Orbital Sciences (ORB) DEFA14A Additional proxy soliciting materials

UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a)

of the Securities Exchange Act of 1934

Filed by the Registrant  þ

Filed by a Party other than the Registrant  o

Check the appropriate box:

o Preliminary Proxy Statement

o Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

o Definitive Proxy Statement

þ Definitive Additional Materials

o Soliciting Material under §240.14a-12

ORBITAL SCIENCES CORPORATION
(Name of Registrant as Specified in Its Charter)
(Name of Person(s) Filing Proxy Statement, if other than the Registrant)
Payment of Filing Fee (Check the appropriate box):
þ	No fee required.
o	Fee computed on table below per Exchange Act Rules 14a-6(i)(1) and 0-11.
	(1)	Title of each class of securities to which transaction applies:
	(2)	Aggregate number of securities to which the transaction applies:
	(3)	Per unit price or other underlying value of the transaction computed pursuant to Exchange Act Rule 0-11 (set forth the amount on which the filing fee is calculated and state how it was determined):
	(4)	Proposed maximum aggregate value of the transaction:
	(5)	Total fee paid:
o	Fee paid previously with preliminary materials.
o	Check box if any part of the fee is offset as provided by Exchange Act Rule 0-11(a)(2) and identify the filing for which the offsetting fee was paid previously.  Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.
	(1)	Amount Previously Paid:
	(2)	Form, Schedule or Registration Statement No.:
	(3)	Filing Party:
	(4)	Date Filed:

*** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Stockholder Meeting to Be Held on April 28, 2011.
ORBITAL SCIENCES CORPORATION You are receiving this communication because you hold shares in Orbital Sciences Corporation and the materials you should review before you cast your vote are now available.  This is not a ballot.  You cannot use this notice to vote these shares. ORBITAL SCIENCES CORPORATION 21839 ATLANTIC BOULEVARD DULLES, VA  20166	Meeting Information
	Meeting Type:		Annual Meeting
	For holders as of:		March 1, 2011
	Date:    April 28, 2011			Time:    9:00 a.m. Eastern Time
	Location:	21839 Atlantic Boulevard Dulles, Virginia 20166
	To obtain directions to attend the meeting and vote in person, please call (703) 406-5000.
	This communication presents only an overview of the more complete proxy materials that are available to you on the Internet.  You may view the company’s Proxy Statement and Annual Report online at www.proxyvote.com. If you want to receive a paper or e-mail copy of these documents, you must request one.  There is NO charge to you for requesting a copy.  Please make your request for a copy as instructed on the reverse side of this notice on or before April 14, 2011 to facilitate timely delivery. We encourage you to access and review all of the important information contained in the proxy materials before voting.
	See the reverse side of this notice to obtain proxy materials and voting  instructions.

— Before You Vote —

How to Access the Proxy Materials

Proxy Materials Available to VIEW or RECEIVE: OUR NOTICE AND PROXY STATEMENT AND ANNUAL REPORT How to View Online:  Have the information that is printed in the boxed marked by the arrow     (located on the following page) and visit: www.proxyvote.com. How to Request and Receive a PAPER or E-MAIL Copy:  If you want to receive a paper or e-mail copy of these documents, you must request one.  There is NO charge for requesting a copy.  Please choose one of the following methods to make your request:
		1) BY INTERNET: 2) BY TELEPHONE: 3) BY E-MAIL*:	www.proxyvote.com 1-800-579-1639 sendmaterial@proxyvote.com
	* If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box
	marked by the arrow    (located on the following page) in the subject line.
Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment
advisor.  Please make your request for a copy as instructed above on or before April 14, 2011 to facilitate timely delivery.

—How To Vote —

Please Choose One of the Following Voting Methods

Vote In Person:If you are a stockholder of record as of the record date, or hold a legal proxy for a stockholder of record, you are invited to attend the annual meeting, and you may vote in person.  At the annual meeting you will need to request a ballot to vote.

Vote By Internet:To vote now by Internet, go to www.proxyvote.com.  Have the information that is printed in the box marked by the arrow   available and follow the instructions.  You may use the Internet to transmit your voting instructions up until 11:59 pm. Eastern Time on Wednesday, April 27, 2011.  

Vote By Mail:You can vote by mail by requesting a paper copy of the materials, which will include a proxy card.

Voting Items
The Board of Directors recommends you vote “FOR” the election of all director nominees:
	1.	To elect four Directors, each to serve for a three-year term ending in 2014.
		Nominees:
		01) 02) 03) 04)	Robert J. Hermann Janice I. Obuchowski Frank L. Salizzoni David W. Thompson
The Board of Directors recommends you vote “FOR” each of the following proposals:
	2.	To ratify the appointment of PricewaterhouseCoopers LLP as the company’s independent registered public accounting firm for the fiscal year ending December 31, 2011.
	3.	To approve, by advisory vote, the compensation paid to our named executive officers, as described in the company’s proxy statement.
The Board of Directors recommends you vote “1 YEAR” on the following proposal:
	4.	To recommend, by advisory vote, the frequency of future advisory votes on our executive compensation.