New Horizons • AP2 Finished

Approach Phase 2, which began when New Horizons was 100 days from Pluto, is just finished.  Some LORRI images from Apr 12-18, showing a full orbit of Charon around Pluto and one revolution of Pluto (like the Earth’s Moon, Charon keeps the same face towards Pluto as it orbits), also reveal possible surface features on Pluto, in particular a possible light colored polar region.

Between April 25 and May 1, at a range of 90 million Kilometers, New Horizons spacecraft photographed Kerberos and Styx, the smallest and faintest of Pluto's five known moons, for the first time.  Following the spacecraft's detection of Pluto's giant moon Charon in July 2013, and Pluto's smaller moons Hydra and Nix in July 2014 and January 2015, respectively, New Horizons can now see of all the known members of the Pluto system.

Styx (green), Nix (yellow), Kerberos (orange) and Hydra (red)

Drawing even closer to Pluto in mid-May, New Horizons will begin its first search for new moons or rings that might threaten the spacecraft on its passage through the Pluto system.

Kerberos and Styx were discovered in 2011 and 2012, respectively, by New Horizons team members using the Hubble Space Telescope. Styx, circling Pluto every 20 days between the orbits of Charon and Nix, is likely just 4 to 13 miles (approximately 7 to 21 kilometers) in diameter, and Kerberos, orbiting between Nix and Hydra with a 32-day period, is just 6 to 20 miles (approximately 10 to 30 kilometers) in diameter. Each is 20 to 30 times fainter than Nix and Hydra.

By the end of AP2, it’s possible that the LORRI camera will reach a point called “BTH” when it will be close enough to take better images of the Pluto system than the Hubble Space Telescope can from Earth orbit.

New Horizons has two cameras. The Long-Range Reconnaissance Imager (LORRI) has a field of view of about one quarter of  degree and takes black-and-white ("panchromatic") images. The Ralph Multispectral Visible Imaging Camera (MVIC) has a field of view nearly six degrees wide and takes both panchromatic and color images. It has panchromatic, near-IR, red, blue, and methane filters, but no green filter.

On May 15, New Horizons will stop its continuous observation of the Pluto system for the second time this year and, again, turn its large antenna toward Earth for 15 days to transmit all the data in its solid state recorders.  Once the recorders are emptied, 47 days before reaching Pluto, Approach Phase 3 will begin, on May 28.  More instruments are being turned on, the imaging campaign shifts from navigation and long distance studies of color and brightness to obtaining higher and higher resolution images of the surface features of Pluto and Charon.

New Horizons • Start of AP2

The scientific goals of the New Horizons mission were established before it was built and a decade before it was launched.  The specific objectives placed demands and constraints on the spacecraft's activity at Pluto, and the development of the timeline for events before, during and after at New Horizons' encounter at Pluto was agreed upon a long time ago.

In the 150 days of the approach, Pluto and Charon would be imaged, every 12 hours for about six and a half days, each time the image resolution improved by 50% (at 150 [Approach Phase 1], 100 [Approach Phase 2], 66, 44, 28, 19, 12 and 6 days before Pluto closest approach. This sequence would provide twelve images, 30 degrees, over one revolution of the Pluto/Charon system.  Early approach imaging will record one complete 30 day orbit of Hydra; imaging another complete orbit will start about 100 days out.

These observations will address time variability, provide airglow spectra, obtain a series of maps at increasingly higher resolution maps, and refine the orbits and (hence the masses) of Pluto, Charon, Nix, and Hydra.  The returned imagery will also be searched for small satellites that had previously escaped detection from Earth. The final set of observations, starting at 6 days out, will only cover half a rotation of Pluto and Charon, and will give the highest spatial resolution images and spectra of the portions of the surface away from the closest-approach hemisphere (including the far-side hemisphere at 3.2 days before closest approach). 

