Ready for the Earth Gravity Assist!

It is hard to believe that OSIRIS-REx has been in space for over one year. We celebrated the first anniversary of our launch on September 8, 2017. I celebrated this milestone by visiting my hometown Boys and Girls Club in Phoenix, AZ, where I grew up. As the plaque in this image shows, I was inducted into the Boys and Girls Clubs of America Alumni Hall of Fame this year. This honor was unexpected and humbling. I look forward to working with this great organization to continue to inspire the next generation to study science and space!

BGC kids - 1

I celebrated the one-year anniversary with the science superstars at the Rosenzweig branch of the Boys and Girls Clubs of Metropolitan Phoenix

The team has worked very hard during this period to check out the spacecraft and prepare for our first big event of the mission – the Earth Gravity Assist (EGA)! After launch, the spacecraft was on a one-year Earth-to-Earth trajectory. The spacecraft approached to within 0.17 AU of the Sun-Earth L4 Lagrangian point on February 16, 2017. We used this opportunity to perform some science and give the team practice operating the science instruments. On ten days between February 9 and 20, the MapCam imager surveyed the sky for Earth-Trojan asteroids. While no near-Earth asteroids were discovered, the search yielded many known main-belt asteroid detections, demonstrating MapCam sensitivity down to magnitude 13.8.


Outbound Cruise

The outbound cruise phase reaches the mid-point with the EGA on September 22, 2017


During Outbound Cruise, the team also commanded two Trajectory Correction Maneuvers (TCMs) and one Deep Space Maneuver (DSM) to refine the spacecraft trajectory and target the correct EGA point. DSM-1 occurred on 28-Dec-2016 and was the main maneuver to target OSIRIS-REx to the Earth flyby. It changed the spacecraft velocity by 431.2 meters per second (965 miles per hour). TCM-2A occurred on 18-Jan-2017 and imparted a 2.6 meter per second (5.8 miles per hour) velocity change to refine the Earth flyby targeting. TCM-3 took place on 23-Aug-2017 (30 days before Earth flyby) with a 0.45 meter per second (1 mile per hour) velocity change and was the final correction needed for Earth flyby. While two other maneuver options existed in the last 30 days before Earth flyby, TCM-3 was so precise that these additional maneuvers were necessary.


EGA distance

OSIRIS-REx will come closer to Earth than many satellites – but only for a short period of time


The Earth-to-Earth trajectory culminates in our EGA on 22-Sep-2017 16:51:45 UTC (about 12:52 pm EDT). The closest approach distance is 17,237 km (10,711 miles) over Antarctica (latitude = 74.73 degrees South, longitude = 271.94 degrees East). This spot is just south of Cape Horn, the southernmost point of South America. During closest approach, the spacecraft is moving at a speed of over 19,058 miles per hour, relative to the Earth. With the closest approach over Antarctica, ground antennas will be out of contact with the spacecraft for about 60 minutes, until the DSN antenna at Goldstone California regains contact.


EGA speed

The closest that OSIRIS-REx will be over Earth’s equator is 13,340 km (8,289 miles) above the 42,164 km (26,199 miles) altitude that is occupied by Geosynchronous Earth-orbiting satellites. This location is special because geosynchronous satellites stay above the same point on Earth’s surface. The spacecraft comes no closer than 265,457 km (164,947 miles) from the Moon about 10 hours after Earth closest approach, just under two-thirds of the average Earth-Moon distance.


Bennu Orbit

Bennu’s orbit is inclined 6 degrees relative to the Earth


The primary purpose of the gravity assist is to change the spacecraft’s orbit, rotating it six degrees into Bennu’s orbit plane. After EGA, the spacecraft will spend another year traveling to Bennu. The effective velocity change imparted to the spacecraft by the angular momentum exchange with Earth as a result of this flyby is 3.778 kilometers per second (8,451 miles per hour), about 90% more than the 1.986 km/s (4,443 miles per hour) velocity change capability in the spacecraft’s onboard propellant! This velocity change does not mean that the spacecraft either speeds up or slows down by this amount, since much of this change is used change the orbit inclination. Thus, this encounter saves the precious rocket fuel onboard the spacecraft.

EGA diagram

The Earth-Moon flyby during the EGA provides an opportunity to calibrate OCAMS, OVIRS, OTES, and TAGCAMS. The Earth will appear dark as the spacecraft approaches from Earth’s night side and will be sunlit after the flyby. After closest approach, the EGA will be used to exercise the science instruments and the team by collecting images and spectra of the Earth and Moon. These data will be used to verify instrument performance and the alignments between them. Earth observations will begin right after closest approach. Additional Earth observations will occur three, six, and ten days after the EGA. For the MapCam, at a range to the target of 100,000 km, the field of view fits within the Pacific Ocean. For the PolyCam, the field of view is much smaller, subtending 500 km on the Earth. Lunar observation conditions are not as favorable. The Moon will be observed on EGA + 3 days when the spacecraft range to the target is around 272,000 km. However, the low phase angles (i.e., close to noon on the Moon) that are optimum for imaging and spectroscopy do not occur until the range to target approaches 1,000,000 km, a factor of 50–60 greater range than that to the Earth.

EGA poster

The team and people around the world are excited about this event. The science team has gathered in Tucson to see the images and data from the Earth and Moon as soon as we downlink them from the spacecraft. Even before that, astronomers around the world trained their telescopes on the skies. The Large Binocular Telescope Observatory in Southern Arizona captured the first image of the spacecraft – when the spacecraft was a faint 25th magnitude!


LBTO spots OREx

The Large Binocular Telescope Observatory was the first to spot OSIRIS-REx inbound


Stay tuned for the images of the Earth and Moon from this amazing encounter. We hope to release the first images on September 26, 2017!



One comment

  1. […] Life on the Asteroid Frontier Life on the Asteroid Frontier: It is hard to believe that OSIRIS-REx has been in space for over one […]

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