Date: June 6, 2011

Title: An Unexplored World: Dawn Approaches Vesta

Podcaster: Emily Lakdawalla

Links: The Planetary Society Blog – http://www.planetary.org/blog/

Description: An unexplored world: Dawn approaches Vesta Next month, the ion-powered Dawn spacecraft will enter orbit around Vesta, the second or third largest member of the asteroid belt. Vesta represents an unexplored class of worlds in the solar system, a partially assembled planet that may have been smashed apart before it had a chance to grow. I’ll preview what we know about Vesta, and what the Dawn mission hopes to find out.

Bio: Emily Lakdawalla is a planetary geologist and writer who works for the world’s largest space interest group, The Planetary Society, as its blogger, web writer, and contributor to the weekly Planetary Radio podcast. She is also a contributing editor for Sky & Telescope magazine. She lives in Los Angeles with a 3-year-old who can list all the planets for you, a new baby who has yet to learn their names, and a husband who likes to pretend he doesn’t know anything about space.

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Transcript:

Hi, I’m Emily Lakdawalla of The Planetary Society, and I’m here to tell you about what I think is the most exciting thing that’s happening in the solar system this summer. It officially began last month, when Dawn returned its first image of the asteroid Vesta. It’s hard to convey just how excited I was last month to see that picture. The image itself isn’t particularly amazing. It’s just bright dot among some stars, a few pixels across. Dawn’s images of Vesta aren’t yet as detailed as what we’ve seen from Hubble. But what this image means! This isn’t just the first sighting of Dawn’s destination; it’s the first sighting of a whole new kind of world that we’ve never seen before. We’ve seen small asteroids but we’ve never visited the great round worlds of the asteroid belt. Over the next few months, as Dawn approaches Vesta, it’ll slowly come in to focus before our eyes, and we’ll get to see a new world, a new kind of world, for the first time. I can’t wait!!

Dawn launched into space on September 27, 2007, and when its mission is over it’ll be the first spacecraft to have orbited two different bodies. It’s approaching the second largest asteroid, Vesta, and after its Vesta mission it’ll go on to the biggest asteroid, Ceres, in 2015. Dawn will study Vesta and Ceres with three main science instruments. It has a color camera, which it’ll use to create photo maps. It has an infrared spectrometer, which will map the infrared colors of the surface and help to figure out the surface mineral composition. And it has a gamma ray and neutron detector, which will detect the elemental composition. Plus the whole spacecraft can be used to measure the mass and gravity field of Vesta and Ceres through precise radio tracking.

Now that Dawn has finally begun its science mission at Vesta, I thought it’d be useful to explain the plans for its mission there. The Vesta Phase of the Dawn mission will last fifteen Earth months or about a third of a Vestian year. The Vesta Phase is divided into five sub-phases.

The Approach Phase began on May 3, 2011, with Dawn still about 1.2 million kilometers away from Vesta. This part will last about three months. Dawn isn’t yet in Vesta orbit. The principal activity of the Approach Phase is to continue thrusting with the ion engines in order to nudge gently into orbit, and to gather data that will help with spacecraft navigation and trajectory planning.

During this time Dawn will take lots of images of Vesta for optical navigation. It’ll measure Vesta’s position against the field of background stars and use that to update its knowledge of where exactly Vesta is. There will be a total of 24 optical navigation sessions. During eight of these sessions, the spectrometer will also take data, mostly to test out its settings and make sure exposure times and so forth are optimal. Next week, the second week of June, it’ll finally get better images of Vesta than Hubble ever has, revealing new detail for the first time.

As Dawn gets closer to Vesta it’ll perform three “rotation characterizations.” This is fancy scientist speak for “getting pictures of the whole globe.” It’ll take images throughout a Vestian day as the world spins in Dawn’s view, to see all longitudes of the asteroid. One Vesta day lasts about 5 hours and 20 minutes. At the time of Dawn’s approach, it will be winter in Vesta’s northern hemisphere, so some of the northern polar regions will not get any sunlight and therefore will not be visible to the camera or spectrometer.

During the second rotation characterization, Dawn will also point away from Vesta and systematically cover the nearby sky in order to search for moons. Lots of asteroids have little satellites, so it’s not unreasonable that Vesta would, but searches from Earth have so far come up empty.