When Approach Phase 1 was done, New Horizons performed a 90 second thruster firing for a trajectory correction maneuver, to slow the spacecraft’s velocity by just 1.14 meters per second.  This shifted its July 14 arrival time by 14 minutes and 30 seconds, back on schedule; and shifted the course “sideways” (looking from Earth) by 3,442 kilometers, sending the spacecraft toward a desired flyby close-approach target point. New Horizons then turned its antenna to Earth to download all the data it has gathered during AP1.

Observations resumed on April 5th more intensive Approach Phase 2, with a repeat of the imaging of one Pluto/Charon revolution and one Hydra orbit.  AP2 adds numerous new and significant observations of the Pluto system, including the first color and spectral observations of Pluto and its moons, and series of long-exposure images that will help the team spot additional moons or rings in the Pluto system. The spacecraft will also make its first ultraviolet observation to study the surface and atmosphere of Pluto and the surface of Pluto's largest moon, Charon, and the spacecraft will conduct a major test of flyby radio science observations. These various activities are critical to developing a fuller picture of that system, and in assessing any hazards New Horizons could face as the spacecraft passes between Pluto and Charon. 

Although New Horizons cannot yet match Hubble's resolution on the Pluto system, New Horizons can now do what Hubble can't: perform continuous observations. By measuring the positions of Nix and Hydra with respect to background stars in these images, scientists will be able to predict the future paths of the moons with much greater precision, and navigators will be able to target New Horizons with less uncertainty. New Horizons' lower resolution, color camera will also start imaging as a backup for navigation in case LORRI fails.  At the end of the forty days of AP2, the spacecraft will again cease observing Pluto, turning to Earth in a spin-stabilized mode to completely empty its data recorders in preparation for the flyby. The team is evaluating new tracking data to decide whether they'll need to carry out a course correction on May 15; a decision is expected about May 1.

New Horizons • AP1

Approach Phase 1 continues as expected, and will last another few days. Starting with one week of OpNav imaging of Pluto and Charon (enough time observe one complete orbit of Charon around Pluto) from January 25 to February 1, and transitioning to imaging with higher sensitivity began with the goal of imaging the next two brightest of Pluto moons, Nix and Hydra.

85 years after the discovery of Pluto, New Horizons to took its first photographs of Nix (red box) and its clearest images of Hydra (yellow box) looking like faint stars in the starry background, faint against the bright, overexposed light of Pluto and Charon.

Images in this series will taken, one every two days till March 6, to observe one full 38 day orbit of Hydra around Pluto.  This will be the end of OpNav 2, and New Horizons will turn its antenna towards the Earth for a long period of data downlink.  Based on the first navigational data returned, New Horizons may adjust its path towards Pluto slightly on March 10.

New Horizons Approach Phases

In mid-January the New Horizons spacecraft entered its scientific data gathering phase. This “encounter” period will last till January 2016, and is divided into three approach phases (AP1, AP2 and AP3), the near Pluto encounter (NEP), and three departure phases (DP1, DP2 and DP3). This post summarizes the approach phases so that my postings here, during the approach, will have a context; I’ll describe the near Pluto encounter and departure phases later.

AP1 started in mid-January 2015, 180 days before New Horizons’ closest approach to Pluto. It is focused mostly on navigation, though two plasma instruments will be measuring the ambient plasma to characterize any differences near Pluto. The first AP1 optical navigation (OpNav) images were taken on January 25th and 27th, and released to public view on Clyde Tombaugh’s birthday, February 4th. These images taken by the Long Range Reconnaissance Imager (LORRI), a high resolution telescope, a range of 200 million Kms.

In July 2014, New Horizons executed its first seven day OpNav campaign. The AP1 OpNav campaign will start by repeating this observation, then continue observations every other day for five more weeks, the four brightest objects in the Pluto system (Pluto, Charon, Nix, and Hydra) will be well separated from one another in LORRI images allowing their orbits to be improved and any variations in brightness to be observed. Pluto will be be barely resolved (about 3 pixels in diameter) by the end of this, the second, OpNav campaign.

For the rest of AP1, New Horizons will change its mode of operation. The imaging requires LORRI to be pointing at Pluto which significantly reduces the ability to communicate to home so, for the last month of AP1, New Horizons will turn to point its large antenna towards Earth, spin slowly to maintain that pointing, and use two transmitters at full power to dump the accumulated navigational data to the ground.

