January 24
24 January 1986
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| Artist’s impression of Voyager passing Uranus, created for NASA in 1981. Credit: NASA/JPL/Don Davis |
Voyager 2 flew by Uranus on January 24, 1986.
At the closest approach, at 17:58:51 UT, the spacecraft came within
81,500 km of the planet's cloudtops. The probe
discovered new rings, eleven previously unknown moons, and a
magnetic field tilted at 55° off-axis and off-center. Voyager
2 remains the only spacecraft to explore Uranus. After the
encounter the probe proceeded on its journey to Neptune.
The spacecraft's observations at the distance of Uranus and Neptune were hampered by the need to take images in low light conditions. The intensity of sunlight on Uranus is about 360 times lower, and on Neptune about 900 times lower, than on Earth. This forced the spacecraft to take images of passing bodies using long exposure times, which risked blurring the images due to the spacecraft's own motion and vibrations. At the same time, the decreasing power of the spacecraft's radio signals reaching Earth limited the data transfer rate — the strength of radio signals received on Earth from the vicinity of Uranus was four times lower, and from the vicinity of Neptune about ten times lower, than during the flyby of Saturn.
During the flight towards Uranus, engineers reprogrammed Voyager 2's computers. New, more efficient image compression and data encoding algorithms were implemented. During the flyby, a camera motion compensation technique was used using the spacecraft's thrusters. Communication with the spacecraft was maintained by Deep Space Network (DSN) stations located in California, Australia, and Spain, equipped with antennas with 64-meter diameter dishes. The incoming signal power was 10−16 W. Signals amplified at the DSN stations were retransmitted via satellite links to the Jet Propulsion Laboratory in California, where a 200-person team of scientists awaited data about Uranus.
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| Voyager 2 trajectory in Uranus System, January 24, 1986. Credit: NASA/JPL |
The first navigational images of the planet were taken in June 1985. The Uranus observation phase began on November 4, 1985, when the probe was 103.5 million km away. On that day, a signal from Earth activated the probe's observation equipment and confirmed its operation. The probe reached Uranus along a curved, kinked path from Earth, covering a distance of 4.8 billion kilometers. At the time of Voyager 2's flyby, the Earth-Uranus distance was approximately 2.96 billion kilometers, resulting in a one-way radio signal travel time of 2 hours and 45 minutes.
Voyager 2 approached from the planet's south pole, almost perpendicular to the plane in which Uranus's ring system and natural satellites orbit. Closest approach to the planet occurred on January 24, 1986, at 17:58:51 UTC, at a distance of 107,100 km from Uranus' center and about 81,500 km from the cloud tops. This was 68 seconds ahead of schedule for the expedition, which began 8.5 years earlier. Voyager reached a speed of 67,820 km/h (41,820 mph). About ten hours earlier, the spacecraft had reached the limits of the planet's magnetosphere. The radio eclipse resulting from the spacecraft's disappearance behind the planet lasted about 1 hour 22 minutes. Most of the close-up observations of the moons and rings were made within 6 hours (from 4 hours before to 2 hours after closest approach). Voyager 2 captured and transmitted to Earth about 6,000 images of Uranus's surface. The spacecraft approached the five previously known moons to the following distances (predicted distances in parentheses): Titania at 365,300 km (372,000 km), Oberon at 471,500 km (472,000 km), Ariel at 130,400 km (127,000 km), Miranda at 28,000 km (29,000 km), and Umbriel at 325,100 km (325,000. km).
On January 29, the spacecraft finally exited the planet's magnetosphere. On February 14, a trajectory correction maneuver lasting over 2.5 hours was performed, which, together with a gravity assist performed during the Uranus flyby, placed Voyager 2 on a hyperbolic orbit toward Neptune. On February 25, 1986, the Uranus observation phase concluded.
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| Uranus in true color compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. Credit: NASA/JPL |
Voyager 2 discovered the planet's magnetic field and radiation belts, studied the structure of its magnetosphere, and discovered auroras. The structure and dynamics of the atmosphere, its helium content, and the content of many chemical compounds were analyzed. The planet's rotation period was determined. Concentric cloud systems were discovered around the south pole, where Uranus faced the Sun and Earth. These were tinged orange — in contrast to the greenish-blue hue of the rest of the planet. This confirmed the assumption that the planet's atmosphere was composed primarily of hydrogen, helium, methane, and ammonia. The probe revealed, for the first time, detailed surface morphology and geology of the planet's largest moons. The most unusual moon turned out to be Miranda, whose surface displays a vast diversity of geology.