Radio tracking during this period will help refine estimates of Vesta’s mass, a parameter that’s crucially important in the planning of Dawn’s orbital mission. After all, how fast Dawn orbits Vesta depends on the gravitational pull of Vesta, which depends upon its mass. The mission has been planned out using a reasonable range of possible masses; determining the mass more definitely might change the dates of later events and might also change the altitude of Vesta’s planned orbit.

On July 16, Dawn will be close enough to Vesta to be captured into orbit. However, “capture” is probably too strong a term. Dawn’s been creeping up on Vesta for years, and it won’t really even notice that it’s orbiting Vesta until another week or two ion engine work lowers its orbit further.

The second Vesta mission phase begins. Survey Orbit will begin when Dawn has reached its planned survey altitude of 2,700 kilometers, which will be on August 8 at the earliest, August 11 at the latest. Dawn will be in a near-polar orbit where it’ll take two and a half to three days to complete each orbit. That means Vesta will rotate under the spacecraft and present all longitudes in sunlight several times on each orbit. This is useful for mapping. The camera will map the whole surface except the dark bits of the north pole about 250 meters per pixel, and the spectrometer about 700 meters per pixel. Since the cameras can only be used on the dayside leg of the orbit, the basic plan is to acquire data on the dayside leg and relay it to Earth on the nightside leg, though there will be a few dayside communications sessions.

Dawn will complete seven of these survey orbits over about 20 days. Then it will continue thrusting to lower its orbit over a period of approximately a month. The rest of the dates in this summary are very approximate.

Once Dawn has reached an altitude of 660 kilometers, somewhen in the vicinity of October 1, it will go into the third phase, High Altitude Mapping Orbit. This orbit has a 12-hour period; combined with Vesta’s 5 hour and 20 minute rotation period means that Dawn will see all of Vesta’s surface after about 10 orbits or five Earth days. The primary goal of the high altitude mapping orbit is what it says — surface mapping with the camera and spectrometer. For 20 orbits Dawn will be pointed straight down in order to map the whole surface; two complete 10-orbit cycles provide redundancy to fill any gaps in coverage that might arise from glitches in data acquisition, transmission, or reception. Then there will be forty more orbits in which Dawn will rotate away from pointing straight down, getting slanting views. When combined with the straight-down views, they’ll yield information on Vesta’s topography, and more importantly for me, will allow us to simulate views of Vesta’s surface in 3D. Then Dawn will again fire its thrusters to reduce its altitude. It will probably take six to eight weeks to arrive at the next phase.

When Dawn has reached an altitude of 180 kilometers and an orbital period of four hours, sometime around or after the end of November, it will have arrived at Low Altitude Mapping Orbit. One main science goal during this period is to use the gamma ray and neutron spectrometer, which isn’t very effective at higher altitudes. Unlike the camera and infrared spectrometer, the gamma ray and neutron spectrometer operates equally well on dayside and nightside, and it will gather data continuously for two months, accumulating better and better counts of flying gamma rays and neutrons to improve the quality of its map.

The other major goal of low-altitude mapping orbit is to determine Vesta’s gravity field through radio science, which will give cluesabout the distribution of mass in Vesta’s interior. The camera and spectrometer will also be used sometimes. Then Dawn will start raising its orbit again.

Dawn will stop thrusting once it has reached the same orbital altitude it had in high-altitude mapping orbit, for a period called, imaginatively, high altitude mapping orbit two. This will be in about a year, in June 2012, which is eight months after the first high-altitude mapping orbit. Eight months is most of a Vesta season, so the north polar terrain that was invisible in winter darkness before will finally be sunlit, allowing Dawn to fill in the north polar parts of the photo maps. Finally, in July of next year, it will depart Vesta for Ceres, which it will reach around February 2015. If 365 Days of Astronomy is still running then, you can bet I’ll tell you about the plans for the Ceres phase of Dawn’s mission! In the meantime, I’ll be posting lots of Dawn images of Vesta at planetary dot org slash blog. This has been Emily Lakdawalla for the Planetary Society. Thanks for listening!

End of podcast:

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