AP2 starts on April 4th, 100 days before closest approach, resuming the plasma measurements and starting the third OpNav campaign. By April 25th, Pluto will be five pixels in diameter in LORRI images, and a second camera, with a quarter the resolution of LORRI but imaging in color, will join the campaign. Sometime in the first week of May, New Horizons will pass a major milestone called “BTH” when Pluto reaches eight pixel in LORRI images; from here on LORRI’s images of Pluto will better than Hubble!

The Pluto system is full of surprises; five moons were not expected. Like the Earth, Pluto has a large moon, and large moons tend to clear anything smaller out of orbit. The four little moons were all discovered in the last ten years, after New Horizons’ launch, right at the limit of visibility from Earth. Concern has grown that there may be more small moons or a ring system around Pluto. When New Horizons can see the Pluto system better than Hubble, it will start searching for previously unseen objects, so that they can be added to the observation plan for flyby, and the spacecraft trajectory can be adjusted, if necessary, to avoid a collision bringing a sudden end to the mission.

AP3 starts 21 days before closest approach. Imaging goals are no longer related to navigation; New Horizons will now focus on getting the best global maps of Pluto and Charon during the three planetary rotations before closest approach. The plasma instruments may detect pickup ions and bow shock. Other instruments will start to look for variability in IR and UV radiation. Imaging will search for, and track, any clouds or hazes and other atmospheric features. At 3.2 days out, long-range imaging will include 40Km mapping of Pluto and Charon. This is half the rotation period of Pluto-Charon and will allow imaging of the side of both bodies that will be facing away from the spacecraft at closest approach. Coverage will repeat twice per day, to search for changes due to snows or cryo-vulcanism. Perhaps most importantly, this phase allows global maps of Pluto and Charon at longitudes other than those seen near closest approach.

There will be particularly photogenic opportunities during AP3, such as images taken of the entire system just filling a LORRI field of view.

New Horizons • Mission Plan

Designing a spacecraft and mission plan to explore the vicinity of Pluto is made difficult by many factors. Not least of these is the time it will take to get there .. without playing orbital tricks and stealing energy from other objects orbiting the Sun, getting to Pluto would take nearly twenty years.

Interplanetary flights, until the last few years, have relied on giving a spacecraft a speed boost as it leaves the the Earth. That extra velocity allows the spacecraft's path to diverge from the Earth’s, either drifting outward to Mars, Jupiter, Saturn, etc, or drifting inwards to Venus or Mercury.  It is surprising, perhaps, that getting an object to Mercury requires extraordinary measures to slow it down enough to get there. But New Horizons had the other problem, gaining energy to raise its orbit so it would reach forty times further from the Sun than the Earth, and still be traveling fast when it got there. In fact, New Horizons, like the long-range Pioneer and Voyager spacecraft before it, will never return.

Even with the most powerful rocket available, the trip to Pluto would take too for long for electronic and mechanical systems to remain reliable by the time Pluto was close enough to observe, but celestial mechanics give us a gift of time. If launched at the highest possible velocity in the last two weeks of January 2006, New Horizons could pass Jupiter just in time to use that planet's massive gravity to accelerate it to a speed that would hurl it to Pluto's orbit in about nine years. That favorable alignment happens every year if all you want to do is travel 35-45AU in a decade; if you want that trajectory to put you there when Pluto is nearby, that chance comes only a very few times every 250 years. The next "launch window" opened in early February 2007, but that delay would add three years to the flight time.

So on January 19, 2006, New Horizons was launched on an augmented Atlas-Centaur three stage rocket (NASA's most powerful at the time), and was the fastest spacecraft ever to leave the Earth. It passed the distance of the Moon in a nine hours (most flights to the Moon last more than three days), and it reached Jupiter in thirteen months.

