July 27, 2010

LORRI Looks Back at “Old Friend” Jupiter

In early 2007 New Horizons flew through the Jupiter system, getting a speed-boost from the giant planet's gravity while snapping stunning, close-up images of Jupiter and its largest moons.

Fast forward to 2010 and New Horizons has given us another glimpse of old friend Jupiter, this time from a vantage point more than 16 times the distance between Earth and the Sun, and almost 1000 times as far away as when New Horizons reconnoitered Jupiter. While the planet is too far for the camera to pick up the swirling clouds and brewing, Earth-sized storms it saw just three years ago, "the picture is a dramatic reminder of just how far New Horizons, moving about a million miles a day, has traveled," says mission Principal Investigator Alan Stern, of the Southwest Research Institute.

The photo, one of three taken on June 24, also marked a successful test. Project Scientist Hal Weaver says the pictures were part of an Annual Checkout activity to have the Long Range Reconnaissance Imager (LORRI) "see" objects relatively close to the Sun (from LORRI's point of view). The angle between Jupiter, New Horizons and the Sun - known as the solar elongation angle - was only 17 degrees, and to the camera eye the nearby Sun was about 460 million times brighter than Jupiter.

LORRI Looks Back: New Horizons had an exciting flyby encounter with Jupiter in early 2007, and the spacecraft has been rapidly moving away from the giant planet ever since. The New Horizons team looked back at Jupiter during Annual Checkout (ACO) 4 to test the Long Range Reconnaissance Imager (LORRI)'s ability to image targets close, in angle, to the Sun. This image was taken on June 24, when New Horizons was 16.3 astronomical units (about 1.5 billion miles) from Jupiter, at a spacecraft-Sun-planet angle of only 17 degrees. Looking like Earth's moon at a quarter phase, Jupiter is clearly resolved, with an apparent diameter of nearly 12 LORRI pixels. LORRI also picks up the moons Ganymede and Europa, even though the exposure time was only nine milliseconds and these Galilean satellites are extremely faint in comparison to Jupiter. (Click on the graphic to enlarge.)

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

We wanted to see how much stray sunlight would creep into these Jupiter pictures, especially since we'll make observations of the Pluto system in a similar geometry after the spacecraft passes Pluto in 2015," says Weaver, of the Johns Hopkins University Applied Physics Laboratory. "We generally prefer to look at targets in the opposite direction from the Sun. In fact, LORRI is calibrated for the low light we'll see in the Pluto system and Kuiper belt. Pointing too close to the Sun could damage the camera, but we decided it was safe to try to observe Jupiter. The observations were successfully executed and the images look great. LORRI was even able to resolve Ganymede, Jupiter's largest moon, and Europa, another of the Galilean satellites."

Although the Sun vastly outshines it, Jupiter was still a bright target for LORRI and the deep-space photo session required fast shutter speeds, with exposure times of only 0.009 seconds. That's why the much smaller satellites appear so faint in the LORRI images.

"This haunting image of Jupiter - far in the distance, back in the Sun's warmer climes from where New Horizons came - reminds us of Voyager's family postcard of the planets taken from beyond Neptune's orbit about 20 years ago,” says Stern. “Perhaps after we fly by Pluto in 2015, we'll try something similar from our perch aboard New Horizons."

LORRI also took aim at Neptune and the M7 star cluster in late June. See the images in the New Horizons Science Gallery.

In Tune with Neptune: The New Horizons Long Range Reconnaissance Imager (LORRI) observed Neptune on June 23, as part of a test of the critical optical navigation during the mission's latest annual checkout (ACO-4). In this 100-millisecond exposure, taken when the spacecraft was 23.2 astronomical units (about 2.15 billion miles) from Neptune, the planet appears slightly larger than a star. At the time of this observation, the solar phase angle (the spacecraft-planet-Sun angle) was 34 degrees and the solar elongation angle (planet-spacecraft -Sun angle) was 95 degrees. Only New Horizons can observe Neptune at such large solar phase angles, which can be used to study the light-scattering properties of Neptune's atmosphere. (Click on the graphic to enlarge.)

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Star Treatment: The New Horizons team calibrates the Long Range Reconnaissance Imager (LORRI) by taking pictures of the open star cluster M7. This is the first LORRI image of M7 taken during Annual Checkout (ACO)-4; the 100-millisecond manual exposure was taken on June 25. A preliminary comparison of this image to a 2008 LORRI picture of M7 indicated no degradation or change in LORRI's performance. (Click on the graphic to enlarge.)

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Annual Checkout Winds Down

Speaking of ACO-4: the mission's fourth annual checkout, which started on May 25, wraps up this week. "We packed a lot of activity into nine weeks," says Mission Operations Manager Alice Bowman, of APL. "It was very successful."

Read about ACO-4.

The final activities included making sure the spacecraft's command and data handling system was in working order, and loading new navigation data into the spacecraft's guidance and control system, based on the June 30 trajectory-correction maneuver that refined New Horizons' path to Pluto. The Venetia Burney Student Dust Counter has also been turned on, now that the other six instruments in New Horizons science payload have been shut down. Working from commands transmitted last week to its computers, New Horizons will enter hibernation on Friday (July 30) and remain in electronic slumber until November. Operators at APL will monitor the craft through a weekly status beacon and a monthly transmission of housekeeping data.