investigation. These onboard cameras snap photos of the sky every quarter of a second to determine Juno’s orientation in space by recognizing star patterns in its images — an engineering task essential to the magnetometer’s accuracy.

But Jørgensen hoped his cameras might also catch sight of an undiscovered asteroid. So he programmed one camera to report things that appeared in multiple consecutive images but weren’t in the catalog of known celestial objects.

He didn’t expect to see much: Nearly all objects in the sky are accounted for in the star catalog. So when the camera started beaming down thousands of images of unidentifiable objects — streaks appearing then mysteriously disappearing — Jørgensen and his colleagues were baffled. “We were looking at the images and saying, ‘What could this be?’” he said.

Jørgensen and his team considered many plausible and some implausible causes. There was the unnerving possibility that the star camera had caught a leaking fuel tank on Juno. “We thought, ‘Something is really wrong,’” Jørgensen said. “The images looked like someone was shaking a dusty tablecloth out their window.”

It wasn’t until the researchers calculated the apparent size and velocity of the objects in the images that they finally realized something: Dust grains had smashed into Juno at about 10,000 miles (or 16,000 kilometers) per hour, chipping off submillimeter pieces. “Even though we’re talking about objects with only a tiny bit of mass, they pack a mean punch,” said Jack Connerney, Juno’s magnetometer investigation lead, and the mission’s deputy principal investigator, who’s based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

As it turned out, the spray of debris was coming from Juno’s expansive solar panels — the biggest and most sensitive unintended dust detector ever built.

“Each piece of debris we tracked records the impact of an interplanetary dust particle, allowing us to compile a distribution of dust along Juno’s path,” Connerney said. Juno launched in 2011. After a deep-space maneuver in the asteroid belt in 2012, it returned to the inner solar system for an Earth gravity assist in 2013, which catapulted the spacecraft towards Jupiter.

Connerney and Jørgensen noticed that the majority of dust impacts were recorded between Earth and the asteroid belt, with gaps in the distribution related to the influence of Jupiter’s gravity. According to the scientists, this was a radical revelation. Before now, scientists have been unable to measure the distribution of these dust particles in space. Dedicated dust detectors have had limited collection areas and thus limited sensitivity to a sparse population of dust. They mostly count the more abundant and much smaller dust particles from interstellar space. In comparison, Juno’s expansive solar panels have 1,000 times more collection area than most dust detectors.

Juno scientists determined that the dust cloud ends at Earth because Earth’s gravity sucks up all the dust that gets near it. “That’s the dust we see as zodiacal light,” Jørgensen said.

As for the outer edge, around 2 astronomical units (AU) from the Sun (1 AU is the distance between Earth and the Sun), it ends just beyond Mars. At that point, the scientists report, the influence of Jupiter’s gravity acts as a barrier, preventing dust particles from crossing from the inner solar system into deep space. This same phenomenon, known as orbital resonance, also works the other way, where it blocks dust originating in deep space from passing into the inner solar system.

The profound influence of the gravity barrier indicates that the dust particles are in a nearly circular orbit around the Sun, Jørgensen said. “And the only object we know of in almost circular orbit around 2 AU is Mars, so the natural thought is that Mars is a source of this dust,” he said.

“The distribution of dust that we measure better be consistent with the variation of zodiacal light that has been observed,” Connerney said. The researchers developed a computer model to predict the light reflected by the dust cloud, dispersed by gravitational interaction with Jupiter that scatters the dust into a thicker disk. The scattering depends only on two quantities: the dust inclination to the ecliptic and its orbital eccentricity. When the researchers plugged in the orbital elements of Mars, the distribution accurately predicted the tell-tale signature of the variation of zodiacal light near the ecliptic. “That is, in my view, a confirmation that we know exactly how these particles are orbiting in our solar system,” Connerney said, “and where they originate.”

While there is good evidence now that Mars, the dustiest planet we know of, is the source of the zodiacal light, Jørgensen and his colleagues cannot yet explain how the dust could have escaped the grip of Martian gravity. They hope other scientists will help them.

In the meantime, the researchers note that finding the true distribution and density of dust particles in the solar system will help engineers design spacecraft materials that can better withstand dust impacts. Knowing the precise distribution of dust may also guide the design of flight paths for future spacecraft in order to avoid the highest concentration of particles. Tiny particles traveling at such high velocities can gouge up to 1,000 times their mass from a spacecraft.

Juno’s solar arrays escaped harm because the solar cells are well protected against impact on the back — or dark — side of the array by the support structure. Juno’s solar arrays escaped harm because the solar cells are well protected against impact on the back — or dark — side of the array by the support structure.

By Lonnie Shekhtman
NASA’s Goddard Space Flight Center, Greenbelt, Md.

## Selected References

• Gemini summary conference, NASA, SP-138, Wash, DC, Feb. 1967.
• Zeitler, E. O., and T. G. Rogers, Gemini program - physical sciences experiments summary, NASA-MSC, TM-X-58075, Houston, TX, Sept. 1971.
• Grimwood, J. M., et al., Project Gemini technology and operations - A chronology, NASA, NASA SP-4002, Wash., DC, 1969.

Diagram of the Gemini capsule. (Courtesy of NASA History Office.)

#### Gemini Books Online

Gemini 8 Images - Catalog of Spaceborne Imaging
More Gemini 8 Images - Kennedy Space Center
More Gemini Diagrams - NASA History Office