The science instruments, cameras, fields and particle detectors, nine instruments in all, are the reason to fly, but the engineering systems to deliver them to the right place, power them, point them in the right direction, keep them warm, and transmit their data back to the home plant are pretty critical too. Spacecraft design and construction is a constant well-managed battle between science and engineering! New Horizons weighed about 480Kg at launch; about 30Kg was science instruments.

Fuel for course correction and pointing accounts for about 75Kg of that; It carries a 210cm dish antenna on its back and, since it will be too far from the Sun for solar cells to work, it also carries a 200watt radioisotope thermoelectric generator (RTG). At Pluto its fastest data transmission rate will be 700 bits/second, and it will take nine months to dump the entire "Pluto Encounter" dataset back to Earth.

Once past Jupiter, New Horizons was placed in 'hibernation mode' which it mostly remained in (woken every year for a system check) till three weeks ago when it was woken to start preparations for Pluto. Pluto "far encounter" starts on January 15, when distant imaging of Pluto against the starry background (for navigation), and other measurements of the interplanetary environment are begun.

New Horizons • Pluto

2015 will be a special year for interplanetary exploration. By the end of the 1970's, humans had sent spacecraft to every planet in the Sun's family, except one.  Missions to orbit Mercury, Venus, Mars, Jupiter and Saturn, and to land four rovers on Mars, have followed. More recently, many of the lesser objects in the solar system have been studied up close: asteroids, the moons of Jupiter and Saturn, and even comets.

In 1930, Pluto was discovered by Clyde Tombaugh, working at the Lowell Observatory in Flagstaff, Arizona. Much smaller than the Earth's Moon, Pluto was found nearly one and a half billion Kilometers beyond Neptune. Pluto has always been been an emigma, its orbit dips inside Neptune's for part of its year and is tipped out of the plane of the other planets, its polar axis is tilted by nearly 90 degrees, its biggest moon, Charon, is about the same size as it is.

In fact, Pluto isn't a planet like the others, it's a member of the Kuiper Belt of objects which has strayed down into the outer regime of the regular planets. The Kuiper Belt represents a cloud of material that never joined in the serious planet making beginnings of the solar system .. some clumping of rocky and icy material happened, but probably none of the violence of melting, bombardment, destruction and collisions that created the inner planets, or the immense gravitational influences that pulled Jupiter, Saturn, Uranus and Neptune into gaseous giants. The Kuiper Belt has been called the dust bunny collection of the solar system.

Following the discovery of more Kuiper Belt objects (KBO's) by Hubble and other powerful telescopes, Pluto joined the new designation of 'dwarf planets' in 2006. Arguments revolve about this reclassification but Pluto is now in a class containing more named objects than there are classical planets (and there are hundreds more observed candidates for the class awaiting confirmation of their orbits and naming).

But Pluto remains unexplored, and enigmatic, to this day. Telescopic instruments have improved since Tombaugh's day; but even the Hubble Space Telescope can magnify the faint point of light that is Pluto barely enough to image it as a fuzzy disk. The HST has also observed four small moons (Nix, Hydra, Kerberos and Styx) in addition to Charon. Pluto is rocky and has a thin atmosphere (mostly nitrogen) that freezes, falling like snow, for the Plutonian 'winter'.

[Pluto takes about 250 years to orbit the Sun. It swings in to about 30AU (closer than Neptune) for some of that time getting close enough to heat its atmosphere into a gas, then back out to about 50AU for a century and a half cold spell. This extreme orbital swing has more impact on Pluto's temperature that its seasons .. because its rotational axis is so tilted, Pluto's "arctic circle" is, essentially, its equator!  Pluto's south pole saw the sun for the first time in 120 years in 1987. Just to completely remove it from your list of vacation spots, the temperature of the surface is 35-55 degree above absolute zero.] -- 1AU (Astronomical Unit) is the distance from the Sun to the Earth ~ 150 million Km.

Planning for a mission to Pluto began in 1989, and the New Horizons spacecraft was launched in January 2006. It will fly though Pluto's space (about 10,000Km above its surface) on July 14, 2015. Recently, after a long search, another KBO was found by Hubble, far beyond Pluto, but reachable, with little expenditure of fuel, by New Horizons in 2019.