What's up in solar system exploration: August 2015 edition
My summer vacation is over. It's time to tear my gaze away from the beach and look upward at the twenty-something planetary spacecraft exploring targets beyond Earth right now. What's going on up there? As ever, Olaf Frohn's diagram provides a helpful summary:
The innermost solar system is a little thin right now. With MESSENGER's demise earlier this year, no active mission is exploring either Mercury or Venus. JAXA's Akatsuki is on the way, hoping to enter orbit in December. Recall that Akatsuki lost its main engine during its first orbit insertion attempt, and has since vented all the fuel that that engine would have used in order to reduce its mass, making it possible for the spacecraft to return to Venus and attempt a second orbit insertion using only the attitude control jets. They posted some happy news last week: on August 4, Akatsuki performed a crucial deep-space course correction maneuver successfully. The maneuver placed Akatsuki on a course that will optimize its ability to perform science if it enters orbit at Venus as hoped.
I don't generally mention the Sun-observing missions in my What's Up updates because I'm biased toward planets. I don't usually mention Earth observing ones because there are too many to keep track of. But I have to make an exception for DSCOVR, which is technically an Earth/Sun mission but proved last week that it's more than capable of making observations of the Moon as well. Wow.
The animation is cool but it's also well worth looking at the still version in order to study its features better. What a beautiful view of the lunar farside! The color is lovely. Note the subtle darkening that marks the south pole-Atiken basin, one of the solar system's largest impact structures.
Mars remains the most active spot beyond Earth in the solar system. This week, Mars Reconnaissance Orbiter reaches its 10th anniversary of service in space, but it's far from the oldest spacecraft in orbit at Mars; Mars Express and Mars Odyssey are still at work up there. Mars Orbiter Mission has ventured into an extended mission and is still returning photos, though apparently none of the full-disk images in a variety of phases that I had hoped for from its 4-Megapixel color camera. Even Mars' newest resident, MAVEN, is three-quarters of the way through its one-year primary science mission, which began on November 16, 2014. MAVEN's mission will undoubtedly be extended long beyond that, as it will be needed to support surface missions if and when Odyssey and Mars Reconnaissance Orbiter eventually fail.
Both Opportunity and Curiosity have been very active lately. Opportunity has finally reached Marathon Valley, a site identified from orbit to have signs of clay chemistry. The team is excited about the science prospects even though the rover's memory problems persist. A. J. S. Rayl has all the details on Opportunity's status in her latest MER Update. Enjoy this recent late-afternoon view from Marathon Valley:
On the other side of Mars, Curiosity has finally been permitted to drill again after concerns about a short circuit in its drill vibration mechanism, and has taken a bite out of Buckskin. I'll have much more detail to report on Curiosity's recent activities later this week. But I can't wait to share this photo, a new self-portrait. Curiosity takes self-portraits -- time permitting -- to document drill locations. But they developed a new, lower angle for this one -- a Sojourner's-eye view of Curiosity.
Cassini's recent activities have included lots of encounters with icy moons. A particularly close one is coming up in a week, when it will zoom past Dione at an elevation of only 474 kilometers. Although the camera will be imaging, the primary focus of the Dione flyby is gravity science; as Jason Perry explains in his Cassini Looking Ahead article for Rev 200, the science team is interested in determining whether Dione's relatively dense interior is differentiated into an icy mantle or rocky core, or not. Cassini will follow the Dione flyby with more distant imaging of Tethys and Enceladus. Later on this year, there will be three close Enceladus flybys: two in October, and one in December. These flybys are all exciting, but they're all bittersweet; increasingly, the mission is doing its last close flyby of target X, Y, or Z. The spacecraft will shift its orbit to an inclined one -- making close flybys of moons rare -- in January. The mission will come to an end in about two years, on September 15, 2017.
There are no spacecraft exploring or cruising to Uranus and Neptune, so next we skip on out to New Horizons, which is (as of the moment that I write this text) nearly 33 million kilometers beyond Pluto, a distance that increases at pretty much the same rate that it did before the flyby -- about 1.2 million kilometers a day. While I was on vacation, only a few new photos appeared on the raw image website, the first five pictures from Departure Phase 1. That's because we're in a planned image drought period, when New Horizons is transmitting the so-called "low-speed" data set and all the spacecraft telemetry. According to what I was told before the Pluto flyby, image transmission will not begin again until September 15. Images that arrive before close of business on September 15 will be released on September 18; anything that comes in the week after that will be released September 25. However, it was also hinted to me that New Horizons might possibly be commanded to return an image or two between now and then. We can hope, but don't expect anything until late September.
Which brings us to the last, distant travelers, the Voyagers. They're still traveling, roughly three times farther from the Sun than New Horizons is, well beyond the outer edge of the Kuiper belt. To answer a frequently-asked question, the two Voyagers will always and forever be farther from the Sun than New Horizons, even though all three are on escape trajectories and lightweight New Horizons left Earth faster. The Voyagers gained more velocity from their multiple giant-planet gravity-assist flybys than New Horizons, which only received one boost, from Jupiter. (Pluto is too small to have affected New Horizons' velocity substantially.) In fact, it's not clear that New Horizons will still be communicating with Earth by the time it reaches the region of space that the Voyagers are now exploring, the boundary between the heliosphere and the interstellar medium. But there is a lot to learn from New Horizons as it probes the region between Pluto and its eventual last-contact point, wherever that might be.
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