Voyager discovered two new rings for the planet and numerous dust lanes in the ring plane, visible only in forward-scattered light. During the flyby, ten of the planet's small moons were discovered. These are: Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck. In 1999, images taken by Voyager 2 revealed another moon, which was named Perdita.
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| Voyager 2 picture of Uranus' rings taken on January 22, 1986, from a distance of 2.52 million kilometers. Nine rings are visible in this image, a 15-second exposure through a clear filter. Credit: NASA/JPL |
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| Miranda, a moon of Uranus, from Voyager 2 on Jan. 24, 1986, around its close approach to the Uranian moon. At least three terrain types of different age and geologic style are evident at this resolution of about 700 meters. Credit: NASA/JPL |
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| This picture is part of the highest-resolution Voyager 2 imaging sequence of Ariel, a moon of Uranus about 1,300 kilometers in diameter. The clear-filter, narrow-angle image was taken Jan 24, 1986, from a distance of 130,000 km. Credit: NASA/JPL |
© 2026, Andrew Mirecki
24 January 1990
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| Hiten spacecraft with Hagoromo subsatellite at the top. Credit: JAXA / ISAS |
Hiten, Japan's first lunar
spacecraft, was launched on January 24, 1990. It was the first lunar probe since
the Soviet Union's Luna 24 in 1976, and also the first lunar probe launched by a country other than the Soviet Union or the United States.
After launch, the spacecraft was put into a highly elliptical orbit around Earth that
intersected with the Moon’s orbit and inserted a small
subsatellite, named Hagoromo, into lunar orbit. Hiten had
carried out a total of 10 lunar flybys, two aerobraking
experiments in Earth’s atmosphere, then was put into a
looping orbit that passed the Earth–Moon L4 and L5
Lagrange points and inserted itself into lunar orbit on
February 15, 1992. The mission ended on April 10, 1993, with
the spacecraft impacting the lunar surface.
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| Lift-off of a Mu-3S-II launch vehicle with Hiten. Credit: JAXA |
Full Description from NASA Space Science Data Coordinated Archive:
Hiten (originally called Muses-A) was an ISAS (Japanese Space Agency) Earth orbiting satellite designed primarily to test and verify technologies for future lunar and planetary missions. The spacecraft carried a small satellite named Hagoromo which was released in the vicinity of the Moon. Hiten itself was put into a highly elliptical Earth orbit which passed by the Moon ten times during the mission, which ended when Hiten was intentionally crashed into the Moon on 10 April 1993. The primary objectives of the mission were to: 1) test trajectory control utilizing gravity assist double lunar swingbys; 2) insert a sub-satellite into lunar orbit; 3) conduct optical navigation experiments on a spin-stabilized spacecraft; 4) test fault tolerant onboard computer and packet telemetry; 5) conduct cis-lunar aerobraking experiments; and 6) detect and measure mass and velocity of micro-meteorite particles. Three follow-on objectives were also added later in the mission: excursion to the L4 and L5 Lagrangian points of the Earth-Moon system, orbit of the Hiten spacecraft around the Moon, and hard landing on the lunar surface. Hiten was named after a flying, music-playing Buddhist angel. Hagoromo was named for the veil worn by Hiten. This mission included Japan's first-ever lunar flyby, lunar orbiter, and lunar surface impact, making Japan only the third nation to achieve each of these goals.
Spacecraft and Subsystems
Hiten was a cylindrically shaped spacecraft, 1.4 m in diameter and 0.8 m high. The small polyhedral-shaped Hagoromo lunar orbiter was mounted on top of the spacecraft. The fully fueled mass of Hiten was 197 kg, this included 42 kg of hydrazine fuel and the 12 kg Hagoromo orbiter. Solar cells on the cylindrical surface of the spacecraft supplied the power requirement of 110 W, backed up by a small onboard battery. The spacecraft was spin-stabilized at 10 - 20.5 rpm.
Spacecraft propulsion and attitude control was provided by eight 23-N and four 3-N hydrazine thrusters, two spin-type Sun aspect sensors, a star scanner, a steerable horizon crossing indicator, three accelerometers, a nutation damper, and control electronics including an onboard processor. An optical navigation subsystem, consisting of two CCD image detectors to detect the Moon and bright stars was also tested.
Communications were accomplished through a medium gain collinear array antenna in both X-band and S-band protruding from the bottom surface of the spacecraft and two cross dipole omni-directional low gain antennas in S-band only, one mounted on the top and one on the bottom. Downlink is via onboard X-band and S-band transmitters, each with two power levels. Two receivers are used for S-band uplink, one connected to the low-gain antennas and the other to the medium gain antenna. Commands were sent from ground stations at 1 kbps. The onboard command computer consists of three independent processor cells with a total of 2 Mbits ROM and 512 Kbits RAM.
Mission Profile
Hiten was launched into highly elliptical Earth orbit on a Mu-3SII-5 rocket from Kagoshima Space Center in Japan at 11:46:00 UT (20:46:00 JST) on 24 January 1990. Injection velocity was 50 m/s less than the nominal value, resulting in an apogee of only 290,000 km compared to the expected 476,000 km. A number of trajectory correction maneuvers were performed and Hiten was put back in a nominal orbit. At 19:37 UT on 18 March 1990 (04:37 19 March JST) as Hiten approached its first lunar flyby, the small Hagoromo spacecraft was released. Although the S-band transmitter aboard Hagoromo had failed on 21 February 1990, the ignition of the Hagoromo deceleration rocket was confirmed by ground observation at 20:04:03 UT, a lunar orbit of 7,400 x 20,000 km with a period of 2.01 days was estimated but never confirmed, so it is unknown if Hagoromo ever went into lunar orbit or escaped into heliocentric orbit. Six seconds later, at 20:04:09 UT (05:04:09 19 March JST) Hiten reached its closest flyby distance to the Moon of 16,472.4 km.
Further manuevers were made to have Hiten simulate the planned trajectory of the future Geotail spacecraft. Hiten completed seven more lunar swingbys by 4 March 1991 and then started the aerobraking portion of its mission. On 19 March at 00:43 UT Hiten flew into the Earth's upper atmosphere at an altitude of 125.5 km over the Pacific at 11.0 km/s. Atmospheric drag lowered the velocity by 1.712 m/s and the apogee altitude by 8665 km. This was the first time aerobraking was used to modify a spacecraft orbit at close to escape velocity. Another aerobraking maneuver was done at 11:36 UT on 30 March at 120 km altitude, reducing velocity by 2.8 m/s and apogee by 14,000 km. This concludied the primary mission and a follow-on mission was started. A ninth lunar swingby was used to increase the apogee to 1,532,000 km. This marked the first use of a low-energy (weak stability boundary) transfer to modify an orbit and the first use of a transfer to the Moon requiring no delta-V for capture. On 2 October 1991 Hiten was temporarily captured by the Moon and then put into a looping orbit which passed through the L4 and L5 stable libration points to look for trapped dust particles. No obvious increase was found. On 15 February 1992 at 13:33 UT (22:33 JST) at a closest approach of 422 km most of Hiten's remaining fuel was used to put it into lunar orbit. The very last fuel was used to have the spacecraft, whose orbit was decaying, crash into the lunar surface on 10 April 1993, although two different times and locations are given in Uesugi, 1996. One is 18:03:25.7 UT (11 April 03:03:25.7 JST)at 34.0 S, 55.3 E, and the other is 18:08:45 UT at 34.3 S, 55.6 E.
Hagoromo Orbiter
The Hagoromo orbiter was a 12 kg, 26-faced polyhedron, 36 cm between opposite faces. A solid propellant (KM-L) retrorocket with a mass of 4 kg was mounted inside the spacecraft for lunar orbit insertion. Sixteen of the surfaces were covered with 1000 sheets of indium-phosphorus solar cells which could generate about 10 W. Two way communications with a ground station were provided by an S-band transponder and an omni-directional cross-dipole antenna mounted on top of the orbiter. No scientific instrumentation was included, only housekeeping data such as temperature was transmitted. The transmitter malfunctioned on 21 February 1990, before the lunar orbit insertion attempt, and no data were transmitted after this time. It is not known whether Hagoromo impacted the Moon or escaped into a heliocentric orbit.
© 2026, Andrew Mirecki
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